Deep-rooted vegetation, Amazonian deforestation, and climate

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Deep-rooted vegetation, Amazonian deforestation, and climate
Global Ecology and Biogeography (1999) 8, 397–405
GCTE/LUCC RESEARCH LETTER
Deep-rooted vegetation, Amazonian deforestation, and
climate: results from a modelling study
AXEL KLEIDON 1 and MARTIN HEIMANN 2 Max-Planck-Institut für Meteorologie, Bundesstraße 55,
20146 Hamburg, Germany
ABSTRACT
The depth of the root system controls the maximum
amount of soil water that can be transpired by the
vegetation into the atmosphere during dry periods.
Water uptake from deep soil layers has been found
to contribute significantly to the dry season
transpiration at some sites in Amazonia and it has
been estimated that large parts of the evergreen
forests in Amazonia depend on deep roots to survive
the dry season. Thus, the presence of deep roots
might provide a significant source of atmospheric
moisture during the dry season, and one which
would be affected by deforestation. We investigate
the role of deep-rooted vegetation and its removal
in the context of Amazonian deforestation using an
atmospheric General Circulation Model (GCM). A
distribution of deep roots is obtained by a numerical
INTRODUCTION
From an atmospheric point of view, land surfaces are
distinct from oceans in that water is not abundantly
available for evaporation but is limited by
precipitation and soil water storage. Only in the
presence of vegetation with a well-developed root
system can considerable amounts of water be extracted
from the soil and transpired into the atmosphere.
This soil storage of plant-available water becomes
1
Correspondence and present affiliation: Department
of Biological Sciences, Stanford University, Stanford,
CA 94305, U.S.A.
e-mail: [email protected]
2
Present
affiliation:
Max-Planck-Institut
für
Biogeochemie, Postfach 10 01 64, 07701 Jena,
Germany. e-mail: [email protected]
optimization approach. The simulated climate with
the use of the calculated deep roots substantially
improves the seasonal characteristics of the GCM.
Three additional simulations are then conducted in
order to isolate the effect of rooting depth reduction
from other parameter changes associated with largescale deforestation. Most of the climatic effects
occur during the dry season and are attributed to
the reduction of rooting depth. Dry periods are
found to last longer, being more intense with drier
and warmer air, while the wet season remains fairly
unchanged. The implications of these climatic effects
for the re-establishment of the natural evergreen
forest are discussed.
Key
words.
Rooting
depth,
albedo,
atmosphere–biosphere interaction, Amazonian
deforestation, land use change, dry season response,
forest recovery, model simulations.
increasingly important with greater seasonality in
precipitation and higher evaporative demands. This
is especially the case in the tropics, where deepreaching root systems have been reported at various
sites (Stone & Kalisz, 1991; Canadell et al., 1996),
and in the Amazon basin (Nepstad et al., 1994) in
particular. Deep rooting systems allow the vegetation
to extract water from deep soil layers for transpiration
throughout the dry season (Edwards, 1979; Nepstad
et al., 1994), thus leading to evapotranspiration near
the potential rate (Edwards, 1979; Shuttleworth,
1988). The importance of deep rooting is amplified
by the low ability of some tropical soils to store
plant available water; despite their often high clay
contents (e.g. Chauvel et al., 1991; Hodnett et al.,
1995). By supplying stored water near the potential
rate during the dry season, deep-rooted vegetation
provides a considerable source of atmospheric
moisture and latent heat to the atmosphere. On the
 1999 Blackwell Science Ltd. http://www.blackwell-science.com/geb
397
398 A. Kleidon and M. Heimann
one hand, the increased latent heat flux leads to
cooler temperatures during this period. On the other
hand, the increased transport of latent heat from the
dry season regions to the convection areas supplies
additional energy for convection, which in turn
enhances the tropical circulation patterns (Milly &
Dunne, 1994; Kleidon & Heimann, 1998).
Consequently, one might expect that the large-scale
removal of deep-rooted vegetation, for instance by
Amazonian deforestation, could have a pronounced
seasonal effect on climate, with implications for the
re-establishment of the original forest.
