ENVIRONMENTAL-REGULATION OF SURFACE CONDUCTANCE FOR EVAPORATION FROM VEGETATION

We examine conductances for evaporation from both vegetation and soil in response to environmental variables. Data from a vertically-structu red pristine forest of Nothofagus are presented as an example of the e ffects of biodiversity on the scaling of conductances between tiers of plant organisation. Available data sets of maximum leaf stomatal cond uctances (g(lmax)) and bulk vegetation surface conductances (G(smax)) are compared. Overall, the ratio G(smax)/g(lmax) is consistently close to 3 for seven major vegetation types of diverse structure. An analyt ical model accounts for this close relationship, and in particular how G(smax) is conservative against changes in leaf area index because of the compensating decrease in plant canopy transpiration and increase in soil evaporation as leaf area index diminishes. The model is also s uccessfully tested by comparison with canopy conductances of emergent trees measured in the Norhofagus forest. The constraint of vegetation surface conductance and evaporation via environmental regulation by ir radiance, air saturation deficit and root zone water supply are discus sed.