Remote sensing of young leaf photosynthetic capacity in tropical and subtropical evergreen broadleaved forests

Abstract. Determining the large-scale Rubisco carboxylation maximum rate (V_c,max25) in relation to leaf age is crucial for assessing the photosynthetic capacity of canopy leaves in global forests. Young leaves (≤180 days) with higher V_c,max25 compared with old leaves (>180 days) largely control the seasonality of leaf photosynthetic capacity in tropical and subtropical evergreen broadleaved forests (TEFs). Nevertheless, it has not yet been adequately quantified across TEFs. In this study, we propose an innovative method that leverages neighborhood pixel analysis with a nonlinear least squares fitting approach to derive the V_c,max25 of the young leaves at 0.25° spatial resolution from satellite-based solar-induced chlorophyll fluorescence (SIF) products spanning from 2001 to 2018, which were reconstructed using both the TROPOMI (Tropospheric Monitoring Instrument) SIF and MODIS reflectance data (RTSIF). Validations against in situ observations show that the newly developed V_c,max25 products accurately capture the seasonality of the young leaf area in South America and subtropical Asia, with correlation coefficients equal to 0.837, 0.661, and 0.952, respectively. Additionally, the V_c,max25 of the young leaves simulated from the RTSIF is effectively correlated (R>0.512) with that dissolved from the gridded gross primary production (GOSIF-derived GPP). Furthermore, the gridded young leaf V_c,max25 dataset effectively detects the green-up region during the dry seasons in the tropics, where the average annual precipitation exceeds 2000 mm/year. The clustering patterns of the young leaf V_c,max25 also effectively match those clustered by climatic variables across the TEFs. Overall, the newly developed V_c,max25 product is the first satellite-based dataset for addressing the V_c,max25 of photosynthetically efficient young leaves and can provide useful information for modeling the large-scale photosynthesis dynamics and thus carbon cycle across the TEFs. Herein, we provide the time series of V_c,max25 derived from RTSIF GPP as the main dataset and GOSIF- and FLUXCOM- derived as supplementary datasets. These V_c,max25 products are available at https://doi.org/10.5281/zenodo.14807414 (Yang et al., 2025).