Numerous studies have investigated the impact of
tropical deforestation on climate using Atmospheric
General Circulation Models (GCMs) (e.g. Nobre
et al., 1991; Henderson-Sellers et al., 1993; Dirmeyer
& Shukla, 1994; Sud et al., 1996; Zhang et al., 1996;
Hahmann & Dickinson, 1997; Lean & Rowntree,
1997). All of theses studies have been conducted in
a similar way, wherein surface parameters such as
albedo, vegetation cover, roughness length and rooting
depth were modified to those representing degraded
grassland in a selected region of the central Amazon
basin in South America. The results were consistent
in that all found a reduction of evapotranspiration,
an increase in sensible heat flux, and an increase in
surface temperature in the deforested region. These
effects have been attributed mainly to the increased
albedo and reduced roughness length (i.e. a smoother
surface, which reduces the turbulent fluxes between
the land surface and the atmosphere). Since the
energy balance at the surface was modified, some
changes of the atmospheric circulation over Amazonia
were also found. However, all studies used land
surface schemes where the tropical evergreen forest
was represented by a rooting depth of less than 2 m,
therefore neglecting the presence and significance of
deep roots.
How much does deep-rooted vegetation affect the
present day climate? And, how much does its removal
contribute to the climate of the deforested landscape?
The aim of this paper is to examine these questions,
focusing on large-scale deforestation in Amazonia
and to discuss the implications for the re-establishment
of tropical evergreen forest. To do so, the next
section gives an overview of the methodology. This
is followed by a section focusing on the climatic
effects of Amazonian deforestation. Subsequently, we
discuss the implications of these changes, the
limitations of the results, and the prospects for future
research.
METHODS
In our earlier work, we developed a methodology which
allowed us to obtain a distribution of rooting depth
from a simulation model using an optimization
principle (Kleidon & Heimann, 1998b). This approach
is based on the assumption that natural vegetation
has adapted to its climatic environment such that the
overall fitness of the vegetation is maximized (e.g.
Schulze, 1982). We implemented this approach by using
a simulation model of land surface hydrology and
vegetation’s net primary productivity (NPP). This
model predicts NPP from plant-available soil moisture
and incoming solar radiation, which are computed
from the global climatology of Cramer & Leemans
(personal communication, updated version of Leemans
& Cramer, 1991) and a global data set on soil texture
(Batjes, 1996). The fitness of the vegetation is measured
by the computed long-term mean NPP as simulated
by the model. A global distribution of rooting depth
is then obtained by maximising long-term NPP in
respect to rooting depth. The maximization is
conducted iteratively, with one complete model
simulation of 10 years during each iteration. The
outcome of this optimization predicts rooting depths
of 8–12 m in wide regions throughout the tropics, in
particular in regions with a pronounced dry season
and in semi-arid regions. More importantly, the
optimization principle ensures that water stress is
minimized by generating sufficiently large storage
capacities of plant-available water in the soil.
Comparison to observations of maximum rooting
depth (Canadell et al., 1996) and river basin discharge
(Dümenil et al., 1993) indicated that the computed
rooting depths are more reasonable compared to values
of 1–2 m, which are commonly being used for rooting
depths in global models of biogeochemistry and
climate.
In a second step, we applied the same methodology
(Kleidon & Heimann, 1998c) to the ECHAM GCM
(Roeckner et al., 1996). This model simulates the
general circulation of the atmosphere, radiation
transfer within the atmosphere, and atmospheric
processes such as clouds and precipitation. An
integrated part is the representation of the land surface
in order to compute the surface energy balance and
the exchange fluxes of water, heat and momentum
between the surface and the atmosphere. In order to
apply the same methodology to the GCM, a
formulation of NPP was incorporated that allowed us
to compute NPP from the simulated soil moisture and
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
Deep-rooted vegetation and Amazonian deforestation 399
solar irradiation. The resulting distribution of rooting
depth is in general agreement with that obtained in
the first study, i.e. the method predicted much larger
rooting depths in many parts of the tropics (in the
order of 10 m compared to standard values of less than
2 m). More importantly, the use of deeper roots led to
considerably increased water availability and
evapotranspiration during the dry season, and more
precipitation during the wet season as the response to
an enhanced atmospheric circulation in the tropics,
thus to an overall enhanced hydrological cycle in the
GCM (Kleidon & Heimann, 1998a). Associated with
the increased evapotranspiration is an enhanced latent
heat flux, leading to enhanced cooling of the surface,
thus resulting in lower surface temperatures by as much
as 8 K in the monthly mean during the dry seasons.
Comparison of other variables to observations, that is,
the persistence of transpiration throughout the dry
season (Shuttleworth, 1988; Nepstad et al., 1994;
Hodnett et al., 1996), basin-wide storage of water
(Matsuyama, 1992), and no seasonality in observed
near surface air temperature (Legates & Wilmott, 1990;
Gash et al., 1996), also showed that the simulated
surface climate considerably improved with the use of
deep roots. We will use this simulation as a
representation of the present-day climate and refer to
it as the ‘deep roots’ simulation in the following.
