a hydraulic conductivity model points to post-neogene

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Ecology, Vol. 85, No. 11
Ecology, 85(11), 2004, pp. 3158-3165
© 2004 by the Ecological Society of America
A HYDRAULIC CONDUCTIVITY MODEL POINTS TO POST-NEOGENE
SURVIVAL OF THE MEDITERRANEAN OLIVE
J.-F. TERRAL,1,4 E. BADAL,2 C. HEINZ,1 P. ROIRON,1 S. THIEBAULT,3 AND I. FIGUEIRAL1
1
Centre de Bio-archéologie et d’Ecologie (CNRS UMR 5059), Institut de Botanique, Université Montpellier 2,
163 rue Auguste Broussonet, 34090 Montpellier, France
2
Dpt. Prehistoria y Arqueologia, Universitat de València, av. Blasco Ibañez 28, 46010 Valencia, Spain
3
Archéologie et Sciences de l'Antiquité (CNRS UMR 7041), Maison de l'Archéologie et de l'Ethnologie,
allée de l’Université, 92023 Nanterre cedex, France
Abstract. Research on the subfossil record and paleoecology of Olea europaea suggests
a new interpretation of its history and ecology with reference to the Mediterranean climate
since the Neogene. New results are based on the wood anatomy of ancient and extant Olea
and a model estimating hydraulic conductance established for wild forms belonging to Olea
europaea subsp. europaea. These suggest that during glacial periods wild olive populations
survived in protected microenvironments, particularly riparian habitats. Thereafter, the postglacial expansion of olive associated with climatic warming took place from these refuge
areas. This new evidence suggests that the continued existence of Olea in Mediterranean
areas since the Neogene was made possible either by preferential survival of Olea lineages
adaptable to the Holocene climate or from enhanced adaptation to extreme environmental
variation, a trait possibly originating from Tertiary predecessors and maintained in postglacial olive populations.
Key words: Last Glacial Maximum; Mediterranean; Neogene; Olea; palaeoecology; quantitative
ecoanatomy; riparian refuge areas
.
THE OLIVE TREE AS A PRODUCT
OF THE NEOGENE FLORA
The olive tree of the Mediterranean Basin is part of
the “Olea europaea complex”, which consists of six
different subspecies (Tab. 1) (Green and Wickens 1989,
Hess et al. 2000, Besnard et al. 2002). The genus Olea
is considered to have existed in the western
Mediterranean area since the Late Paleogene
(Arambourg et al. 1953, Suc 1984, Bessedik 1985,
Palamarev 1989).
Olea pollen (Bessedik 1985) and macrofossils (Palamarev 1989) are recorded as early as the Aquitanian
(25-20 Ma, Early Miocene). In the northwestern
Mediterranean area, Olea has been recorded unequivocally in strata as old as 3.2 Ma (million years) from the
Middle Pliocene (Arambourg et al. 1953, Palamarev
1989). During the beginning of the Pliocene, palynological data show that this area was dominated by subtropical species such as palms, laurels, swamp cypresses and magnoliaceous dicotyledons, indicating a warm
and humid climate (Suc et al. 1995, Fauquette et al.
1999). Certain western Mediterranean elements such as
evergreen and deciduous Quercus, Carpinus, Fagus
and eastern Mediterranean species such as Zelkova and
Manuscript received 22 May 2003; revised 3 March 2004;
accepted 1 April 2004. Corresponding Editor:C.C. Labandeira.
4
E-mail: [email protected]
Celtis, also were present. In view of this floristic association, it is possible that Olea was present in western
and central Europe as early as the Eocene to Oligocene
(Mai 1989), and distributed around the Tethys Basin
since at least the Miocene (Figueiral et al. 1999). Presumably, this early genus was an ancestor of modern
Olea and well adapted to a subtropical climate, probably growing on drained soils and sunlit slopes. The
Eocene to Oligocene transition was marked by important climatic cooling, possibly coinciding with the decline of laurophyll tropical and subtropical plants (Mai,
1989). The tectonic convergence between northwestern
Africa and southwestern Europe, which led to the
Messi-nian salinity crisis (5.59 to 5.33 Ma, Krijgsman
et al. 1999), may have resulted in the Pliocene migration of African species into Europe, along with Olea
(Médail et al. 2001, Quézel and Médail, 2003). During
this period, the Macaronesian archipelagos emerged
(except the islands of La Palma and El Hierro) and
Olea may have been progressively introduced by birds
that colonized dry areas (Hess et al. 2000). The subspecies O. cerasiformis and O. guanchica (Table1, Fig.
