August 02, 2017

Global Photosynthesis Modeling is Stymied by Competing Hypotheses on Scaling of Plant Traits

Uncertainty in how maximum photosynthetic rates scale across the Earth leads to substantial uncertainty predictions of terrestrial carbon uptake.

The Science

A major source of uncertainty in modeling of global photosynthesis, and associated carbon cycle dynamics, is the calculation of maximum photosynthetic carboxylation rate, which is one of two plant traits that closely determines photosynthetic rate. Various methods are used in terrestrial biosphere models to calculate these traits, each representing a different theory about how these traits scale, but the resultant errors have not yet been quantified.

The Impact

This research highlights the need for robust estimates of global photosynthesis and a better understanding of how maximum photosynthetic rates scale across the Earth’s surface.

Summary

The impact on global patterns of photosynthesis of four trait-scaling hypotheses (plant functional type, nutrient limitation, environmental filtering, and plant plasticity) was investigated by an international team of researchers. Led by a U.S. Department of Enerby researcher at Oak Ridge National Laboratory, the study finds that global photosynthesis estimates from the different trait-scaling hypotheses ranged between 108 and 128 petagrams of carbon per year (Pg C yr^–¹), representing around 65% of the uncertainty range found in photosynthesis model intercomparison exercises. The uncertainty propagated through to a 27% variation in net biome productivity, the net amount of carbon removed from the atmosphere by land ecosystems. All hypotheses produced global photosynthesis estimates that were highly correlated with proxies of global photosynthesis. Nevertheless, nutrient limitation appeared to be marginally the best method to simulate the scaling of maximum photosynthetic rates. The comparison of model photosynthesis with “observed” photosynthesis was stymied by the fact that no robust methods exist to measure photosynthesis at the global scale. For this reason, researchers used three proxies of global photosynthesis to compare with the model estimates. Interestingly, photosynthesis in agricultural regions of Earth were much higher in the satellite-based photosynthesis proxies that measure solar-induced fluorescence of the photosynthetic machinery in a leaf. Higher photosynthesis in these regions when measured from space suggests that models and other photosynthesis proxies may be missing an important component of global photosynthesis in these managed ecosystems.

Principal Investigator

Anthony Walker
Oak Ridge National Laboratory
walkerap@ornl.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

Next-Generation Ecosystem Experiments (NGEE)–Tropics project of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science.

References

Walker, A. P., T. Quaife, P. M. van Bodegom, and M. G. De Kauwe, et al. "The impact of alternative trait-scaling hypotheses for the maximum photosynthetic carboxylation rate (Vcmax) on global gross primary production." New Phytologist 215 (4), 1370–1386 (2017). https://doi.org/10.1111/nph.14623.