Winter-Deciduous versus Evergreen Habit
in Mediterranean Regions: A Model1
Mark A. BIumIer2
Abstract: Although winter-deciduous species are presumed to be
"out-of-phase" with the mediterranean climate regime, distri­
butional evidence suggests some taxa may be more tolerant of
summer drought than evergreen sclerophylls. Deciduous spe­
cies possess several features that confer advantage in extreme
summer dry regions: drought-deciduousness, an efficient re­
sponse to severe seasonal drought; high photosynthetic rates
especially during the optimum late spring period; and a phenol­
ogy similar to evergreens (but with more rapid growth) during
the critical first year. I present a preliminary model of the
relative importance of deciduous and evergreen plants in rela­
tion to climate, soils, and biotic influences.
The association of broadleaved evergreen sclerophyllous
shrubs and trees with mediterranean-type (winter-wet, summerdry) regions is generally regarded as the classic example of
convergent evolution in plant physiognomy in relation to cli­
mate (Box 1981, Cody and Mooney 1978, Schimper 1898).
Paradoxically, winter-deciduous species also grow where summer
drought is severe, although this fact is scarcely recognized; in
fact, the association of mediterranean climate with evergreen
sclerophyllous habit is so strongly engrained that deciduous taxa
are sometimes misclassified as evergreen (Specht 1969). At
first glance, winter-deciduous species appear to be "out-ofphase" with the mediterranean climate, since they are leafless
during part of the rainy season. On the other hand, it is often
argued that evergreen sclerophylls have a growth rhythm that is
in tune with and evolved in response to the mediterranean
climatic regime (Cody and Mooney 1978, Mooney and Dunn
1970a). But most sclerophylls evolved prior to the development
of summer-dry climates (Axelrod 1973, Minnich 1985), and
many of the important mediterranean sclerophylls, or their
nearest relatives, grow also where summer rain is significant
(Minnich 1985, Suc 1984); so it is probably more accurate to
speak of them as pre-adapted (Minnich 1985).
This paper describes how winter-deciduousness can be an
effective adaptation to extreme summer drought, and examines
a preliminary model of the relative likelihood of occurrence of
evergreen and deciduous species, depending on climatic, edaphic,
and biotic conditions.
1
2
Presented at the Symposium on Oak Woodlands and Hardwood Rangeland
Management, October 31-November 2, 1990, Davis, California.
Graduate Student, Department of Geography, University of California, Berke­
ley, California.
194
DISTRIBUTIONAL EVIDENCE
Surprisingly, winter-deciduous species are often dominant
where summer drought is especially severe. In California, the
blue oak (Quercus douglasii) is distributed on sites that are often
below, i.e., more xeric than, the chaparral belt (Baker and others
1981), and that also tend to be too dry for all other tree species
(Barbour 1988, Griffin 1971, 1977, Vankat 1982). Shrubby
evergreen oaks also grow in dry regions, but usually where there
is a certain amount of summer rain. The "semi-evergreen"
engelmann oak (Q. engelmannii) may be analogous to blue oak,
as it is found primarily in interior regions that are hotter in
summer than the southern California coast that supports the
evergreen coast live oak (Q. agrifolia) (Barbour 1988, Lathrop
and Osborne 1990). Even the valley oak (Q. lobata) may be
more tolerant of summer drought than evergreen trees (Cooper
1926, Griffin 1971, 1973, 1976).
An even more striking pattern occurs in the Fertile Crescent
and in Near Eastern mountain ranges generally. The region is
vegetated almost exclusively by deciduous species, including
oaks as well as many other genera (Zohary 1963, 1973). Yet
summer drought is often even more severe than in California
(Naveh 1967). The Fertile Crescent, especially its southern tips,
experiences the hottest, longest, and driest summers of any
winter rain region. Hence, in a sense, it has the most mediterranean
climate on earth — although it is sometimes regarded as non­
mediterranean simply because sclerophylls are rare (Blumler
1984). Winter-deciduous oaks and other taxa also can be found
within the Mediterranean Basin in areas that experience rela­
tively severe summer drought, such as southwestern Turkey
(Quezel 1986), southern Italy, southern Spain, and North Africa.
