Coexistence of tropical forest tree species along the demographic buffering spectrum

Organisms have evolved diverse adaptive strategies to cope with environmental fluctuations. Slow-growing long-lived species tend to exhibit low temporal variability in population fitness (strongly buffered demographically), whereas fast-growing short-lived species optimize fitness in favorable years (weakly buffered). These patterns set up the expectation that demographic differentiation may reduce differences in long-term fitness among species and promote coexistence in variable environments. Yet, this expectation has never been empirically tested for trees. Here, we quantified fitness inequalities among 204 co-occurring tropical trees spanning a life-history spectrum from strongly to weakly buffered. We found predictable variation in long-term population fitness for species at low densities (λ_{s_inv}), pointing to demographic differentiation as a key for coexistence in fluctuating environments. Simulated increases in temperature, precipitation, and drought variability led to divergent impacts on species' fitness and increased fitness differences. Together, these findings provide a novel perspective on mechanisms underpinning the astounding diversity of tropical forests.