Lr` ["`£Ci - ERI Library - Northern Arizona University

SOIL SEED BANK IN SOUTHWESTERN PONDEROSA PINE:
IMPLICATIONS FOR ECOLOGICAL RESTORATION
By Judith D. Springer
A Thesis
Submitted in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
in Forestry
Northern Arizona University
May 1999
W. Wallace Covington, Ph.D. '
_ )~/Lr' ["' £Ci~{~
L~ura E. DeWald, Ph.D.
/~ ~
Thomas E. Kolb, Ph .D.
~~
ABSTRACT
SOIL SEED BANK IN SOUTHWESTERN PONDEROSA PINE:
IMPLICATIONS FOR ECOLOGICAL RESTORATION
.nJDITH D. SPRINGER
The objectives of my study were to: 1) obtain a baseline estimate of
the plant species in the soil seed bank in a southwestern ponderosa pine
forest prior to ecological restoration treatments (such as tree thinning and
prescribed burning) ; 2) determine the relationship between soil seed bank
species and the aboveground vegetation; and 3) determine species
composition and seed density in the forest floor organic matter (0
horizon) versus mineral soil.
Results from the baseline study indicate that overstory canopy type
has a significant influence on the species in the soil seed bank. In general,
annual and biennial species are found in greater numbers in the soil seed
bank than perennials. From a total of 38 species, the most common
species in the soil seed bank were Verbascum thapsus, Leonurus cardiaca,
and Conyza canadensis. Six species were non-native and few perennial
grasses emerged in seed emergence trials.
ii
In an area undergoing ecological restoration, 14 species emerged in
seed emergence trials from seed bank samples collected after overstory
trees were thinned and prior to prescribed burning, with an estimated seed
density of3,152 seeds/mi. The most common species were Collinsia
parviflora and Verbascum thapsus, which accounted for 45% and 30% of
the germinants, respectively.
Fifteen species were observed in the aboveground vegetation at this
same site after thinning. Of these, Collinsia parviflora was the most
common species. Although Verbascum thapsus accounted for 30% of the
viable seeds in the seed bank, it accounted for only 3% of the plants in the
aboveground vegetation. There were approximately 101 plants/m' in the
thinned area, but only 0.4 plants/m' in the nearby unthinned control.
There was a significant correlation between aboveground vegetation and
the seed bank in the thinned area, but not in the control.
There was a significantly larger number of seeds in mineral soil than
in the 0 horizon. However, a significantly greater number of non-native
species occupied the 0 horizon versus the mineral soil. There was a
significantly greater number of viable seeds at the 0-5 em depth compared
to the 5-10 em depth (including both organic matter and mineral soil).
III
ACKNOWLEDGEMENTS
I would like to thank my major professor Dr. Margaret M. Moore
for her ideas, understanding, support, and sense of humor. I would also
like to thank my other committee members Dr. W. Wallace Covington,
Dr. Laura E. DeWald and Dr. Thomas E. Kolb for their insight, assistance
with new ideas, and review of this thesis. Special thanks are warranted for
Dr. Pete Fule, Amy Waltz, Doc Smith, Gina Vance, Becky Kerns, Tom
Heinlein, Brad Blake, Rebecca Sayers, and Dr. Mike Kearsley for fielding
my questions, offering advice, and helping me navigate around various
hurdles; for Dr. Tina Ayers, Dr. H. David Hammond, Dr. Randy Scott,
Nancy Brian, Lee Hughes, and John Anderson for assistance with plant
identification; and for Greg Taylor and the other members of the BLM
prescribed burn crew.
I would also like to acknowledge the student workers, staff and
graduate students at the NAU School of Forestry Ecological Restoration
ProgramlEcology Lab who assisted with plot establishment, data
collection, greenhouse weeding and watering, and other tasks related to
my projects. They include Julie Blake, Jason Brooks, Codey Carter,
Cheryl Casey, Walker Chancellor, Janelle Clark, Liza Comita, Marcy
DeMillion, Mike Elson, Brandon Harper, Barbara Kent, Julie Korb,
IV
Lauren Labate , Tammy Lesh, Lisa Machina, Ted Ojeda, John Paul
Roccaforte, Scott Schaff, Stacey Sprecher, Mike Stoddard, Dave
Vanderzanden, Alex Viktora, Patty West, and Margot Wilkinson-Kaye.
Funding for this study was provided by McIntire-Stennis grant
ARZZ-NAU-MS53 . The BLM Arizona Strip District also provided
funding for the overall restoration project at Mt. Trumbull.
Finally, I would like to thank my husband Abe for his unflagging
encouragement and support and also for his vigilance in watering the
plants in my greenhouse study while I was away on fieldwork.
v
T ABLE OF CONTENTS
ii
ABSTRACT
ACKNOWLEDGEMENTS
.iv
vi
TABLE OF CONTENTS
LIST OF TABLES
viii
LIST OF FIGURES
x
DEDICATION
xi
PREFACE
xii
CHAPTER 1. INTRODUCTION
1
CHAPTER 2. COMPREHENSIVE LITERATURE REVIEW
4
CHAPTER 3. MANUSCRIPT: SOIL SEED BANK IN FIVE CANOPY
TYPES AT Mr. TRUMBULL, MORAVE COUNTY, AZ.
Abstract
23
Introduction
24
Study Area
26
Methods
29
Results
, .. ,
34
Discussion
44
Conclusion
50
CHAPTER 4. MANUSCRIPT: EFFECTS OF TREE TIllNNING ON
THE SOIL SEED BANK IN SOUTHWESTERN PONDEROSA PINE
Abstract
51
Introduction
52
VI
Study Area
54
Methods
57
Results
61
Discussion
73
Conclusion
81
CHAPTER 5. CONCLUSIONS AND MANAGEMENT
IMPLICATIONS
83
LITERATURECITED
87
APPENDIX A: FLORA IN THE VICINITY OF MT. TRUMBULL,
MOHAVE COUNTY, ARIZONA
96
APPENDIX B: BIOGRAPHICAL STATEMENT
vii
102
LIST OF TABLES
Table
Page
2.1 Number of seeds and the predominant species present in the seed
pools of various canopy types
6
3.1
Ranges of litter depth in five canopy types at Mt. Trumbull
31
3.2
Total number of viable seeds in the soil seed bank from samples
collected in five canopy types at Mt. Trumbull
35
3.3
Viable seeds/rrr in the soil seed bank from samples collected from
37
five canopy types at Mt. Trumbull
3.4
A comparison ofbetween-subjects effects of canopy type on five plant
functional groups and families at Mt. Trumbull, AZ
39
3.5
Total number of seedlings that emerged from organic and mineral soil
samples in five canopy types at Mt. Trumbull
40
3.6
Number of seedlings of each species that emerged from mineral soil
samples collected from five canopy types at Mt. Trumbull
41
3.7
Number of seedlings of each species that emerged from 0 horizon
samples collected from five canopy types at Mt. Trumbull
42
4.1
Number of viable seeds that emerged from samples collected at Mt.
Trumbull, September 1996, after thinning and prior to burning ....62
4.2
Estimated seed density/rrr' of species that emerged from seed bank
samples collected at Mt. Trumbull, September 1996, after thinning
and prior to burning
62
4.3
Species observed growing aboveground in three different treatments
at Mt. Trumbull, 1997
64
4.4
Number of plants of each species/rrr observed in the aboveground
vegetation at Mt. Trumbull, 1997. Averaged over 15 plots
66
Vlll
4.5
Relationship of species found in the soil seed bank at Mt. Trumbull
after thinning and in the aboveground vegetation in the following
growing season
67
4.6
Species observed growing aboveground in 3 different treatments at
Mt. Trumbull, 1998
69
4.7
Number of plants of each species/m' observed in the aboveground
vegetation at Mt. Trumbull, 1998
70
4.8
Comparison ofthe aboveground vegetation in the two years following
thinning
71
ix
LIST OF FIGURES
Figure
2.1 Diagram of soil seed bank dynamics
Page
8
3.1
Study area near Mt. Trumbull in northern Arizona
26
3.2
Principal component analysis
38
4.1
Study area near Mt. Trumbull in northern Arizona
55
4.2
Example of 2 x 3 m herbaceous restoration plot
58
4.3
Number of plants of each species in the aboveground vegetation in
1997, by treatment
65
4.4
A comparison of the aboveground vegetation in the two years
following overstory thinning
72
4.5
The forest floor prior to overstory thinning
77
4.6
Example of understory plot number 17 after thinning of overstory
trees, August 1997
77
4.7
Understory plot number 17 in September, 1997
78
4.8
Understory plot number 17 in August, 1998
78
x
To Isaac
Xl
PREFACE
This thesis was written in manuscript format. It contains an
introduction, comprehensive literature review, two journal manuscript
chapters, and a conclusion. Methods are contained in each of the manuscript
chapters. The result is some redundant material, which hopefully I have
kept to a minimum. Chapter 3 "Soil Seed Bank in Five Canopy Types at
Mt. Trumbull, Mohave County, Arizona" will be submitted to the Canadian
Journal ofBotany and Chapter 4 "Effects of Thinning on the Soil Seed Bank
in Southwestern Ponderosa Pine" will be submitted to Restoration Ecology.
Citations throughout the thesis follow the format of the Canadian Journal of
Botany. Scientific names were taken mainly from Utah Flora (Welsh et aI.
1993) . Scientific names for those species not found in Utah Flora were
taken from Intermountain Flora (Cronquist et
at 1972, 1977, 1984, 1989,
1994, 1997) and Arizona Flora (Kearney and Peebles 1960), respectively.
These sources were also used to determine native versus non-native species.
Where nativity is questionable, plants are indicated as native to the United
States.
xii
CHAPTER 1
INTRODUCTION
"1 took in February three tablespoonfuls of mud from a little
pond .. J kept it covered up for six months, pulling up and
counting each plant as it grew; the plants were of many kinds,
and were altogether 537 in number; and yet the viscid mud
was all contained in a breakfast cup!" (Darwin 1859).
Southwestern ponderosa pine herbaceous, shrub, and forb species
(hereafter collectively referred to as understory) are essential for many
ecological reasons including serving as food and habitat for animal species,
serving a role in nutrient cycling and soil formation and stabilization, and
providing a source offuel for low-intensity fires. In addition, the greatest
biodiversity of species in southwestern ponderosa pine forests is contained
in this understory layer of vegetation. Southwestern ponderosa pine
understory species composition and cover have been adversely modified
since Euro-American settlement (presettlement), due mainly to livestock
overgrazing in the late nineteenth century and active fire suppression
(Arnold 1950, Weaver 1951, Cooper 1960, Covington and Moore 1994,
Touchan et al. 1995, Covington et al. 1997). Production of understory
species has probably also declined due to the two previous factors as well as
to an increasing ponderosa pine tree density (Cooper 1960, Covington et al.
1994, Covington and Moore 1994, Covington et al. 1997).
Restoring the overstory tree structure to densities and patterns similar
to pre-Euro-American conditions is important for restoring native
herbaceous and shrub species to their historical levels in southwestern
ponderosa pine ecosystems. Basic restoration treatments in these forests
include protecting the remaining old-growth trees by removing organic
matter from around the base, thinning postsettlement trees to presettlement
patterns and densities, and reintroducing a fire regime that is similar to
historical fires in frequency and intensity (Fule et al. 1997). Where natural
regeneration of the understory is inadequate after overstory treatments,
restoration through seeding and planting may be required.
Knowledge of seed bank ecology is essential for understanding
successional patterns, plant community development, and population
dynamics (perez et al. 1998). Key to restoring the understory in ponderosa
pine ecosystems is an understanding of the past and present on-site plant
material, including aboveground vegetation composition and biomass as
well as viable seeds in the soil seed bank. Information on the quantity and
type of species in the seed bank will give an indication ofthe species that
will emerge following a disturbance, such as fire or thinning (Pratt et al.
1984, Vose and White 1991, Brown 1992, Warr et aL 1993), and can
possibly provide clues to the historical species composition.
2
The objectives of my study were to: 1) obtain a baseline estimate of
the species found in the soil seed bank beneath five canopy types of
overstory vegetation at the study site; 2) determine the species composition
and seed density in forest floor organic matter versus mineral soil; 3)
examine how ecological restoration thinning practices affect seeds in the soil
seed bank; and 4) relate aboveground vegetation to viable seeds in the soil
seed bank.
I hypothesized that: 1) seed bank species will be strongly associated
with dominant canopy types across the landscape; 2) more viable seeds will
be found in the 0 horizon than in the mineral soil; 3) the number of viable
seeds will decrease with depth in the soil; 4) ecological restoration
treatments such as thinning of overstory trees will change the distribution
and number of viable seeds that will be able to germinate after restoration
treatments; and 5) different species will be found in the aboveground
herbaceous vegetation than in the soil seed bank of mature ponderosa pine
forests. Ultimately this information will help in the decision making process
to determine if seeding of native understory species or control of non-native
species is necessary to meet target goals of ecological restoration.
3
CHAPTER 2
COMPREHENSIVE LITERATURE REVIEW
Introduction
Seed banks may be found in the canopy of trees and shrubs (aerial
seed banks) or belowground in the mineral soil or vegetative litter. The
former type of seed bank often consists of serotinous cones in genera such
as Banksia which may persist in the canopy for long periods of time,
awaiting a fire or severe drought to open the cones (Enright and Lamont
1989, Baskin and Baskin 1998). The latter type, the soil seed bank or seed
P004 is defined by Simpson et at (1989) as all viable seeds present on or in
the soil or associated litter. Another accumulation ofpotential plants in the
soil is the bud bank and is defined as the accumulation of bulbs, bulbils, and
buds on rhizomes, corms, and tubers (Harper 1977, Baker 1989, Simpson et
aI. 1989). The soil seed bank. in southwestern ponderosa pine is the primary
subject ofthis thesis, although portions of the bud bank. are also included.
This literature review focuses on the soil seed bank. and discusses the
soil seed bank across ecosystems, the natural history and ecology of soil
seed banks, and soil seed banks in several southwestern ecosystems. In
addition, it includes methods of measuring and estimating the species and
seeds found in the soil seed bank.
4
Life history may dictate whether a plant species utilizes the soil for
storage of its seeds. In annuals, the dormant seeds in the soil seed bank
serve as a "genetic memory" of past selective conditions and act as a
mechanism for storing genotypes for future environmental variability
(Templeton and Levin 1979, White 1979, Eriksson 1992, Pake and Venable
1996, Bai and Romo 1997). A single species can have more than one
genotype to capitalize on variability in different habitats and thus enhance
its ability to withstand environmental variability (Cavers 1995). In contrast
to annuals, the adult population of most perennial species contains a genetic
memory to withstand environmental variability, and in general, perennials
do not rely on the soil seed bank. as a means of maintaining genetic variation
(Bazzaz 1996).
Since Charles Darwin first recorded his observations on the soil seed
bank in 1859, an enormous amount of literature on soil seed banks has been
accumulated - a literature far too extensive to be discussed in detail here.
For in-depth information, there are comprehensive discussions of seed bank.
ecology (Harper 1977, Roberts 1981, Silvertown 1982, Leek et al1989,
Thompson 1992, Baskin and Baskin 1998), literature reviews (Warr et al.
1993, Thompson et al. 1997), bibliographic reviews (Vyvey 1989), and
discussions on the relationship between seed banks and restoration ecology
(Bakker et al. 1996). There are also numerous detailed discussions of
sampling techniques (Champness 1949, Bigwood and Inouye 1988, Gross
1990, Brown 1992, Warr et al, 1993, Ter Heerdt et al. 1996), and of the
5
appropriate size and number of samples to be collected (Benoit et al. 1989,
Benoit et al. 1992, Dessaint et al. 1996).
Soil Seed Banks across Ecosystems
Soil seed banks are found in most terrestrial ecosystems, although
their importance varies within and among ecosystems (Table 2.1).
Table 2.1. Number of seeds and the predominant species present in the seed
pools of various vegetation types.
