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Abstract: Upscaling water use of individual trees to stands using sap flux techniques is a common method for partitioning site water balance, but few such studies have occurred in the tropics. Increasing interests in the role of tropical forests in global cycles have spurred upscaling studies in natural tropical forests, which present challenges from greater tree species and functional diversity, and potential factors that would reduce transpiration, such as frequent cloud cover and wet canopy conditions. In a premontane wet tropical forest in central Costa Rica, sap flow was measured in 15 trees stratified into 5 size classes based on tree diameters. None of the trees belonged to the same species, genus, or even family. We also accounted for potential radial variation in sap flux density. Data were scaled to estimate transpiration within a small 2.2‐ha watershed using stand surveys of sapwood area. Stand transpiration averaged only 1.4 ± 0.7 mm day−1 within this forested watershed due to persistent low radiation, evaporative demand, and frequent wet canopy conditions. Our systematic approach used tree size attributes to scale water use to the stand, given difficulties to quantify species differences in such a diverse ecosystem. Contrary to previous evidence on temperate trees, the large trees sampled did not exhibit flow reductions in deeper sapwood, which warrants further study. These results highlight some unique aspects of measuring transpiration in wet tropical forests that are important to consider for future studies in diverse stands. [ABSTRACT FROM AUTHOR]

Depending on severity, wildfire alters stand biomass, tree species distribution, and age, which may modify stand transpiration (Et) and the amount of water available to other parts of the hydrologic cycle. Our objective was to determine how wildfire severity affected Et in mixed pine/oak (Pinus taeda L./Quercus stellata Wangehn., Quercus marilandica Muenchh.) stands in the Lost Pines eco‐region (Bastrop, TX, USA). Transpiration was estimated for mature pines and oaks at unburned and moderately burned sites and oak resprouts and pine saplings at a severely burned plot. On average, mature pines had 36% greater sap flux rates (Js) than mature oaks in the unburned and moderately burned stands. Under low moisture stress, regenerating pines had greater Js than resprouting oaks, but Js quickly decreased as soil moisture declined. By contrast, mature pines were unaffected by dry periods. Pines contributed most to Et at the unburned and moderate stands. Conversely, oak Et dominated the severely burned stand, contributing over 95%. Transpiration was greatest at the moderately burned stand (2.02 mm day−1), followed by the unburned (1.44 mm day−1), and the severely burned stands (0.46 mm day−1). Despite greater Js in resprouts and saplings, reductions in total sapwood area resulted in lower stand‐level daily Et at the severe site. Although severe fire decreased stand transpiration through reductions in vegetation density, individual oak resprouts appear to thrive, undeterred by high vapour pressure deficit. Without pine planting, oaks will likely dominate severely burned stands that could result in shifts to local hydrology and microclimate. [ABSTRACT FROM AUTHOR]