A revised two-leaf light use efficiency model for improving gross primary production estimation at a tropical evergreen broadleaved forest site

Accurate quantification of terrestrial gross primary production (GPP) is essential for enhancing our in-depth understanding of the global carbon budget and climate change [1] [2] . The two-leaf light use efficiency (TL-LUE) model, considering more deeply the disparities of photosynthesis capacity between sunlit and shaded leaves, has been proven to be a more efficient and potent approach than the big-leaf light use efficiency (BL-LUE) model for global GPP simulations [3] . However, the TL-LUE model is theoretically applicable for sunny days and fails to reflect the real configuration of the canopy under overcast and cloudy days, since the direct radiation could be shaded by clouds and thus all the leaves within the canopy are in reality shaded leaves (i.e., no sunlit leaves should exist). This mismatch between the theory and reality could definitely introduce a certain degree of systematic errors into the GPP simulations by the TL-LUE model. Here, we proposed a revised two-leaf light use efficiency (RTL-LUE) model for improving GPP estimation through better quantifying the sunlit and shaded leaf area index (LAI) under different sky conditions.