Changes of Light Components and Impacts on Interannual Variations of Photosynthesis in China Over 2000-2017 by Using a Two-Leaf Light Use Efficiency Model

Diffuse radiation produces a higher light use efficiency (LUE) than does direct radiation. However, light components and their impacts on interannual variations of photosynthesis in China have not been intensively studied. We estimated gross primary production (GPP) from a two-leaf LUE model that incorporates a diffuse radiation effect and compared it with a big-leaf LUE model that ignores the diffuse radiation impact. The results show that a dimming occurred in China-the estimated annual photosynthetically active radiation (PAR) declined by -1.79 MJ m(-2) yr(-1). However, the diffuse fraction and diffuse PAR increased over this interval. Both GPP models simulated an increasing trend of GPP in China from 2000 to 2017. The big-leaf GPP model underestimated the increasing trend of GPP from the two-leaf GPP model by 6.4%. The diffuse fraction change during the dimming exerted an embedded positive impact relative to the big-leaf GPP trend in China. To evaluate the impact of total PAR versus its components on GPP, annual anomaly percentage (A(p)) of GPP shows that the big-leaf GPP model had a positive correlation between A(p) of GPP and PAR across China. Considering the diffuse radiation impact, A(p) of two-leaf GPP was negatively correlated with PAR in southern China, which had a higher leaf area index (2.02) and a greater diffuse fraction (0.63). Strong diffuse radiation effects in southern China produced GPP anomalies in opposition to the direction of the PAR changes. These findings highlight the distinctive impact of light components on interannual variations of plant photosynthesis.