Abstract: Tropical forests contribute over half of the global primary productivity, playing a critical role in regulating the global carbon cycle. In recent decades, global tropical forests have shown a widespread increase in vegetation cover. However, how tropical forests respond to spatiotemporal climate change under increasing vegetation cover remains a critical question, limiting the development of effective conservation strategies. To address this, we evaluated the sensitivity of tropical forests to spatiotemporal climate variability using the vegetation sensitivity index (VSI) across greening in 2000–2021 and identified dominant climate drivers based on solar-induced chlorophyll fluorescence, enhanced vegetation index, and leaf area index data. Results indicate that over 84% of global tropical forests show an increase in vegetation cover, while a decrease in vegetation cover appeared in the southern Congo and southeastern Amazon. VSI showed a latitudinal gradient, with high values (> 60) near the equator and lower ones (< 40) in higher latitudes. Tropical forests in the Congo had the highest VSI, followed by the Amazon and Southeast Asia. VSI decreased in over half of the Amazon tropical forests, whereas the Congo and Southeast Asian forests showed comparable proportion of pixels with decreasing and increasing VSI trends. Notably, tropical forests typically exhibited increasing variability, indicated by the detrended variance along the greening trend gradient. In addition, tropical forests are highly vulnerable to climate change in the western Amazon, equatorial Congo Basin, and equatorial Southeast Asia based on the aspects of VSI magnitude and VSI trend. Precipitation is the dominant climate driver regulating global tropical forest variability to climate change, followed by temperature and solar radiation. Temperature dominated tropical forests variability in Southeast Asia. Our findings highlight the instability and vulnerability of tropical forests under climate change despite widespread increase in vegetation cover.

Key words: Tropical forests, Greening, Climate variability, Sensitivity