Seasonal Variations in Leaf Maximum Photosynthetic Capacity and Its Dependence on Climate Factors Across Global FLUXNET Sites

The maximum carboxylation rate (Vcmax) is an important parameter affecting the photosynthesis rate of plant leaves. In terrestrial ecosystem models (TEMs), Vcmax at 25 degrees C (Vm25 degrees) 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 degrees 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 degrees has not been systematically studied. Here, we generated a Vm25 degrees 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 degrees from EC data. We investigated seasonal variations of Vm25 degrees 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 degrees values derived from flux data using BEPS-LRC correlate well with Vm25 degrees of the reference data set (R-2 = 0.74, slope = 0.77, and root mean square error = 24.45 mu mol m(-2) s(-1), p < 0.001). Leaf Vm25 degrees 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 degrees, followed by SWC. The interactive effects of Ta and SWC on Vm25 degrees vary among different biomes. The seasonal variation of Vm25 degrees was strongly dependent on LCC. After the correction of temperature effect, the contribution of LCC to the seasonal variation of Vm25 degrees averaged 26% among eight biomes. These findings provide useful information for better parameterization of Vm25 degrees in TEMs.