Tropical Leaves Adjust Water Use Over the Day, Not Over Their Lifetime

The Science

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 water use efficiency (WUE). There are still significant uncertainties associated with the dynamics of WUE over different timescales, such as a day to the full lifespan of a leaf. Researchers collected data to assess possible physiological and mechanistic factors that influence WUE dynamics. While WUE does differ between leaves of different phenological stages, the trend was not consistent across species. However, researchers identified a unidirectional increase in WUE of approximately 2.5 times over the course of the day in five of the six species studied.

The Impact

Understanding physiological factors that most strongly contribute to variation in leaf-level WUE is a major roadblock to accurate transpiration representation in climate models. In this study, researchers demonstrate that including leaf age as a primary driver of WUE did not improve or explain variation in modeled transpiration. However, models which accounted for diurnal changes in WUE improved representation of transpiration. These findings provide a roadmap for future investigation into the physiological traits that most strongly influence transpiration over space and time. Future studies need to closely consider model assumptions, like constant WUE, implicit in many models that project the future of tropical forests.

Summary

The relationship between carbon dioxide assimilation and water loss via stomatal conductance is a primary source of uncertainty in terrestrial biosphere model projections of ecosystem-scale carbon uptake and water cycling. 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 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.

Results show stomatal response curves and survey-style measurements produce statistically different estimations of stomatal parameters, which result in large (between 26% and 125%) differences in simulated fluxes of water. Furthermore, g₁ varies both diurnally and to a lesser degree with leaf age. These results show models using stomatal parameters derived from response curves significantly underestimate canopy level transpiration. While leaf traits do vary among leaf phenological stages, models that only include mature vegetation parameterizations perform similarly to those that explicitly simulate three leaf age stages.

Principal Investigator

Shawn Serbin
NASA Goddard Space Flight Center
[email protected]

Co-Principal Investigator

Kenneth Davidson
American Forests
[email protected]

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

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 U.S. Department of Energy’s (DOE) Office of Science, and through DOE contract no. DE-SC0012704 to Brookhaven National Laboratory.

References