1) may have been differentiated by reproductive isolation. During the Pliocene, extreme climatic events
(4.5 and 3.6 Ma) led to the desertification of the Sahara
and the expansion of a Sahelian vegetation (Suc et al.
November 2004
NOTES
3159
TABLE 1. Present-day distribution and ecological range of Olea.
Olea L. material studied
Olea europaea L.
subsp. europaea
Location
Mediterranean Basin
subsp. maroccana
Western High Atlas
subsp. cerasiformis
Madeira Islands
subsp. guanchica
Canary Island
subsp. laperrinei
central Sahara and Sahel
subsp. cuspidata
africana phenotype
chrysophylla phenotype
cuspidata phenotype
Olea capensis L.
Olea woodiana Knobl.
Olea perrieri Chev.
Olea lancea Lam.
South Africa (Capetown
region)
from South Africa to
Kenya
from Ethiopia to the
Arabian Peninsula
from Iran to western
China
central, southern, and
eastern Africa
eastern and southern africa
Madagascar
Mauritius
Vegetation belt
sclerophyllous shrub and
forest (matorral)
sclerophyllous shrub and
preforest
sclerophyllous shrub and
forest (matorral)
sclerophyllous shrub and
forest (matorral)
Saharo-sahelian mountain
shrub
Bioclimatic context†
thermomesomediterranean/
humid to semiarid
arid inframediterranean
dry infracanarian
dry infracanarian
Saharian and Sahelian/semiarid to hyperarid
thermomediterranean/dry
sclerophyllous shrub and
forest (matorral)
evergreen and semideciduous shrub and forest
mountain evergreen forest
subtropical/subhumid to arid
mountain evergreen forest
subtropical/dry-semiarid
mountain evergreen forest
evergreen forest
tropical-subtropical/humid
to semiarid
tropical/dry-semiarid
evergreen forest
evergreen mountain forest
tropical/humid-subhumid
tropical/dry-semiarid
subtropical/dry-semiarid
†Definitions are as follows: inframediterranean and infracanarian, T (mean annual temperature; °C) > 19; thermomediterranean, 17 ≤ T < 19; mesomediterranean, 13 ≤ T < 17. Ombroclimatic parameters are : hyperhumid, P (mean annual
precipitation; mm) ≥ 1600; humid, 1000 ≤ P < 1600; subhumid, 600 ≤ P < 1000; dry: 350 ≤ P < 600; semiarid, 200 ≤ P <
350; and arid, p < 200. The mediterranean bioclimatic context is based on thermic parameters defined by Rivas Martinez
(1987).
1995). Thus, the Hoggar (Algeria) and Marra (Sudan)
mountains, and the eastern part of the Anti-Atlas Cordillera (Morocco) may have represented relictual areas
for O. europaea laperrinei and O. europaea maroccana, respectively (Quézel and Médail, 2003). The Middle Pliocene (3.15 to 2.85 Ma) corresponded to a period
of widespread climatic and floral change with the gradual disappearance of subtropical plants and the establishment of Mediterranean elements (Michaux et al.
1979). The emerging northwestern Mediterranean flora
included relatively thermophilic and xerophytic elements such as Olea (possibly Olea europaea subsp.
europaea), Pistacia and evergreen Quercus, as well as
mesic taxa such as deciduous Quercus. The ecological
success of these relatively xeric elements resulted from
drier climatic conditions following the expansion of the
polar ice sheets (Shackleton et al. 1988). The onset of a
drier temperate climate may have been triggered by two
factors: (1) a change in atmospheric circulation and
deflection of anticyclone systems to lower latitudes
linked to the establishment of the Gulf Stream (Haywood et al. 2000), and (2) increasing CO2 levels (Raymo et al. 1996).
Around the Late Pliocene (c. 2.8 to 2.6 Ma) the first
major glaciation in northern Europe marked the begin-
ning of Pleistocene climatic oscillations (1.88 Ma to 10
Ka), during which Olea managed to survive, becoming
a major element of the Mediterranean sclerophill
vegetation over the last 10 millennia. But the question
remains as to how and where did olive survive during
the Last Glacial Würm Maximum (LGM) ?
THE SURVIVAL OF THE OLIVE TREE DURING
THE LAST GLACIAL MAXIMUM (LGM)
In the northwestern Mediterranean, Olea has been
identified only sporadically in the pollen record up to
8000 yr BP (Pons and Reille 1988, Carrión and Dupré
1996, Carrión et al. 1995, Carrión and Van Geel 1999).
