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 growth (strongly buffered demographically), whereas fast-growing short-lived species optimize growth in favorable years (weakly buffered). These patterns set up the expectation that differentiation in demographic buffering may reduce disparities in long-term fitness among species, enhancing the potential for coexistence in variable environments. Yet, this expectation has never been empirically tested for trees. Here, we quantified differences in long-term population growth among 204 co-occurring tropical trees spanning a life-history spectrum from strongly to weakly buffered. We found that interspecific differences in demographic buffering reduced disparities in long-term population fitness at low densities, highlighting demographic differentiation as a key mechanism promoting coexistence in fluctuating environments. However, simulated increases in temperature, precipitation, and drought variability produced divergent fitness responses among species and exacerbated interspecific fitness disparities. Together, these findings provide a novel perspective on the mechanisms that underpin the astounding tree diversity in tropical forests.