Comparison of big-leaf and two-leaf light use efficiency models for GPP simulation after considering a radiation scalar

Light use efficiency (LUE) models, mainly including the big-leaf (BL) and two-leaf (TL) categories, are efficient approaches to simulate gross primary productivity (GPP). Recently, a TL LUE model considering a radiation scalar (RTL-LUE) was developed, which improved the GPP simulation and unified the same model structure with the BL models using only one maximum LUE. However, whether the radiation scalar is suitable for BL models is still unknown yet, and the core parameters of maximum LUE in the BL and TL models have not been fairly compared before. In this study, we step forward to modifying the MOD17 model using the radiation scalar (RMOD17), and compare the GPP simulations and maximum LUEs in BL (MOD17, RMOD17) and TL (RTL-LUE) models at global 169 FLUXNET eddy covariance (EC) sites. Results indicate that the GPP estimation from RMOD17 (R2=0.72) matches better with EC GPP than those from the original MOD17 (R2=0.65), because the radiation scalar effectively corrects the underestimations or overestimations in low or high photosynthetically active radiation (PAR) ranges caused by the radiation-independent LUE in MOD17. The RTL-LUE can further improve the accuracy (R2=0.74) by alleviating the GPP underestimation of BL RMOD17 in high productivity ranges, which is mainly caused by the insufficient consideration of the shaded leaves contributions. The maximum LUE from RMOD17 and RTL-LUE both show more reasonable values and lower diurnal variations than MOD17, and further analysis proves the radiation scalar is the main reason for this. Besides, RTL-LUE presents a lower model parameter sensitivity compared to RMOD17, indicating that TL is a more robust strategy than BL to simulate GPP. This study highlights the importance of considering a radiation scalar and two-leaf strategy in GPP simulation in LUE models, to better describe the multi-order impacts of radiation on vegetation photosynthesis.