In order to investigate the large-scale climatic effects
of Amazonian deforestation, we modify land surface
parameters in the model from those representative of
an evergreen forest (i.e. a dark, rough surface with the
optimum soil water storage capacities as described
above) to those of a degraded grassland (i.e. a light,
smooth surface with little soil water storage capacity)
in the region indicated in Fig. 1. The values
representative of grassland are taken from Nobre et al.
(1991). There, rooting depth was reduced from 2 m to
60 cm and albedo increased from values of 0.12–0.14
to 0.20. Other parameters were also modified, such as
leaf area index and roughness length. Here, we repeat
this deforestation layout and compare it to the
simulated climate in the presence of evergreen forest
(i.e. the ‘deep roots’ simulation) with special emphasis
to the role of deep root removal. Specifically, we
perform three simulations to assess the extent to which
the climatic effects of deforestation can be attributed
solely to the removal of deep roots. In a first run,
the ‘shallow roots’ simulation, only rooting depth is
reduced to 0.60 m and is converted to total available
water capacity by using the data set on plant-available
water produced by Batjes (1996). In a second run,
Fig. 1. A map of tropical South America showing the
approximate representation of the land area in the model
(squares). Also indicated is the region in which land surface
characteristics are changed in order to simulate the climatic
effects of deforestation in Amazonia (grey boxes with thick
outline). The size of each square is roughly 550 km×550 km.
the ‘increased albedo’ simulation, only the albedo is
increased. The other parameters are kept at values
representative of an evergreen forest in these two
simulations. In a final run, the ‘deforestation’
simulation, we include all modifications in order to
represent degraded grassland (i.e. albedo set to 0.20,
rooting depth to 0.60 m, leaf area index to 2.2,
vegetation coverage to 85%, and the roughness length
of the vegetation to 0.08 m). The layout of the
simulations is summarized in Table 1. We also
investigate changes in the length and intensity of dry
periods. Dry periods are defined here as continuous
periods in which daily precipitation is less than 1 mm/
day (a sensitivity study in respect to this threshold
showed that the following results are fairly independent
of this value). The intensity of a dry period is defined
as the integrated net radiation over the dry period. The
frequency distribution of the length and intensity of
dry periods is obtained from daily output over 15 years
from the model simulation.
RESULTS
When only rooting depth is reduced (‘shallow roots’
simulation), less soil water is accessible to the vegetation
during dry periods, leading to a considerable reduction
in evapotranspiration (Fig. 2a). This occurs mainly
during the dry season of the southern hemisphere, in
June to September. Associated with the reduction in
evapotranspiration is a reduction in the latent heat
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
400 A. Kleidon and M. Heimann
Table 1. Set-up of the simulations in order to estimate the
importance of deep root removal in the overall climatic
response to large-scale Amazonian deforestation (taken as the
transformation of evergreen forest into grasslands). The ‘deep
roots’ simulation serves as the baseline simulation, in which
land surface properties representative of an evergreen forest
are used (i.e. a dark, rough surface with optimum water
storage). In contrast, grasslands are associated with a light,
smooth surface with little water storage capacity. The
simulations ‘shallow roots’ and ‘increased albedo’ allow us to
investigate the effect of these isolated properties of grasslands
on the simulated climate. Comparing these to the
‘deforestation’ simulation then allows us to assess the
importance of rooting depth in the overall response. Other
land surface parameters include roughness length, vegetation
coverage, and leaf area index.
Simulation
Rooting
depth
Surface albedo Other
(reflectance)
land surface
parameters
‘deep roots’
‘shallow roots’
‘increased albedo’
‘deforestation’
Forest
Grassland
Forest
Grassland
Forest
Forest
Grassland
Grassland
Forest
Forest
Forest
Grassland
flux, which eliminates an effective means of cooling
the surface, in turn resulting in increased near-surface
temperatures (Fig. 2b). The strong reduction of
atmospheric moisture (Fig. 2c) can also be seen as a
consequence
of
decreased
dry
season
evapotranspiration. However, since precipitation is low
during the dry season, precipitation remains mainly
unaffected (Fig. 2d). Less atmospheric moisture causes
reduced cloud formation, resulting in increased
absorption of solar radiation at the surface (Fig. 2e).
However, net radiative loss through emission of longwave (LW) radiation at the surface increases (Fig. 2f),
mainly as a consequence of a warmer surface and the
increased transmissivity of the atmosphere (due to
reduced atmospheric moisture content). This decrease
in net LW radiation counteracts, but does not
compensate the increase in solar radiation on an annual
basis (Table 2).
