Old-growth tropical forests are not in a steady state but show changes in productivity, species composition, and functional trait composition. These shifts may have a range of causes, such as climate change and disturbances. Tropical forests can respond to these multiple drivers at instantaneous to millennial time scales, which cannot be disentangled using short-term studies. Moreover, their responses can vary with forest type. For 8 lowland and highland Neotropical forest landscapes, we assess their long-term responses in functional trait composition to past climate change and (human) disturbances. We link vegetation dynamics from fossil pollen records to functional trait data to assess how shifts in community-mean functional traits can be explained by climate change and disturbance events. We find that tropical forests show clear, predictable responses to environmental changes. Temperature was most important, with warmer climates providing more favorable conditions for taller taxa with soft wood at high elevations and for smaller taxa with soft wood at low elevations. Furthermore, increasing rainfall and decreasing drought favors fast-growing, drought-vulnerable taxa that are tall and have large leaves, whereas increased human disturbances favor more disturbance-adapted species that are short with a fast life cycle and have small leaves to cope with increased heat and insolation in open areas. The predicted future increases in temperature and droughts may cause shifts in tropical forest composition and may reduce their carbon storage and sequestration capacity, thus potentially providing a positive feedback on climate change.
human disturbance, fossil pollen, functional traits, fire, climate change