Compositional response of Amazon forests to climate change

Most of the planet’s diversity is concentrated in the tropics, which includes many regions undergoing rapid climate change. Yet, while climate-induced biodiversity changes are widely documented elsewhere, few studies have addressed this issue for lowland tropical ecosystems. Here we investigate whether the floristic and functional composition of intact lowland Amazonian forests have been changing by evaluating records from 106 long-term inventory plots spanning 30 years. We analyse three traits that have been hypothesized to respond to different environmental drivers (increase in moisture stress and atmospheric CO₂ concentrations): maximum tree size, biogeographic water-deficit affiliation and wood density. Tree communities have become increasingly dominated by large-statured taxa, but to date there has been no detectable change in mean wood density or water deficit affiliation at the community level, despite most forest plots having experienced an intensification of the dry season. However, among newly recruited trees dry-affiliated genera have become more abundant, while the mortality of wet-affiliated genera has increased in those plots where the dry season has intensified most. Thus, while changes in compositional dynamics (recruits and mortality) are consistent with climate-change drivers, these have yet to significantly impact whole-community composition. Overall, we find that the increase in atmospheric CO₂ appears to be driving a shift within tree communities to large-statured species and that climate changes to date will eventually impact Amazon forest composition, but long generation times of tropical trees mean that the speed of the floristic response is lagging behind the rate of climatic change.