Ecohydrological Response of Trees to Leaf Wetness Gradients Under Wet and Dry Canopy Conditions in a Montane Tropical Forest

Abstract:

Wet canopy evaporation is a significant component of the water budget in rainforests. Frequent precipitation events followed by drying produce spatial and temporal variation in wet/dry forest canopy conditions that influence processes such as photosynthesis and growth. Upper canopies contribute a disproportionately large fraction of transpiration and carbon assimilation relative to lower canopy layers as exposed leaves dry more rapidly following each rain event. However, the partitioning between wet canopy evaporation and dry canopy transpiration has not been extensively studied in tropical forests. As part of a larger study in central Costa Rica aimed at improving land-surface modeling of evapotranspiration (ET) processes in tropical montane forests, we compared transpiration among trees with exposed and shaded crowns under both wet and dry canopy conditions. Transpiration was measured using 33 sap flow sensors in 20 trees (7 dominant and co-dominant, 8 mid-story, and 5 suppressed) in a mature forest stand surrounding a 40-m tower equipped with eddy covariance and micrometeorological sensors. During the wet month of July, leaves were dry 53% of the time in the upper canopy compared with only 16% of the time in the lower canopy. Dominant and co-dominant trees contributed 68% to total transpiration at this site (23% mid-story, 9% suppressed). Under dry conditions, sap flow rates of dominant and co-dominant trees were about double that of suppressed trees, while suppressed trees differed little between wet and dry days. On partially wet days, all trees had similar sap flow rates, regardless of canopy exposure, with rates nearly as low as days that were entirely wet. This behavior is due to rapid individual tree responses to the transition between wet and dry conditions, indicating that persistently wet tropical environments are uniquely adapted to short-term dryness. ET has important influence on precipitation in tropical forests through land-atmosphere interactions, particularly in regions where “precipitation recycling” is high. Hence improvements in ET parameterizations can lead to changes in land-atmosphere feedbacks simulated by coupled atmosphere-land models and alter the role of recycling in the water cycle over tropical forests.