Light use efficiency in China’s forests: identifying spatiotemporal dynamics and disentangling relative contributions of driving factors

Vegetation productivity is an important indicator of the terrestrial carbon cycle, which is controlled by both biological and non-biological factors and is commonly modelled by parameters of the rate (e.g., light use efficiency; LUE) and duration (e.g., growing season length) of carbon uptake. Although recent research has found that the increase in vegetation productivity is driven primarily by the rate of carbon uptake, studies have shown that approximately 60% of the spatiotemporal variability in LUE remains unexplained, highlighting the urgent need to better understand the factors controlling the LUE. Previous studies focused mainly on the independent effects of land-surface environmental factors on LUE, whereas the roles of biological factors and their interactions with environmental factors have been less explored. Based on site-level observations, we integrated Random Forest modelling with six conventional LUE models and observation-based analyses to investigate the spatiotemporal dynamics and relative contributions of biological and environmental factors to forest LUE across China. The results showed that the LUE of forests in China was 0.303 ± 0.019 g C MJ −1 , with an increasing rate of 0.003 g C MJ −1 yr −1 ( p < 0.001). In areas with increasing LUE, the model-derived attribution assigned 15% of the relative contribution to land-surface environmental factors and 85% to the combined biological factors + atmospheric CO 2 component; a paired EC-LUE CO 2 sensitivity experiment further partitioned the latter into approximately 16% atmospheric CO 2 and 69% remaining biological component as a first-order, module-specific estimate. Notably, forest LUE is constrained by moisture availability in China, with old forests (particularly deciduous broadleaf forests) showing greater sensitivity to increases in soil moisture, highlighting the importance of interactions between biological and environmental factors. Overall, under the warming and humidifying trends observed in China, old forests may utilize light more efficiently than young forests. Maintaining forest integrity and protecting old forests could therefore contribute to sustaining forest carbon sinks, although their future performance will depend on regional climate trajectories and forest characteristics.