Gas Exchange and Water Relations
139
1995). In holm oak, the critical predawn xylem potential for stomatal closure
in summer is -3.0 to -3.5 MPa (Aussenac and Valette 1982).
10.5 Ecophysiology of Stool Sprouts
Holm oak forests in NE Spain are frequently thinned or burned, and holm
oak resprouts vigorously after disturbance (Chap. 5). During much of the
between-disturbance period, holm oak forests are made up of young stool
sprouts, whose ecophysiological responses may differ from those of adult
trees. In particular, the much larger root to shoot ratio of sprouts, which rely
upon the pre-existing root system of the stool, results in a much larger availability of soil resources per unit of leaf area in sprouts compared with mature trees. This can greatly modify the ecophysiological behaviour of holm
oak. To test this, water relations and gas exchange in mature holm oak and in
young sprouts produced by experimentally clipped stools were studied at
Collserola by Castell et al. (1994). This site lies in the coastal range near Barcelona, and it has a drier and warmer climate than the permanent plot at
Montseny. Shoot elongation of mature holm oak occurs mainly in spring, and
stops in mid-August, while few individuals show a new flush in autumn. This
pattern seems closely related to the decrease in predawn water potential
during summer drought (Fig. 10.2). In contrast, midday water potential
changes very little, remaining above -3.0 MPa, the potential below which extensive cavitation occurs (Lo Gullo and Salleo 1993; see Sect. 10.8). Similar
patterns of gas exchange in summer have been found for Quercus suber in
Portugal under comparable climate conditions (Oliveira 1995).
In summer, mature holm oaks at Collserola maintain a relatively high xylem water potential due to a 50% reduction in stomatal conductance from
spring values. At the same time, net photosynthesis is reduced by ca. 85%
(Fig. 10.2). These differences suggest some inhibitory effects of high light intensities at the chloroplast level. After the first autumn rains, xylem water
potential, leaf conductance and net photosynthesis in mature trees quickly
recover to the spring levels (Fig. 10.2). Although the rates of net photosynthesis are fairly high in autumn, there is no or low shoot elongation at this
time, probably because production is dedicated to ripening fruits, as happens
in other species (Bazzaz et al. 1987). Winter cold does not seem to affect
markedly holm oak ecophysiology in this area (mean December temperature
is 7°C), and net photosynthesis is quite high in sunny winter days.
Seasonal patterns of water relations, gas exchange and growth have also
been evaluated for young sprouts. In spring, the elongation rates of sprouts
are much higher than those observed in mature plants, then they decrease in
August, and vigorously increase again in autumn. High sprout growth rates
are closely related to their greater relative water availability, since the preexisting root system gives them a high root to shoot ratio. High water poten-
139
1995). In holm oak, the critical predawn xylem potential for stomatal closure
in summer is -3.0 to -3.5 MPa (Aussenac and Valette 1982).
10.5 Ecophysiology of Stool Sprouts
Holm oak forests in NE Spain are frequently thinned or burned, and holm
oak resprouts vigorously after disturbance (Chap. 5). During much of the
between-disturbance period, holm oak forests are made up of young stool
sprouts, whose ecophysiological responses may differ from those of adult
trees. In particular, the much larger root to shoot ratio of sprouts, which rely
upon the pre-existing root system of the stool, results in a much larger availability of soil resources per unit of leaf area in sprouts compared with mature trees. This can greatly modify the ecophysiological behaviour of holm
oak. To test this, water relations and gas exchange in mature holm oak and in
young sprouts produced by experimentally clipped stools were studied at
Collserola by Castell et al. (1994). This site lies in the coastal range near Barcelona, and it has a drier and warmer climate than the permanent plot at
Montseny. Shoot elongation of mature holm oak occurs mainly in spring, and
stops in mid-August, while few individuals show a new flush in autumn. This
pattern seems closely related to the decrease in predawn water potential
during summer drought (Fig. 10.2). In contrast, midday water potential
changes very little, remaining above -3.0 MPa, the potential below which extensive cavitation occurs (Lo Gullo and Salleo 1993; see Sect. 10.8). Similar
patterns of gas exchange in summer have been found for Quercus suber in
Portugal under comparable climate conditions (Oliveira 1995).
In summer, mature holm oaks at Collserola maintain a relatively high xylem water potential due to a 50% reduction in stomatal conductance from
spring values. At the same time, net photosynthesis is reduced by ca. 85%
(Fig. 10.2). These differences suggest some inhibitory effects of high light intensities at the chloroplast level. After the first autumn rains, xylem water
potential, leaf conductance and net photosynthesis in mature trees quickly
recover to the spring levels (Fig. 10.2). Although the rates of net photosynthesis are fairly high in autumn, there is no or low shoot elongation at this
time, probably because production is dedicated to ripening fruits, as happens
in other species (Bazzaz et al. 1987). Winter cold does not seem to affect
markedly holm oak ecophysiology in this area (mean December temperature
is 7°C), and net photosynthesis is quite high in sunny winter days.
Seasonal patterns of water relations, gas exchange and growth have also
been evaluated for young sprouts. In spring, the elongation rates of sprouts
are much higher than those observed in mature plants, then they decrease in
August, and vigorously increase again in autumn. High sprout growth rates
are closely related to their greater relative water availability, since the preexisting root system gives them a high root to shoot ratio. High water poten-