Coexistence of tropical forest tree species along the demographic buffering spectrum

Organisms have evolved a wide range of demographic strategies to cope with environmental variability. At the one end of the continuum are slow-growing conservative species that exhibit low temporal variability in population fitness (i.e. strongly buffered demographically), while at the other end are fast-growing short-lived species that optimize fitness during favorable years (i.e. weakly buffered). These emergent patterns set up the expectation that demographic differentiation is an equalizing mechanism that prevents competitive exclusion for trees in variable environments. Yet, this expectation has never been empirically tested. Here, we quantified long-term fitness inequalities among 208 co-occurring tropical trees that span the life-history continuum from fast-growing short-lived trees to slow-growing long-lived trees. We find predictable variation in the long-term population fitness for species at low densities, pointing to demographic differentiation as a key mechanism that promotes species co-existence in variable environments. We also find that a 1% increase in temperature or precipitation variability relative to observed conditions has a pronounced negative effect on the long-term population fitness for most species and intensified fitness inequalities among them. Together, these results provide a novel perspective on the mechanisms that underpin the astounding diversity of tropical forests and a backdrop for modeling shifts in the assembly of forest communities in a changing world.