Modelling Canopy Gas Exchange During Summer Drought
151
are diurnal courses of leaf conductance, transpiration and net photosynthesis for sun and shade leaves in different canopy layers. Total daily canopy
carbon assimilation and transpiration are calculated from the hourly contributions of all leaves. While the model considers light interception by stems,
values of canopy carbon uptake include only the gas exchange of leaves and
ignore respiration by twigs and stems.
11.3 Model Implementation at the Avic Catchment
Simulations of canopy gas exchange in holm oak were carried out at two sites
(ridge top and valley bottom) along a slope and microclimate gradient at the
Avic catchment (Prades Mountains, NE Spain; Sala et al. 1994; Sala and Tenhunen 1996; see Chap. 2 for site description).
Canopy structural characteristics model inputs (leaf area index, stem area
index, leaf size, leaf inclination angle and stem inclination angle) were derived from measurements described in Chapter 9 and Sala et al. (1994). The
degree of leaf clustering was chosen to allow the best agreement between
predicted light values and measured data in different canopy levels (Eckardt
et al. 1978; A. Sala et al. unpubl.). Leaf optical properties were derived from
reported values in the literature (Eckardt et al. 1978) and were assumed to be
constant throughout the canopy.
Parameters describing leaf photosynthetic characteristics in leaves in the
uppermost canopy layer were set as reported by Tenhunen et al. (1990) for
the closely related scrub oak species Quercus coccifera. These parameters
were changed as a function of cumulative leaf area index such that photosynthetic capacity of the leaves at different canopy layers was linearly related
to the cumulative LAI and to leaf nitrogen (Field 1983; Sabate et al. 1995;
Chap. 9). The model assumes that photosynthetic capacity in each canopy
layer does not change during the year (Kaiser 1987; Epron and Dreyer 1993).
CO2 partial pressure was set equal to 350 ppm.
11.3.1 Incorporating the Effects of Seasonal Water Deficits
on Stomatal Regulation
According to the model of Ball et al. (1987), a scaling factor (gF) relates stomatal conductance to a "stomatal index". This is, in turn, directly proportional to the net assimilation rate and the relative humidity within the leaf
boundary layer and inversely proportional to the CO2 partial pressure within
the boundary layer. By only changing gF Tenhunen et al. (1990) were able to
incorporate the integrated effects of long-term water stress on stomatal
regulation and produce realistic simulations of gas exchange during the year.
Sala and Tenhunen (1996) optimized the value of gF to allow the closest fit
151
are diurnal courses of leaf conductance, transpiration and net photosynthesis for sun and shade leaves in different canopy layers. Total daily canopy
carbon assimilation and transpiration are calculated from the hourly contributions of all leaves. While the model considers light interception by stems,
values of canopy carbon uptake include only the gas exchange of leaves and
ignore respiration by twigs and stems.
11.3 Model Implementation at the Avic Catchment
Simulations of canopy gas exchange in holm oak were carried out at two sites
(ridge top and valley bottom) along a slope and microclimate gradient at the
Avic catchment (Prades Mountains, NE Spain; Sala et al. 1994; Sala and Tenhunen 1996; see Chap. 2 for site description).
Canopy structural characteristics model inputs (leaf area index, stem area
index, leaf size, leaf inclination angle and stem inclination angle) were derived from measurements described in Chapter 9 and Sala et al. (1994). The
degree of leaf clustering was chosen to allow the best agreement between
predicted light values and measured data in different canopy levels (Eckardt
et al. 1978; A. Sala et al. unpubl.). Leaf optical properties were derived from
reported values in the literature (Eckardt et al. 1978) and were assumed to be
constant throughout the canopy.
Parameters describing leaf photosynthetic characteristics in leaves in the
uppermost canopy layer were set as reported by Tenhunen et al. (1990) for
the closely related scrub oak species Quercus coccifera. These parameters
were changed as a function of cumulative leaf area index such that photosynthetic capacity of the leaves at different canopy layers was linearly related
to the cumulative LAI and to leaf nitrogen (Field 1983; Sabate et al. 1995;
Chap. 9). The model assumes that photosynthetic capacity in each canopy
layer does not change during the year (Kaiser 1987; Epron and Dreyer 1993).
CO2 partial pressure was set equal to 350 ppm.
11.3.1 Incorporating the Effects of Seasonal Water Deficits
on Stomatal Regulation
According to the model of Ball et al. (1987), a scaling factor (gF) relates stomatal conductance to a "stomatal index". This is, in turn, directly proportional to the net assimilation rate and the relative humidity within the leaf
boundary layer and inversely proportional to the CO2 partial pressure within
the boundary layer. By only changing gF Tenhunen et al. (1990) were able to
incorporate the integrated effects of long-term water stress on stomatal
regulation and produce realistic simulations of gas exchange during the year.
Sala and Tenhunen (1996) optimized the value of gF to allow the closest fit