However, Olea pollen has been recorded during the
LGM, in archaeological sites located in southeastern
Spain (Carrión et al. 2003). Other thermophilic and
xerophytic plants such as Pistacia lentiscus, which today is associated with Olea europaea, also have been
identified during the early Holocene in this area (Pons
and Reille 1988).
Identification of archaeological charcoals has provided evidence of early occurrences of Olea associated
with human settlements since ca. 24000 yr BP in Portugal (Zilhão et al. 1995, Figueiral 1998, Figueiral and
3160
NOTES
Ecology, Vol. 85, No. 11
FIG. 1. Distribution of Olea species and Olea europaea subspecies, location of plant material studied, and archaeological
sites and layers. Radiocarbon dates (calibrated according to CALIB 4.2 program [Stuiver et al. 1998] based on charcoal
fragments are also presented. Circles represent the distribution of the species and subspecies studied.
Terral 2002), and ca. 7500 yr BP in Spain (Barton et al.
1990) and France (Solari and Vernet 1992). Olive wood
has been used as fuel long before its cultivation, domestication, and widespread nutritional and economic
use in the Mediterranean Basin.
Quantitative eco-anatomical analyses were carried
out on charcoal fragments assigned to Olea europaea,
from the sites of Buraca Grande (Upper Palaeolithic to
Neolithic, Portugal), Cova de les Cendres (Neolithic,
Spain) and Giribaldi (Neolithic, France) (Fig. 1). These
investigations combine measurements of wood characters following the methodological and analytical protocols developed previously by Terral and Arnold-Simard (1996) and Terral and Mengüal (1999). Data obtained also were compared to those measured from the
anatomical analysis of material from living subspecies
of the O. europaea complex and from other Olea species (Table 1; Fig. 1).
Anatomical correlates of ecological parameters revealed a very heterogeneous conductance surface of
vessels. Results indicate that in some archaeologically
documented intervals, the olive charcoal assemblage
was formed from two statistically distinct populations
(Fig. 2). These populations correspond to two different
ecological habitats from where wood fuel was collected
by humans around settlements. In fact, while some
specimens present small surface vessels (mean =
730.89 µm², 95% CI = 15.78), as expected from plants
growing on slopes and/or well drained soils, others
show very large vessels (mean = 1038.13 µm², 95%
CI = 27.03) and consequently very high calculated
water conductivity, representing highly efficient sap
transport through xylem vessels. These plants must
have developed in riparian habitats. So far, modern
analogues for these subfossil specimens have not been
found in the extant material studied of Olea europaea
subsp. europaea.
A recent study based on living wild olive populations
from the western Mediterranean (Table. 2) that correlates anatomical features with climatic data indicates
that “vessel conductivity” depends on annual mean
rainfall (Terral and Mengüal 1999; Fig. 3). Variations
in “conductivity” in relation to mean annual rainfall
also may be modulated by factors such as soil fertility
November 2004
NOTES
3161
FIG. 2. Distribution of frequencies of olive charcoal from archaeological assemblages (see insert in Fig. 1) in relation to
“vascular conductivity.” Results of normality tests (Kolmogorov-Smirnov) are presented for each population identified
corresponding to a distinct habitat. Dashed lines indicate the range of conductivities for each population. For all tests shown,
P > 0.01.
and humidity. This model, when applied to other Olea
europaea subspecies and other Olea species (Table 1,
Fig. 1), allows us to distinguish trees growing in typical
conditions and belonging to Olea europaea subsp. europaea (defining the interpolation zone) from olive
adapted to arid conditions (extrapolation zone 1) vs.
olive growing under hyperhumid conditions
(extrapolation zone 2) (Fig. 3).
Today, olive (O. europaea subsp europaea) grows
mostly on well draining soils and in bioclimatic conditions ranging from semiarid to subhumid. In semiarid
areas, wild olive trees can also grow beside ephemeral
streams called Oued in the Maghreb and Barrancos in
the Iberian Peninsula (Bensettiti and Lacoste 1999), as
seen in Northern Africa and Southern Spain. Their analysis (Fig. 3) allows an interpretation of why some of
the subfossils from supposedly two different habitats
have such high calculated values of “hydraulic conductivity”. A riparian habitat alone can explain high
“vessel conductivity” values in terms of ecophysiological and adaptive responses of wood growth to water
availability. At the end of winter and early spring, temporary streams fed by abundant rain would have supplied growing olive trees with an added water supply
and thus affected the spring wood growth. Interestingly, these samples also exhibited similar values of conductance to those of tropical species, such as O. capensis (see Plate 1) and O. perrieri, growing under
humid to very humid conditions, from central, southern
to eastern Africa, and in Madagascar respectively, (Fig.