Vegetation type
Arable fields
Annual grassland
Arable fields
Freshwater marsh
Arable fields
Mojave desert
Meadow grassland
Ponderosa pine
Arable fields
Oak woodlands
100 yr. Picea forest
Subalpine forest
Chaparral
Upland coniferous forest
Zoysia grassland
Coniferous forest
Prairie grassland
Salt marsh
Sagebrush semi-desert
Ponderosa pine
Sand pine scrub
Subarctic pinelbirch forest
Seedslm2
1,600-86,000
9,000-54,000
1,000-40,000
6,405-32,000
5,000-23,000
800-18,750
280-16,980
13,052-14,463
7,620
5,760-6,850
1,200-5,000
3,100
700-3,000
500-2,650
1,980
1,000
300-800
31-566
3-92
8-22
20
0
Location
England
California
Minnesota
New Jersey
Canada
Nevada
USSR
Washington
Honduras
California
USSR
Oregon
California
Alberta
Japan
Canada
Kansas
Wales
Utah
Arizona
Florida
Canada
Predominant speeiesIVegetation types
Agricultural weeds
Annual grasses
Agricultural weeds
Annuals and perennials
Agricultural weeds
Winter annuals
Bromus inermis, Geranium pratense
Annual forbs, non-natives
Agricultural weeds
Stellaria media
Early successional species
Juncus sp.
Perennial shrubs
Carex sp.• Salix sp.
Zoysia japonicus
Alnus rubra
Annuals
Sea rush
Grasses, forbs
Chenopodium sp., Gnaphalium sp.
Cyperus sp., herbs
No viable seeds present
From Biswell (1974), Harper (1977), Nelson and Chew (1977), Silvertown
(1982), Pratt et al. (1984), Hassan and West (1986), Vose and White (1987),
Fyles (1989), Marafion and Bartolome (1989), Ingersoll and Wilson (1993),
Carrington (1997), Baskin and Baskin (1998).
Thompson (1978) suggests that buried seed numbers increase with the
frequency and intensity of disturbance and decrease with stress. Stress
6
occurs when the productivity of all or part of the vegetation is insufficient to
produce enough seeds to maintain the seed bank, and may be caused by a
variety of factors including drought, temperature fluctuations, herbivory,
and competition (Thompson 1978). The highest seed densities are found in
temperate arable soils, primarily because most plants that are considered
weeds form large soil seed banks, and also due to the amount and frequency
of soil disturbance associated with most modem agricultural practices
(Thompson 1978, Thompson et a1. 1993, Warr et al. 1993, Cavers 1995).
Wetlands also typically have a very high buried seed density, again due to
the frequency and intensity of disturbance (Cavers 1995), and also because
they serve as catchments for wind and water dispersed seeds. The lowest
numbers of seeds in the soil seed bank generally occur in tropical rain forest,
subarctic, arctic and alpine systems, and in some temperate woodlands and
grasslands (Thompson 1978, Warr et al. 1993).
Natural History of Soil Seed Banks
In this section I outline the trends observed in soil seed banks, how
seeds enter the soil, where seeds are located in the soil, dormancy, and
losses from the seed bank, and I also discuss how disturbance relates to the
soil seed bank. A diagram of seed bank dynamics is shown in Figure 2.1.
In general, there are many more potential plants in the form of seeds
that lie dormant below the soil surface than there are in the aboveground
vegetation. Seeds will often accumulate in the soil in areas of favorable
7
microclimates for each species (Cardina and Sparrow 1996), and patches of
aboveground vegetation can serve as seed entrapment sites and provide
micro sites conducive to seedling establishment (safe sites). Thus, buried
seeds may accumulate differentially between vegetated and bare soils
(Ingersoll and Wilson 1993) and will often be found aggregated in the soil
around the mother plant, existing vegetation, or sites which formerly
contained vegetation.
Seed Rain
Predation
~
Dormant
Predation ~
I
I
I
Active
Seed
Stimuli
Seed
Bank
I
Bank
~
Germmation
Seedlings
..
~
"
Death
I
-,Ir
Death
(Decay, Dessication, Senescence,
Pathogen Attack)
Figure 2.1 Diagram of soil seed bank dynamics (adapted from Harper
1977).
There is a well-documented lack of correspondence in both
abundance and species composition, and between the aboveground and
belowground vegetation, particularly in sites that have not recently
experienced disturbance (Thompson and Grime 1979, Donelan and
8
Thompson 1980, Silvertown 1982, Ingersoll and Wilson 1993, Warr et al,
1993, Warr et al. 1994, Dutoit and Alard 1995). This lack of
correspondence is especially noticeable in grasslands, wetlands, and
woodlands (Warr et al. 1993). In frequently disturbed habitats, the species
composition aboveground and belowground is generally similar (Warr et al.
1993). The addition of nitrogen is also responsible for decreasing the
proportion of species shared between the soil seed bank and the
aboveground vegetation, due primarily to the suppression of seed
germination (Kitajima and Tilman 1996).
Another widely observed trend is a dearth of dominant tree, and
sometimes also shrub, seeds in the soil seed bank of many temperate forest
ecosystems, especially coniferous systems (Thompson 1978, Kramer and
Johnson 1987, Ingersoll and Wilson 1990, Warr et al. 1994, Ferrandis et al.
1996). However, Archibold (1979) found a large number of woody species
seeds in the seed bank of a mixed conifer boreal forest in Canada.
Seeds can be dispersed in space as well as time if they retain their
viability for long periods. The majority of buried seeds die within a few
years (Cavers 1995), but significant numbers of seeds of some species can
survive for decades or even 100 years or more (Kivilaan and Bandurski
1981, Warr et al. 1993). The soil seed bank contains both persistent and
transient seeds (Thompson and Grime 1979). Persistent seeds tend to
survive in the soil for a year or more, while transient seeds rarely survive in
the soil for more than a year. Transient seeds are normally larger and either
9
flattened or elongated, which tends to complicate burial (Thompson et al.
1993) . Persistent seeds tend to be small and compact (which probably leads
to their ease of burial), are generally from annual species, and are commonly
found in arable and fire-prone habitats (Harper 1977, Eriksson 1992,
Thompson et aI. 1993, Bakker et aL 1996). Several other factors also lead to
persistence in the soils including germination cues and requirements,
dormancy mechanisms, and resistance to pathogens (Thompson et al. 1993).
Another consistent characteristic of species with persistent seeds is the
inhibition of germination by darkness (Grime 1979).
Seed Distribution in the SoiJ
How seeds enter the soil is not a well-understood process. It may be a
gradual process by which seeds are incorporated into the litter and
eventually buried, or it may be rapid due to freeze-thaw cycles, rainwater
percolation, soil faunal activity, burrowing rodents, or trampling activity by
large mammals (Warr et aI. 1993). Dung beetles are also known to bury
seeds by incorporating them into balls of dung (Wicklow et aI. 1984).
The general trend is toward a decreasing seed density with increasing
soil depth. Deeply buried seeds are assumed to be older than seeds near the
surface (Warr et aL 1993, Warr et aL 1994), and their chances of
germination decrease with depth (Cavers 1983, Cavers 1995). Seeds of
short-lived (though not necessarily transient) species tend to be viable only
near the surface. In contrast, viable long-lived (persistent) seeds may be
10
found throughout the soil profile. In order to germinate, these deeply buried
seeds must be brought nearer to the surface by plowing, road building ,
windthrow, or other soil disturbances. Species that are lost from the
aboveground vegetation may still sometimes be found deep in the soil, while
those that have recently colonized an area may be found only near the
surface (Milberg 1995).
As would be expected, the soil layers (both organic and mineral)
nearest the surface hold the greatest number of viable seeds, and the litter
layers tend to contain the greatest density of seeds (Strickler and Edgerton
1976, Granstrom 1982). Moore and Wein (1977) found the greatest seed
viability in the upper 0-2 em ofthe 0 horizon, and Perez et aI. (1998) in the
upper 5 em of mineral soil. Kramer and Johnson (1987) found the greatest
viability in the upper 0-5 em, which included both organic matter and
mineral soil.
Dormancy
Seeds utilize a number of survival strategies until a time when
conditions are favorable for germination and growth initiation. The most
common strategy is seed dormancy. A dormant reserve ofseeds in the soil
is a means whereby plants employ rapid germination to capitalize on
conditions after a disturbance. Silvertown (1982) suggests that dormant
seeds are only produced in large numbers by species whose growing
populations are subject to periodic local extinction. Such is the case for
11
grassland annuals, arable weeds and many other early successional species .
However, not all early-successional species produce dormant seeds.
There are numerous factors that cue seeds to break dormancy and
germinate. These include light intensity, photoperiod, light quality,
temperature, nitrates, oxygen and carbon dioxide levels, pH, moisture,
physical abrasion of the seed coat (Silvertown 1982), smoke, heat
scarification and high soil temperatures after fire, and presence of chemicals
found in charred wood (Christensen and Muller 1975, Keeley et al. 1985,
Auld and Bradstock 1996, Keeley and Fotheringham 1997). In addition,
there is some evidence that germination may be triggered by temperature
fluctuations in the soil brought about by removal ofthe insulating effect of
foliage, litter, or other organic matter (Grime 1979). Enforced dormancy
may occur in species whose seeds are subjected to changes in the spectral
composition of light rays that have been fihered through a dense leaf canopy
(Grime 1979).
Losses from the Soil Seed Bank
Only a small percentage of buried seeds will ever germinate, the
possibility of germination being dependent mostly on a favorable microsite
climate. At least 95% of all plant mortality typically occurs at the seed stage
(Cavers 1983). The loss of seeds from the soil seed bank is mainly due to
predation, germination, fungal attack, desiccation, senescence, and high fire
temperatures (Harper 1977, Clark and Wilson 1994, also see Figure 2.1).
12
Most types of disturbance will deplete the seed bank through germination,
High temperatures from fire can result in decreased seed density in the soil
seed bank by killing some seeds and by stimulating germination in others
(Clark and Wilson 1994). Closure of the overstory tree or shrub canopy
may also result in impoverishment of the aboveground herbaceous or shrub
vegetation which leads to a depleted or depauperate seed bank (Warr et al.
1994). Seeds may also physically remain in the soil seed bank even though
they have lost their viability and will never germinate.
Disturbance
As mentioned earlier, sites that undergo frequent disturbance usually
have a large soil seed bank due to favorable conditions for, and increases in,
germination following disturbance, and the subsequent reproduction and
production of seeds to reoccupy the soil (Thompson 1978). In addition, the
seeds in the seed bank in these systems do not have to lie dormant for long
periods of time while awaiting favorable conditions for germination. The
seed dormancy period is a time of high mortality for many species, and the
loss of many seeds occurs during this time (Cavers 1995, Figure 2.1).
However, one type of system which experiences frequent disturbance, the
intensively grazed system, is an exception to the trend toward large seed
banks. Intensive grazing can cause decreases in seed rain that will lead to
decreases in the soil seed bank of some species (Dutoit and Alard 1995,
Bertiller 1996, Ghermandi 1997). If severe enough, the grazing can
13
eliminate palatable perennial grasses from the above and belowground
vegetation (Bertiller 1996). In contrast, Milberg (1995) found that the
number of species in the soil seed bank and in the aboveground vegetation
can actually increase due to the transport of seeds by grazing animals.
The closer temporally an ecosystem is to a disturbance, the smaller
the soil seed bank due to depletion by germination (Warr et aI. 1993,
Mulugeta and Stoltenberg 1997). As seeds from the seed bank germinate,
mature and reproduce, the seed bank continues to increase in size.
Eventually succession may reach a point where the overstory vegetation
begins to eliminate some understory species through shading or other
competitive means. In frequently disturbed habitats, species composition
between aboveground vegetation and the soil seed bank is generally similar,
but there is little correspondence in undisturbed habitats as succession
proceeds and the aboveground species composition changes (Warr et aI.
1993). Communities with large persistent seed banks are characterized by a
regime of severe, unpredictable disturbances such as cultivation, high­
intensity fire, or large fluctuations in water level (Thompson 1992).
Predictable seasonal and small-scale disturbances, such as ice storms,
windthrow, or small animal activity, do not promote the accumulation of
large seed banks. In plant communities where disturbance is either
intermediate in size and frequency or locally variable, there may be a
combination of both short-lived and long-lived species in the seed bank.
Systems such as old-growth Douglas-fir (Psuedotsuga menziesiiy forests,
14
which do not experience frequent disturbance, may have a decreased
number of germinating seedlings following disturbance such as fire or
logging (Ingersoll and Wilson 1990).
The intensity and frequency of disturbance may also affect the
density, composition, and location of viable seeds in the soil. Low-intensity
fires may kill only seeds in the 0 horizon while allowing those in the
mineral soil to survive. However, high intensity fires may heat the surface
of the mineral soil enough to cause destruction of the entire seed bank (Flinn
and Wein 1977, Flinn and Wein 1988). Some species utilize a strategy by
which other reproductive plant parts, such as rhizomes, in the mineral soil
are able to survive fire, sometimes even severe fire. These species may have
a competitive advantage over species that must establish by seed (Flinn and
Wein 1977, Flinn and Pringle 1983). Whether seeds survive a fire can also
be contingent on conditions prior to the fire, i.e., whether fuels were moist
or dry (Cushwa et al. 1968).
As succession continues after disturbance, seed bank density and
species diversity in the seed bank decrease so that late-successional
communities often have relatively few buried viable seeds (Donelan and
Thompson 1980, Warr et al 1993, Dutoit and Alard 1995). Early­
successional species are often short-lived, produce many more seeds than
species that appear later in the successional sequence, and rely on a long­
term seed viability strategy for continued survival ofthe species in systems
with low frequency of disturbance (Dutoit and Alard 1995). In temperate
15
mature woodlands, the light-demanding, early successional species often
have been lost and replaced by shade-tolerant species, which generally
produce few viable seeds and must re-invade sites by new propagules
following disturbance (Warr et al. 1993, Dutoit and Alard 1995). Late
successional ecosystem soils often possess the seeds of all the preceding
seral stages (Warr et al. 1993).
Seed Banks in Ponderosa Pine and Selected Southwestern Ecosystems
There is a vast amount of literature available on soil seed banks, and
some of this research has occurred in certain ecosystems that also occur in
the southwestern United States. In general, however, there have been very
few studies actually conducted in the Southwest. I will only discuss those
ecosystems that are included in my study area or adjacent to it,
encompassing ponderosa pine forests, pinyon woodlands, sagebrush
communities, and grasslands.
Ponderosa Pine Forests
In general, very little is known about seed bank ecology in ponderosa
pine forests. At a ponderosa pine/Gambel oak study site near Flagstaff: AZ,
Vose and White (1987) found about 8 viable
seeds/nr' in burned plots and 22
seeds/nr' in control plots. Ceanothus fendleri, a shrub species, sprouted
prolifically by vegetative means after fire, but no seeds of this species were
found in the seed bank. The conclusion was that seed rain is a more
16
important source of new seedlings after prescribed burning than is buried
seed.
In contrast, buried seeds may be a significant source of colonization
in ponderosa pine forests of east central Washington. Pratt et al. (1984)
estimated that there were approximately 13,052
±1481 viable seeds/rrr' in the
top 10 ern ofthe mineral soil and associated litter during the spring. In the
autumn, this number increased to approximately 14,463 ± 1356. Seeds of 57
species were present in the soil seed bank and 21 ofthese species were not
found in the aboveground vegetation. Woody species accounted for only
about 1% ofthe seed bank even though they dominated the aboveground
vegetation. Two non-native species (Stel/aria media and Poa pratensis)
accounted for 50% ofthe buried viable seed. The majority of viable grass and
annual forb seeds were found in the litter, whereas perennial forb species
were mainly found in the mineral soil.
Pratt et al, (1984) suggest three possible reasons for the elevated
estimates they found compared to most coniferous forests : 1) the ponderosa
pine-snowberry community that they studied has high understory diversity
and abundance (the site is a forest community with a meadow-steppe
understory), 2) the study area was in an intermediate successional stage with
both climax and early successional species present, and 3) the surface area
sampled was larger than in most other studies.
17
Pinyon Woodlands
Species diversity and abundance decrease with successional stage in
soil seed banks of Southern California pinyon woodlands (Pinus edulis) as
stands approach a closed canopy condition (Koniak and Everett 1982). This
decrease corresponds with a decline in species frequency and composition in
aboveground vegetation. Soil seed banks differ spatially in this ecosystem
according to the micro site type. The duff (0 horizon or litter) microsite acts
as a seed trap; however, the micro site with the highest seedling emergence is
beneath shrub canopies. Dense stands of shrubs act as windbreaks and
accumulate both litter and seeds. Annual species accounted for 89% of the
seedling emergence from four microsites (grass-forb, shrub-tree, tree-shrub,
and tree). Collinsia parviflora dominated the seed bank. Other dominant
species were Eriogonum vimineum, Cryptantha circumseissa, and Bromus
teetorum (Koniak and Everett 1982).