In contrast, an increase in albedo (‘increased albedo’
simulation) reduces absorbed solar radiation
throughout the year (Fig. 2d). Since, in the presence
of deep roots, water is sufficiently available, this
decrease in absorbed solar radiation mainly leads to a
continuous
reduction
in
evapotranspiration
throughout the year (Fig. 2a) and has no effect on air
temperature (Fig. 2b). Atmospheric moisture (Fig. 2c)
and net LW radiation (Fig. 2f) are only marginally
affected relative to the ‘deep roots’ simulation.
When all land surface parameters are changed
(‘deforestation’ simulation, i.e. rooting depth, albedo,
and the other parameters as mentioned above), the
combined effect of these parameter changes can be
compared with the isolated effects (i.e. the simulations
‘shallow roots’ and ‘increased albedo’). It is apparent
from Table 2 that the effects of the ‘deforestation’
simulation are closest to the ‘shallow roots’ simulation,
so that the reduction of rooting depth dominates the
overall response. On an annual basis, the reduction of
absorbed solar radiation resulting from the albedo
increase is more than compensated for by the reduction
of cloud cover and the associated increase in incoming
solar radiation. This is in contrast to most other studies
on the climatic effects of Amazonian deforestation
(see above). However, the total amount of absorbed
radiation at the surface decreases because of an
enhanced loss through the increase in net LW radiation.
We also find a substantial increase in runoff as a
consequence of the reduced evapotranspiration during
the dry period (while precipitation remains unchanged).
Figure 3 shows the frequency distributions of dry
periods for all four simulations. Extreme dry periods
(in terms of length, Fig. 3a, and intensity, Fig. 3b) are
more frequent in the ‘shallow roots’ simulation and in
the ‘deforestation’ simulation, but not in the ‘increased
albedo’ simulation. Thus, these changes in the
frequency distributions can be attributed to the
reduction in rooting depth. Note that this result is not
apparent from the differences in the mean monthly
response shown in Fig. 2.
DISCUSSION
For a complete assessment of the climatic consequences
of Amazonian deforestation one should also consider
the reversibility of the changes. In other words, how
persistent will the changes in land surface
characteristics be (and thus of the climatic effects) under
the simulated climate of the deforested landscape?
Certainly, there are important, ecological processes
which will be affected in the absence of the forest,
such as the need for source populations, limitations to
dispersal mechanisms, degradation of soils through
increased erosion, and changes in the fire regime (e.g.
caused by longer dry seasons). However, when
inspecting this question from an optimistic climatic
viewpoint (and neglecting the ecological implications),
one might expect vegetation to regrow under the
deforested climate since the character of the wet season
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
Deep-rooted vegetation and Amazonian deforestation 401
Fig. 2. Effect of albedo increase (white bars), rooting depth reduction (grey bars) and deforestation (solid bars) on: (A)
evapotranspiration (B) 2 m air temperature (B), total atmospheric moisture (C), precipitation (D), net shortwave radiation (E),
and net longwave radiation (F). A negative change in net long-wave radiation corresponds to more net loss of available energy
at the surface through enhanced upward long-wave radiative flux. Values are averaged over the deforested region.
is hardly affected. However, for the re-establishment
of the original evergreen forest after deforestation, it
seems that the key component lies in the dry season,
since the demand for evapotranspiration (and thus for
water storage) is considerably increased for a series
of reasons: (1) less atmospheric moisture (drier air)
increases the water vapour pressure deficit; (2) higher
air temperatures (warmer air) further enhances the
dryness of the air and the water vapour pressure deficit;
and (3) longer dry periods will require more storage
of plant available water in the soil. All of these factors
could slow down the succession of evergreen species
after deforestation because of longer periods of drought
stress, which could favour more drought-resistant
species (e.g. grasses, dry-deciduous forest vegetation).
Ultimately, the speed of succession towards the original,
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
402 A. Kleidon and M. Heimann
Table 2. Annual mean changes of actual evapotranspiration (DET), precipitation (DP), runoff (DR), near surface air
temperature (DT), net shortwave (solar) and longwave (thermal) radiation (DSRAD, DLRAD) for each of the sensitivity
simulations in respect to the simulation with deep roots. Note that the net shortwave radiation in the ‘deforestation’ simulation
increases despite the increase in albedo due to decreased cloud cover.
