Modelling Canopy Gas Exchange During Summer Drought
159
which changes in leaf arrangement cannot compensate for increases in selfshading associated with increases in LAI.
At all levels of water stress, a reduction of canopy LAI from 6 to 2 m 2 m- 2
resulted in an annual water saving of approximately 35% (Fig. 11.5). However, at any LAI, larger water savings (up to 41 %) were simulated by stomatal
regulation of water loss (imposed via the parameter gF). These results indicate that short-term functional responses (stomatal control) characteristic of
holm oak and other Mediterranean sclerophylls may be more effective than
long-term structural responses (changes in leaf area) in reducing annual
water loss (Tenhunen et al. 1990). Thus, even if stands exhibited relatively
high LAI, stomatal regulation of water loss would serve to prevent excessive
water loss during periods of summer drought. However, the results also indicate that LAI values greater than 5 m 2 m- 2 do not result in significant increments of annual carbon input (Caldwell et al. 1986). Furthermore, WUE decreases approximately 5% with each successive increase in LAL Thus, increases in LAI up to 5 m 2 m- 2 are associated with increases in carbon input
but also with decreases of WUE. This is because as LAI increases light penetration within the canopy decreases and the contribution of leaves lower in
the canopy to the total canopy carbon input decreases.
11.5 Conclusions
The model used here allowed effective predictions of diurnal and seasonal
courses of gas exchange in holm oak. The effects of summer drought on
stomatal regulation were successfully introduced by changing a single model
parameter which can be estimated from pre-dawn water potential. Because
of the mechanistic nature of the model, a direct linkage with a hydrological
model (e.g. Beven and Kirkby 1979) would allow us to estimate spatial and
seasonal patterns in canopy water use and to assess the impacts of climate
change, at least in the short term (Reynolds et al. 1992), on the hydrology of
the entire catchment.
The analysis of the relative effects of structural vs. functional adjustments
as mechanisms to cope with reduced site water availability suggests a delicate
trade-off between water loss and carbon input for Mediterranean sclerophylls. This trade-off may shift depending on biotic and abiotic interactions.
Stands supporting LAIs below 3 m 2 m- 2 are found in dry sites where, according to model simulations, strong reductions of carbon input are compensated for by substantial increases in WUE. Under conditions of reduced
water availability, optimization of water use may be crucial to survive and
coexist with other xerophytic Mediterranean sclerophylls characteristic of
dry areas such as Quercus coccifera. In sites where the water balance is more
favourable, LAls up to 5 m 2 m- 2 may be found, suggesting that maximizing
canopy carbon input (even if at the expense of increased water use and reduced WUE) may allow holm oak to outgrow potential competitors.
159
which changes in leaf arrangement cannot compensate for increases in selfshading associated with increases in LAI.
At all levels of water stress, a reduction of canopy LAI from 6 to 2 m 2 m- 2
resulted in an annual water saving of approximately 35% (Fig. 11.5). However, at any LAI, larger water savings (up to 41 %) were simulated by stomatal
regulation of water loss (imposed via the parameter gF). These results indicate that short-term functional responses (stomatal control) characteristic of
holm oak and other Mediterranean sclerophylls may be more effective than
long-term structural responses (changes in leaf area) in reducing annual
water loss (Tenhunen et al. 1990). Thus, even if stands exhibited relatively
high LAI, stomatal regulation of water loss would serve to prevent excessive
water loss during periods of summer drought. However, the results also indicate that LAI values greater than 5 m 2 m- 2 do not result in significant increments of annual carbon input (Caldwell et al. 1986). Furthermore, WUE decreases approximately 5% with each successive increase in LAL Thus, increases in LAI up to 5 m 2 m- 2 are associated with increases in carbon input
but also with decreases of WUE. This is because as LAI increases light penetration within the canopy decreases and the contribution of leaves lower in
the canopy to the total canopy carbon input decreases.
11.5 Conclusions
The model used here allowed effective predictions of diurnal and seasonal
courses of gas exchange in holm oak. The effects of summer drought on
stomatal regulation were successfully introduced by changing a single model
parameter which can be estimated from pre-dawn water potential. Because
of the mechanistic nature of the model, a direct linkage with a hydrological
model (e.g. Beven and Kirkby 1979) would allow us to estimate spatial and
seasonal patterns in canopy water use and to assess the impacts of climate
change, at least in the short term (Reynolds et al. 1992), on the hydrology of
the entire catchment.
The analysis of the relative effects of structural vs. functional adjustments
as mechanisms to cope with reduced site water availability suggests a delicate
trade-off between water loss and carbon input for Mediterranean sclerophylls. This trade-off may shift depending on biotic and abiotic interactions.
Stands supporting LAIs below 3 m 2 m- 2 are found in dry sites where, according to model simulations, strong reductions of carbon input are compensated for by substantial increases in WUE. Under conditions of reduced
water availability, optimization of water use may be crucial to survive and
coexist with other xerophytic Mediterranean sclerophylls characteristic of
dry areas such as Quercus coccifera. In sites where the water balance is more
favourable, LAls up to 5 m 2 m- 2 may be found, suggesting that maximizing
canopy carbon input (even if at the expense of increased water use and reduced WUE) may allow holm oak to outgrow potential competitors.