ENVIRONMENTAL AND PHYSIOLOGICAL REGULATION OF TRANSPIRATION IN TROPICAL FOREST GAP SPECIES - THE INFLUENCE OF BOUNDARY-LAYER AND HYDRAULIC-PROPERTIES

Environmental and physiological regulation of transpiration were exami ned in several gap-colonizing shrub and tree species during two consec utive dry seasons in a moist, lowland tropical forest on Barro Colorad o Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential a nd environmental variables were measured concurrently. This allowed co ntrol of transpiration (E) to be partitioned quantitatively between st omatal (g(s)) and boundary layer (g(b)) conductance and permitted the impact of individual environmental and physiological variables on stom atal behavior and E to be assessed. Wind speed in treefall gap sites w as often below the 0.25 m s(-1) stalling speed of the anemometer used and was rarely above 0.5 m s(-1), resulting in uniformly low g(b) (c. 200-300 mmol m(-2) s(-1)) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than g(b). This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g(s) when g(s) was near its mean value was predicted to yield only a 2.5% change in E. Porometric estim ates of E, obtained as the product of g(s) and the leaf-bulk air vapor pressure difference (VPD) without taking g(b) into account, were up t o 300% higher than actual E determined from sap flow measurements. Por ometry was thus inadequate as a means of assessing the physiological c onsequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeare d to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light a nd leaf-air VPD was taken into account. This result suggests that cont rasting stomatal responses to similar leaf-bulk air VPD may be governe d as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g(s) initially increased sharpl y with increasing leaf area-specific total hydraulic conductance of th e soil/root/leaf pathway (G(t)), becoming asymptotic at higher values of G(t). For both E and g(s) a unique relationship appeared to describ e the response of ail species to variations in G(t). The relatively we ak correlation observed between g(s) and midday leaf water potential s uggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf wa ter status.