136
Robert Save, Carles Castell and Jaume Terradas
We then summarize two of our own studies in NE Spain that illustrate two
revealing situations: water relations of holm oak in a mesic site, and the ecophysiology of stool sprouts. We go on to address some of the major mechanisms involved, and finally consider the ecosystem and biogeographical implications of the observed behaviour.
10.2 Sderophylly and Water Stress
The double-stress climate is associated with some recurrent or convergent
plant characteristics in the five Mediterranean-climate regions of the world.
Among these traits, sclerophylly has received much attention (e.g. for holm
oak: Ilijanic and Gracanin 1972; Larcher 1960; Aussenac and Valette 1982;
Salleo et al. 1997). Sclerohyllous leaves, such as those of holm oak, are useful
to minimize water losses and to resist those losses that do occur. Holm oak
leaves can tolerate leaf water saturation deficits of up to 37% (Larcher 1960),
which are lethal to most mesophytic trees. However, sclerophylly is not a
universal feature in Mediterranean-climate plants, neither is it restricted to
them. In Mediterranean-climate areas, sclerophylly is found in isomorphic
evergreen plants, such as holm oak, but, when the intensity and length of
summer drought increase, heteromorphic evergreen and even summerdeciduous plants replace the isomorphic sclerophylls (Margaris 1981). Then,
the advantages of sclerophyllous leaves are probably restricted to a limited
portion of these Mediterranean-climate areas. In addition, the main advantage of sclerophylly for holm oak probably does not lie in reducing water
losses since its stomata remain open much longer than in deciduous oaks
(Salleo and Lo Gullo 1990). So, holm oak is not a drought-avoider but a
drought-tolerant species. Furthermore, sclerophylly can also be an evolutionary response to low nutrient availability (Loveless 1961) and to herbivory. In
fact, sclerophylly can result from selection for increased leaf longevity under
situations of resource shortage (Turner 1994). Once acquired, sclerophylly
can serve a protective function useful in facing a variety of abiotic and biotic
stresses.
10.3 Transpiration and Water Status of Leaves in a Mesic Site
Our first study in the mesic permanent plot at La Castanya in the Montseny
experimental area (Chap. 2) yielded information on the patterns of water use
of holm oak growing at relatively high soil water availability (Save 1986).
This plot lies at the base of a steep NW-facing slope where the trees benefit
from a deep soil, lateral soil water inputs, relatively high rainfall, and reduced
evaporative demand through topographic shading. Even under these favour-
Robert Save, Carles Castell and Jaume Terradas
We then summarize two of our own studies in NE Spain that illustrate two
revealing situations: water relations of holm oak in a mesic site, and the ecophysiology of stool sprouts. We go on to address some of the major mechanisms involved, and finally consider the ecosystem and biogeographical implications of the observed behaviour.
10.2 Sderophylly and Water Stress
The double-stress climate is associated with some recurrent or convergent
plant characteristics in the five Mediterranean-climate regions of the world.
Among these traits, sclerophylly has received much attention (e.g. for holm
oak: Ilijanic and Gracanin 1972; Larcher 1960; Aussenac and Valette 1982;
Salleo et al. 1997). Sclerohyllous leaves, such as those of holm oak, are useful
to minimize water losses and to resist those losses that do occur. Holm oak
leaves can tolerate leaf water saturation deficits of up to 37% (Larcher 1960),
which are lethal to most mesophytic trees. However, sclerophylly is not a
universal feature in Mediterranean-climate plants, neither is it restricted to
them. In Mediterranean-climate areas, sclerophylly is found in isomorphic
evergreen plants, such as holm oak, but, when the intensity and length of
summer drought increase, heteromorphic evergreen and even summerdeciduous plants replace the isomorphic sclerophylls (Margaris 1981). Then,
the advantages of sclerophyllous leaves are probably restricted to a limited
portion of these Mediterranean-climate areas. In addition, the main advantage of sclerophylly for holm oak probably does not lie in reducing water
losses since its stomata remain open much longer than in deciduous oaks
(Salleo and Lo Gullo 1990). So, holm oak is not a drought-avoider but a
drought-tolerant species. Furthermore, sclerophylly can also be an evolutionary response to low nutrient availability (Loveless 1961) and to herbivory. In
fact, sclerophylly can result from selection for increased leaf longevity under
situations of resource shortage (Turner 1994). Once acquired, sclerophylly
can serve a protective function useful in facing a variety of abiotic and biotic
stresses.
10.3 Transpiration and Water Status of Leaves in a Mesic Site
Our first study in the mesic permanent plot at La Castanya in the Montseny
experimental area (Chap. 2) yielded information on the patterns of water use
of holm oak growing at relatively high soil water availability (Save 1986).
This plot lies at the base of a steep NW-facing slope where the trees benefit
from a deep soil, lateral soil water inputs, relatively high rainfall, and reduced
evaporative demand through topographic shading. Even under these favour-