3; Table 1). From both ecological and functional viewpoints, wood anatomy in Olea could be considered a
dynamic compromise between water transport efficiency and structural support (Bass et al. 1988). These fea-
3162
NOTES
Ecology, Vol. 85, No. 11
TABLE 2. Bioclimatic context and plant associations of Olea europaea subsp. europaea populations from which wood
samples were collected for quantitative anatomical analysis.
Bioclimatic
context†
No.
Station (population)
Modern olive trees growing in typical conditions
La Clape (France)
1
Teulada (Spain)
2
Montroy (Spain)
3
Les Baux (France)
4
Frontignan (France)
5
St. Jean de Védas (France)
6
Nyons (France)
7
Remoulin (France)
8
Simat de Valldigna (Spain)
9
Quissac (France)
10
Cuers ‘France)
11
meso/dry
meso/dry
meso/dry
meso/subhumid
meso/subhumid
meso/subhumid
meso/subhumid
meso/subhumid
meso/subhumid
meso/humid
meso/humid
12
13
14
Sauve (France)
Vic-le-Fesq (France)
Crevillente (Spain)
meso/humid
meso/humid
thermo/semiarid
15
16
17
18
19
20
21
22
23
24
25
26
27
Elche (Spain)
Elda (Spain)
San Juan de Alicante (Spain)
Villajoyosa (Spain)
Moraira (Spain)
Dos Aguas (Spain)
Sierra Parenchiza (Spain)
Cabo de San Antonio (Spain)
Gandía (Spain)
Jara (Spain)
Játiva (Spain)
Monduber (Spain)
Pego (Spain)
thermo/semiarid
thermo/semiarid
thermo/semiarid
thermo/semiarid
thermo/dry
thermo/dry
thermo/dry
thermo/subhumid
thermo/subhumid
thermo/subhumid
thermo/subhumid
thermo/subhumid
thermo/subhumid
Modern olive trees growing in riparian conditions
Jorox (Spain)
28
29
Menzel (Tunisia)
thermo/semiarid
thermo/semiarid
Plant associates
Quercus coccifera, Q. ilex, Pistacia lentiscus
Q. coccifera, P. lentiscus, Erica multiflora
Q. coccifera, Q. ilex, P. lentiscus
Q. ilex, Q. coccifera, J. oxycedrus
Q. ilex, Q. coccifera, P. terebinthus
Q. ilex, J. oxycedrus, P. terebinthus
Q. ilex, Q. coccifera, P. terebinthus
Q. ilex, Q. coccifera, P. terebinthus
Q. ilex, Q. coccifera, P. terebinthus
Q. ilex, J. oxycedrus, P. terebinthus
Q. coccifera, Rosmarinus officinalis, E. multiflora
Q. ilex, J. oxycedrus, P. terebinthus
Q. ilex, Q. coccifera, J. oxycedrus
Stipa tenacissima, Rhamnus lycioides, R. officinalis
S. tenacissima, R. lycioides, R. officinalis
S. tencissima, Pinus halepensis, R. lycioides
S. tencissima, Pinus halepensis, Q. coccifera
S. tencissima, Pinus halepensis, R. lycioides
P. lentiscus, Chamaerops humilis, R. officinalis
P. lentiscus, E. multiflora, Q. coccifera
P. lentiscus, R. officinalis, E. multiflora
P. lentiscus, C. humilis, R. officinalis
P. lentiscus, C. humilis, R. officinalis
P. lentiscus, C. humilis, R. officinalis
P. lentiscus, Q. coccifera, P. halepenis
Q. coccifera, P. lentiscus, R. officinalis
Q. coccifera, P. lentiscus, R. officinalis
Nerium oleander, Rubus ulmifolius, Q. coccifera
N. oleander, Tamarix africana, Atriplex halimus
N. oleander, T. africana, A. halimus
thermo/semiarid
Meknes (Morocco)
30
Note: see Table 1 for bioclimatic definitions.
† Abbreviations are: meso, mesomediterranean; thermo, thermomediterranean.
tures may have allowed Olea to adapt to different
humidity levels, below 45° N. latitude.