Shrublands/Sagebrush Communities
Seed banks in Utah sagebrush communities differ both temporally
and spatially. Soil seed bank densities are lower in burned sites immediately
after fire based on soil seed bank data collected from a site in western Utah
(Juab County). However, these densities increased to pre-fire levels within a
year (85.5
± 29.8 viable seeds/m' in unburned plots versus 38.7 ± 8.5 viable
seeds/or' in burned plots) (Hassan and West 1986). Like other studies in
semidesert and desert ecosystems, Hassan and West (1986) conclude that
18
seed bank densities are greater under shrub canopies than in the areas in
between. However, "hot spots," areas occupied by shrubs prior to destruction
by fire, may have almost complete depletion of the seed bank, probably due
to germination in an area with higher organic matter and other favorable
microsite conditions.
Grasslands
Grassland seed banks in Europe and North America are generally
small, with the exception of grasslands that have been cultivated (Warr et al.
1993). Soil seed banks in annual grasslands may be comparatively large. An
annual grassland in coastal California contained 15,980 to 40,730 seeds/m'
(Maraiion and Bartolome 1989). Annual grasslands are thought to be an
unnatural artifact of intensive grazing and drought combined with the
invasion of non-native grasses and forbs (Keeley 1981) . Seed reserves in
annual grasslands are important as a dormancy mechanism for seedlings to
emerge during the rainy season, rather than after fire, as in many other
Southwest ecosystems. Regeneration from basal buds following
aboveground destruction is well-developed in perennial grasses (Keeley
1981) and may compensate for destruction of seeds by fire and/or herbivory.
Quantification oftbe Soil Seed Bank
The two most common methods utilized in soil seed bank studies for
determining the number of seeds in the soil seed bank are the seed extraction
19
and the seed emergence methods (Brown 1992). Seed extraction is used to
count the number of seeds in the soil, usually by separating the seeds from
the soil by washing and sieving , and then by identifying the seeds that
remain. Its main weaknesses are that it does not ascertain which seeds are
viable, and it is extremely time- and labor-intensive. In the seed emergence
method, samples are spread in trays and maintained under those conditions
thought to promote as high a germination rate as possible, usually in a
greenhouse (Bakker et at. 1996). The seed emergence method relies on
germination of seeds and will therefore only approximate the number and
type of viable seeds in the seed bank. The main limitation to the emergence
method is that it detects only the readily germinable fraction of the seeds.
Those species with seeds that require a certain environmental signal or cue to
initiate germination will remain undetected if the required cue does not occur
in the emergence test (Brown 1992).
Ter Heerdt et at. (1996) have developed a seedling emergence method
which involves concentrating (washing and sieving) the seeds prior to placing
them onto sterilized soil. They found that this method increases the number
of seedlings that emerged as compared to unconcentrated samples, and it
reduces the greenhouse space needed. Another method is to analyze the soil
following seed emergence for seeds that did not germinate because they are
no longer viable.
A larger number of seeds will typically germinate under greenhouse
conditions than in the field. However, seedling emergence under field
20
conditions can also be compared to emergence under greenhouse conditions
if established vegetation is first removed in the field. Most seeds that
germinate in the field are those which reside in a thin light-penetrable layer
of soil under prevailing weather conditions. Advantages of experimentation
in the field are that a large area can be covered, and rare species and those
with a patchy distribution will be revealed (Thompson et al. 1997).
In general, when estimating the number of seeds in the soil seed bank,
it is more advantageous to collect a large number of small samples than a
small number of large samples (Warr et al. 1993, Thompson et al. 1997).
Many plant species tend to have a patchy distribution of seeds in the soil over
space and time. In order to capture this patchiness, it is necessary to collect
many samples over a fairly wide area.
Analysis of Soil Seed Bank Data
Data obtained from seed bank studies are often highly skewed and
non-normally distributed primarily due to the clustered seed distribution of
many species in the soil (Warr et al. 1993, Thompson et al. 1997). This leads
to a large number of zeroes that accompany this distribution in studies that
examine the presence/absence or frequency of species. In addition, standard
statistical transformations often do not achieve normality, and thus non­
parametric statistical tests must be used . Even so, non-parametric tests can
only be applied to the most common species found in the soil seed bank due
to the loss of power that occurs with analysis of uncommon or absent species
21
(Warr et al. 1993, Thompson et al. 1997). Data may also be examined using
various multivariate techniques and ordinations to search for patterns.
Software packages that are commonly used include TWINSPAN,
DECORANA (Warr et al. 1993) and DECODA (peter Minchin 1999,
personal communication, unreferenced). However, because ofthe difficulties
in analysis, soil seed bank data are most often summarized descriptively
(Warr et al. 1994). The specific analyses used in this thesis are described in
Chapters 3 and 4.
22
CHAPTER 3
MANUSCRIPT
SOIL SEED BANK IN FIVE CANOPY TYPES AT
MT. TRUMBULL, MOHAVE COUNTY, ARIZONA
Abstract
A soil seed bank study was conducted in northern Arizona to
determine species composition prior to ecological restoration treatments,
such as tree thinning and prescribed burning, and to relate the seed bank in
the soil to the overstory canopy type. In addition, data were also collected
to determine seed quantity and composition in the 0 horizon and in
mineral soiL
The five canopy types chosen for the baseline were old-growth and
pole-sized ponderosa pine (Pinus ponderosa), Gambel oak (Quercus
gambelii), New Mexico locust (Robinia neomexicanai, and big sagebrush
(Artemisia tridentata). Soil seed bank samples were collected in the
autumn and placed in a greenhouse for seed emergence trials. Thirty-eight
species germinated in the study. The three most common species were
Verbascum thapsus, Leonurus cardiaca, and Conyza canadensis. A
principal component analysis revealed that overstory canopy type
influenced soil seed bank species. The New Mexico locust canopy type is
characterized by a soil seed bank dominated by non-native species and
23
members of the Scrophulariaceae and Chenopodiaceae families, while
having few grasses, sedges or species in the Asteraceae family. The seed
bank ofthe big sagebrush canopy type varied between sampling locations.
Seed banks of the other three canopy types tend to be characterized by few
grasses and sedges, and the number of non-native species is variable. A
statistically significant difference in species composition based on overall
canopy type was found. Numbers of non-native species and species in the
Chenopodiaceae family differed significantly among canopy types.
Seed germination was higher from mineral soil samples (61 %)
than from the 0 horizon (39%). When non-native species were separated
from native species, a significantly larger number ofnon-native species
germinated from the 0 horizon rather than from the mineral soil.
Introduction
Extensive changes in species composition, herbaceous productivity,
fire frequency and intensity and other factors have occurred in the ponderosa
pine forests ofthe southwestern United States since Euro-American
settlement (hereafter called presettlement) (Cooper 1960, Covington et al.
1997). Increasing tree densities, overgrazing, introduction of non-native
plant species, and fire suppression are the main factors responsible for
alterations in these ecosystems, especially in herbaceous and shrub
understory composition and cover (Cooper 1960, Covington et aI. 1997).
Knowledge of presettlement herbaceous vegetation is limited due to a
reduced capacity for preservation of non-woody vegetation in the soil, and
24
limited historical records. A variety of methods, techniques, and tools have
been used to determine historical herbaceous species composition and
frequency. These methods and tools include historical records and
photographs, packrat middens, palynology, relict sites (Kaufmann et al.
1994), diaries, surveys, military expeditions (Dick-Peddie 1993), and opal
phytoliths (Fisher et al. 1987, Piperno 1993, Fisher et al. 1995). Analyses of
seeds in the soil seed bank can also be used as tools to reconstruct historical
species composition, although their usefulness may be limited to the past 50
years or so (Warr et al. 1994), and perhaps to only a few species.
Nevertheless, seeds of some species found in the soil can be quite old
(Granstrom 1982), and may prove useful in determining past frequency and
composition of these species. Ultimately, the analyses of soil seed banks may
be used in vegetation reconstruction, but they may be more useful in
ecological restoration for determining how rapidly the understory vegetation
of an area may recover following ecological restoration treatments,
identifying which species may germinate from the soil seed bank, and
determining whether seeding of native species may be necessary.
Some studies have shown that there are many more seeds in forest
floor material or
a horizon (also known as duffand litter) than in the
underlying mineral soil in coniferous forests (Moore and Wein 1977, Kramer
and Johnson 1987). This study was also designed to determine if the same
principle is true for mature ponderosa pine forests in the Southwest.
25
Few studies exist of soil seed banks in present-day ponderosa pine
(Pratt et al. 1984, Vose and White 1987). The objectives of my study are to
obtain a baseline estimate of the species presently found in the soil seed bank
in a ponderosa pine forest to assist in our understanding of seed bank ecology
in these systems, to compare the soil seed bank species occurring under
different dominant overstory species, and to determine whether there are
differences in viable seed number between forest floor material (0 horizon)
and mineral soil. My study is associated with a large-scale ecological
restoration project, and the soil seed bank data will be used to aid in restoring
understory species that historically occurred in the study area.
My hypotheses are that: 1) similar seed bank species will occur with
the same dominant overstory species across the landscape, and 2) there will
be more seeds in the 0 horizon than in the mineral soil.
In this chapter I will describe the baseline soil seed bank in five
canopy types and examine the viable seeds in the 0 horizon versus those in
the mineral soil.
Study Area
My study site is located within the Arizona Strip, the portion of
northern Arizona that lies between Utah to the north and the Colorado River
to the south. My study was conducted within an area between the Mt.
Trumbull and Mt. Logan Wilderness Areas (Figure 3.1). The approximate
26
location ofthe study area is 36°22'30" N latitude and 113°10' W longitude.
The elevation in the study area ranges from approximately 2,080 m to 2,290
m. Much of the topography in the region is comprised of relatively recent
volcanic rocks on top of older sedimentary rocks (Koons 1945), and the
parent material for the soils in the vicinity ofMt. Trumbull is primarily
derived from volcanic materials. The main soil type is Lozinta, an ashy
skeletal over fragmental or cindery, mixed, mesic, Vitrandic Ustochrept
(Alfred Dewall 1996, personal communication, unreferenced), although there
are several other soil types in and around the study area.
Climatological data for the Arizona Strip are limited due to sparse
human settlement ofthe area. Most of the data currently available were
collected from cities surrounding the Arizona Strip beginning in the early to
mid-1900s. The regional climate differs from year to year because the
Arizona Strip lies on the monsoon boundary. As in much ofthe Southwest,
precipitation occurs annually in a bimodal distribution pattern. Thirty-four
percent of annual precipitation falls in the winter months of
November through February. In years of summer monsoon, an average of
35% of annual precipitation falls during the months of July through
September (Altschul and Fairley 1989). Elevation also controls climate to a
large degree in the Arizona Strip. Annual precipitation in the ponderosa pine
type varies between 38-64 em (USDI BLM 1990). There are no perennial
streams in the study area, but there are several springs in the vicinity ofMt.
Trumbull and Mt. Logan.
28
The majority of the study area is of the ponderosa pine forest type,
which is bordered by pinyon-juniper woodland. The ponderosa pine forest
type in the Arizona Strip occurs at elevations of approximately 1,830-2,440
m (USDI BLM 1990). Over 250 species of forbs, grasses, and shrubs have
been identified in the ponderosa pine type and its transition zones in the
vicinity ofMt. Trumbull and surrounding mountains (Appendix A).
Preliminary data show that the last widespread, naturally occurring fire in the
vicinity ofMt. Trumbull was in 1870. Fire scars collected from individual ponderosa
pine trees indicate that fires occurred every four to six years prior to Euro-American
settlement and use of the area (Pete Fule 1997, personal communication,
unreferenced).
Soil Seed Bank in Five Canopy Types
Field and Greenbouse Metbods
I chose three areas, each separated by several km, in which to collect
baseline seed bank samples (EB-2, EB-3, and EB-5; see Figure 3.1). The
same five canopy types were chosen in each area based on the dominant
overstory species. The canopy types were old-growth ponderosa pine (Pinus
ponderosa), pole-sized ponderosa pine, New Mexico locust (Robinia
neomexicana), big sagebrush (Artemisia tridentatai, and Gambel oak
(Quercus gambe/ii) . The old-growth ponderosa pine canopy type consisted
of large diameter ponderosa pines with red-yellow bark, an indicator of old­
age trees in this species (Elias 1987). Pole-sized ponderosa pine stands were
29
dense stands of smaller diameter trees (less than approximately 40 em in
dense stands of smaller diameter trees (less than approximately 40 em in
diameter) that were younger and had not undergone a change in bark color.
The remaining three canopy types only occur in relatively small and distinct
patches in the study site. Therefore, a minimum patch size of 0.5 ha was
chosen, but patches ranged in size to a maximum of 4 or more ha
(approximately 10 acres or more).
Soil seed bank samples were collected in mid-September, 1997. A 15
meter line transect was established through the approximate center of each
canopy type, and a sample was collected at one meter increments, for a total
of 45 samples per canopy type, and a total of225 samples overall (n=225) .
Samples were collected with a 5 em diameter bulk density hammer (slide
hammer) and included a 5 em diameter forest floor or 0 horizon sample (at
varying depths) plus the underlying mineral soil to a depth of 5 em,
The average depth of 0 horizon material taken with the samples
varied among the five vegetative types. The mean depth of the 0 horizon
material in the old-growth ponderosa pine stands was 7.76 cmand the depths
were 6.56, 3.18, 1.05, and 0.16 ern in pole stands, oak stands, locust stands,
and sagebrush flats, respectively. The ranges of litter depth (0 horizon) are
given in Table 3.1.
Samples were temporarily stored in plastic bags and were placed in
cold storage (4° C) for 6 weeks to satisfy chilling requirements for some of
the seeds. Samples were removed from cold storage in mid-November, 1997
and moved to the greenhouse.
30
The seed emergence method was used because of its ability to
determine the viable fraction of seeds. Methodology relating to sampling,
Table 3.1. Ranges of litter depth in five canopy types at Mt. Trumbull.
Canopy type
Old-growth ponderosa pine
Ponderosa pine poles
Oak
Locust
Sagebrush
Range of litter depth (ern)
0.2 -18.5
1.0-14.7
0.4 - 6.8
0-3.5
0-0.5
treatment of samples, and seedling inventory was adapted from Gross (1990),
Brown (1992), and Warr et aI. (1993).
Samples from each transect per canopy type (n=15) were composited
and spread thinly over a layer of approximately 5 em of sterile soil in
approximately 11 x 11 ern plastic flats. These flats were arranged in random
order on a greenhouse bench. Six flats holding only a mixture of sterilized
soil were randomly placed among the sample flats to account for species
germinating due to possible greenhouse contamination. Flats were fertilized
approximately every two weeks with a dilute nutrient solution (Miracle-Gro®
and Miracid®, alternatively). Seedlings were inventoried at 10-14 day
intervals. As seedlings germinated and could be identified, they were
removed from the plastic flats to prevent competition among seedlings. Flats
were watered as needed to maintain moist soil conditions until the end of the
study in May, 1998 (6 months). Although seeds have been shown to
germinate in the second year after initiation of seed bank studies, most
31
studies have shown that the majority of seeds germinate within the first two
months (Warr et al. 1993). According to Warr et al. (1993) there is little to
be gained by prolonging seed bank studies for longer than six months.
Statistical Methods
A principal component analysis (PCA) was performed on the species
data obtained from the five canopy types. Seed bank species that formed
large families or functional groups were placed into five categories for the
analysis: non-native species, grasses and sedges, Chenopodiaceae family,
composite family (Asteraceae) and the Scrophulariaceae family. The
objective of the PCA was to reduce the data to enable me to interpret patterns
and trends and to determine under which canopy type the various seed bank
species might be found. I then performed a multivariate analysis of variance
(MANOVA) to determine if there were between-group differences within the
five functional groups and families.
Scientific Nomenclature
Scientific names were taken mainly from Utah Flora (Welsh et al.
