We compare independent data sets of the maximum stomatal conductance (
g(smax), for single leaves) and bulk surface conductance (G(smax), for
a vegetated surface including the plant canopy and soil) for evaporat
ion. Data were obtained from field measurements, restricted to periods
with plentiful soil water, adequate light, high relative humidity and
moderate temperature. The data encompass most major vegetation types
and a wide range of leaf area index (Lambda). Observed G(smax) is not
systematically dependent on Lambda; and takes average values of 20 and
33 mm s(-1) for natural vegetation and agricultural crops. A similar
pattern exists in the g(smax) data, which yield remarkably consistent
average values of 6 and 12 mm s(-1), respectively, for natural vegetat
ion and crops. Overall, the ratio G(smax)/g(smax) is consistently clos
e to 3, for seven major vegetation types of diverse structure. A simpl
e model accounts for the close relationship between g(smax) and G(smax
), and in particular how G(smax) is conservative against Lambda becaus
e of the compensating decrease in plant canopy evaporation and increas
e in soil evaporation as Lambda diminishes. The results are important
for development of parameters for biosphere-atmosphere interactions in
models.