150
AnnaSala
function, which improve the light environment within the canopy and canopy carbon balance (Sala et al. 1994; Rambal et al. 1996; Sala and Tenhunen
1996; Chap. 9). Sensitive stomatal control of water loss in holm oak is also a
well documented short-term response (hours to days) by which canopy water
use is reduced and the damaging effects of transient or prolonged drought
are avoided (e.g. Tenhunen et al. 1987; Terradas and Save 1992; Tetriach 1993;
Sala and Tenhunen 1994).
Here, I will use a process-based simulation model that incorporates
physiological behaviour and vegetation structure (Tenhunen et al. 1990;
Harley and Tenhunen 1991; Reynolds et al. 1992; Sala and Tenhunen 1996) to
examine the relative effects of structural and physiological adjustments to
drought on the annual water loss and carbon assimilation of holm oak canopies. This type of analysis, performed under varying intensities of summer
drought, will provide a better understanding of the complex interactions
between canopy structure and function of Mediterranean sclerophylls such
as holm oak. It will also ,provide better insight into the trade-off between optimization of water use vs. carbon uptake in Mediterranean sclerophylls, a
topic that needs further investigation (Rambal 1993). I will first provide a
brief description of the model and the main results obtained from its application in the holm oak forest of the Avic catchment (Prades Mountains, NE
Spain).
11.2 Model Description
A full description of the model used and its parameterization is provided in
Sala and Tenhunen (1996). The model incorporates a canopy model of light
interception and microclimate (Caldwell et al. 1986; Tenhunen et al. 1990;
Reynolds et al. 1992) which estimates detailed microclimatic data within the
canopy. Microclimatic conditions and leaf photosynthetic properties in different canopy layers are used to calculate photosynthesis rates following a
mechanistically based model of C3 leaf photosynthesis (Farquhar and Von
Caemmerer 1982; Tenhunen et al.1990; Harley and Tenhunen 1991). Stomatal
conductance is then calculated as a function of photosynthesis rates with the
empirical model described by Ball et al. (1987; see below). Transpiration rates
are calculated from stomatal conductance. Initially, the model assumes leaf
temperature equal to air temperature. Since leaf temperature depends on the
transpiration rate, final equilibrium leaf temperature is solved by an energy
balance approach with successive within- and between-canopy layer iterations.
Inputs to the model include I-h time steps of the driving variables (global
short-wave radiation, air temperature, relative humidity and wind velocity
above the canopy, and soil surface temperature) and structural, optical, and
physiological properties of leaves in each canopy layer. Outputs of the model
AnnaSala
function, which improve the light environment within the canopy and canopy carbon balance (Sala et al. 1994; Rambal et al. 1996; Sala and Tenhunen
1996; Chap. 9). Sensitive stomatal control of water loss in holm oak is also a
well documented short-term response (hours to days) by which canopy water
use is reduced and the damaging effects of transient or prolonged drought
are avoided (e.g. Tenhunen et al. 1987; Terradas and Save 1992; Tetriach 1993;
Sala and Tenhunen 1994).
Here, I will use a process-based simulation model that incorporates
physiological behaviour and vegetation structure (Tenhunen et al. 1990;
Harley and Tenhunen 1991; Reynolds et al. 1992; Sala and Tenhunen 1996) to
examine the relative effects of structural and physiological adjustments to
drought on the annual water loss and carbon assimilation of holm oak canopies. This type of analysis, performed under varying intensities of summer
drought, will provide a better understanding of the complex interactions
between canopy structure and function of Mediterranean sclerophylls such
as holm oak. It will also ,provide better insight into the trade-off between optimization of water use vs. carbon uptake in Mediterranean sclerophylls, a
topic that needs further investigation (Rambal 1993). I will first provide a
brief description of the model and the main results obtained from its application in the holm oak forest of the Avic catchment (Prades Mountains, NE
Spain).
11.2 Model Description
A full description of the model used and its parameterization is provided in
Sala and Tenhunen (1996). The model incorporates a canopy model of light
interception and microclimate (Caldwell et al. 1986; Tenhunen et al. 1990;
Reynolds et al. 1992) which estimates detailed microclimatic data within the
canopy. Microclimatic conditions and leaf photosynthetic properties in different canopy layers are used to calculate photosynthesis rates following a
mechanistically based model of C3 leaf photosynthesis (Farquhar and Von
Caemmerer 1982; Tenhunen et al.1990; Harley and Tenhunen 1991). Stomatal
conductance is then calculated as a function of photosynthesis rates with the
empirical model described by Ball et al. (1987; see below). Transpiration rates
are calculated from stomatal conductance. Initially, the model assumes leaf
temperature equal to air temperature. Since leaf temperature depends on the
transpiration rate, final equilibrium leaf temperature is solved by an energy
balance approach with successive within- and between-canopy layer iterations.
Inputs to the model include I-h time steps of the driving variables (global
short-wave radiation, air temperature, relative humidity and wind velocity
above the canopy, and soil surface temperature) and structural, optical, and
physiological properties of leaves in each canopy layer. Outputs of the model