Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
09 August 2020

Global Evidence for the Acclimation of Ecosystem Photosynthesis to Light

Ecosystems acclimate their maximum photosynthesis to the available light.

Science

Photosynthesis is the primary energetic and carbon uptake pathway of ecological systems, affecting all aspects of ecosystem function. Over longer timescales plants adapt to environmental changes, adjusting the amount of nitrogen they invest in leaves in order to optimize maximum photosynthesis to the available light. Despite being observed at the leaf scale, this slower acclimation process is not typically represented in terrestrial biosphere models.

Impact

The results show that the acclimation of photosynthesis to light is a widespread phenomenon in natural ecosystems, one that most current terrestrial biosphere models (TBMs) fail to accurately represent. Our results provide an observationally-inferred benchmark of light acclimation, which can be used to test and improve large-scale estimates of photosynthesis. The results suggest that recently developed theory of optimal ecological function can be used to incorporate photosynthetic light acclimation into TBMs, and improve global photosynthesis estimates.

Summary

Global photosynthesis is the largest carbon flux in the global carbon cycle, removing CO2 from the atmosphere and thus contributing to climate change mitigation. The amount of carbon assimilated by photosynthesis is dependent on the instantaneous and non-instantaneous responses of vegetation to light. Photosynthesis responds quickly to changes in light, increasing with incoming photosynthetic photon flux density (PPFD) until leaves become light-saturated. This instantaneous response to PPFD, which is widely studied and incorporated into models of photosynthesis, is overlaid on non-instantaneous photosynthetic changes resulting from the acclimation of plants to average PPFD over intermediate timescales of a week to months ( ). Such photosynthetic light acclimation is not typically incorporated into models, due to the lack of observational constraints. Here, we use eddy covariance observations from globally distributed and automated sensor networks, along with photosynthesis estimates from 9 terrestrial biosphere models (TBMs) to quantify and assess photosynthetic acclimation to light in natural environments. In addition, we use recent theoretical developments to incorporate light acclimation in a TBM. Our results show widespread light acclimation of ecosystem photosynthesis and shows that the TBMs examined either neglect or substantially underestimate light acclimation. By updating a TBM to include photosynthetic acclimation, successfully reproducing the -Amax relationship, we provide a robust method for the incorporation of photosynthetic light acclimation in future models.

Contact
Trevor Keenan
Lawrence Berkeley National Laboratory
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