Leaf Energy Balance in the Wet Lowland Tropics

Abstract

The interplay between macroclimate and vegetation in the wet tropics creates leaf microclimates in which light intensity, humidity, air temperature, and wind velocity tend to be correlated. Along a vertical transect through the forest, this creates an increase in vapor pressure deficit (VPD) with increasing height. The correlated changes in environmental factors act to narrow the range of microclimates encountered along a vertical transect through the forest, making it possible to simulate leaf energy balance for “typical” microhabitats. Basing the simulations on leaves of the size most frequently encountered and with stomatal responses to light and VPD, leaf energy balance along a vertical gradient from forest floor to canopy can be summarized as follows: As a result of high (>90%) humidities and low radiation in the understory, leaf temperatures are generally very close to air temperature. As radiation increases and humidity decreases, leaf overtemperatures rise to about 6°C, and transpiration rates increase. Simulations and several observations suggest that leaf temperatures near the top of the canopy or in clearings can exceed 40°C under moderately high radiation. Leaf size has received considerable study in the wet tropics. Trends toward leaves larger than the predominant size (the mesophyll) occur with increasing moisture and increasing shade, but gap species often have large leaves. Optimization models have predicted that leaf size should be maximal in either the lowest or the intermediate strata. Simulations with varying leaf size and stomatal conductance suggest that leaf size has little effect on energy balance in understories and that interactions with conductance determine the consequences of leaf size in open conditions. General trends in leaf energy balance for a number of sites and habitat types are not yet available. Further studies are needed of stomatal conductance and responses to VPD, leaf temperature relations, and absorptance properties.