In interior central Chile, which has a long, dry, but relatively cool
summer, deciduous legumes are sometimes dominant; but these
maybe phreatophytic (Rundel 1981).
The suggestion that mediterranean winter-deciduous spe­
cies must be phreatophytic (Box 1981) could apply to some taxa
in the Old World, such as Styrax officinalis, and to the valley oak
in California (Cooper 1926, Griffin 1976, 1977). However, it
clearly does not apply to the blue oak, which does not need to rely
upon the water table (Griffin 1973); nor does it apply to impor­
tant Mediterranean/Near Eastern oaks such as the tabor (Q.
ithaburensis), Q. brantii, and Q. macrolepis (Eig 1933,
Oppenheimer 1950). On the other hand, evergreen sclerophylls
often grow where they can tap ground water year-round (Arkley
1981, Griffin 1973, Mooney 1983). An alternative hypothesis
that winter cold limits dominance by evergreens (Larcher 1981,
Raven and Axelrod 1978) cannot apply where deciduous and
USDA Forest Service Gen. Tech. Rep. PSW-126. 1991
evergreen species are sympatric, as in Israel and California. For
instance, evergreen chaparral is often found upslope from blue
oak woodland (Baker and others 1981, Vankat 1982), where
conditions are cooler and wetter. In Israel, the tabor is thought to
be less cold tolerant than the evergreen kermes oak (Q.
calliprinos), which ranges to higher elevations.
Blumler (1984) pointed out that the "edaphic climate"
exerts a critical control on plant distribution where summer
drought is extreme. If edaphics moderate drought seasonality
(as on coarse, rocky, or infertile substrates), one typically finds
evergreen shrub associations. If not, one encounters deciduous
park forest, bunchgrass/annual communities, or, in the most
extreme cases such as vernal pools, pure stands of annuals. For
instance, although evergreen sclerophylls often grow intermixed
with blue oak, the latter is more important on relatively finetextured soils and open slopes that dry out most completely in
summer (Gartner and others 1957, Griffin 1977). On the Santa
Rosa Plateau, coast live oak is more common around rock
outcroppings and less common in open soil than the engelmann
oak (Snow 1972). In Israel, the major control on distribution in
the lowlands is edaphic: the tabor and other winter-deciduous
species occur primarily on heavier, more fertile soils that often
dry out to a greater degree in summer than those supporting
maquis (Eig 1933, Kutiel and others 1979, Litav and Orshan
1971, Oppenheimer 1950, Rabinovitch-Vin 1983).
ADVANTAGES OF DECIDUOUS
HABIT
How is it possible that winter-deciduous species could
achieve a competitive advantage under severe summer drought
conditions? First, they seem to be drought-deciduous, dropping
their leaves in late summer or even earlier if under extreme stress
(Diamantoglou and others 1989, Griffin 1973, McCreary 1990).
Sclerophylls do not lose their leaves so readily (Griffin 1973),
perhaps because there would be a greater cost to the plant than
in a deciduous species that is programmed to withdraw nutrients
prior to abscission; but once the deciduous species has lost its
leaves, it is in a better position to resist drought than the
evergreen. Where summer drought is severe, optimum growing
conditions prevail in late spring, and conditions become pro­
gressively less favorable until soil moisture recharges with the
next winter's rains; fall-winter photosynthesis in evergreen
species may be limited not only by low light but also by low
moisture, and a positive carbon budget may not always be
possible. Thus, leaf loss in late summer or fall may be more
efficient than sclerophylly (Griffin 1973). This is especially
likely if cool winter temperatures further reduce potential pho­
tosynthetic rate (even if there is no actual damage to plant tissue
from frost).
Second, deciduous species inherently have more rapid
maximum photosynthetic rates (Mooney 1983), and may be able
USDA Forest Service Gen. Tech. Rep. PSW-126. 1991
to grow as rapidly on an annual basis as evergreens, despite
being leafless for part of the year (Mooney and Dunn 1970b).
