Comparison of tree transpiration under wet and dry canopy conditions in a Costa Rican premontane tropical forest

Spatial and temporal variation in wet canopy conditions following precipitation events can influence processes such as transpiration and photosynthesis, which can be further enhanced as upper canopy leaves dry more rapidly than the understory following each event. As part of a larger study aimed at improving land surface modelling of evapotranspiration processes in wet tropical forests, we compared transpiration among trees with exposed and shaded crowns under both wet and dry canopy conditions in central Costa Rica, which has an average 4200mm annual rainfall. Transpiration was estimated for 5months using 43 sap flux sensors in eight dominant, ten midstory and eight suppressed trees in a mature forest stand surrounding a 40-m tower equipped with micrometeorological sensors. Dominant trees were 13% of the plot's trees and contributed around 76% to total transpiration at this site, whereas midstory and suppressed trees contributed 18 and 5%, respectively. After accounting for vapour pressure deficit and solar radiation, leaf wetness was a significant driver of sap flux, reducing it by as much as 28%. Under dry conditions, sap flux rates (J(s)) of dominant trees were similar to midstory trees and were almost double that of suppressed trees. On wet days, all trees had similarly low J(s). As expected, semi-dry conditions (dry upper canopy) led to higher J(s) in dominant trees than midstory, which had wetter leaves, but semi-dry conditions only reduced total stand transpiration slightly and did not change the relative proportion of transpiration from dominant and midstory. Therefore, models that better capture forest stand wet-dry canopy dynamics and individual tree water use strategies are needed to improve accuracy of predictions of water recycling over tropical forests. Copyright (c) 2016 John Wiley & Sons, Ltd.