Impact of Photosynthesis and Transpiration Seasonality on Models

The Science

Photosynthesis is a necessary process for plant growth. However, photosynthesis requires a considerable release of water vapor via transpiration, and the ratio of photosynthesis to transpiration fluxes may change over the season or life of a given leaf. The dynamics of these fluxes over a growing season are not well characterized in Earth system models. Researchers found that photosynthesis and water use efficiency (WUE) are dynamic over a leaf’s lifetime and may not be synchronized. Photosynthesis increases slowly with leaf age and is driven primarily by changes in leaf biochemistry. In contrast, transpiration increases quickly with leaf age and is driven by changes in leaf anatomy.

The Impact

Earth system models poorly represent seasonality in tree physiology, particularly concerning the efficiency with which plants acquire carbon at the cost of water loss. To address this, researchers measured photosynthesis, transpiration, and leaf traits of temperate trees throughout a growing season to evaluate the patterns and drivers of photosynthesis and transpiration. Results were incorporated into simple models of forest function to evaluate the impact of this new understanding of seasonality, which increased predictive capacity, particularly in the spring and fall phenological periods. Overall, the updated model approach predicts a 16% higher seasonal transpiration and a 3% higher seasonal carbon assimilation.

Summary

Stomatal conductance to water vapor directly affects the potential rates of transpiration and photosynthetic carbon assimilation. Through variation in stomatal behavior, stomata dictate the marginal WUE of a plant. Stomatal behavior is known to vary seasonally and with leaf ontogeny. However, land surface models of vegetation do not currently represent this process. In this study, a team of researchers investigated leaf-level physiological, hydraulic, and anatomical properties as they changed throughout a growing season. Researchers paid particular interest to the stomatal slope parameter, which is inversely proportional to WUE.

Photosynthetic capacity and WUE were both found to be seasonally variable, yet their patterns were not synchronized. Parameters related to photosynthesis tracked seasonal trends in leaf structural and nutritional characteristics, while stomatal parameters lagged and tracked changes in anatomy and photosynthetic potential. Research also showed that when stomatal slope is modeled as a seasonally dynamic parameter, computed seasonal transpiration increases by 16%. Simulations indicate a clear need for models to account for seasonality more explicitly in photosynthetic and stomatal parameters.

Principal Investigator

Shawn Serbin
Brookhaven National Laboratory
[email protected]

Program Manager

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

Funding

Funding for this project came from the Black Rock Forest Consortium through the David Redden Conservation Science Fund. Additional funding was provided by Stony Brook University’s Department of Ecology and Evolution through the Lawrence B. Slobodkin Research Fund. Support was also received from the Next-Generation Ecosystem Experiments Tropics project, which is supported by the U.S. Department of Energy (DOE), Office of Science, Biological and Environmental Research program, and through the U.S. DOE contract no. DE-SC0012704 to Brookhaven National Laboratory.

References