Abstract
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 gradients in Peru and Bolivia, we assessed
community-level shifts in species composition over a 40+ year time span. We tested the
thermophilization hypothesis, which predicts an increase in the relative abundances of species
from warmer climates through time. Additionally, we examined the relative contributions of tree
mortality, recruitment, and growth to the observed compositional changes. Mean
thermophilization rates across the Amazon-to-Andes gradient were slow relative to regional
temperature change. Thermophilization rates were positive and more variable among Andean
forest plots compared to Amazonian plots but were highest at mid-elevations around the cloud
base. Across all elevations, thermophilization rates were driven primarily by tree mortality and
decreased growth of highland (cool adapted) 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, the high tree mortality, and the slower-than-warming rates of
compositional change, we conclude that most tropical tree species, and especially lowland
Amazonian tree species, will not be able to escape current or future climate change through
upward range shifts, causing fundamental changes to composition and function in Earth’s highest
diversity forests.