1993). Scientific names for those species not found in Utah Flora were taken
from Intermountain Flora (Cronquist et aI. 1972, 1977, 1984, 1989, 1994,
and 1997) and Arizona Flora (Kearney and Peebles 1960).
32
Seeds in 0 Horizon versus Mineral Soil
Field and Greenhouse Methods
Soil seed bank samples were collected in mid-September 1997 from
five canopy types based on the dominant overstory canopy: old-growth
ponderosa pine, pole-sized ponderosa pine, New Mexico locust, Gambel oak,
and big sagebrush. Three 0.0929 m2 (1 :ft?) 0 horizon samples were
collected from each canopy type in EB-3 using a steel square and a serrated
knife (n=15, see Figure 3.1). The soil beneath the 0 horizon samples was
then collected to a depth of 5 em with a bulk density hammer (slide hammer)
and a trowel. These samples were stratified at 4° C for eight weeks and
placed in a greenhouse. Approximately 1 cm of sterilized soil was placed in
each flat beneath the samples. Upon germination and identification,
seedlings were removed, and samples were stirred periodically to bring new
viable seeds to the surface. Other seed emergence methodology is as outlined
in the previous section.
Statistical Methods
Data were first transformed using a square root transformation to
achieve a more normal distribution of the data. A t-test was then performed
to determine if there were significant differences between the mean number
of seeds found in the 0 horizon versus those in the mineral soil.
33
Results
Soil Seed Bank in Five Canopy Types
Thirty-eight species germinated over six months in the greenhouse
seed emergence experiment (Table 3.2). The only two species found in all
five canopy types were Verbascum thapsus and Conyza canadensis. Conyza
canadensis is found both in the aboveground vegetation at Mt. Trwnbull and
as a greenhouse contaminant, so these results may not be representative.
Three species were found in four of the canopy types (Chenopodium album
var. berlandieri, Euphorbia serpyllifolia, and Poa pratensisi, and 16 species
(42%) were found in only one canopy type. The old-growth ponderosa pine
canopy type had the fewest germinating seedlings (38), whereas the locust
canopy type had the most (575). However, 65% ofthe seedlings in the locust
type were Verbascum thapsus. The highest species richness was in the
sagebrush canopy type (19 species), although the oak and locust canopy types
were very similar with 18 species each. The old-growth ponderosa pine type
had 16 species. The lowest species richness was found in the pole-sized
ponderosa pine canopy type with only 7 species. Again, Verbascum thapsus
comprised a high number of seedlings in this canopy type (81 %).
Overall, the three most common species were Verbascum thapsus,
which comprised nearly half of the seedlings in the study, Leonurus cardiaca,
and Conyza canadensis (note that this species was also a greenhouse
contaminant. Numbers may be inflated by as much as 3-5%). Six ofthe 38
34
Table 3.2. Total number of viable seeds (producing seedlings) in the soil seed
bank from samples collected in five canopy types at Mt. Trumbull, AZ.
Old-growtb Pole-sized
Ponderosa Ponderosa
Pine
Pine
Aro~.
Arenaria /anuginosa ssp.
saxosa
Argemone munita
Artemisia tridentata
Asteraceae ­ unknown"
Bromus tectorum
Carex occidetualis
Chenopodium a/bum var.
ber/andieri
Chenopodium botrys
Chenopodium graveo/ens
Chenopodium /eptophyl/um
Co//insia parviflora
Conyza canadensis**
Oak
Locust
Sagebrush
5
Total
5
6
6
1
22
23
9
17
I
8
1
2
3
6
9
17
3
I
5
5
5
9
127
1
3
6
6
10
~~fu~a
Dracocepha/um parviflorum
E/ymus e/ymoides
Erigeron divergens
Euphorbia serpyl/ijo/ia
Ga/ium bifolium
Gayophytum diffusum
Leonurus cardioca
Lepidium sp.
Lupinus argerueus
Microsteris gracilis
Mimulus rubel/us
Muhlenbergia mimaissima
Nama dichotomum
Penstemon barbatus
Poa pratensis
Po/ygonum convolvulus
Po/ygonum doug/asii var.
johnstonii
Senecio eremophi/us var.
macdouga/ii
Senecio midtilobaius
Sporobo/us cryptandrus
Typha sp.
Verbascum thapsus
Verbena bracteata
Viola canadensis
-
Total
38
7
8
7
1
2
76
3
2
1
2
3
124
2
1
1
3
2
6
78
28
82
37
2
7
172
3
48
4
2
3
26
I
5
3
3
2
5
7
2
I
6
2
I
7
I
32
14
I
48
5
1
5
6
4
I
I
5
3
I
2
3
4
4
2
509
10
7
1,157
I
81
100
14
377
35
I
1
8
4
189
3
574
256
* Identification based on vegetative characteristics - plant died before
flowering.
** Conyza canadensis was also a greenhouse contaminant
35
species (16%) are believed to be non-native to the United States: Bromus
tectorum, Chenopodium botrys, Leonurus cardiaca, Poa pratensis,
Polygonum convolvulus, and Verbascum thapsus. The Typha sp. is also
potentially non-native. Pratt et a1. (1984) found that two non-native species
comprised almost 50% of the viable seeds in seed bank samples from a
ponderosa pine forest in Washington.
Species were evenly divided between biennials/perennials and
annuals (50% of each) for all canopy types combined. Artemisia tridentata
was the only woody species that germinated from the samples. Six species
were grasses/sedges and three ofthese species were perennial grasses. They
include Poa pratensis - a non-native, Elymus elymoides, and Sporobolus
cryptandrus.
Twenty families were represented in the samples for all canopy types
combined. The most represented families were the Asteraceae (6 species),
Poaceae (5 species), Chenopodiaceae (4 species) and Scrophulariaceae (4
species).
Results from soil seed bank studies are normally expressed as a mean
number of seeds/IIi (Roberts 1981). The total number of seeds/m' for all
canopy types in this study is given in Table 3.3. Results are the same as for
Table 3.2, except expressed as the number of viable seeds per square meter.
Results of the Principal Component Analysis (PCA) indicate that the
first two factors explain approximately 67% of the variance in the data.
Factor 1 indicates that grasses and sedges tend to be associated with members
36
Table 3.3. Viable seeds/m' in the soil seed bank from samples collected from
five canopy types at Mt. Trumbull.
Old-growth Pole-sized Oak
Ponderosa Ponderosa
Pine
Arabis sp.
Arenaria lanuginosa ssp.
saxosa
Argemone munita
Artemisia tridentata
Asteraceae - unknown"
Bromus tectorum
Carex occidentalis
Chenopodium album var.
berlandieri
Chenopodium botrys
Chenopodium graveolens
Chenopodium leptophyllum
Collinsia parviflora
Conyza canadensis
Corydalis aurea
DracocephaJum parviflorum
Elymus elymoides
Erigeron divergens
Euphorbia serpyllifolia
Galium bifolium
Gayophytum diffusum
Leonurus cardiaca
Lepidium sp.
Lupinus argenieus
Microsteris gracilis
Mimulus rubellus
Muhlenbergia mimaissima
Nama dichotomum
Penstemon barbatus
Poa pratensis
Polygonum convolvulus
Polygonum douglasii var.
johnstonii
Senecio eremophilus var.
mocdougalii
Senecio multilobatus
Sporobolus cryptandrus
Typha sp.
Verbascum thapsus
Verbena bracteata
Viola canadensis
Total seeds/m 2
Locust
Sagebrush
Pine
57
57
68
68
102
23
102
II
260
II
193
34
192
II
57
68
56
57
II
249
II
II
91
II
II
68
II
68
II
34
­
113
79
79
91
24
860
103
294
II
34
-
II
23
23
II
23
34
1403
II
II
II
33
883
317
929
419
23
79
1946
56
34
23
57
79
79
34
543
45
II
23
45
II
II
II
23
68
362
158
II
11
II
57
11
57
68
II
45
428
917
1,132
542
57
II
34
11
23
1437
II
34
23
68
34
23
34
Total
II
158
4267
II
II
396
91
45
2,138
34
6,497
2,897
34
45
22
5761
113
79
13,092
*Identification based on vegetative characteristics - plant died before
flowering.
** Conyza canadensis was also a greenhouse contaminant
37
ofthe Asteraceae family, while the Chenopodiaceae family, Scrophulariaceae
family and the non-native species tend to be associated with each other.
Factor 2 indicates that the Scrophulariaceae and Chenopodiaceae families are
often associated with each other but not with non-native species. Although
these interpretations appear to be in contrast, it may be true that the
Chenopodiaceae and Scrophulariaceae families are often associated with non­
native species, but not always .
Factor loadings were then plotted in order to associate the two factors
with canopy type (Figure 3.2). The locust canopy type is characterized
mainly by the non-native species, and the Chenopodiaceae and
Scrophulariaceae families. There is overlap between the locust, oak and pole­
sized canopy types on Factor 2, and some overlap in Factor 1. Old-growth
ponderosa pine, pole-sized ponderosa pine, and the oak canopy types were
not associated with grasses and sedges. No trends were obvious in the
sagebrush canopy type.
The MANOVA reveals that canopy cover has a significant effect on
the composition of the species found in the soil seed bank (Hotelling's Trace
test; p<O.OOl). The significance of between-group effects is given in Table
3.4. The Chenopodiaceae family and the non-native species differ
significantly (p<0.1 0) among canopy cover types, whereas there was no
difference for graminoids, Asteraceae species, or Scrophulariaceae species.
38
Seeds in 0 Horizon versus Mineral Soil
Thirty-eight species also emerged from samples taken from the
mineral soil and 0 horizon (or forest floor) in five vegetative types (although
these species are not all the same taxa as the 38 species that emerged from
soil seed bank samples in the baseline analysis). A total of 2, 160 seedlings
germinated. Sixty-one percent of the seedlings germinated from mineral soil
samples (1,327 seedlings) and 39% germinated from the 0 horizon (833
seedlings). The total number of seedlings in each canopy type is given in
Table 3.5.
Table 3.5. Total number of seedlings that emerged from organic and mineral
soil samples in five canopy types at Mt. Trumbull.
Canopy type
Mineral
Organic
Old-growth ponderosa pine
Ponderosa pine poles
Oak
Locust
Sagebrush
Total
44
40
52
690
380
161
1327
7
282
470
34
833
The oak canopy type had the most viable seeds in the mineral soil,
while the old-growth ponderosa pine canopy type had the least. The locust
canopy type had the most viable seeds in the 0 horizon, and the ponderosa
pine pole type had the least. In all canopy types, there were more viable
seeds in the mineral soil, with the exception ofthe locust canopy type. The
number of each species, broken down by organic and mineral soils, is given
in Tables 3.6 and 3.7.
40
Table 3.6. Number of seedlings of each species that emerged from mineral
soil samples collected from five canopy types at Mt. Trumbull.
Speeies
Old-growtb
Ponderosa
pine
Agropyron sp.
Allium bisceptrum"
Arenaria lanuginosa ssp. saxosa
Artemisia carruthii.
Bromus tectorum
Chenopodium a/bum var. berlandieri
Chenopodium botrys
Chenopodium graveo/ens
Chenopodium /eptophy//um
Convolvulus arvensis
Conyza canadensis **
Corydalis aurea
Erigeron divergens
Euphorbia serpyllifolia
Ga/ium bifolium
Gayophytum diffusum
Leonurus cardioca
Lotus sp.
Mimulus rubellus
Nama dichotomum
Penstemon pachyphyllus
Po/ygonum convolvulus
Po/ygonum douglasii varjohnstonii
Poa pratensis"
Senecio eremophi/us var. macdougalii
Senecio multilobatus
Typhasp.
Verbascum thapsus
Vio/a canadensis
Unknown Asteraceae
Total
Pole-sized Oak
Ponderosa
Pine
Loeusr
Sagebrush Total
4
4
4
2
5
9
4
2
4
1
12
2
10
10
2
25
1
1
10
1
1
1
1
3
4
82
3
3
580
1
17
1
3
3
10
590
1
1
20
2
1
10
15
10
8
7
3
5
3
1
6
89
6
7
10
19
3
1
44
44
52
54
2
345
690
380
50
499
161
2
1
1327
* Includes portions ofthe bud bank
** Conyza canadensis was also a greenhouse contaminant (none detected in
this component of the study).
Thirty species germinated from the mineral soil. The most common
species were Leonurus cardiaca and Verbascum thapsus; both non-natives.
No other species came close to these two in number of germinants. Besides
the two species mentioned above, there were at least five other non-native
species that germinated in the mineral soil. These species were Bromus
41
Table 3.7. Number of seedlings of each species that emerged from 0 horizon
samples collected from five canopy types at Mt. Trumbull.
Species
Old-growtb
Ponderosa
Pine
Agropyron sp.
Allium bisceptrum"
Arenaria lanug inosa ssp.
saxosa
Artemisia sp.
Bromus tectorum
Chenopodium album var.
berlandieri
Conyza canadensis"
Dracocephalum parviflorum
Elymus elymoides
Euphorbia serpyllifolia
Galium bifolium
Gayophytum diffusum
Leonurus cardiaca
Machaeranthera canescens
Mirabilis linearis
Muhlenbergia mtmaissima
Penstemon pachyphyllus
Polygonum convolvulus
Polygonum douglasii
Poa pratensis
Senecio eremophilus var.
macdougaJii
Senecio multilobatus
Typha sp.
Verbascum thapsus
Vicia canadensis
Unknown Poaceae
Unknown Boraginaceae
Total
Pole-sized
Ponderosa
Pint
Oak
Locu st
Sagebr ush
3
Total
3
4
4
2
3
6
7
I
7
8
2
I
3
2
3
17
2
2
8
18
I
I
215
15
II
5
8
16
16
I
I
40
7
222
I
I
I
I
I
15
17
6
29
6
5
16
2
I
4
7
6
I
282
444
6
I
I
470
461
I
I
I
34
833
* Includes portions of the bud bank
** Conyza canadensis was also a greenhouse contaminant (none detected in
this component of the study).
tectorum, Chenopodium botrys, Convolvulus arvensis, Polygonum
convolvulus, and Poa pratensis. The Agropyron sp. is potentially non-native.
There are six Agropyron and Elymus species at Mt. Trumbull (both genera
were grouped under Agropyron until recently), and four ofthem are non­
42
native. Although only 23% percent of the species were non-native, these
species comprised 84% ofthe genninants. No woody species were present.
Twenty-seven species germinated from the 0 horizon samples. The
two most common species were again Leonurus cardiaca and Verbascum
thapsus. Together they comprised 86% of the genninants, and Verbascum
thapsus seedlings accounted for 94% of the genninants in the locust canopy
type, while 76% ofthe seedlings in the oak canopy type were Leonurus
cardiaca. At least five species (or 22%) were non-native (Bromus tectorum,
Leonurus cardiaca, Polygonum convolvulus, Poa pratensis, and Verbascum
thapsus (Agropyron sp. is also potentially non-native). One species, an
unknown Boraginaceae, died before it could be keyed to species. Another
unknown grass species did not flower and could not be identified based on its
ligules or other vegetative characteristics.
There was a significantly higher number (p<0.1 0) of viable seeds
found in the mineral soil versus in the 0 horizon. However, when non-native
species were separated from native species, there was a significantly higher
number of non-natives in the 0 horizon. These results are suggestive,
however, because of the differing volume between organic matter samples
and mineral soil samples.
43
Discussion
Soil Seed Bank in Five Canopy Types
Each canopy type at Mt. Trumbull likely has different rnicrosite
conditions, including differing decomposition rates, and variable temperature
and moisture regimes, as well as numerous other biotic and abiotic factors.
These differences may promote or impair the growth of certain species.
Factors other than canopy type can also have significant effects on plant
growth, in particular the soil type, which also varies across the landscape at
Mt. Trumbull. These factors may in tum determine the type of overstory
vegetation that can survive in the different soil types. The low species
richness in the pole-sized ponderosa pine type is most likely a function of
canopy closure which may cause a lack of moisture and nutrients as well as
increased accumulation of needles at the soil surface (Covington et al. 1997).
The high number of viable Verbascum thapsus seeds is related to a high seed
production as well as the ability of its seeds to remain viable for long periods
of time (Thompson et al. 1997).
