From seasons to decades: Solar radiation, cloud cover, and CO$_2$ shape young leaf phenology in a tropical forest over 26 years

Abstract

1. Climate change is altering plant phenology globally with potential deleterious impacts on animal species and entire ecosystems, yet the long-term effects of climate change on tropical leaf production remain poorly understood. 2. We analyzed 26 years of young leaf phenology field data from Kibale National Park, Uganda, focusing on 12 tree species consumed by leaf-eating mammals. We examined seasonal and long-term patterns and how they are related to climatic variables using Bayesian hierarchical generalized additive mixed models (GAMMs). 3. The tree community and most species exhibited peaks in young leaf production during the two annual rain seasons, with seasonal changes primarily associated with diffuse light availability through solar radiation and cloud cover, as well as rainfall and minimum temperature. Long-term variations in leaf production was primarily linked to long-term changes in atmospheric CO2, solar radiation, and cloud cover. 4. Our results support the role of CO2 fertilization, though decreasing levels of solar radiation resulting from the ending of the recent solar cycle may be slowing this effect. 5. Synthesis: This study highlights the critical role of diffuse light, solar radiation, and the solar cycle in predicting tropical leaf production, emphasizing that interpretations of greening trends must consider solar radiation alongside atmospheric CO2 levels. Furthermore, our findings emphasize the complex relationship between climate and young leaf phenology, highlighting the importance of integrating species-specific long-term data to better understand the effects of climate change on food availability for tropical folivores and tropical forest ecosystems in general.