Minimum carbon uptake controls the interannual variability of ecosystem productivity in tropical evergreen forests

Tropical evergreen forests contribute an important part to the interannual variability (IAV) of the global terrestrial gross primary productivity (GPP). Due to its year-round growing-season, high minimum carbon uptake (GPP_min) and dry season greening-up, the key processes driving the GPP variability across seasonal to interannual scale are still in debate. Here, we analyzed the time-series of FLUXCOM GPP (1980–2013), sun-induced fluorescence (SIF; 2001–2013) and site-level GPP measurements in three tropical evergreen forests regions (i.e., Amazon, Africa, and Southeast Asia). We decomposed the annual accumulated GPP into the basic and recurrent GPP, which represent the accumulated minimum and seasonal vegetation productivity, respectively. Then we quantified the proportion of each component and estimated the contribution to the IAV of GPP. We find that the basic GPP overwhelmed the recurrent GPP with the averaging ratio of 4.2:1 across the global tropical regions, and dominated the IAV of annual total GPP in 83.7% of the tropical evergreen forest areas. The high contribution of the basic GPP resulted from the great sensitivity of GPP_min to rainfall changes among years. The decomposition of the basic and recurrent GPP sheds new light on the understanding of tropical GPP variability in responding to climate change at seasonal and annual scale. Our study highlights the critical role of the GPP_min in shaping temporal dynamics of the annual GPP in tropical forests and emphasizes the importance of managing tropical forest of the shifting periods between wet-dry seasons in global tropical regions.