The number of seeds per square meter in this study was comparable to
other studies in coniferous forests, but results from two other studies in ponderosa
pine were quite different from mine. Pratt et aI. (1984) found a total of 57 species
in their seed bank study located in eastern Washington (4 shrubs, 8 graminoids,
19 annual forbs, 25 perennial forbs and 1 tree) and a total of 8,822 germinating
seedlings. The number of viable seeds/m' was 13,052±1,481 for spring-collected
samples and 14,463±1,356 for autwnn-collected samples. Vose and White (1987)
44
found a total of 3 species and 10 viable seeds in their seed bank samples collected
in northern Arizona. There were 8.4 viable seeds/m' in burned samples and 22.1
viable seeds/m' in control samples. None of their species were identified to taxa,
so a breakdown of life history is not possible. Sample treatments varied as well.
Similar to my study, Pratt et aI. (1984) stratified autumn-collected samples and
churned samples periodically. However, their samples were heat-treated as well.
Vose and White (1987) did not heat or stratify samples, and collected samples in
the spring.
In a ponderosa pine forest in Washington, over 60% of the species in the
soil seed bank were non-native (Pratt et aI. 1984). Many ofthe species in my
study were also non-natives. Two particularly numerous non-native species were
Verbascum thapsus and Leonurus cardiaca. Verbascum thapsus can produce large
numbers of seed that remain viable in the soil for long periods of time, often for
more than 40 years, and perhaps well over a hundred years (Thompson et al.
1997). In studies from western Europe, where Verbascum thapsus is a native
species, the number of viable seeds in the soil has ranged as high as 2,096 seeds/m'
(Thompson et al. 1997), compared to a range of 23 - 4,267 seeds/nr' , depending
on canopy type, in my study. Verbascum thapsus is also a prolific seed producer
(Kivilaan and Bandurski 1981). Both species have medicinal uses; however, they
may be of concern in ecological restoration, based on target restoration goals. If
this is the case, their numbers (as well as those of other non-natives) in the soil
seed bank should be monitored over time.
45
Within ponderosa pine forests, the aboveground vegetation is
scattered or patchy, and this distribution is reflected in the soil seed bank
vegetation as welL Animal species are also know to cache seeds (Harper
1977, Mayer and Poljakoff-Mayber 1982). These phenomena make it
difficult to capture a representative species distribution in soil seed bank
samples. Seeds also tend to congregate around mother plants. This scattered
distribution of species is particularly apparent in much of the Mt. Trumbull
area due to a large number of small-diameter ponderosa pines and a dense
canopy that discourages growth of many of the understory species. In much
of the area, there is also a lack of native perennial grass species. This lack of
plant species, the scattered distribution of species, or the time of collection
may explain the dearth of native perennial grasses as genninants from my soil
seed bank samples. Because perennial grass species did germinate from
some of my seed bank samples, and because there is a small number of
perennial grasses across the landscape, the seed bank samples most likely
indicate a general lack of perennial grass seeds in the soil seed bank.
Intensive grazing, such as occurred in much ofthe Southwest at the turn of
the century (Touchan et al. 1995), may also cause a decrease in the number of
grass species in the soil seed bank (Bertiller 1996). Therefore, these species
may need to be seeded following restoration practices if there are no
colonizing sources.
The majority of species in my study originated from buried seeds;
however, a number of plants also originated from buried rhizomes (Poa
46
pratensis) and corms (Allium bisceptrum) (personal observation), and thus
include portions of the bud bank. While all Allium bisceptrum seedlings
germinated from corms, it was difficult to determine ifthe origin ofthe Poa
pratensis seedlings was from pieces of buried vegetation or from buried
seeds. In a boreal forest in northern Saskatchewan, Archibold (1979) found
that after fire 87% of plants originated from seed and 13% from remnant
roots and rhizomes. Although propagules are not technically considered to be
part of the soil seed bank, but rather a portion of the bud bank (Harper 1977,
Baker 1989, Simpson et al. 1989), I have included them as part of my thesis
because of their importance as a component ofthe vegetation following
ecological restoration.
Because of differing germination requirements, not all species in the
soil seed bank will have been captured in my samples. Although samples
were stratified beforehand to simulate winter temperatures, it was not feasible
to simulate fluctuating temperature, light, and moisture regimes, or fire.
Conditions in the greenhouse provided maximum light, ample moisture, and
adequate nutrients. These conditions most likely promoted the growth of
early successional species that capitalize on decreased competition and
removal ofthe canopy. Many annual species have a number of different
genotypes within an ecosystem to withstand environmental variability
(Bazzaz 1996).
My samples did capture a number of perennials that are not numerous
across the landscape. One or two individuals represented most of these
47
species. Typha sp., for example, was an unusual occurrence in the seed bank
of a ponderosa pine forest. The greenhouse was ruled out as a source of
seeds (Brad Blake, 1998, Northern Arizona University, personal
communication), so the seeds were either persistent in the mineral soil for a
long period of time (two genninants came from the 0 horizon and one from
the mineral soil) or were transient and arrived at the sampling site by airborne
means.
One or a few individuals also represented several other species.
Continued monitoring in the field will determine if these few individuals in
the soil seed bank will act as a sufficient colonizing source, and if control of
non-natives (recruited from the soil seed bank) will be necessary.
Seeds in 0 Horizon versus Mineral Soil
The composition ofthe organic material overlying the mineral soil has
a substantial effect on the seeds that are found within it. Most types of litter
act as seed traps and litter can also inhibit germination (Kitajima and Tilman
1996). Coniferous litter is acidic and requires a greater length of time to
decompose than oak or locust litter (both are deciduous species) (Kimmins
1987). In addition, deep forest floors can be excessively wet, acidic, and cold
(Kimmins 1987). All of these factors can contribute greatly to seed mortality,
and if the litter layer is deep, a large number of seeds will never reach mineral
soiL In addition, some types of pine, including ponderosa, are known to have
an allelopathic effect on grasses and other species (Jameson 1968).
48
In contrast to several other studies in coniferous forests (Archibold
1989), the number of viable seeds at Mt. Trumbull was highest in the mineral
soil. This trend may be because ofthe properties ofthe litter discussed above
or perhaps due to the decreasing number of plants over time, which normally
contribute seeds to the upper layers of soil. Pratt et al. (1984) found a higher
number of viable seeds in a Washington ponderosa pine forest in the litter
than in the mineral soil. More specifically, most grass and annual forb
species seeds were contained in the litter, while those of perennial forb
species were primarily found in the mineral soil. In contrast, no seeds in the
litter germinated in a study by Vose and White (1987) in southwestern
.:
ponderosa pine.
In my study, the species richness of annualslbiennials and perennials
was fairly evenly divided between the mineral soil and 0 horizon (13 species
of perennials in the mineral soil and 14 in the 0 horizon, and 17 species of
annualslbiennials in the mineral soil and 12 in the 0 horizon). In the mineral
soil, approximately 47% of the seeds were from perennial species, and
approximately 32% of the seeds in the 0 horizon were from perennial
species. However, when the two most common species (Leonurus cardiaca
and Verbascum thapsus) were removed from the totals, these numbers were
15% and 32%, respectively.
49
Conclusion
Mt. Trumbull and the neighboring mountains are ponderosa pine sky
islands, isolated from the nearest ponderosa pine forests on the Kaibab
Plateau by nearly 50 km. Opportunities for seed dispersal to and from the
area were historically fairly limited. With Euro-American settlement of the
Arizona Strip has come an increased opportunity for seed dispersal on
motorized vehicles (Clifford 1959, Schmidt 1989), livestock (Dutoit and
Alard 1995), and footwear (Clifford 1956). Concomitant with EuroAmerican settlement has been an influx of non-native species of vegetation
throughout the Southwest, either inadvertently or purposely through use of
non-native species as forage or for soil stabilization. Protecting native
vegetation and preventing introduction of non-native species (particularly
those that are aggressive) is a high priority in ecological restoration ofthese
areas. Seeding or transplanting native species may be necessary to meet
target restoration goals for understory herbaceous and shrub vegetation.
50
CHAPTER 4
MANUSCRIPT
EFFECTS OF TREE THINNING ON THE SOIL SEED BANK IN
SOUTHWESTERN PONDEROSA PINE
Abstract
I examined the soil seed bank in an area of a ponderosa pine forest in northern
Arizona that was undergoing ecological restoration treatments to determine the
relationship between viable seed composition and density in the soil seed bank and
the corresponding aboveground vegetation composition and density that emerges
after tree thinning. My second objective was to determine if there are differences in
the composition and density of the soil seed bank with soil depth following thinning
of overstory trees.
Fourteen species emerged in samples collected after overstory trees were
thinned and prior to prescribed burning, with an estimated seed density of3,152
seeds/mi. Eleven of the species were annuals or biennials and three were perennials.
All were forbs; no perennial grasses or woody species emerged. One of the species
was the non-native Verbascum thapsus. The two most common species were
Collinsia parviflora and Verbascum thapsus, which accounted for 45% and 30% of
the germinants, respectively. There was a significantly greater number of viable
seeds at the 0-5 em depth compared to the 5-10 cm depth (including both organic
51
matter and mineral soil). Aboveground vegetation was also measured in the area.
Fourteen species (and at least one unknown species) occurred in the aboveground
vegetation after tree thinning . Six of these species did not germinate from soil seed
bank samples. Collinsia parviflora was the most common species in the aboveground
vegetation. Although Verbascum thapsus accounted for 30% of the viable seeds in
the soil seed bank samples, it accounted for only 3% of the plants in the aboveground
vegetation one year following thinning. There were approximately 101 aboveground
plants/m' in the thinned area (and this number increased nearly fourfold by the
second year following thinning), but only 0.4 plants/rrr in the unthinned control area.
There was also a significant correlation between the number of plants in the
aboveground vegetation and the number of viable seeds in the soil seed bank in the
thinned area.
Introduction
Several studies have shown that mature temperate forests contain fairly small
soil seed banks (Warr et al. 1993). However, relatively little research has been
conducted on buried seed reserves in coniferous forests (Archibold 1989). Because
disturbance (principally fire) is characteristic of many drier coniferous forests in the
western United States, early successional species are usually well represented in the
seed bank (Archibold 1989), while late-successional species are virtually nonexistent.
Two previous soil seed bank studies from ponderosa pine forests have very
contrasting results. Vose and White (1987), in a study conducted in northern
Arizona, found buried seed densities of 8.4 seeds/rrr' for burned areas and 22.1
52
2
seeds/m in an unburned control. Overstory tree density ranged from approximately
120 stems per hectare in their below-canopy sawtimber stratum to approximately
10,070 stems per hectare in the sapling stratum. Chenopodium sp., Gnaphalium sp.,
and one or more unidentified dicot species were the only species represented. No
seeds germinated from the litter. The authors felt that seed rain is potentially more
important than the buried seed component in contributing new seedlings after burning
in this forest type.
Pratt et a1. (1984) conducted a study in a ponderosa pine forest in eastern
Washington in which 57 species germinated. The estimated seed density was 13,052
± 1481 seeds/rrr for spring-collected samples and 14,463 ± 1356 seeds/m' for
autumn-collected samples. Forty-five percent of the seeds were annual forb species,
and 24 of the species were non-native. Three of these non-native species accounted
for over halfof the seed bank (Poa pratensis, Stellaria media, and Cerastium
vulgatum). The high number ofviable seeds in this study was attributed to several
factors. The most important was probably the location of the study, which was in a
forest-steppe transition zone. The stand was also in an intermediate successional
stage.
My study was initiated to further our understanding of the soil seed bank in
ponderosa pine forests and to determine: 1) the species composition of the soil seed
bank in an area undergoing ecological restoration (primarily thinning of post­
settlement ponderosa pine trees); 2) whether there are differences in soil seed bank
composition with depth in the soil following overstory thinning treatments, which
may cause some soil disturbance; 3) the short-term effects of thinning practices on
53
the soil seed bank and the aboveground vegetation; and 4) the relationship of seeds in
the soil seed bank to plants in the aboveground vegetation.
Study Area
The study site is located within the Arizona Strip, the portion of northern
Arizona that lies between Utah to the north and the Colorado River to the south. My
study was conducted within an area between the Mt. Trumbull and Mt. Logan
Wilderness Areas, in the treatment unit marked as 96-1, a few hundred meters
southwest ofEB-l (Figure 4.1). The approximate location of the study area is
36°22'30" N latitude and 113°10' W longitude. The elevation in the study area
ranges from approximately 2,080 m to 2,290
rn,
Much of the topography in the
region is comprised of relatively recent volcanic rocks on top of older sedimentary
rocks (Koons 1945), and the parent material for the soils in the vicinity ofMt.
Trumbull is primarily derived from volcanic materials. The main soil type is Lozinta,
an ashy skeletal over fragmental or cindery, mixed, mesic, Vitrandic Ustochrept
(Alfred Dewall 1996, personal communication, unreferenced), although there are
several other soil types in and around the study area.
Climatological data for the Arizona Strip are limited due to sparse
human settlement of the area. Most of the data currently available were
collected from cities surrounding the Arizona Strip and were gathered
beginning in the early to mid-1900s. The regional climate differs from year
to year because the Arizona Strip lies on a monsoonal boundary. As in much
of the Southwest, precipitation occurs annually in a bimodal distribution
54
pattem Thirty-four percent of annual precipitation falls in the winter months
of November through February. In years of summer monsoon, an average of
35% of annual precipitation falls during the months of July through
September (Altschul and Fairley 1989). Elevation also controls climate to a
large degree in the Arizona Strip. Annual precipitation in the ponderosa pine
type varies between 38-64 em (USDI BLM 1990). There are no
perennial streams in the study area, but there are several springs in the
vicinity ofMt. Trumbull and Mt. Logan.
The majority of the study area is the ponderosa pine forest type,
which is bordered by pinyon-juniper woodland. The ponderosa pine forest
type in the Arizona Strip occurs at elevations of approximately 1,830-2,440
m (USDI BLM 1990). Over 250 species of forbs, grasses, and shrubs have
been identified in the ponderosa pine type and its transition zones in the
vicinity of Mt. Trumbull and the surrounding mountains (Appendix A).
Preliminary data show that the last widespread, naturally occurring fire in the
vicinity ofMt. Trumbull was in 1870. Fire scars collected from individual ponderosa
pine trees indicate that fires occurred every four to six years prior to Euro-American
settlement and use of the area (pete Fule 1997, personal communication,
unreferenced).
An ongoing ecological restoration project, which encompasses my study area,
contains several treatment units, based on the year in which ecological restoration
thinning and burning treatments are scheduled for initiation. The area utilized for this
study was a treatment unit approximately 13 hectares in size (33 acres). This area
56
was thinned in the summer of 1996 to recreate presettlement tree forest structure
(density and pattern). A demonstration plot nearby had an overstory tree density in
1995 of approximately 1396 trees/ha. These trees were thinned to approximately 104
trees/ha in 1996 (Pete Fule and Amy Waltz 1996, Northern Arizona University,
personal communication, unreferenced). A prescribed broadcast burn was conducted
in the autumn of 1996.
Methods
Effect of Thinning Practices on the Soil Seed Bank
Field Methods
Twenty-seven herbaceous understory plots were established in 1996, after
thinning of overstory postsettlement trees and prior to prescribed burning. These
plots were located across a northeast-facing slope, and were 2 x 3 m in size with six 1
m2 subplots contained within each plot (Figure 4.2). Five of these 2 x 3 m plots were
established on skid trails, five in undisturbed areas (areas within the thinned unit
where the surface was not directly impacted by thinning) and twelve plots in thinned
areas that were covered with thinning slash. The twelve plots covered with moderate
levels of thinning slash would later be divided into two groups - one halfwould be
burned and one half would be left unburned. Two oftbese plots were used as
"insurance" in case the prescribed burn jumped the fireline into a plot that was to
remain unburned, and would be discarded if not needed (seed bank samples were
collected in these plots, however). An additional five plots were also established in a
nearby control area which was unthinned and unburned.
57
Bouteloua gracilis
C - Control
Poa fendleriana
X
X
X
X
X
X
Carex occidetalis
Elymus elymoides
Empty - (soil seed bank)
X
X
X
X
-
X
X
Figure 4.2. Example of2 x 3 m herbaceous sampling plot. Note six 1 m2 subplots.
