Amazonian and Andean tree communities are not tracking current climate warming

Climate change is shifting species distributions, leading to changes in community composition and novel species assemblages worldwide. However, the responses of tropical forests to climate change across large-scale environmental gradients remain largely unexplored. Using long-term data over 66,000 trees of more than 2,500 species occurring over 3,500 m elevation along the hyperdiverse Amazon-to-Andes elevational gradient, we assessed community-level shifts in species composition through a 44-year time span. We tested for the predicted increase in relative abundances of species from warmer climates (thermophilization) along the Amazon-to-Andes elevational gradients in Peru and Bolivia. Additionally, we examined the relative contributions of tree mortality, recruitment, and growth to observed compositional changes. Mean thermophilization rates across the Amazon-to-Andes gradient were slow relative to concordant changes in regional temperatures. Thermophilization rates were positive and more variable among Andean forest plots compared to Amazonian plots but were fastest at mid-elevations around the cloud base. Across all elevations, thermophilization rates were driven primarily by tree mortality and decreased growth of highland species rather than an influx of lowland species with higher thermal optima. Given the high variability of community-level responses to warming along the elevational gradients and the generally slower-than-warming rates of compositional change, we conclude that most tropical tree species, and especially Amazonian tree species, will not be able to escape current or future climate change through upward range shifts.