The Science

Plants absorb light to fix carbon dioxide; the efficiency of this process is termed as light-use efficiency and can be calculated based on different light absorption definitions. Among the light being absorbed by plants, only a fraction is captured by chlorophyll and can be further used for photosynthesis. In this study, scientists from Brookhaven National Laboratory (BNL) used satellite data and derived an estimation of the fraction of light that is absorbed by chlorophyll. The scientists found that different plants have a similar efficiency using chlorophyll absorbed light to fix carbon dioxide; this efficiency is also found to be stable throughout the season in tropical forest. The results of this study can be used to improve models’ capability to estimate the total carbon fixed by plants at global scale.

The Impact

This analysis resolves the much-debated concept of satellite-derived ecosystem-scale maximum daily light-use efficiency by showing a spatio-temporal convergence of maximum daily light-use efficiency based on radiation absorption by canopy chlorophyll. These results of the convergent relationship between ecosystem-scale maximum light-use efficiency and canopy-scale chlorophyll content also provide an improved satellite-based parameterization of large-scale vegetation models to improve the capability to estimate the total carbon fixed by plants at global scale.

Summary

Seasonal variation of ecosystem-scale maximum daily light-use efficiency (approximated by the light-use efficiency under the reference environmental condition) was derived from one eddy covariance tower site, the Tapajos K67 site, in central Amazon. The eddy covariance derived maximum light use efficiency terms (PC) were used as ground truth and then compared with three versions of satellite indices, including Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), and MERIS Terrestrial Chlorophyll Index (MTCI). Since MTCI is an indicator of canopy-scale chlorophyll content, the close match between the seasonality of MTCI and ecosystem-scale light-use efficiency of the reference environment suggests that satellite-derived canopy-scale chlorophyll content can track the photosynthetic capacity in the tropical forests. The similar finding, but across diverse ecosystems across the globe, is also found in this study. As such, this study demonstrates a convergent relationship between canopy chlorophyll (e.g., satellite-derived MTCI) and maximum daily light-use efficiency across both spatial and temporal scales.

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

References