Satellite retrievals of information about the Earth’s surface are widely used to monitor global terrestrial photosynthesis and primary production and to examine the ecological impacts of droughts. Methods for estimating photosynthesis from space commonly combine information on vegetation greenness, incoming radiation, temperature and atmospheric demand for water (vapour-pressure deficit), but do not account for the direct effects of low soil moisture. They instead rely on vapour-pressure deficit as a proxy for dryness, despite widespread evidence that soil moisture deficits have a direct impact on vegetation, independent of vapour-pressure deficit. Here, we use a globally distributed measurement network to assess the effect of soil moisture on photosynthesis, and identify a common bias in an ensemble of satellite-based estimates of photosynthesis that is governed by the magnitude of soil moisture effects on photosynthetic light-use efficiency. We develop methods to account for the influence of soil moisture and estimate that soil moisture effects reduce global annual photosynthesis by ~15%, increase interannual variability by more than 100% across 25% of the global vegetated land surface, and amplify the impacts of extreme events on primary production. These results demonstrate the importance of soil moisture effects for monitoring carbon-cycle variability and drought impacts on vegetation productivity from space.
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P-model outputs from site-scale and global simulations are available on Zenodo at: https://doi.org/10.5281/zenodo.1423484.
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We thank C. Jian and Y. Ryu for sharing BESS model outputs, and Y. Zhang for sharing VPM model outputs and all for supporting the use and interpretation of their data. We thank Z. Zhu for sharing updated FPAR3g data, D. Sandoval Calle for preparing soil data, and W. Han, R. Thomas and T. Davis for their contributions to the development of the P-model. B.D.S. was funded by ERC Marie Sklodowska-Curie fellowship H2020-MSCA-IF-2015, project FIBER, grant no. 701329. J.P. was funded by ERC Synergy grant no. ERC-SyG-2013-610028 IMBALANCE-P, the Spanish Government grant no. CGL2016-79835-P and the Catalan Government grant SGR-2017-1005. T.F.K was supported by the NASA Terrestrial Ecology Program IDS Award No. NNH17AE86I. This work is a contribution to the AXA Chair Programme in Biosphere and Climate Impacts and the Imperial College initiative on Grand Challenges in Ecosystems and the Environment (I.C.P.). S.I.S acknowledges support from the EU FP7 programme, through the ERC DROUGHT-HEAT project (contract no. 617518). This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux-TERN, TCOS-Siberia and USCCC. The processing and harmonization of the FLUXNET eddy covariance data was carried out by the European Fluxes Database Cluster, AmeriFlux Management Project and the Fluxdata project of FLUXNET, with the support of the CDIAC and ICOS Ecosystem Thematic Center, and the OzFlux, ChinaFlux and AsiaFlux offices.
Supplementary Description, Supplementary Figures 1–13 and Tables 1,2