10 Gas Exchange and Water Relations
Robert Save, Carles Castell and Jaume Terradas
10.1 Introduction
Mediterranean environments are often characterized by a double stress:
summer drought and winter cold (Mitrakos 1980; Miller 1981; Terradas and
Save 1992). Summer drought results from the coincidence of low summer
precipitation with high temperature, high irradiance, and high water vapour
pressure deficit (Di Castri and Mooney 1973); it has been traditionally recognized as the main climate constraint characterizing Mediterranean-type
ecosystems. However, some degree of stress can be also due to winter cold,
which may be determinant in montane and/or continental sites. This is reflected in the seasonal patterns of plant activity. Photosynthetic activity is
typically relatively high in spring, decreases strongly in summer due to
drought (Oechel et al. 1981, Tenhunen et al. 1990), increases again after the
first autumn rains, and decreases or ceases during the winter months. Plant
growth and leaf transpiration may follow similar patterns.
Effects of drought and cold stresses on the water relations of holm oak
(Quercus ilex L.) have been long studied (Larcher 1960, 1981; Kyriakopoulos
and Larcher 1976; Kyriakopoulos and Richter 1977; Lossaint and Rapp 1978;
Save 1986; Comfn et al. 1987; Ramball992; Castell et al. 1994; Sala and Tenhunen 1994, 1996). Compared to temperate deciduous trees, holm oak follows a conservative strategy with low rates of gas exchange per unit of leaf
area. Stomatal regulation is considered to be efficient in this species (Larcher
1960). However, under moderate water stress, holm oak maintains higher
stomatal conductance for longer periods than deciduous oaks (Acherar and
Rambal 1992) and than shallow-rooted Mediterranean woody species. So,
transpiration continues and low xylem water potentials develop. High capacity to adjust stomatal conductance and to maintain low water potentials,
combined with deep roots (Chap. 4) allowing access to deep soil water, are
the clues to holm oak responses to water availability.
In this chapter we analyze the ways in which holm oak copes with this
double stress. We first deal briefly with the adaptive meaning of sclerophylly.
Ecological Studies, Vol. 137
Ferran Roda et al. (eds) Ecology of Mediterranean
Evergreen Oak Forests
© Springer-Verlag, Berlin Heidelberg 1999
Robert Save, Carles Castell and Jaume Terradas
10.1 Introduction
Mediterranean environments are often characterized by a double stress:
summer drought and winter cold (Mitrakos 1980; Miller 1981; Terradas and
Save 1992). Summer drought results from the coincidence of low summer
precipitation with high temperature, high irradiance, and high water vapour
pressure deficit (Di Castri and Mooney 1973); it has been traditionally recognized as the main climate constraint characterizing Mediterranean-type
ecosystems. However, some degree of stress can be also due to winter cold,
which may be determinant in montane and/or continental sites. This is reflected in the seasonal patterns of plant activity. Photosynthetic activity is
typically relatively high in spring, decreases strongly in summer due to
drought (Oechel et al. 1981, Tenhunen et al. 1990), increases again after the
first autumn rains, and decreases or ceases during the winter months. Plant
growth and leaf transpiration may follow similar patterns.
Effects of drought and cold stresses on the water relations of holm oak
(Quercus ilex L.) have been long studied (Larcher 1960, 1981; Kyriakopoulos
and Larcher 1976; Kyriakopoulos and Richter 1977; Lossaint and Rapp 1978;
Save 1986; Comfn et al. 1987; Ramball992; Castell et al. 1994; Sala and Tenhunen 1994, 1996). Compared to temperate deciduous trees, holm oak follows a conservative strategy with low rates of gas exchange per unit of leaf
area. Stomatal regulation is considered to be efficient in this species (Larcher
1960). However, under moderate water stress, holm oak maintains higher
stomatal conductance for longer periods than deciduous oaks (Acherar and
Rambal 1992) and than shallow-rooted Mediterranean woody species. So,
transpiration continues and low xylem water potentials develop. High capacity to adjust stomatal conductance and to maintain low water potentials,
combined with deep roots (Chap. 4) allowing access to deep soil water, are
the clues to holm oak responses to water availability.
In this chapter we analyze the ways in which holm oak copes with this
double stress. We first deal briefly with the adaptive meaning of sclerophylly.
Ecological Studies, Vol. 137
Ferran Roda et al. (eds) Ecology of Mediterranean
Evergreen Oak Forests
© Springer-Verlag, Berlin Heidelberg 1999