During the Last Glacial Maximum and possibly during other dry-glacial periods, wild olive trees would
have survived in well protected areas, such as closed
valleys and south-facing slopes. Riparian forests would
have acted as ecological buffer zones maintaining a
microclimate free from drastic temperature fluctuations. Riverbanks thus would have played a major role
as refuge areas for Mediterranean taxa; olive trees
could have grown alongside riparian taxa such as Salix,
Populus, Alnus, and probably Nerium oleander, in
warmer Mediterranean latitudes. By the end of the glacial episodes, these refuges would have facilitated the
spread of olive into additional habitats. An initial rapid
expansion of olive related to climatic warming (Terral
and Mengüal 1999), could have been extended by human activities (Terral and Arnold-Simard 1996, Terral
2000). During the Holocene, olive trees retained their
underlying riparian characteristics as shown by charcoal
data from the Neolithic period, such as the sites of
Buraca Grande, Cova de les Cendres and Giribaldi.
NEW PERSPECTIVES ON THE ECOLOGY
OF THE OLIVE TREE AND RELATIVES
The evolution and distribution of Olea are closely
linked to tectonic processes and climatic change. Recent phylogenetic studies support the hypothesis of an
ancient segregation of the genus into two subgenera in
Asia, Oceania and Africa. These studies also suggest an
African origin for the O. europaea complex (Besnard et al., 2002b). As a result, we understand that the
ancestral Miocene Eurasian Olea either had to decline
or adapt to the Neogene environmental changes. By the
Pliocene to Pleistocene transition, the Olea europaea
complex might have assumed its current geographical
November 2004
NOTES
3163
PLATE 1. Tranverse section of olive wood
(Olea capensis L.; Cape of Good Hope, South
Africa). Key to abbreviations: V, vessel section;
PP, paratracheal parenchyma; R, biseriate ray;
F, fiber. The thin section was prepared at the
AMAP laboratory (Montpellier, France)and
photographed by S. Ivorra (CBAE, Montpellier)
range and genetic structure. Currently, Olea covers a
broad ecological range from O. europaea subspecies
well adapted to dryness, to species such as O. capensis
and O. perrieri, able to grow under humid bioclimatic
conditions (Fig. 3).
In the northwestern Mediterranean areas, wild olive
ultimately disappeared from riparian habitats, only surviving in dry and uncultivated locations apart from its
cultivated descendants. At present, wild olive populations are sparse, owing mostly to human impact on
FIG. 3. Predictive model of mean annual precipitation according to vascular conductivity ([vessel area/π]²/vessel density),
based on wild olive trees (Olea europaea subsp. europaea) growing in typical modern ecological conditions. Data from (1)
olive trees (O. e. subsp. europaea) growing in riparian conditions, (2) other Olea species and O. europaea subspecies, and
(3) archaeological charcoal appear to be outside of the interpolation zone of the model. The positioning in the extrapolation
zones may be explained by either the influence of climatic parameters (Olea species and Olea europaea subspecies) or edaphic
conditions from archaeological charcoal. Abbreviations are: BG, Buraca Grande; CC, Cova de les Cendres; and GI, Giribaldi.
3164
NOTES
the environment and shrinking primary plant communities (Lumaret and Ouazzani 2001). It has been
assumed that the distribution of the cultivated olive
tree coincides with that of the Mediterranean bioclimatic zone (Baldy 1990), which is characterized by hot
and dry summers and humid winters. However, it now
appears that the olive tree was capable of adapting to
more diverse environmental conditions, particularly
humidity, and more so than its present distribution
might suggest. Consequently, the current distribution
of olive should be interpreted not only in terms of
Mediterranean climate, but also as a consequence of
anthropogenic influence.
We have demonstrated that ecophysiological interpretations from ancient fossil charcoal material from
western Mediterranean can indicate a more complex
history of this plant prior to being heavily influenced
by cultivation and domestication.
ACKNOWLEDGEMENTS
We thank T. Aubry, J. Bernabeu, D. Binder, and H. Moura
for permission to study the material from their archaeological
sites. We are grateful to N. Chatti, L. Fabre and J.-C. Auffray,
for their assistance during field work, and to N. Rowe and P.
Vargas for extensive comments and discussion. We also thank
S. Ivorra (CBAE, Montpellier) for her technical assistance.
Our reference collection has been enriched with Olea specimens offered by P. Schafer (Herbarium of the Institut de
Botanique, Montpellier), P. Gasson (Royal Botanic Gardens,
Kew), M. Thinon (IMEP, Marseille) and T. Otto (CICT, Toulouse). Their help is gratefully acknowledged. Constructive
suggestions from C. Labandeira and two anonymous reviewers enabled us to improve the final version of this article.
This work was supported by the CNRS-GDR 2474 “Morphométrie et Evolution des Formes.”
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