Representing diurnal shifts in leaf-level water use efficiency may be key to modeling tropical forest gas exchange
To understand how tropical ecosystems will respond to global change, researchers must correctly represent the relationship between water loss and carbon gain in leaves, known as the water use efficiency (WUE). There are still significant uncertainties associated with the dynamics of WUE over different timescales, from over the day, to the changes experienced over the full lifespan of a leaf. Here we collected data to assess the possible physiological and mechanistic factors which influence WUE dynamics. While WUE does differ between leaves of different phenological stages, the trend was not consistent across species. However, we identified a unidirectional increase in WUE of approximately 2.5 times over the course of the day in five of the six species studied.
One of the major roadblocks to accurate representation of transpiration in climate models is an understanding of the physiological factors which most strongly contribute to variation in leaf level WUE. In this study, we demonstrate that including leaf age as a primary driver of WUE did not help to improve or explain variation in modeled transpiration. However, models which accounted for diurnal (within-day) changes in WUE improved the representation of transpiration. These findings provide a roadmap for future investigation into the physiological traits which most strongly influence transpiration over space and time and underscore the need to closely consider the model assumptions, like constant WUE, implicit in many of the models used to project the future of tropical forests.
A primary source of uncertainty in terrestrial biosphere model projections of ecosystem-scale carbon uptake and water cycling is the relationship between CO2 assimilation and water loss via stomatal conductance. In models, this relationship is governed by two terms, the stomatal slope (g1) and intercept (g0). Accurate mechanistic representation of how the g1 and g0 parameters vary over time is crucial, particularly in wet tropical broadleaf forests where trees have a near consistent annual pattern of leaf production and senescence, and precipitation and humidity are strongly seasonal. These stomatal parameters are estimated using leaf-level gas exchange by two alternative methods: (1) a response curve where the environmental conditions are modified for a single leaf, or (2) a survey approach, where repeated measurements are made on multiple leaves over a diurnal range of environmental conditions.
In this study we found that stomatal response curves and survey style measurements produce statistically different estimations of stomatal parameters, which resulted in large (between 26% and 125%) differences in simulated fluxes of water. Furthermore, we found that g1 varies both diurnally and to a lesser degree with leaf age. Taken together, these results show that models which use stomatal parameters derived from response curves significantly underestimated canopy level transpiration, and that while leaf traits do vary among leaf phenological stage, models, which tend to only include mature vegetation parameterizations, perform similarly to those that explicitly simulate three leaf age stages.
Contact: Kenneth Davidson, Brookhaven National Lab (email@example.com)
This work was supported by the Next-Generation Ecosystem Experiments (NGEE) – Tropics project, which is funded by the Biological and Environmental Research (BER) Program within the US Department of Energy’s (DOE) Office of Science and through DOE contract no. DE-SC0012704 to Brookhaven National Laboratory.
Davidson KJ, Lamour J, Rogers A, Ely KS, et al. “Short-term variation in leaf-level water use efficiency in a tropical forest” New Phytologist, 237, 2069-2087, (2023), [DOI: 10.1111/nph.18684]