Effect
DET
(mm)
DP
(mm)
DR
(mm)
DT
(K)
DSRAD
(W/m2)
DLRAD
(W/m2)
Rooting depth reduction only
(‘shallow roots’ simulation
-‘deep roots’ simulation)
Albedo increase only
(‘increased albedo’ simulation
-‘deep roots’ simulation)
All modifications
(‘deforestation’ simulation
-‘deep roots’ simulation)
−383
+50
+433
+2.5
+16
−12
−106
−194
−88
−0.0
−8
−1
−474
−140
+334
+2.5
+6
−14
evergreen forest would depend to a large extent on the
growth of the root system, especially on the time needed
to re-establish deep roots. In principle, this analysis
could be extended by using a dynamic vegetation model
to assess this question. However, the dynamic aspects
of root growth (and carbon allocation to roots) and
their relationships to water uptake are poorly
represented in these models, mainly attributable to the
lack of knowledge about these relationships.
The simulation results are subject to some
limitations, amongst which the more important are as
follows (for a fuller discussion, see Kleidon & Heimann,
1998a,b). The model used only contains a simple
treatment of vegetation aspects and land surface
processes (which should nevertheless be appropriate
for the large scale considered here). In particular, the
model represents soil hydrology by a budget equation
which certainly has its limits. However, the
improvement of the model simulation when deep roots
are incorporated, originated from the continuous
evapotranspiration during the dry season. For such
behaviour to occur in regions with a pronounced
seasonality in precipitation, sufficient access through
roots to water stored in the soil is mandatory. This
conclusion should be independent of whatever model
is used. Nevertheless, the soil water storage capacity
also depends on soil textural information (more
precisely, information on plant available water, PAW=
field capacity – permanent wilting point), so that the
use of accurate soil information is important. While
the use of the optimization approach guarantees soil
water storage sizes large enough to maintain
evapotranspiration (and thus does not require soil
texture information), accurate soil texture information
is crucial for the correct sensitivity of land surface
processes to rooting depth change. This aspect is
especially important considering that the reported PAW
values for many regions of the tropics (and the Amazon
basin in particular) are very small despite high clay
contents (e.g. Chauvel et al., 1991; Hodnett et al.,
1996). We tried to accommodate this aspect by explicit
use of a recent data set of plant-available water by
Batjes (1996).
Also not considered here is the effect of seasonally
flooded regions on the basin hydrology. These areas
could make up to about 10% of the Brazilian Amazonia
(as cited in Matsuyama, 1992). While the results of
this study may be affected by this to some extent, the
main result, i.e. the role of rooting depth, should
remain unaffected since the important effects take place
towards the end of the dry season.
CONCLUSIONS
We conclude that the possession of deep roots by the
vegetation plays an important role in the Amazonian
climate. Only in the presence of deep roots can the
observed seasonal course of air temperature be
understood and adequately reproduced with a climate
model. The water availability during the dry season –
which is determined by the depth of the root system
to a large extent – is then a key component for
understanding the modelled climatic effects of
Amazonian deforestation. Most of the modelled
climatic changes arising from large-scale deforestation
can be attributed to the removal of deep roots. The
strong response to rooting depth is a consequence of
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
Deep-rooted vegetation and Amazonian deforestation 403
Fig. 3. Frequency distribution of (A) length and (B) intensity of dry periods for the control (solid), increased albedo (dashed),
reduced rooting depth (dotted), and deforestation (dash-dotted) simulation. A dry period is defined as a continuous period with
daily precipitation below 1 mm/day. The figures show the combined frequency distributions for the whole region.
 1999 Blackwell Science Ltd, Global Ecology and Biogeography, 8, 397–405
404 A. Kleidon and M. Heimann
both sufficient water availability in the ‘deep roots’
simulation due to the deep roots, and low water
availability in the ‘deforestation’ simulation resulting
from shallow rooting depth. The climate in the
deforested landscape has more frequent long dry
periods which are warmer, drier and more intense. All
of these factors impose a larger demand for water (and
thus storage) on the vegetation. We may therefore
conclude that the recovery of the land surface
functioning to its original state (i.e. in the presence of
the natural evergreen forest) would depend largely
on root growth/re-establishment of deep roots. More
information from field studies in respect to growth of
root systems, especially in respect to the establishment
of deep roots, would be necessary, as also would be
additional simulations with other models. This is one
of the aims of the REACH project on rooting properties
and global models (Jackson et al., 1999). In addition,
the ‘Large scale Biosphere atmosphere experiment in
Amazonia’ (LBA) would provide a good opportunity
to obtain more field observations on the role and
functioning of roots.
ACKNOWLEDGMENTS
The simulations were performed at the German Climate
Computing Centre (DKRZ). This paper is based on a
presentation presented at the GCTE-LUCC Science
Conference in Barcelona/Spain in March 1998. It
contributes to the activities of the REACH project
on rooting properties and global models currently at
NCEAS under the auspices of R. B. Jackson. We
thank the editor for his stimulating comments on the
manuscript.
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