Soil seed bank samples were collected in all 27 understory plots in September
1996, after thinning and prior to burning. Two soil seed bank samples (to a depth of
5 em) were extracted with a 5 em diameter bulk density hammer (slide hammer) from
a randomly selected 1 m2 subplot (labelled "Empty") (n=54) (Figure 4.2). Fifty-four
additional seed bank samples were collected from each 2 x 3 m plot and divided into
depths of 0-5 em and 5-10 cm. Organic material was not intentionally included or
removed; therefore, samples often included organic material because of the soil
churning action ofthe mechanical thinning equipment. The subplot labelled
"Control" would later be used to inventory aboveground vegetation (discussed in the
next section).
58
Greenhouse Methods
For a discussion of the various methods used for quantification of the seeds in
the soil seed bank, see the discussion in the Literature Review. For this study, I used
the seed emergence method outlined in Warr et al. (1993), Brown (1992), and Gross
(1990) because of its ability to determine the viable fraction of seeds; mainly those
that would emerge following a disturbance such as thinning of the forest canopy.
Samples were placed in cold storage (4° C) for 8 weeks and then spread thinly over a
layer of approximately 5 ern of sterile soil in 15 x 20 cm plastic flats which were
placed in a greenhouse. Six flats holding only a mixture of sterilized soil were
randomly placed among the flats to account for species germinating due to possible
greenhouse contamination. All flats were fertilized approximately every two weeks
with a dilute nutrient solution (Miracle-Gro® and Miracid®, alternatively).
Seedlings were inventoried at 10-14 day intervals. As seedlings germinated and
could be identified, they were removed from the flats to prevent competition
between seedlings. Flats were watered as needed to maintain moist soil until the end
of the study in May 1997. Although seeds have been shown to germinate in the
second year after initiation of seed bank studies, most studies have shown that the
majority of seeds germinate within the first two months (Warr et al. 1993).
According to Warr et aI. (1993) there is little to be gained by prolonging seed bank
studies for longer than six months.
59
Statistical Methods
A paired sample t-test was conducted between the two depths at which seeds
were collected (0-5 em and 5-10 ern) to determine if there was a significant
difference in the mean number of seeds at these two depths. A square root
transformation was necessary to normalize the data.
Scientific Nomenclature
Scientific names were taken mainly from Utah Flora (Welsh et aI. 1993).
Scientific names for those species not found in Utah Flora were taken from
Intermountain Flora (Cronquist et aI. 1972, 1977, 1984, 1989, 1994, 1997) and
Arizona Flora (Kearney and Peebles 1960), respectively.
Relationship of Seed Bank to Aboveground Vegetation
Field Methods
Aboveground vegetation was tallied in a randomly chosen 1 m2 subplot in
each 2 x 3 m herbaceous understory plot (labeled "Control") (Figure 4.2). The
aboveground vegetation was sampled three times during the 1997 growing season to
determine ifthere was a relationship between the species in the current aboveground
species composition and the species in the soil seed bank. Although the herbaceous
understory plots were subjected to five treatments, only the three unburned treatments
will be discussed here (control, skid trail, unburned slash). Aboveground vegetation
was also inventoried in the same subplots in August 1998 to gain information on the
role of the soil seed bank in plant succession.
60
Statistical Methods
The data for this portion of the study were relatively normally distributed and
a square root transformation failed to improve the distribution. A Pearson correlation
was used to determine if there was a significant relationship between the
aboveground vegetation and belowground soil seed bank in the thinned area and in
the control. A regression equation was developed to determine if the quantity of
aboveground vegetation could be predicted from viable seeds collected in seed bank
samples. Wilcoxon Signed Ranks Tests were also used to determine if there were
significant differences between the thinned area and the control in the number of
seeds in the soil seed bank and in the aboveground vegetation.
Results
Effect of Thinning Practices on the Soil Seed Bank
Fourteen species germinated in this study, with an estimated density of3,152
seeds/m' in the samples collected after overstory trees were thinned and prior to
prescribed burning. These species, their total number of viable seeds, and their
estimated seed denslty/rn' are given in Tables 4.1 and 4.2.
Eleven ofthe fourteen species (or 75%) that emerged in the study were
annuals or biennials. The three perennial species that germinated were Erigeron
divergens, Senecio eremophilus var. maedougalii, and Verbena braeteata. No
perennial grasses emerged. The three most common species were Collinsia
61
Table 4.1. Nwnber of viable seeds that emerged from samples collected at Mt.
Trwnbull, September 1996, after thinning and prior to burning (0-5 em samples n=81,
5.1-10 ern, samples n=27).
Species
Chenopodium album var. berlandieri
Chenopodium graveolens
Chenopodium leptophyllum
Collinsia parviflora
Conyza canadensis *
Erigeron divergens
Euphorbia serpyllifolia
Gayophytum diffusum
Mimulus rubellus
Muhlenbergia minutissima
Polygonum douglasii var. johnstonii
Senecio eremophilus var. macdougalii
Verbascum thapsus
Verbena bracteata
Total
* Conyza canadensis was also a greenhouse
Sampling Depth
0-5 em
5-10 em
4
1
2
110
38
I7
2
2
2
1
2
58
II
Total
5
1
2
148
19
4
I
2
69
I
1
3
I
132
3
337
contaminant
3
I
139
3
398
7
61
Table 4.2. Estimated seed density/rrr of species that emerged from seed bank
samples collected at Mt. Trumbull, September 1996, after thinning and prior to
burning.
Sampling Depth
0-5 em
5-10 em
25
19
6
12
687
717
99
44
13
6
II
324
198
6
17
6
811·
132
Species
Chenopodium album var. berlandieri
Chenopodium graveolens
Chenopodium leptophyllum
Collinsia parviflora
Conyza canadensis*
Erigeron divergens
Euphorbia serpyllifolia
Gayophytum diffusum
Mimulus rubellus
Muhlenbergia minutissima
Polygonum douglasii var. johnstonii
Senecio eremophilus var. macdougalii
Verbascum thapsus
Verbena bracteata
1
Total
2,042
* Conyza canadensis was also a greenhouse contaminant
62
Total
44
6
12
1,404
143
13
6
11
522
6
17
6
943
1
1,110
3,152
parviflora, Mirabilis rubellus, and Verbascum thapsus. One species (Verbascum
thapsusy was non-native. Collinsia parviflora and Verbascum thapsus accounted for
45% and 30% of the genninants, respectively. Note that Conyza canadensis was also
a greenhouse contaminant, and these numbers may be inflated by as much as 5-10%.
There were also differences between the species with viable seeds in the seed
bank between the thinned area and the control. Only three species germinated from
samples collected in the control (Gayophytum diffusum, Mirabilis rubellus, and
Verbascum thapsusy; versus thirteen species in the thinned area
Although all fourteen species were found at the 0-5 cm depth, only six of
these species (36%) were also found at the 5-10 cm depth, and these species were all
annuals or biennials, with the exception of Erigeron divergens. Seeds found at the
greater depth can be assumed to consist entirely of persistent species and are assumed
to be older than those higher up in the soil profile (Warr et aI. 1993), unless their
presence is due to contamination from previous sampling or from movement through
the soil profile. The paired samples t-test had a significance ofp<0.004. Thus, there
was a significantly greater number of seeds at the 0-5 em depth, compared to the 5.1­
10 em depth.
Aboveground Vegetation
Fifteen species (one was an unidentifiable annual) were inventoried in the
aboveground vegetation in the growing season following restoration treatments in the
thinned and unburned plots. Only one species, Pinus ponderosa, germinated in the
63
untreated control area. These species are listed in Table 4.3 and their numbers are
shown in Figure 4.3.
Table 4.3. Species observed growing aboveground in three different treatments at
Mt. Trumbull, 1997.
Species
Skid trail
Argemone munita
Chenopodium album var. berlandieri
Chenopodium leptophyllum
Collinsia parviflora
Elymus elymoides
Euphorbia serpyllifolia
Euphorbia lurida
Gayophytum diffusum
Mimulus rubellus
Nicotiana attenuata
Penstemon barbatus
Pinus ponderosa
Polygonum douglasii var. johnstonii
Verbascum thapsus
Unknown annuals
X
X
X
X
X
X
X
X
X
X
Unburned slash
Control
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Thirteen species were inventoried on plots located on skid trails, with a total
of 699 plants. Ten species were inventoried on plots which contained unburned slash
from thinning, and there were 320 plants. There were approximately 101 plants/rn' in
the thinned area and 0.4 plants/m' in the control (Table 4.4). The most common
species in the thinned area were Collinsia parviflora and one or more unknown
annuals, which I was unable to identify at the time of inventory without destructive
sampling. Collinsia parviflora comprised 45% of the genninants in the seed bank
samples and 57% of the plants in the aboveground vegetation in the treated area.
While Verbascum thapsus had a high number of viable seeds in the seed bank
samples, its aboveground vegetation was only 3% ofthe total vegetation.
64
492
200
-...
1"1
,,
,
,,
1\
180
160
140
b
~
Q)
~
100
]
80
....0
0-
VI
~
~
~
~
~
60
~
~
40
~
"
20
~I ~
0
~
~
....
~
Q,~
#
~f:>
~
~
~~
G~Qj
\Q,~
:(.
~'If:>
~\V>~
o~o
~Q,~
G
\()
~\~
c
.$>
¢'..f
~o~
'9V>~
.).~~
~
.().,Qj
o~'I;,
~. ~I ~
~
-
fWI
/ ~ # (
# ~ ,
o~Q,~ 'Cl'~ ~o~~4, ~~f' ~lO ~\()\V ~~~\
~~
o Control
c
1\
1\
0'
s
t5.
Skid Trail
• Slash/No Bum
1\
120
~
~
~
1\
~~
~~4,
6~
~~ Ei
"...
....
....
....
, ~ /
~ ~ ;
~
)
~\~V> . e,~~ ()\\Q,~ 'C~'C o~~Q, . (f~"'\ ~~(J.~ #' f;}~~
:c~\
~~()
. ~~
\Cp~
~
o~
l~
~\Q,~
<t-~
~Q,
.~
~ .'i
e,\\'l
it
~ov
~
~
~<t4,
Figure 4.3. Number of plants of each species in the aboveground vegetation in 1997, by treatments, Mt. Trumbull, AZ.
~
l
Q,f
rj
0
\)~
Table 4.4. Mean number of plants of each species/rrr observed in the aboveground
vegetation at Mt. Trumbull, 1997. These numbers are averaged over 15 plots (treated
area n= I0, control n=5).
Species
Argemone munita
Chenopodium album var. berlandieri
Chenopodium leptophyllum
Collinsia parviflora
Elymus elymoides
Euphorbia serpyllifolia
Euphorbia lurida
Gayophytum diffusum
Mimulus rubel/us
Nicotiana attenuata
Penstemon barbatus
Pinus ponderosa
Polygonum douglasii var. johnstonii
Verbascum thapsus
Unknown annuals
Total
Treated area/m 2
0.1
4.3
1.2
57.6
0.1
1.5
0.2
5.0
3.5
1.7
0.1
ControVm 2
0.40
2.3
2.9
20.5
101
0.40
Out of a total of 17 species (not including the unknown annual), 8 species
were found in both the aboveground vegetation and the soil seed bank (47%). Six of
the species found in the aboveground vegetation did not germinate from soil seed
bank samples collected in the study (35%) (Table 4.5). These species were
Argemone munita, Elymus elymoides, Euphorbia lurida, Nicotiana attenuata,
Penstemon barbatus, and Pinus ponderosa (Argemone munita, Elymus elymoides,
and Penstemon barbatus did germinate in the baseline study (see Chapter 3». Of
these six species, four are perennials; Elymus elymoides (grass), Pinus ponderosa
(tree), and Euphorbia lurida and Penstemon barbatus (both perennial forbs).
Nicotiana attenuata is known to have higher germination rates in the presence of
smoke than in unburned sites (Baldwin et al. 1994) .
66
Three species found in the seed bank samples, but not in the aboveground
vegetation were Conyza canadensis, Erigeron divergens, and Muhlenbergia
minutissima. These three species have been located in the vicinity of the plots, so
Table 4.5. Species found in the seed bank in an approximately 13 ha (33 acre) area at
Mt. Trumbull, after thinning and in the aboveground vegetation in the following
growmg season.
Species
Abovrground ngdatio Seed bank post-thinnin,
Argemone munita
Chenopodium album var. berlandieri
Chenopodium leptophyl/um
Collinsia parviflora
Conyza canadensis
Elymus elymoides
Erigeron divergens
Euphorbia serpyllifolia
Euphorbia lurida
Gayophytum diffusum
Mimulus rubel/us
Muhlenbergia minutissima
Nicotiana attenuata
Penstemon barbatus
Pinus ponderosa
Polygonum douglasii var. johnstonii
Verbascum thapsus
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
their absence in the aboveground vegetation is probably due merely to the scattered
distribution of vegetation which did not encompass the subplots.
The Pearson correlation between the aboveground and belowground soil seed
bank vegetation in the thinned area was significant at the 0.01 level (r 2=O.775),
indicating a positive relationship between the number of seeds in the soil seed bank
and the number of plants in the aboveground vegetation. However, there was not a
significant correlation between aboveground vegetation and soil seed bank vegetation
in the unthinned control.
67
A regression equation was developed for the data to determine if the number
of aboveground plants can be predicted from the number of viable seeds collected in
seed bank samples. The regression equation is Y=1.094 + 0.027X; where X= the
number of viable seeds in the soil seed bank based on seed emergence, and Y = the
number of plants in the aboveground vegetation in thinned areas. Further refinement
of this equation is necessary before it can be used to predict aboveground vegetation
in other areas because of the microsite conditions at the study site. Using the
equation mentioned above, a significant relationship was found in the thinned area
between number of seeds in the soil seed bank and the number of plants in the
aboveground vegetation (R2=0.948; p<O.OOOl). Again, the relationship between the
aboveground vegetation and belowground soil seed bank in the unthinned control was
not significant. A slightly significant difference was found between the aboveground
vegetation and the seeds in the soil seed bank in the thinned area and in the control
(both at p=O.l05) using the Wilcoxon signed ranks test.
To gain greater insights into the value ofthe soil seed bank and its role in
plant succession, I inventoried the aboveground vegetation on the understory plots
two years after thinning. The following tables (Tables 4.6 and 4.7) list the species
found in the aboveground vegetation in the same subplots that were inventoried the
previous year. Many of these species probably germinated from seeds produced by
the previous year's vegetation, or from seed rain, but some may have germinated
from seeds in the soil seed bank that required a longer period of time for the
appropriate germination cues to occur. The value of this data is to show the path of
succession separate from the species that originated from the soil seed bank the first
68
year after the thinning disturbance. These data also give some indication of species
richness and abundance under field conditions versus the species that will germinate
from samples in the greenhouse.
Table 4.6. Species observed growing aboveground in three different treatments at Mt.
Trumbull, 1998 (two years after thinning).
Species
Skid Trail
Bromus tectorum
X
Chenopodium album var. berlandieri
X
Chenopodium leptophyllum
X
Conyza canadensis
X
Elymus elymoides
X
Erigeron divergens
X
Gayophytum diffusum
X
Lactuca serriola
X
Lotus wrightii
Microsteris gracilis
x
Pinus ponderosa
Polygonum douglasii
X
Senecio eremophi/us var. macdougalii
X
Senecio multi/obatus
X
Verbascum thapsus
X
Unburned Slash Control
X
X
X
X
X
X
X
X
X
X
X
Fourteen species were recorded in the thinned area and one (Pinus ponderosa)
in the controL There were approximately 391 plants/nr' in the treated area and 0.2
plants/m' in the control (Table 4.7).
A comparison of Tables 4.4 and 4.7 reveals that, out ofa total of22 species,
there were eight species found only in 1997, eight species found only in 1998, and
seven species found in both years (Table 4.8).
69
Table 4.7. Average number of plants of each species/rrr observed in the aboveground
vegetation at Mt. Trumbull, 1998 (treated area n=10, control n=5).
