Lars Eklundh

Tropical rainforests have complex responses to seasonal climatic variations. Compared to the Amazon and Asian rainforests, the Congo rainforest exhibits a more widespread bimodal seasonal pattern, yet it remains understudied. Here, we use three independent satellite-based vegetation indices to investigate the seasonal variations of the Congo rainforest, including solar-induced chlorophyll fluorescence (SIF), the two-band enhanced vegetation index (EVI2), and vegetation optical depth (VOD), which represent vegetation canopy photosynthesis, greenness, and water content, respectively. We find widespread asynchronous bimodal seasonality among the three vegetation indices, suggesting alternating physiological and structural variations of the Congo rainforest. These bimodal seasonality shifts are driven by temporally diverse environmental factors. The variations in photosynthesis are predominantly driven by temperature from January to June of the first growing season, and by precipitation and temperature from July to December of the second growing season. The bimodal seasonal pattern of vegetation greenness is primarily constrained by precipitation across most of the region. In contrast, variations in vegetation water content are closely related to terrestrial water storage, with a temporal lag of 1 to 2 months. These findings highlight the intertwined seasonality of the vegetation canopy traits and their environmental controls in the Congo rainforest, providing an improved understanding of the vegetation-climate feedback for predicting rainforest responses to climate change.