Regulation of stomatal conductance and transpiration in forest canopies.

Processes regulating stomatal conductance, g(s), and transpiration, E, from forest canopies are reviewed. The first section deals with the response of g(s) to environmental variables. Phenomenological models have been used to interpret field data and predict diurnal and seasonal variability in g(s), but models that couple stomatal conductance to photosynthesis at the leaf scale are now being used more widely. The vertical distribution of foliar nitrogen concentration is helpful for scaling these processes from leaves to canopies, and the analysis of data from many studies has led to the emergence of simplified, general relationships for estimating evaporation and carbon uptake by forests at stand and regional scales. Evidence for the regulation of stomatal conductance by hydraulic and chemical signals is presented in the second section. Rapid and reversible changes in g(s) following a perturbation to the water potential gradient in the flow pathway suggest that stomata respond directly to hydrostatic signals. Other evidence supports the contention that signals are transmitted by abscisic acid (ABA), possibly originating in the roots. For large woody plants, the short-term responses of stomata are probably brought about by hydraulic signals that affect g(s) by triggering the release of ABA in the leaves. Tardieu and Davies (1993) developed an interactive model that incorporates hydraulic and chemical effects to describe the response of stomata to soil drying and evaporative demand. In the third section, evidence is presented that short-term changes in g(s) are linked closely to the hydraulic properties of the conducting system to minimize loss of hydraulic conductivity through xylem by cavitation. Examples of homeostatic mechanisms that operate to ensure the long-term balance between evaporative demand and the potential hydraulic conductivity of trees growing in different environments are described. Two hypotheses are examined: (1) height growth in trees is limited by the capacity of the conducting system; and (2) the decline in productivity with stand age is attributable to a decrease in conductivity.