Abstract
The maximum rate of carboxylation (Vcmax), a key parameter indicating photosynthetic capacity, is commonly fixed as a constant by vegetation types and/or varies according to empirical scaling functions in Earth system models (ESMs). As such, the setting of Vcmax results in uncertainties of estimated carbon assimilation. It is known that the coupling between leaf chlorophyll and Rubisco (ribulose-1,5-biphosphate carboxylase-oxygenase) contents can be applied to estimate Vcmax. However, how this coupling is affected by environmental changes and varies among plant functional types (PFTs) has not been well investigated yet. The effect of varying coupling between chlorophyll and Rubisco contents on the estimation of Vcmax is still not clear. In this study, we compiled data from 76 previous studies to investigate the coupling between Chlorophyll (Chl) and Rubisco (Rub), in different PFTs and under different environmental conditions. We also assessed the ability of a Rub-based semi-mechanistic model to estimate Vcmax normalized to 25 °C (Vcmax25) based on the Rub–Chl relationship. Our results revealed strong, linear Rub-Chl relationships for different PFTs (R 2 = 0.73, 0.67, 0.54 and 0.72 for forest, crop, grass and shrub, and C4 plants, respectively). The Rub–Chl slope of natural C3 PFTs was consistent and significantly different from those of crops and C4 plants. A meta-analysis indicated that reduced light intensity, elevated CO2, and nitrogen addition strongly altered Rub/Chl. A semi-mechanistic model based on PFT-specific Rub–Chl relationships was able to estimate Vcmax25 with high confidence. Our findings have important implications for improving global carbon cycle modeling by ESMs through the improved parameterization of Vcmax25 using remotely sensed Chl content.
Highlights
Photosynthetic carbon assimilation is the principal driver of land carbon sink and represents the largest flux in the global carbon cycle (Schulze 2006, Keenan and Williams 2018, Luo et al 2019)
The leaf Chl content of all C3 plants was significantly correlated with leaf Rub content (R2 = 0.64, p < 0.001, N = 90) in area units
The relationships between leaf Chl and Rub content, in different plant functional types (PFTs) and under different environmental conditions were investigated by compiled data from 76 previous studies
Summary
Photosynthetic carbon assimilation is the principal driver of land carbon sink and represents the largest flux in the global carbon cycle (Schulze 2006, Keenan and Williams 2018, Luo et al 2019). Accurate estimation of photosynthetic carbon assimilation is a prerequisite for predicting land carbon cycle and carbon-climate feedbacks (Collins et al 2012). One of the key parameters used within the FvCB model to indicate photosynthetic capacity is the maximum rate of carboxylation by the enzyme ribulose-1,5-biphosphate carboxylase-oxygenase (Rubisco), normalized to 25 ◦C (Vcmax). Accurate estimation of Vcmax is essential to improve the performance of ESMs. Vcmax is determined by the amount and kinetics of the Rubisco (Rub) enzyme (Sage et al 2008, Bar-On and Milo 2019). Vcmax is commonly fixed as a constant by vegetation types and/or varies according to empirical scaling functions in ESMs (Rogers 2014, Luo et al 2019). Because of the difficulty in modeling Vcmax values over a complete range of spatial and temporal scales, uncertainty remains in the parameterization of the photosynthetic capacity and in the incorporation of interannual variability and longterm trends into ESMs (Piao et al 2013, Anav et al 2015, Baldocchi et al 2016)
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