A Review of Wet Tropical Forest Transpiration: Lessons Learned and a Path Forward

Abstract:

Tropical rainforests contribute significantly to global water vapor fluxes, yet there are few empirical studies that address critical issues such as how evaporative fluxes are affected by land use change and feedbacks to precipitation recycling. We conducted a review of global studies concentrating on energy-limited wet tropical forests to ask (1) What are the primary mechanisms that determine maximum transpiration of a site, given variation in forest density and composition, rainfall, and available energy; (2) Where are the global hot spots for transpiration; and (3) What are the critical knowledge gaps? For this analysis, we considered empirical estimates of evaporation and transpiration based on sapflow and eddy correlation, rather than modeled from meteorological variables. Sites were restricted to those with greater than 2000 mm precipitation and a PET/P <1. Tree plantations were distinguished from natural stands. Where possible, we examined the ratio of evaporation to transpiration and made estimates of leaf wetness duration based on rainfall frequency. We also determined maximum transpiration (T_max) and vapor pressure deficit (D_max) for each site. Based on the relationship between observed T_max and D_max, we estimated a theoretical maximum transpiration for wet tropical forests around the globe. Accurate estimates of tropical forest transpiration are particularly important under future climate scenarios where increased temperatures are expected to increase water vapor storage in the atmosphere. More frequent droughts in tropical ecosystems are leading to forest die-offs. Therefore a better understanding of physiological thresholds or functional safety margins is needed.