Deciduous oaks typically produce soft leaves in spring, which
become progressively tougher (more drought resistant) over the
course of the summer. Thus, they are able to take full advantage
of the abundant moisture and warm temperatures of late spring.
Evergreen sclerophylls necessarily photosynthesize less rapidly
at that time, although they are able to maintain a relatively stable
rate throughout the year. This suggests that evergreens are best
adapted to relatively mild temperature fluctuations and absence
of a pronounced, predictable, annual boom and bust cycle of
potential photosynthesis. If this is so, they also should be best
adapted to semi-arid conditions in which dry spells are relatively
short, while deciduous taxa may be better adapted to long,
seasonal droughts. Hence, one can find sclerophyllous shrublands
also in non-mediterranean, subtropical semi-arid climates (e.g.,
Freitag 1971, Keeley and Keeley 1988, Minnich 1985).
Finally, deciduous taxa, unlike some evergreen sclerophylls,
seem to germinate and grow through the critical first winter
(Griffin and Muick 1990, Matsuda and McBride 1986, Matsuda
and others 1989). Their high growth rate probably increases
their competitive ability against fast-growing herbs. Most
woody seedlings do poorly in direct competition with annual
grasses on fertile soils (Blumler 1984, da Silva and Bartolome
1984, Gordon and others 1989, Kaplan 1984, Litav and others
1963, Schultz and others 1955), but evergreen sclerophylls
presumably are even weaker seedling competitors than deciduous
taxa. Hence, one might predict that they could be completely
excluded if summer-dry woodlands are open enough to support
a vigorous, herbaceous vegetation. For example, Griffin (1976)
found that valley oak seedlings were killed primarily by browsing
and only slightly affected by grass competition, while evergreen
oaks were little affected by browsing but established primarily
in shade. Because it has very large acorns, valley oak is probably
more competitive than blue oak (studied by Gordon and others).
However, it is a plausible hypothesis that even the blue oak is
more resistant to herbaceous competition than are evergreen
species.
Environmental history (climatic change) also has probably
played an important role. Much of the Near East would have
experienced extremely cold winters during glacial periods (van
Zeist 1967, Wright 1962), and perhaps sclerophylls would be
more widespread today if they had not been eliminated at those
times. Similarly, the present distribution of California oaks may
be partly a function of Pleistocene climates. However, envi­
ronmental history cannot explain the failure of evergreen spe­
cies to replace deciduous ones where distributions have been
sympatric for thousands of years. For instance, evergreen and
deciduous oaks were both present in Israel by the Late Pleistocene
(Henry 1989, Horowitz and Gat 1984). In any event, it is
unlikely that winter-deciduousness evolved in response to ex­
treme mediterranean climate; rather, species such as the blue oak
were presumably pre-adapted to summer drought, just as were
the evergreen sclerophylls that make up the chaparral and
maquis.
Additional considerations determine sclerophyll distribu­
tion. Sclerophylls are stress-tolerators in Grime's (1977, 1979)
195
tripartite system. Thus, photosynthetic rate in shade is higher
than that of deciduous species, while it is much less in sun. Also,
as is well-known, sclerophylly is associated with soil infertility
(Beadle 1966, Chapin 1980, Mooney 1983). Of course, summer
drought is moderated in shade and on infertile soils (Blumler
1984). Finally, adult sclerophylls have much better sprouting
ability than deciduous species; however, this may not be true of
seedlings (Snow 1980).
A PRELIMINARY MODEL
A preliminary model of the conditions favoring winterdeciduous and broadleaved evergreen sclerophyllous shrubs
and trees in mediterranean regions is presented here:
Environment
Cold winters
Mild winters
Hot summers
Mild summers
Summer rain
No summer rain
Heavy soil texture
Coarse soil
Fertile soil
Infertile soil
Rock
Herbaceous cover
Shade
Sun
Browsing of dense stands
open stands
Fire in dense stands
open stands
Deciduous
x
Evergreen
The major management implications of the model are that
dense woodlands are likely to become dominated by evergreen
species, while frequent fire may be a means of maintaining
deciduous park forest. These conclusions seem to be in line with
available evidence on regeneration patterns in California (Griffin
1971, 1976, 1977, McClaren and Bartolome 1989, Muick and
Bartolome 1987). At the same time, it is also possible that
establishment occurs primarily during recruitment "windows"
that open when conditions suddenly change. For instance,
removal of cattle from an area that had been heavily grazed
might be followed by an interval of several years during which
herbaceous vegetation recovered and browsers and acorn seed
eaters increased in numbers. During that short period of time,
establishment might be excellent. Such possibilities are ignored
in this model.