Species
Bromus tectorum
Chenopodium album var. berlandieri
Chenopodium leptophy/lum
Conyza canadensis
Elymus elymoides
Erigeron divergens
Gayophytum dijfusum
Lactuca serriola
Lotus wrightii
Microsteris gracilis
Pinus ponderosa
Polygonum douglasii var. johnstonii
Senecio eremophilus var. macdougalii
Senecio multi/obatus
Verbascum thapsus
Total
Treated area/nr'
0.1
264.5
0.9
0.6
1.7
Coutrol/nr'
2.8
69.2
6.8
0.2
17.1
0.2
22.7
1.4
0.2
2.8
391.0
0.2
The number of plants in the aboveground vegetation/m' is shown in Figure
4.4. Some species had a small number of plants in the first year, but produced
enough seeds to increase greatly in number in the second year (Chenopodium album
var. berlandieri and Gayophytum diffusum). Some seeds may also have entered the
plots through seed rain The majority of species were found only in year one or year
two.
70
Table 4.8. A comparison ofthe aboveground vegetation thinned area in 1997 and
1998, Mt. Trumbull, AZ.
Species
Argemone munita
Bromus tectorum
Chenopodium album var. berlandieri
Chenopodium leptophyllum
Collinsia parviflora
Conyza canadensis
Elymus elymoides
Erigeron divergens
Euphorbia lurida
Euphorbia serpyllifolia
Gayophytum diffusum
Lactuca serriola
Lotus wrightii
Microsteris gracilis
Mimulus rubellus
Nicotiana attenuata '
Penstemon barbatus
Pinus ponderosa
Polygonum douglasii var. johnstonii
Senecio eremophi/us var. macdougalii
Senecio multi/obatus
Verbascumthapsus
Unknown annuals
71
1997
1998
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
265
~
70
~
60
1.1997 1
1998 [
It!
~
~ 50
I
(
8
f.
~
g!!l
0'
g
40
fa
--l
IV
c.
....
0
] 30
E
:l
,
Z
l
Ilii
20
I
I
10
-.J
rr
o
i:-'I,I,~
~:~
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....
~
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~~~ ~I,~tf! t-\~<' ~~~~
\~
<It'
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~
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. 'bot>
e,~
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Yoc,
o~
o~
. c,e,
t$~ '9~o ~~~ ~'I,.f ~~.. (J'l! ~~ {:o~ ~v~ ~'I,~
~V'
rf
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if
o
(J'
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t
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~'I,-'
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r
~I,v-'
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(j~
o~~
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'J'''
'l /
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y~
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$Qo" ,*v-'
'bo~
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~\
c,'boovOJ
. o. : XJ~ . #
o~
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~'I,c;
~~ ."o'¢-Oo ....I,~ ...~~ ~~.
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(f
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Figure 4.4. A comparison of the aboveground vegetation in the two years following overstory thinning, Mt. Trumbull, AZ.
~l
v~
~~?JO ~:s-~ #' ~~~
;#
'0<'
Discussion
The most numerous species in the soil seed bank samples from Mt. Trumbull
appears to be the non-native biennial Verbascum thapsus. The other most common
species are annuals, in particular the Chenopodium species, Collinsia parviflora,
Conyza canadensis, and Mimulus rubel/us. Conyza canadensis is found both in the
field in the aboveground vegetation and as a greenhouse contaminant, and thus its
numbers in the seed bank in this study should be viewed with some amount of
caution (numbers may be inflated by as much as 5-10%). Some ofthese species,
such as Verbascum thapsus can have a fairly high longevity in the soil, perhaps over
100 years (Kivilaan and Bandurski 1981), although its longevity in many studies has
been questioned. This trait may allow Verbascum thapsus to occupy the soil seed
bank for decades following ecological restoration treatments.
Verbascum thapsus, and other common species in my study have been found
to have a high seed density in the soil in other studies. Thompson et aL (1997) give
densities from Europe as high as 23,300 seeds/m' for Chenopodium album, 346
seeds/rrr' for Conyza canadensis, and 2,096 seeds/m' for Verbascum thapsus. Vose
and White (1987) found a small number of Chenopodium sp. in the soil seed bank
near Flagstaff: AZ. The most common species found in my study were not shared by
Pratt et al. (1984) in their Washington study. They found approximately 50
Chenopodium album seeds/m', 33 Verbascum thapsus seeds/nr', 17 Conyza
canadensis seeds/rn', and 117 Collinsia parviflora seeds/rrr'. It appears from my
73
study and that of Pratt et al. (1984) that there can be major differences in viable seed
density within the soil seed bank across the landscape in ponderosa pine forests.
The importance of various life forms represented in the seed bank varies
across ecosystems. Herbaceous species tend to dominate temperate soil seed banks
(Warr et al. 1993). However shrubs and trees can be well represented in the soil seed
bank in some conifer-dominated ecosystems (Archibold 1979, Morgan and
Neuenschwander 1988). My results indicated that perennial grasses, shrubs, and
trees are not well represented in the soil seed bank at Mt. Trumbull (see Chapter 3).
Pratt et al. (1984) had similar results from a ponderosa pine forest in eastern
Washington. The soil seed bank there was composed of 45% annual forbs, and
woody species accounted for only 1%. A relatively high number of perennial grasses
were found in soil seed bank samples collected in May from a ponderosa pine forest
in northern Arizona (Brandon Harper and Margaret Moore, 1998, Northern Arizona
University, personal communication, unreferenced). Tree stands had been thinned
approximately five years earlier, and part ofthe site had been burned by prescription
approximately three years earlier. Species included Blepharoneuron tricholepis,
Festuca arizonica, Muhlenbergia montana, Elymus elymoides, Poafendleriana, Poa
pratensis, and others. The presence of these grasses indicates that these species are
capable of utilizing the soil seed bank. Absence of perennial grass species in my
study at Mt. Trumbull may be due to the time of collection (mainly autumn), missed
germination cues in the greenhouse, or most likely to the loss of these species from
the aboveground vegetation and the soil seed bank over time.
74
The majority of seeds in the thinned area were in the upper 5 ern of soil,
which generally included some organic material. In most studies, the greatest
numbers of seeds are in the 0 horizon (litter) (Moore and Wein 1977, Pratt et al.
1984, Rydgren and Hestmark 1997). Pratt et al. (1984) found the viable seeds of
most grass and annual species were contained primarily in the litter, probably due to
either the short longevity of these species or to their recent introduction into the litter
where the litter acts as a seed trap. Conversely, the seeds of perennial forbs were
contained primarily in the mineral soil, suggestive of earlier introduction of these
species and their increased longevity. Germinating seeds in studies where litter is
removed mainly consist of the persistent seeds. Seeds from the 5-10 cm samples in
my study were probably persistent because they were found in the mineral soil. One
difficulty with sample collection is the presence or absence of litter. Litter can be
included in a 0-5 em sample, or it can be removed prior to sampling. Thus the
volume of samples at 0-5 cm may differ. If litter is included, the question arises as to
what depth the sample should be extracted. If litter is removed before a sample is
taken, a whole host of seeds will not be included in the sample: These factors should
be kept in mind when interpreting results that compare differences in the soil seed
bank with soil depth.
The number of viable seeds decreases with soil depth in my study, which is
consistent with other studies (Kellman 1970, Moore and Wein 1977, Pratt et al. 1984,
Kramer and Johnson 1987, Archibold 1989, McGee and Feller 1993). Note that the
previous manuscript chapter involved determining differences between organic
matter and mineral soil, while this chapter describes differences in depth. Earthworms
75
playa significant role in vertical transport of seeds in many ecosystems (Willems and
Huijsmans 1994). However, because of a relative dearth of earthworms and other
soil fauna in coniferous forests, vertical transport of seeds by these organisms is
probably insignificant (McGee and Feller 1993, Rydgren and Hestmark 1997). In
southwestern ponderosa pine forests undergoing ecological restoration (including
thinning of post-settlement ponderosa pine trees), or those that have been logged in
the past , thinning equipment probably has a much greater impact on the movement of
seeds in the soil than do most soil organisms.
Thinning the postsettlement pole-sized trees had a significant impact on the
emergence of species from the soil seed bank into the aboveground vegetation.
Thinning allows sunlight to reach the forest floor which stimulates germination of
many species (Baskin and Baskin 1998). Figures 4.5 through 4.8 depict the
conditions available at my study site for germination prior to and after thinning, and
show the emergence of vegetation in the aboveground vegetation. Figure 4.5 shows a
heavy layer of organic matter on top of the mineral soil and a dense tree canopy.
After thinning of postsettlement trees, there is increased light (Figure 4.6); however,
the dense organic layer will remain until a fire or some other disturbance removes it.
The successful establishment of vegetation from the soil seed bank in
coniferous forests requires a reduction in surface organic matter, which occurs
naturally with fire (Archibold 1989). However, mechanical thinning produces some
dispersion ofthis organic matter and may bring seeds to the surface that would
otherwise never germinate, while at the same time burying others. Certain seeds
76
buried at depth, which are perhaps now uncommon in the aboveground vegetation,
may be brought to the surface where conditions are more amenable to germination.
There is generally a poor correspondence between the aboveground
vegetation and the seeds in the soil seed bank (Whipple 1978, Pratt et al. 1984,
McGee and Feller 1993, Rydgren and Hestmark 1997). I found the same result in the
unthinned control area of my study. However, I found a high correlation between
these two variables following thinning. Litter generally has a viable seed species
composition more similar to the aboveground vegetation than does the mineral soil
(Rydgren and Hestmark 1997) for reasons outlined earlier. In nitrogen-deficient
sites, there may also be a high correspondence between the species in the
aboveground vegetation and in the soil seed bank, because nitrogen addition
suppresses germination of some species (Kitajima and Tilman 1996).
I also found some species in the aboveground vegetation that were not in the
soil seed bank. Possible reasons for this phenomenon include spottiness of seeds in
the soil (which was not captured by my samples), season of collection, the seeds
never existed in the soil seed bank (and entered the system through seed rain), and/or
required germination cues that did not occur in the greenhouse. Germination cues
provided a whole suite of problems. Ideally, all samples should have been subjected
to numerous cues including excess water as well as periods of insufficient moisture,
varying amounts of light, smoke, varying temperatures, etc. Covering the whole suite
of germination cues is not possible in a study ofthis type. Therefore, it is difficult to
determine how many viable seeds are actually present in a sample of soil. The
79
presence or absence of aboveground vegetation and also the seed rain at the site will
give some indication of the species that may be in the soil seed bank. Another option
is to use the seed extraction method to count seeds.
Those species present in the aboveground vegetation only in 1997 (the fIrst
year following thinning) are most likely dependent on disturbance for germination
and were able to restock the seed bank after reproduction. Those present only in
1998 probably require a longer period oftime to germinate from the soil seed bank,
required some germination cue that occurred in the second year, or their seeds were
introduced from off-site by wind, water or animals. Vose and White (1987) found
seed rain to be an important source of colonization following disturbance, such as
fire. The majority of species were found in only one year. A large number of
Collinsia parviflora plants were observed in 1997 and none in 1998. This could be
due to the time of sampling. An inventory was conducted three times in 1997
(including spring), but only once in 1998 (autumn). Since this annual species
flowers early in the season, it may have been overlooked in 1998.
I found few plants to act as a colonizing source outside of the thinned area.
The ability of an area to become revegetated following disturbance may be limited to
the dispersal ability of species, with edges often having the highest diversity (Schott
and Hamburg 1997). Probably only sites contiguous to the established community
receive enough seed rain to produce much new vegetation. After logging and
burning in a Douglas-fir forest (Clark and Wilson 1994), relatively few
individuals established from seed after the first year. However, the second year after
disturbance, there was a tremendous increase in individuals establishing from seed.
80
Conclusion
Most of the viable seeds are found in the upper few centimeters of the 0
horizon and mineral soil at the thinned portion of my study site at Mt. Trumbull, AZ.
Thinning practices did not significantly alter the distribution of seeds in the soil
profile. However, thinning the postsettlement trees in southwestern ponderosa pine
produces a tremendous increase in the composition and frequency of aboveground
vegetation from the soil seed bank. Whether this increase is attributable to increased
light or moisture or some other environmental factor, or combination of factors, was
not measured or determined by this study.
The results of my study suggest that the soil seed bank is important for
recolonizing a site after a disturbance such as thinning. The most common species in
the soil seed bank at Mt. Trumbull are early successional annuals and biennials.
These species help to stabilize the soil following disturbance, act as fuel for fire, and
may prevent the establishment of non-natives. However, certain non-natives are also
very common in the soil seed bank, particularly Verbascum thapsus, in the upper 0-5
em of the soil. It may be important to follow any thinning practice by a prescribed
burn to remove some non-native seeds and for germination cues of species that
require heat or smoke. The greatest number of viable seeds in the soil seed bank at
Mt. Trumbull (based on seed emergence) are in the top 5 em ofthe mineral soil.
Thus fire might be used as a tool to manipulate the species in the soil seed bank or the
aboveground vegetation. Fire also decreases accumulated litter, releases nutrients,
and stimulates germination of some species. Fire (heat or smoke) may be required as
81
a germination cue for some species (Baldwin and Morse 1994, Keeley and
Fotheringham 1997), and may be necessary for long-term germination success of
some species, such as Nicotiana attenuata in this study
82
CHAPTERS
CONCLUSIONS AND MANAGEMENT IMPLICATIONS
The importance of the soil seed bank at a particular site will vary across
southwestern ponderosa pine forests, and is dependent on current and past land use,
disturbance history, and past and current vegetation at the site. Continuous heavy grazing
and high intensity fires can destroy seed sources, both in the soil seed bank and in the
aboveground vegetation (Bertiller 1996, Ghermandi 1997, Trabaud et al. 1997).
My results indicate that although perhaps less than 5% of the Verbascum thapsus
in the soil seed bank will germinate in the first year following thinning (based on
comparisons between numbers of viable seeds in the soil seed bank and number ofpJants
in the aboveground vegetation), this species may account for as much as 45% of the
viable seeds in the soil seed bank. In addition, this species is opportunistic in its ability to
germinate in stages to capitalize on new environmental conditions, such as when the
herbaceous vegetation around it is removed (personal observation). This habit allows it
to easily colonize a site after disturbance. My results also indicate that non-native species
may be associated with particular overstory tree species, especially New Mexico locust.
Also of concern is the Jack of native perennial grasses in the soil seed bank at Mt.
Trumbull, as well as in the aboveground vegetation. These grasses are important for
nutrient cycling, as forage, and as a source of fuel for low-intensity fires. They also
function to stabilize the soil and protect it from erosion. Whether seeding is required
83
after ecological restoration treatments at a particular site will be dependent on the on-site
vegetation, both aboveground and in the soil seed bank. Because perennial grasses are
not well represented in the seed bank at Mt. Trumbull. it may be necessary to seed native
grass species to meet particular management goals and to achieve target restoration goals.
Another desirable restoration goal may be to seed other native species in order to
return vegetation at the site to presettlement conditions. The species chosen for a
particular seed mix will be based on such criteria as cost, functional role in the
ecosystem, whether it serves as a food source, the location of collection, use as an erosion
control. and other factors. Disadvantages of seeding are the potential not only of
introducing new genetic strains of species already existing at the site, but also the
potential of introducing entirely new species which have never been native to a region, or
even native to the U.S.
The greatest number of viable seeds in the soil seed bank at Mt. Trumbull (based
on seed emergence) are in the top 5 em ofthe mineral soil. Only the seeds in the top few
centimeters will germinate unless some type of disturbance or soil fauna moves seeds
closer to the surface and therefore closer to the light levels required for germination of
most species. Although thinning the overstory trees will allow increased light levels to
reach the ground, it may be necessary to remove organic matter through prescribed
burning to permit light to reach the viable seeds in the mineral soil. Thus fire might be
used as a tool to manipulate the species in the soil seed bank or the aboveground
vegetation. Of particular interest is whether fire can be used to control non-native species
by destroying viable seeds in the top few centimeters of soil or litter. Fire also decreases
accumulated litter, releases nutrients, and stimulates germination of some species. Fire
84
(or smoke) may be required as a germination cue for some species (Baldwin and Morse
1994, Keeley and Fotheringham 1997), and may be necessary for long-term germination
success of some species, such as Nicotiana attenuata, in this study. The heat of a low
intensity fire will penetrate, at most, only a few centimeters into the mineral soil (Moore
and Wein 1977). A large number of seeds will remain viable in the mineral soil to
potentially recolonize an area. The severity of a burn can become a selective force in seed
germination (Morgan and Neuenschwander 1988). The location of seeds in the soil
profile can also have a significant effect on survival. For instance, large accumulations of
seed in the 0 horizon may be destroyed by fire (Moore and Weinn 1977, Morgan and
Neuenschwander 1988). Thus, unburned areas, such as skid trails, can provide areas for
recruitment and act as safe sites from fire for both native and non-native species. This
allows species, which might otherwise be harmed by fire, to germinate, reproduce, and
replenish the soil seed bank. However, soil compaction may also be a problem in these
areas.
