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

Abstract

Determining the large-scale RuBisCO carboxylation maximum rate (Vc,max25) in relation to leaf age is essential for evaluating the photosynthetic capacity of canopy leaves in global forests. Young leaves (≤ 180 d), which exhibit higher Vc,max25 compared to old leaves (> 180 d), are key to controlling the seasonality of leaf photosynthetic capacity in tropical and subtropical evergreen broadleaved forests (TEFs). Nevertheless, quantifying the leaf photosynthetic capacities of different ages across TEFs remains challenging, especially when considering continuous temporal variations at continental scales. In this study, we propose a novel methodology that leverages neighborhood pixel analysis with nonlinear least-squares optimization to derive the Vc,max25 of young leaves at 0.25° spatial resolution. This approach utilizes satellite-based solar-induced chlorophyll fluorescence (SIF) products spanning the period from 2001 to 2018, which were reconstructed using both TROPOMI (Tropospheric Monitoring Instrument) SIF and MODIS reflectance data (RTSIF). Validations against in situ observations demonstrate that the newly developed Vc,max25 products accurately capture the seasonality of young leaves in South America and subtropical Asia, with correlation coefficients of 0.84, 0.66, and 0.95, respectively. The Vc,max25 of the young leaves simulated from the RTSIF-derived gross primary production (GPP) is effectively correlated (R > 0.51) with that dissolved from the global Orbiting Carbon Observatory-2 (OCO-2)-based SIF (GOSIF) GPP. Furthermore, the gridded Vc,max25 dataset for young leaves successfully detects the green-up regions during the dry seasons in the tropics. Overall, this study presents the first satellite-based Vc,max25 dataset specifically targeting photosynthetically efficient young leaves, providing valuable insights for modeling large-scale photosynthetic dynamics and carbon cycles in TEFs. Herein, we provide the Vc,max25 time series derived from RTSIF GPP as the primary dataset, supplemented by GOSIF-derived and FLUXCOM products. These Vc,max25 products are available at https://doi.org/10.5281/zenodo.14807414 (Yang et al., 2025).