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
The predictions of the model can be treated as a series of
hypotheses to be tested, since only a few have been verified
sufficiently. Deciduous plants are predicted to perform well
where summers are hot, winters cold, summer drought severe,
soil fertile or heavy, competition from herbaceous vegetation
intense, and/or stand density is low; evergreen sclerophylls
should be favored if summers and winters are mild, some
summer rain occurs, soils are infertile, herbaceous competition
is limited, and/or seedlings are shaded. In addition, fire in dense
stands should favor sclerophylls because adults resprout better
than deciduous plants. Fires may favor the latter if woody
vegetation is sparse because adult mortality will be low and
because immature deciduous plants may resprout as well as or
better than evergreens. Browsing or cutting should favor adult
sclerophylls, but may also favor immature deciduous plants.
196
MANAGEMENT IMPLICATIONS
REFERENCES
Arkley, R. J. 1981. Soil moisture use by mixed conifer forest in a summer-dry
climate. Soil Science Society of America Journal 45:423-427.
Axelrod, D. I. 1973. History of the mediterranean ecosystem in California. In:
di Castri, F.; Mooney. H. A., eds. Mediterranean-type ecosystems: origin and
structure. Berlin: Springer-Verlag; 225-277.
Baker, G. A.; Rundel, P. W.; Parsons, D. J. 1981. Ecological relationships of
Quercus douglasii (Fagaceae) in the foothill zone of Sequoia National Park,
California. Madroño 28:1-12.
Barbour, M. G. 1988. Californian upland forests and woodlands. In: Barbour,
M. G.; Billings, W. D., eds. North American terrestrial vegetation. Cambridge: Cambridge University Press; 131-164.
Beadle, N. C. W. 1966. Soil phosphate and its role in molding segments of the
Australian flora and vegetation with special reference to xeromorphy and
sclerophylly. Ecology 47:992-1007.
Blumler, M. A. 1984. Climate and the annual habit. Berkeley, CA: University
of California; 119 p. Thesis.
Box, E. O. 1981. Macroclimate and plant forms: an introduction to predictive
modeling in phytogeography. The Hague: Dr W. Junk.
Chapin, F. S. 1980. The mineral nutrition of wild plants. Annual Review of
Ecology and Systematics 11:233-260.
Cody, M. L.; Mooney, H. A. 1978. Convergence versus nonconvergence in
Mediterranean-climate ecosystems. Annual Review of Ecology and System­
atics 9:265-321.
Cooper, W. S. 1926. Vegetational development upon alluvial fans in the vicinity
of Palo Alto, California. Ecology 7:1-30.
Da Silva, P. G.; Bartolome, J. W. 1984. Interactions between a shrub, Baccharis
pilularis subsp. consanguinea (Asteraceae), and an annual grass, Bromus
mollis (Poaceae), in coastal California. Madroño 31:93-101.
Diamantoglou, S.; Rhizopoulou, S.; Kull, U. 1989. Energy content, storage
substances, and construction and maintenance costs of Mediterranean decidu­
ous leaves. Oecologia 81:528-533.
Eig, A. 1933. A historical-phytosociological essay on Palestinian forests of
Quercus aegilops L. ssp. ithaburensis (Desc.) in past and present. Beihefte
zum Botanischen Zentralblatt 51B:225-272.
Freitag, H. 1971. Studies in the natural vegetation of Afghanistan. In: Davis,
USDA Forest Service Gen. Tech. Rep. PSW-126. 1991
P. H.; Harper, P. C.; Hedge, I. C., eds. Plant life of South West Asia.
Edinburgh: Royal Botanical Garden; 89-106.