Most studies indicate that there is little correlation between the aboveground
vegetation and the soil seed bank. My results imply that although this may be true prior
to thinning, there is a high correlation the first year following thinning. Aboveground
vegetation may increase four-fold by the second year following thinning, and species
composition may change dramatically between the first and second year following the
disturbance.
Baseline inventories of species that utilize the soil for storage of seeds
can be used as indicators of the vegetation that will emerge following
disturbances, including ecological restoration treatments (tree thinning,
85
prescribed burning, etc.). Future inventories can then monitor changes in the
soil seed bank with elapsed time since these restoration treatments. Pre- and
post-restoration analyses ofthe seed bank may give some indication ofthe
historical composition ofthe seed bank, the relationship of the seed bank to
other components in the ecosystem, and can aid in our understanding of
population dynamics and succession in these systems. Seed bank studies can
also be used to determine if additional seeding of native species or control of
non-native species may be necessary to restore diversity and structure to
areas undergoing ecological restoration.
Future research needs include examining the role of fire after thinning on the
soil seed bank, competition between the seedlings emerging from the soil seed bank
and those in seeding mixtures (as well as interspecific competition in the soil seed
bank), the distribution of seeds with depth in areas which have not been thinned , and
further refinement of the regression equation to predict aboveground vegetation from
the soil seed bank.
86
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95
APPENDIX A:
FLORA IN THE VICINITY OF MT. TRUMBULL, MOHAVE COUNTY,
ARIZONA
96
Family
Aceraceae
Amaranthaceae
Anacardiaceae
Apiaceae
Asclepiadaceae
Asteraceae
Berberidaceae
Boraginaceae
Species
Acer grandidenuuum
Amaranthus albus
Rhus glabra
Rhus aromatica var. trilobata
Cymopterus multinervatus
Cymopterus purpureus
Lomatium foeniculaceum var. macdougalii
Asclepias asperula var. asperula
Asclepias subverticillata
Agoseris glauca
Antennaria sp.
Artemisia carruthii
Artemisia dracunculus
Artemisiafrigida
Artemisia tridentata
Brickellia grandijlora
Chaenaclis douglasii
Chrysothamnus depressus
Chrysothamnus viscidijlorus
Cirsium wheeleri
Conyza canadensis
Erigeron bellidiastrum
Erigeron divergens
Erigeron jlageliaris
Erigeron speciosus
Eupatorium herbaceum
Gutierrezia sarothrae
Helianthus anomalus
Hymenopappus filifolius
Hymenoxys acaulis
Hymenoxys cooperi
Kuhnia chlorolepis
Lactuca serriola
Machaeranthera canescens
Onopordum acanthium
Rudbeckia sp.
Sanvitalia abertii 1
Senecio eremophilus var. macdougalii 2
Senecio multilobatus
Solidago sparsijlora
Stephanomeria tenuifolia
Taraxacum officinale
Tetradymia canescens
Townsendia incana
Tragopogon dubius
Verbesina encelioides
Viguiera multiflora
Mahonia fremomii
Mahonia repens
Cryptamha cinerea var. cinerea
Hackellia sp.
Lappula occidentalis
97
Common Name
bigtooth -maple
pale amaranth
smooth sumac
skunkbush sumac
purple-nerved spring-parsley
variable spring-parsley
Macdougal's biscuitroot
spider milkweed
whorled milkweed
mountain dandelion
pussytoes
Carruth's wormwood
tarragon
fringed sagebrush
big sagebrush
tasselflower
Douglas'dustymaiden
dwarfrabbitbrush
viscid rabbitbrush
Wheeler's thistle
horseweed
pretty daisy
spreading daisy
trailing daisy
Oregon daisy
white thoroughwort
broom snakeweed
sand sunflower
hyalineherb
stemless woollybase
Cooper's bymenoxys
false bonset
prickly lettuce
hoary aster
Scotch thistle
coneflower
Abert's creeping zinnia
Macdougal's groundsel
Uinta groundsel
alcove goldenrod
slender wirelettuce
common dandelion
gray horsebrush
silvery townsendia
yellow salsify
crownbeard
showy goldeneye
Fremont's mahonia
Oregon-grape
James's cryptanth
stickseed
western stickseed
Brassicaceae
Capparaceae
Caprifol iaceae
Caryophyllaceae
Chenopodiaceae
Convulvulaceae
Cupressaceae
Cyperaceae
Ericaceae
Euphorbiaceae
Fabaceae
Fagaceae
Fumiariaceae
Mertensia macdougalii I
Arabis holboellii var. fendleri
Chorispora tenella
Descurainia p innata var. iruermedia
Draba sp.
Lepidium densiflorum
Streptanthus cordatus
Cleome lutea
Cleome serrulata
Polanisia dodecandra
Sambucus caerulea
Symphoricarpos oreophi/us
Arenariafendleri var. eastwoodiae
Arenariafendleri var. glabrescens
Arenaria lanuginosa ssp. saxosa
Si/ene scouleri
Stellaria jamesiana
Chenopodium album var. berlandieri
Chenopodium botrys
Chenopodium leptophyllum
Chenopodium graveolens (incisum) I
Salsola pestifer
Convolvulus arvensis
.Juniperus osteosperma
Carex occidentalis
Carex. sp.
Arctostaphylos patula
Arctostaphylos pungens
Euphorbia brachycera
Euphorbia lurida I
Euphorbia serpy//ifolia
Astragalus argophyllus
Astragalus henrimontanensis
Astragalus castoneaeformis I
Astragalus oophorus var. caulescens
Dalea oligophylla
Dalea polygonoides 1
Dalea searlsiae
Lathyrus lanzswertii var. laetiviren
Latus humistratus
Latus plebeius
Lotus iaahensis
Lotus wrightii
Lupinus argenieus
Lupinus kingii
Medicago sativa
M elilotus oficinalis
Phaseolus angustissimus 1
Psoralidium tenuiflorum
Robinia neomexicana
Trifolium sp.
Vida americana
Quercus gambelii
Quercus gambelii x turbinella
Quercus turbinella
Corydalis aurea
98
Macdougal 's bluebells
Fendler's rockcress
musk-mustard
western tansymustard
whitlowgrass
densecress
twistflower
yellow spiderflower
Rocky Mtn, bee plant
clammy-weed
blue elderberry
mountain snowberry
Eastwood's sandwort
basin sandwort
sprawling sandwort
Scouler's campion
James' chickweed
Berlandier's pigweed
Jerusalem-oak
narrowleaf goosefoot
fetid goosefoot
Russian thistle
bindweed
Utah juniper
western sedge
sedge
greenleaf manzanita
mexican manzanita
shorthorn spurge
San Francisco Mt. spurge
thyme-leaved spurge
meadow milkvetch
Dana's milkvetch
chestnut milkvetch
pallid egg miJkvetch
western prairie-clover
sixweeks prairieclover
Searles priarie-clover
largeflower sweet-pea
low trefoil
longbracted trefoil
Utah trefoil
Wright's trefoil
silvery lupine
King's lupine
alfalfa
yellow sweet-clover
slimleafbean
prairie scurfpea
New Mexico locust
clover
American vetch
Gambel's oak
hybrid
turbinella live-oak
golden corydalis
Garryaceae
Gentianaceae
Geraniaceae
Grossulariaceae
Hydrophyllaceae
Lamiaceae
Liliaceae
Linaceae
Loasaceae
Malvaceae
Monotropaceae
Nyctaginaceae
Onagraceae
Orobanchaceae
Papaveraceae
Pinaceae
Poaceae
Garrya jlavescens
Swertia a/bomarginata
Swertia radiata
Erodium cicutarium
Geranium caespitosum
Geranium richardsonii
Ribes cereum
Ribes sp.
Nama dichotomum I
Phacelia crenulata var. corrugata
Phacelia magellanica 1
Phacelia sp.
Dracocephalum parviflorum
Leonurus cardiaca
Marrubium vulgare
Monardella odoratissima
Allium bisceptrum
Calochortus nuttallii
Fritillaria atropurpurea
Linum australe
Linum perenne ssp. lewisii
Mentzelia dispersa
Mentzelia pumila
Sphaera/cea munroana
Sphaera/cea parvifolia
Pterospora andromedea
Mirabiltslinearis var. decipiens
Mirabilis linearis
Mirabilis oxybaphoides
Epilobium angustifolium
Gayophytum diffusum
Oenothera caespitosa
Oenothera coronopifolia
Oenothera jlava
Orobanche fasciculata
Argemone munita
Eschscholtzia mexicana
Pinus eduJis
Pinus ponderosa
Agropyron cristatum
Agropyron intermedium var. imermedium
Agropyron intermedium var. trichophorum
Aristida purpurea
Bouteloua curtipendula
Bouteloua gracilis
Bromus ciliatus
Bromus inermis
Bromus carinatus
Bromus tectorum
Elymus cinereus
Elymus elongatus
Elymus elymoides
Elymus trachycaulus
Eragrostis mexicana I
Festuca ovina var. arizonica
Koeleria macrantha
99
silk-tassel bush
white-margined swertia
elkweed
storksbill
James' geranium
Richardson 's geranium
wax currant
currant
wishbone fiddleleaf
corugate phacelia
varileaf phacelia
phacelia
smallflower dragonhead
motherwort
common horehound
stinking horsemint
Palmer's onion
sego lily
leopard-lily
small yellow-flax
blue flax
entire mentzelia
Wyoming stick leaf
Munro's globemallow
smalleaf globemallow
pinedrops
New Mexico umbrellawort
narrowleaf umbrellawort
spreading four o'clock
fireweed
diffuse groundsmoke
tufted evening-primrose
rootstock evening-primrose
yellow evening-primrose
cluster cancerroot
armed pricklypoppy
Mexican eschscholtzia
two-needle pinyon
ponderosa pine
crested wheatgrass
intermediate wheatgrass
int. wheatgrass
purple threeawn
side-oats grama
bluegrama
fringed brome
smooth brome
mountian brome
cheatgrass
Great Basin wildrye
tall wheatgrass
squirreltail
slender wheatgrass
Mexican lovegrass
Arizona fescue
Junegrass
Polemoniaceae
Polygonaceae
Polypodiaceae
Portulacaceae
Rammculaceae
Rhamnaceae
Rosaceae
Rubiaceae
Rutaceae
Salicaceae
Santalaceae
Saxifragaceae
Muhlenbergia mimuissima
Muhlenbergia repens
Muhlenbergia wrightii
Munroa squarrosa
Oryzopsis micrantha
Poa fendleriana
Poa pratensis
Schizachyrium scoparium
Secale cereale
Sporobolus cryptandrus
Stipa comata var. comata
Stipa hymenoides
Gilia aggregata
Gilia aggregata var. arizonica
Gilia multiflora 1
1vficrosteris gracilis
Navarretia breweri
Phlox austromontana
Phlox longifolia
Eriogonum alatum
Eriogonum cernuum
Eriogonum corymbosum var. corymbosum
Eriogonum pharnaceoides var. cervinum
Eriogonum racemosum
Eriogonum trichopes
Eriogonum umbellatum
Polygonum convolvulus
Polygonum douglasii var. johnstonii
Cheilanthes feei
Cystopteris fragilis
Woodsia scopulina
Portulaca oleracea
Clematis ligusticifolia
Delphinium nuttallianum
Ranunculus oreogenes
Ranunculus testiculatus
Thalictrum fendleri
Ceanothus fendleri
Ceanothus martinii
Amelanchier alnifolia
Amelanchier utahensis
Cercocarpus ledifolius
Fallugia paradoxa
Petrophytum caespitosum
Prunus emarginata
Purshia mexicana var. stansburyana
Purshia tridentata
Rosa woodsii var. ultramomana
Galium bifolium
Galium wrightii
Kelloggia galioides
Ptelea trifoliata ssp . pal/ida
Populus tremuloides
Comandra umbellata var. pal/ida
Heuchera parvifolia
Heuchera rubescens
100
annual muhly
creeping muhly
spike muhly
false buffalograss
little-seed ricegrass
muttongrass
Kentucky bluegrass
little bluestem
cultivated rye
sand dropseed
needle-and-thread grass
Indian ricegrass
scarlet gilia
Arizona skyrocket
manyflowered gilia
little polecat
Brewer's pincushion
desert phlox
longleaf phlox
winged buckwheat
nodding buckwheat
Fremont's buckwheat
wirestem buckwheat
redroot buckwheat
tanglefoot buckwheat
sulphur buckwheat
black bindweed
Douglas' knotweed
Fee's lipfem
brittle-fem
Rocky Mountain woodsia
purslane
white virgins-bower
Nelson's larkspur
mountain buttercup
bur buttercup
Fendler's meadowrue
Fendler's mountain-lilac
Martin's ceanothus
serviceberry
Utah serviceberry
mountain mahogany
Apache plume
rock spiraea
bitter cherry
Stansbury cliffrose
bitterbrush
Arizona wildrose
twinleaf bedstraw
Wright's bedstraw
Kelloggia
common hoptree
quaking aspen
bastard toadflax
little leaf alumroot
red alumroot
Scrophulareaceae
Solanaceae
Verbenaceae
Violaceae
Viscaceae
1
2
Castilleja integra I
Castilleja linariifolia
Collinsia parvijlora
Cordylanthus parvijlorus
Mimulus bigelovii var. cuspidatus
Mimulus rubel/us
PedicuJaris centranthera
Penstemon barbatus
Penstemon eatonii
Penstemon linarioides var. sileri
Penstemon linarioides ssp.tv. unknown
Penstemon ophianthus
Penstemon pachyphyllus
Penstemon palmeri
Penstemon rostrijlorus
Penstemon thompsoniae
Verbascum thapsus
Datura wrightli
Nicotiana attenuata
Physalis hederifolia
Physalis hederifolia var. palmeri
Solanumjamesii
Solanum rostratum
Solanum triflorum
Verbena bracteata
Verbena gooddingii
Viola canadensis
Viola matallii
Phoradendron juniperinum
wholeleaf indian paintbrush
linearleaf paintbrush
blue-eyed Mary
small-flower birdbeak
Bigelow's monkeyflower
reddish monkeyflower
pinyon-juniper lousewort
beardlip penstemon
Eaton's penstemon
Siler's penstemon
mat penstemon
Loa penstemon
thickleaf penstemon
Palmer's penstemon
Bridges' penstemon
Thompson's pensternon
woolly mullein
angels-trumpet
coyote tobacco
ivy-leaved ground-cherry
Palmer's ground-cherry
James ' potato
buffalobur
cutleaf nightshade
prostrate vervain
Goodding's vervain
Canada violet
Nuttall's violet
juniper mistletoe
from Kearney and Peebles (1960)
from Cronquist et al. (1972, 1974, 1984, 1989, 1994, and 1997)
101
APPENDIX B:
BIOGRAPHICAL STATEMENT
102
Name of Author:
Judith Diane (Brazis) Springer
Place of Birth:
Wooster, Ohio
Date of Birth:
September 7, 1966
Educational Institutions Attended
Northern Arizona University, School of Forestry, MS Program, 1996- I999
Northern Arizona University, Non-degree Graduate Program, 1994- I995
The Ohio State University, Continuing Education, 1990- I994
The Ohio State University, College of Food, Agriculture, and the Environment, B.S. 1988
Professional Positions Held
Research Technician, School of Forestry, Ecology Lab, Northern Arizona University,
1998- present
Graduate Research Assistant, School of Forestry, Northern Arizona University, 1996­
1997
Student Research Aide, Ecology Lab, School of Forestry, Northern Arizona University,
1994-1995
Environmental Specialist II, Technical Assistance Section, Division of Hazardous Waste
Management, Ohio Environmental Protection Agency, 1990-1994
Volunteer Positions
The Nature Conservancy, Northern Arizona Field Office, 1994-1995
The Nature Conservancy, Ohio Chapter, 1990-1994
Professional Affiliations
American Penstemon Society
Arizona Native Plant Society
Ecological Society of America
Society for Ecological Restoration
103