Gross primary productivity from leaf-age-dependent light use efficiency (LA-LUE) model over pantropical evergreen broadleaved forests

Tropical and subtropical evergreen broadleaved forests (TEFs) contribute more than one-third of terrestrial gross primary productivity (GPP). However, the continental-scale leaf phenology-photosynthesis nexus over TEFs is still poorly understood to date. This knowledge gap hinders most light use efficiency (LUE) models from accurately simulating the GPP seasonality in TEFs. Leaf age is the crucial plant trait to link the dynamics of leaf phenology with GPP seasonality. Thus, here we incorporated the seasonal leaf area index (LAI) of different leaf age cohorts into a widely-used LUE model (i.e., EC-LUE) and proposed a novel leaf-age-dependent LUE model (denoted as LA-LUE model). At the site level, the LA-LUE model (average R2 = 0.59, average RMSE = 1.23 gC m−2 d−1) performs better than the EC-LUE model in simulating the GPP seasonality across the nine TEFs sites (average R2 = 0.18; average RMSE = 1.87 gC m-2 d-1). At the continental scale, the monthly GPP estimates from the LA-LUE model are consistent with FLUXCOM GPP data (R2 = 0.80; average RMSE = 1.74 gC m-2 d-1), and satellite-based GPP data retrieved from the global Orbiting Carbon Observatory-2 (OCO-2) based solar-induced chlorophyll fluorescence (SIF) product (GOSIF) (R2 = 0.64; average RMSE = 1.90 gC m-2 d-1) and the reconstructed TROPOspheric Monitoring Instrument SIF dataset using machine learning algorithms (RTSIF) (R2 = 0.78; average RMSE = 1.88 gC m-2 d-1). Typically, the estimated monthly GPP not only successfully represents the unimodal GPP seasonality near the Tropics of Cancer and Capricorn, but also captures well the bimodal GPP seasonality near the Equator. Overall, this study for the first time integrates the leaf age information into the satellite-based LUE model and provides a feasible implementation for mapping the continental-scale GPP seasonality over the entire TEFs.

Methods
On the basis of the theory of light use efficiency, the LA-LUE model was proposed by incorporating the seasonal leaf area index (LAI) of different leaf age cohorts into the EC-LUE (Eddy Covariance – Light Use Efficiency) model. To do this, three LAI cohorts (i.e., LAIyoung, LAImature, and LAIold) were introduced to reconstruct the dynamic maximum light use efficiency (εmax) and absorbed photosynthetically active radiation (APAR) for young leaves, mature leaves, and old leaves. After that, the product of εmax and APAR of sunlit and shaded leaves were accumulated to each leaf age cohort, respectively, and then to simulate the total GPP. It utilizes a new leaf-age-dependent leaf area index (Lad-LAI) product in combination with ERA5 air temperature, dew temperature, and mean surface downward short-wave radiation flux data and NOAA Earth System Research Laboratories CO2 concentration product to produce a coherent seasonal data record of GPP over pantropical evergreen broadleaved forests. The mapped extent is longitude (-180, 180) and latitude (-30, 30). Unit: gC m-2 month-1; Fill value: -1.