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Improved estimates of global terrestrial photosynthesis using information on leaf chlorophyll content

Luo, Xiangzhong, Croft, Holly, Chen, Jing M., He, Liming, Keenan, Trevor F.
Global change biology 2019 v.25 no.7 pp. 2499-2514
biosphere, broadleaved evergreen forests, carbon dioxide, carboxylation, chlorophyll, climate change, cropland, deciduous forests, eddy covariance, environmental factors, fluorescence, grasslands, greenhouse gas emissions, leaves, mixed forests, models, nitrogen content, photosynthesis, remote sensing, savannas, shrublands, temporal variation, uncertainty, wetlands
The terrestrial biosphere plays a critical role in mitigating climate change by absorbing anthropogenic CO₂ emissions through photosynthesis. The rate of photosynthesis is determined jointly by environmental variables and the intrinsic photosynthetic capacity of plants (i.e. maximum carboxylation rate; Vcmax25). A lack of an effective means to derive spatially and temporally explicit Vcmax25 has long hampered efforts towards estimating global photosynthesis accurately. Recent work suggests that leaf chlorophyll content (Chlₗₑₐf) is strongly related to Vcmax25, since Chlₗₑₐf and Vcmax25 are both correlated with photosynthetic nitrogen content. We used medium resolution satellite images to derive spatially and temporally explicit Chlₗₑₐf, which we then used to parameterize Vcmax25 within a terrestrial biosphere model. Modelled photosynthesis estimates were evaluated against measured photosynthesis at 124 eddy covariance sites. The inclusion of Chlₗₑₐf in a terrestrial biosphere model improved the spatial and temporal variability of photosynthesis estimates, reducing biases at eddy covariance sites by 8% on average, with the largest improvements occurring for croplands (21% bias reduction) and deciduous forests (15% bias reduction). At the global scale, the inclusion of Chlₗₑₐf reduced terrestrial photosynthesis estimates by 9 PgC/year and improved the correlations with a reconstructed solar‐induced fluorescence product and a gridded photosynthesis product upscaled from tower measurements. We found positive impacts of Chlₗₑₐf on modelled photosynthesis for deciduous forests, croplands, grasslands, savannas and wetlands, but mixed impacts for shrublands and evergreen broadleaf forests and negative impacts for evergreen needleleaf forests and mixed forests. Our results highlight the potential of Chlₗₑₐf to reduce the uncertainty of global photosynthesis but identify challenges for incorporating Chlₗₑₐf in future terrestrial biosphere models.