Gas Exchange and Water Relations
137
able conditions, results show that water-loss behaviour in holm oak is intermediate between those of xerophytic and mesophytic plants. Daily leaftranspiration rates, measured in this plot by the quick-weigh method, are
above those of conifers, but well below those of some Mediterranean sclerophyllous broadleaved trees (e.g. Pistacia lentiscus, Laurus nobilis or Phillyrea
latifolia; Ilijanic and Gracanin 1972), and nearly 50% of those found on average in a beech forest at Montseny (Save 1986). Transpiration rates of holm
oak fluctuate seasonally, being on average 44% lower at La Castanya in winter than in summer. Transpiration declines significantly when soil temperature decreases in autumn. Cuticular transpiration rates at La Castanya are on
average between 2 and 8% of maximum stomatal transpiration. This confirms the significance of the thick cuticles of sclerophyllous leaves in reducing water loss. In contrast, cuticular transpiration rates are much higher in
mesophytic, deciduous leaves: e.g. 34% in Quercus humilis and 32% in Fagus
sylvatica (Larcher 1960; Rabella 1991).
Stomatal conductances measured with a steady state porometer by Save
(1986) in the permanent plot at La Castanya are within the range given by
Berger et al. (1977) for holm oak (0.02-0.2 cm S-I). Total stomatal closure was
not frequent in this site, where we generally found only midday stomatal
regulation. This may be due to the favourable water balance in this mesic
plot. In southern France, holm oak shows an earlier decline of transpiration
in the morning, and longer periods of stomatal closure (Berger et al. 1977)
than those we observed in our permanent plot.
10.4 Within-Canopy Variations
Variations along the vertical canopy profile in leaf morphology and leaf age
structure (Chap. 9) and in canopy microclimate promote significant changes
in leaf transpiration rates. On average, sun leaves in the permanent plot at La
Castanya show transpiration rates 19% higher than shade leaves. Maximum
daytime conductance is usually found at the top of the canopy (10m above
the ground; Save 1986). However, in summer stomatal conductance decreases
at midday in sun leaves, and a peak of conductance appears at 8 m
(Fig. 10.1). These differences arise in part because stomatal regulation does
not proceed simultaneously over the whole canopy, resulting in large variations in stomatal conductance, water content and gas exchange along vertical
profiles, mainly when the canopy is under high irradiance (Rabella et al.
1983). In contrast, much smaller within-canopy differences have been found
in cork oak (Quercus suber L.), a closely related species but having a sparser
crown (Oliveira et al.1992; Oliveira 1995).
The above patterns must be interpreted in terms of the morphological and
functional traits of sun leaves that increase their resistance to stress. Furthermore, sun leaves maintain shaded canopy leaves under a lower stress
137
able conditions, results show that water-loss behaviour in holm oak is intermediate between those of xerophytic and mesophytic plants. Daily leaftranspiration rates, measured in this plot by the quick-weigh method, are
above those of conifers, but well below those of some Mediterranean sclerophyllous broadleaved trees (e.g. Pistacia lentiscus, Laurus nobilis or Phillyrea
latifolia; Ilijanic and Gracanin 1972), and nearly 50% of those found on average in a beech forest at Montseny (Save 1986). Transpiration rates of holm
oak fluctuate seasonally, being on average 44% lower at La Castanya in winter than in summer. Transpiration declines significantly when soil temperature decreases in autumn. Cuticular transpiration rates at La Castanya are on
average between 2 and 8% of maximum stomatal transpiration. This confirms the significance of the thick cuticles of sclerophyllous leaves in reducing water loss. In contrast, cuticular transpiration rates are much higher in
mesophytic, deciduous leaves: e.g. 34% in Quercus humilis and 32% in Fagus
sylvatica (Larcher 1960; Rabella 1991).
Stomatal conductances measured with a steady state porometer by Save
(1986) in the permanent plot at La Castanya are within the range given by
Berger et al. (1977) for holm oak (0.02-0.2 cm S-I). Total stomatal closure was
not frequent in this site, where we generally found only midday stomatal
regulation. This may be due to the favourable water balance in this mesic
plot. In southern France, holm oak shows an earlier decline of transpiration
in the morning, and longer periods of stomatal closure (Berger et al. 1977)
than those we observed in our permanent plot.
10.4 Within-Canopy Variations
Variations along the vertical canopy profile in leaf morphology and leaf age
structure (Chap. 9) and in canopy microclimate promote significant changes
in leaf transpiration rates. On average, sun leaves in the permanent plot at La
Castanya show transpiration rates 19% higher than shade leaves. Maximum
daytime conductance is usually found at the top of the canopy (10m above
the ground; Save 1986). However, in summer stomatal conductance decreases
at midday in sun leaves, and a peak of conductance appears at 8 m
(Fig. 10.1). These differences arise in part because stomatal regulation does
not proceed simultaneously over the whole canopy, resulting in large variations in stomatal conductance, water content and gas exchange along vertical
profiles, mainly when the canopy is under high irradiance (Rabella et al.
1983). In contrast, much smaller within-canopy differences have been found
in cork oak (Quercus suber L.), a closely related species but having a sparser
crown (Oliveira et al.1992; Oliveira 1995).
The above patterns must be interpreted in terms of the morphological and
functional traits of sun leaves that increase their resistance to stress. Furthermore, sun leaves maintain shaded canopy leaves under a lower stress