Legacy effects of fire on tree diversity, structure and aboveground carbon in tropical semi-arid naturally assembled communities

Seasonal fires are integral to the ecological functioning of tropical savannas; however, increasing fire frequency threatens the integrity of these ecosystems by reducing their productivity and the ecosystem services they support. Understanding how species population demography, functional traits and phylogenetic structure regulate plant community responses to repeated fires is crucial in this context. Combining plot-based tree inventory data with two decades (2001−2021) of remotely sensed fire history across West African tree and shrub savannas, we examined climate-fire feedback, and assessed how multiple components of tree diversity – taxonomic, structural, functional and phylogenetic – as well as key stand characteristics, relate to fire frequency, climate variability, and aboveground tree carbon stock. Fire frequency was closely associated with rainfall and its seasonality, highlighting the role of prolonged dry spells in fuel load accumulation. Although repeated fires did not significantly impact tree diversity in any dimension, they were negatively related to both the density of the largest trees (top 5 % by size) and aboveground carbon stock. Notably, plots that burned more than five times over two decades, showed substantial declines in tree carbon storage. Among all measured variables, the density of the largest trees emerged as the strongest correlate of aboveground carbon, alongside species richness and phylogenetic diversity. However, it was the only variable consistently mediating the negative pathway between fire frequency and aboveground carbon. Our findings (i) show a convergence of multiple tree diversity dimensions in their response to frequent fires, pointing to the resilience of savanna tree diversity to repeated burning, potentially due to long-term adaptations of the local species pool, but also (ii) underscore the vulnerability of biomass accumulation and carbon storage to fire-induced demographic shifts. Increased fire frequency propelled by both climate change and human interactions in the surrounding agricultural lands would jeopardise the capacity of these plant communities to maximize carbon storage, highlighting the need for fire management strategies that sustain ecosystem functions under future environmental pressures. • Fire frequency was primarily driven by rainfall and its seasonality. • Tree diversity was not associated with fire frequency. • Carbon stock decreased in plots that burned more than five times in 20 years. • Fire regulates tree carbon through demographic bottleneck and reduction in the number of large trees.