Biophysical controls on canopy water balance.

摘要

Observations and modeling of terrestrial hydrologic-cycle processes ranging in scale from the leaf-level to the whole catchment are presented. Measurements of transpiration rates at the leaf surface and at the canopy level along a topographic gradient in a temperate forest are analyzed first. The results show mid-day leaf-level transpiration rates for the dominant species in this forest changed dramatically along the topographic gradient. However the observations also indicate a close coupling between leaf-level fluxes and the difference in water potential from the leaves to the soil. As a result, the leaf area-specific hydraulic conductance for the dominant species in this forest was remarkably consistent along the topographic gradient despite significant spatial variability in stand structure and available soil moisture. The simulation model developed for this thesis reproduces the observed spatial trends in transpiration rates and hydraulic conductance in this forest. Model predictions of catchment stream flow also compare favorably with observations. Sensitivity analyses on the model results show the overall water balance in this system is most responsive to changes in total leaf area, root biomass, and leaf water status. In a related study, this model is also used to simulate the water-relations of Populus deltoides grown under near-ambient (430 μmol mol −1) and elevated (1200 μmol mol−1) carbon dioxide concentrations [CO₂]. Modeling results confirmed observations showing leaf-level transpiration rates did not change with growth [CO ₂] during periods marked by high soil moisture conditions. Total canopy water use during these periods was higher in elevated [CO₂] because of the increased leaf production in these trees relative to the trees grown in near ambient [CO₂]. Observations and modeling results also show that decreasing soil moisture content had a significantly greater effect on leaf-level fluxes in the trees grown in elevated [CO₂] compared to those grown in near-ambient [CO₂]. Consequently, the total canopy water use was approximately equal across [CO₂] treatments during drought stress periods. Together these results suggest the effects of elevated [CO₂] on the water-relations of this species are mediated primarily by changes in morphology and the physiological requirement to maintain favorable leaf water status during drought stress.