Seasonal Variations in Leaf Maximum Photosynthetic Capacity and Its Dependence on Climate Factors Across Global FLUXNET Sites
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AbstractThe maximum carboxylation rate (Vcmax) is an important parameter affecting the photosynthesis rate of plant leaves. In terrestrial ecosystem models (TEMs), Vcmax at 25°C (Vm25°) is often assigned as constants according to plant functional types (PFTs), while its variations with leaf temperature and nutrient contents are described using empirical functions. However, Vm25° could itself vary seasonally due to changes in the leaf physiological state that cannot be described by the empirical functions, potentially causing large uncertainties in simulated water and carbon fluxes. So far, the seasonal variation in Vm25° has not been systematically studied. Here, we generated a Vm25° data set of eight main biomes from 2000 to 2020 using eddy covariance (EC) measurements at globally distributed 176 sites. The boreal ecosystem productivity simulator was combined with a light response curve model (BEPS‐LRC) to invert Vm25° from EC data. We investigated seasonal variations of Vm25° and analyzed how different environmental and physiological factors, such as physiological (leaf chlorophyll content, LCC and Rubisco or RuBP) and climatic environment factors, including air temperature (Ta), solar shortwave radiation, CO2 concentration (CO2), and soil water content (SWC), influence this parameter. Vm25° values derived from flux data using BEPS‐LRC correlate well with Vm25° of the reference data set (R2 = 0.74, slope = 0.77, and root mean square error = 24.45 μmol m−2 s−1, p < 0.001). Leaf Vm25° has strong seasonal variations in all PFTs except for in evergreen broadleaf forests, but its seasonal variation patterns differ greatly among eight biomes. Air temperature (Ta) is the most important determinant of Vm25°, followed by SWC. The interactive effects of Ta and SWC on Vm25° vary among different biomes. The seasonal variation of Vm25° was strongly dependent on LCC. After the correction of temperature effect, the contribution of LCC to the seasonal variation of Vm25° averaged 26% among eight biomes. These findings provide useful information for better parameterization of Vm25° in TEMs.
