The Science

Tropical forest photosynthesis varies with the environment and with biotic changes in photosynthetic infrastructure, but the scientific understanding of the relative effects of these factors across timescales is limited. Here, the scientists used a statistical model to partition the variability of seven years of eddy covariance–derived photosynthesis in a central Amazon evergreen forest into two main causes (i.e., environmental versus biological) and identified the differential regulation of tropical forest photosynthesis at different timescales.

The Impact

This study has three important implications for the broader ecology, evolutionary biology, plant physiology, and modeling communities: (1) challenges modeling approaches that assume tropical forest photosynthesis is primarily controlled by the environment at both short and long timescales; (2) advances ecophysiological understanding of resource limitation (i.e., light versus water) and the temperature sensitivity of tropical evergreen forest; and (3) highlights the importance of accounting for differential regulation of tropical forest photosynthesis at different timescales and of identifying the underlying feedbacks and adaptive mechanisms.

Summary

Canopy-scale photosynthesis (i.e., gross ecosystem productivity, GEP) of a central Amazonian evergreen forest in Brazil was derived from the k67 eddy covariance tower (2002–2005 and 2009–2011), using the standard approach to partition ecosystem respiration from eddy covariance measurements of net ecosystem exchange . This work used statistical models to partition the variability of seven-year eddy covariance–derived GEP into two causes: variation in environmental drivers (solar radiation, diffuse light fraction, and vapor pressure deficit) and biotic variation in canopy photosynthetic light-use efficiency. The “full” model was driven by both environmental and biotic factors and the “Env” model was driven by environmental factors only. The models were trained by using the hourly data of years 2003, 2005, 2009, and 2011, and validated by the independent data of years 2002, 2004, and 2010, including the aggregation to different timescales (i.e., daily and monthly). Study results showed that both models (full versus Env) simulated photosynthetic dynamics well at hourly timescales; however, when aggregating the model results into other timescales (i.e., daily, monthly, and yearly), the Env model showed continuous decline in model performance. By contrast, the full model consistently simulated the photosynthetic dynamics across all timescales. The results thus suggest that environmental variables dominate photosynthetic dynamics at shorter timescales (i.e., hourly to daily) but not at longer timescale (i.e., monthly and yearly), and they highlight the importance of accounting for differential regulation of GEP at different timescales and of identifying the underlying feedbacks and adaptive mechanisms.

Principal Investigator

Jin Wu
Brookhaven National Laboratory
jinwu@bnl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

J. Wu and B. Christoffersen were supported in part by the Next-Generation Ecosystem Experiments (NGEE)–Tropics project. The NGEE-Tropics project is supported by the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.

References