Gartner, F R.; Schultz, A. M.; Biswell, H. H. 1957. Ryegrass and brush seedling
competition for nitrogen on two soil types. Journal of Range Management
10:213-220.
Gordon, D. R.; Welker, J. M.; Menke, J. W.; Rice, K. J. 1989. Competition for
soil water between annual plants and blue oak (Quercus douglasii) seedlings.
Oecologia 79:533-541.
Griffin, J. R. 1971. Oak regeneration in the upper Carmel Valley, California.
Ecology 52:862-868.
__.1973. Xylem sap tension in three woodland oaks of central California.
Ecology 54:152-159.
__.1976. Regeneration in Quercus lobata savannas, Santa Lucia Mountains,
California. American Midland Naturalist 95:422-435.
__.1977. Oak woodland. In: Barbour, M. G.; Major, J., eds. Terrestrial
vegetation of California. New York: John Wiley and Sons; 383-415.
Muick, P. C. 1990. California native oaks: past and present. Fremontia
18(3):410.
Grime, J. P. 1977. Evidence for the existence of three primary strategies in
plants and its relevance to ecological and evolutionary theory. American
Naturalist 111:1169-1194.
__.1979. Plant strategies and vegetation processes. Chichester: John Wiley
and Sons.
Henry, D. O. 1989. From foraging to agriculture. Philadelphia: University of
Pennsylvania.
Horowitz, A.; Gat, J. R. 1984. Floral and isotopic indications for possible
summer rains in Israel during wetter times. Pollen et Spores 26:61-68.
Kaplan, Y. 1984. The ecosystem of the Yahudia Nature Reserve with emphasis
on dynamics of germination and development of Quercus ithaburensis
Decne. Wageningen: University of Wageningen. Dissertation.
Keeley, J. E.; Keeley, S. C. 1988. In: Barbour, M. G.; Billings, W. D., eds.
Chaparral. North American terrestrial vegetation. Cambridge: Cambridge
University Press; 165-207.
Kutiel, P.; Danin, A.; Orshan, G. 1979. Vegetation of the sandy soils near
Caesarea, Israel. I. Plant communities, environment and succession. Israel
Journal of Botany 28:20-35.
Larcher, W. 1981. Low temperature effects on Mediterranean sclerophylls: an
unconventional viewpoint. In: Margaris, N. S.; Mooney, H. A., eds. Com­
ponents of productivity of Mediterranean-climate regions: basic and applied
aspects. The Hague: Dr W. Junk; 259-266.
Lathrop, E. W.; Osborne, C. D. 1990. From acorn to tree: ecology of the
engelmann oak. Fremontia 18(3):30-35.
Litav, M.; Kupernik, G.; Orshan, G. 1963. The role of competition as a factor
determining the distribution of dwarf shrub communities in the Mediterranean
territory of Israel. Journal of Ecology 51:467-480.
Litav, M.; Orshan, G. 1971. Biological flora of Israel. I. Sarcopoterium
spinosum (L.) Sp. Israel Journal of Botany 20:48-64.
Matsuda, K.; McBride, J. R. 1986. Difference in seedling growth morphology
as a factor in the distribution of three oaks in central California. Madroño
33:207-216.
Matsuda, K.; McBride, J. R.; Kimura, M. 1989. Seedling growth form of oaks.
Annals of Botany 64:439-446.
McClaren, M. P.; Bartolome, J. W. 1989. Fire-related recruitment in stagnant
Quercus douglasii populations. Canadian Journal of Forest Research 19:580585.
McCreary, D. D. 1990. Blue oaks withstand drought. California Agriculture
44(2):15-17.
Minnich, R. A. 1985. Evolutionary convergence or phenotypic plasticity?
Responses to summer rain by California chaparral. Physical Geography
6:272-287.
USDA Forest Service Gen. Tech. Rep. PSW-126. 1991
Mooney, H. A. 1983. Carbon-gaining capacity and allocation patterns of
mediterranean-climate plants. In: Kruger, F. J.; Mitchell, D. T.; Jarvis, J. U.
