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.