Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties

Abstract

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (g_s) and boundary layer (g_b) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was 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 estimates 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 to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences 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 appeared 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 and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g_s initially increased sharply with increasing leaf area-specific total hydraulic conductance of the 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 describe the response of all species to variations in G_t. The relatively weak correlation observed between g_s and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.