M., eds. Mediterranean-type ecosystems: the role of nutrients. Berlin:
Springer-Verlag; 103-119.
__;Dunn, E. L. 1970a. Convergent evolution in mediterranean-climate
evergreen sclerophyll shrubs. Evolution 24:292-303.
__. 1970b. Photosynthetic systems of mediterranean-climate shrubs and trees
of California and Chile. American Naturalist 104:447-453.
Muick, P. C.; Bartolome, J. W. 1987. Factors associated with oak regeneration
in California. In: Plumb, T. R.; Pillsbury, N. H., tech. coords. Proceedings
of the symposium on multiple-use management of California's hardwood resources; 1986 November 12-14; San Luis Obispo, CA. Gen. Tech. Rept.
PSW-100. Berkeley, CA: Pacific Southwest Forest and Range Experiment
Station, Forest Service, U.S. Department of Agriculture; 86-91.
Naveh, Z. 1967. Mediterranean ecosystems and vegetation types in California
and Israel. Ecology 48:445-459.
Oppenheimer, H. R. 1950. The water turn-over of the Valonea oak. Palestine
Journal of Botany (Rehovot) 7:177-179.
Quézel, P. 1986. The forest vegetation of Turkey. Proceedings of the Royal
Botanical Society of Edinburgh 89B: 113-122.
Rabinovitch-Vin, A. 1983. Influence of nutrients on the composition and
distribution of plant communities in mediterranean-type ecosystems of Israel.
In: Kruger, F. J.; Mitchell, D. T.; Jarvis, J. U. M., eds. Mediterranean-type
ecosystems: the role of nutrients. Berlin: Springer-Verlag; 74-85.
Raven, P. H.; Axelrod, D. I. 1978. Origin and relationships of the California
flora. University of California Publications in Botany 72:1-134.
Rundel, P. W. 1981. The matorral zone of central Chile. In: di Castri, F.;
Goodall, D. W.; Specht, R. L., eds. Mediterranean-type shrublands. New
York: Elsevier; 175-201.
Schimper, A. F. W. 1898. Pflanzengeographie auf physiologischen grundlage.
Jena: G. Fischer.
Schultz, A. M.; Launchbaugh, J. L.; Biswell, H. H. 1955. Relationship between
grass density and brush seedling survival. Ecology 36:226-238.
Snow, G. E. 1972. Some factors controlling the establishment and distribution
of Quercus agrifolia Nee and Quercus engelmannii Greene in certain southern
California oak woodlands. Corvallis: Oregon State University; 105 p.
Dissertation.
__.1980. The fire resistance of engelmann and coast live oak seedlings. In:
Plumb, T. R., tech. coord. Proceedings of the Symposium on the ecology,
management, and utilization of California oaks; 1979 June 26-28; Claremont,
CA. Gen. Tech. Rept. PSW-44. Berkeley, CA: Pacific Southwest Forest and
Range Experiment Station, Forest Service, U.S. Department of Agriculture;
62-66.
Specht, R. L. 1969. A comparison of the sclerophyllous vegetation character­
istic of mediterranean type climates in France, California, and southern
Australia. I. Structure, morphology, and succession. Australian Journal of
Botany 17:277-292.
Suc, J: P. 1984. Origin and evolution of the Mediterranean vegetation and
climate in Europe. Nature 307:429-432.
Vankat, J. L. 1982. A gradient perspective on the vegetation of Sequoia National
Park, California. Madror5o 29:200-214.
Van Zeist, W. 1967. Late Quaternary vegetation history of western Iran.
Review of Palaeobotany and Palynology 2:301-311.
Wright, H. E. 1962. Pleistocene glaciation in Kurdistan. Eiszeitalter und
Gegenwart 12:131-164.
Zohary, M. 1963. On the geobotanical structure of Iran. Bulletin of the Research
Council of Israel 11D (suppl.):1-113.
__.1973. Geobotanical foundations of the Middle East. 2 Vols. Stuttgart:
Gustav Fischer.
197