Tropical forests in the Americas are changing too slowly to track climate change

Introduction : Tropical land regions are experiencing rapid climate change, with some scenarios for the tropical americas projecting temperature increases of up to ~4°c and precipitation reductions of close to 20% by 2100. this would expose current species assemblages to climates that they have never experienced before, potentially selecting for future plant communities adapted to such climates but unlike those currently observed. community responses to climate change will thus likely depend on underlying mechanisms and geographical context. in the face of threats from climate change, it is both critical and urgent to understand the ability of these complex systems to adapt to change and survive. the relationships among environmental conditions, plant performance, and distribution are mediated by species' functional traits. therefore, a trait-based approach provides a promising framework for predicting the impacts of climate change and resilience across forest ecosystems. Rationale : Climate change is already affecting the survival and distribution of tropical american plant communities. if species respond to climate change through migration, then we would expect montane communities to track changes in climate better than those in the lowland forests because mountains have different climate conditions occurring at shorter distances and thus are potentially easier to migrate across than lowlands. given exposure to a drying and warming climate, we could expect increased abundance of species exhibiting more drought-tolerance traits. drought-avoidance traits, notably deciduousness, could also become more prominent in the future as an adaptation to increasing drought. it is as yet unclear how shifts in the abundance and distribution of species translate into changes in functional composition and what functional changes have occurred as a response to the onset of a warmer, drier, and more variable climate across the tropical americas. it is uncertain if these functional shifts match the direction of climate change and, if so, whether the rate of functional trait change keeps pace with climate change or lags behind. here, we address these knowledge gaps by analyzing tree community trait shifts that have occurred across the past 40 years in tropical forests of the americas due to the dynamics of survivor, recruit, and fatality tree assemblages. the survivor tree assemblages consist of trees with traits potentially better suited to existing climatic conditions, and the recruit assemblages are composed of individuals with traits adapted to emerging climatic conditions. the fatality assemblages could represent individuals with less resilient traits that cause their inability to cope with climatic shifts. we also quantify if the observed changes in trait composition have been enough to track climate change to date. Results : Overall, we found that lowland forests show significant and larger changes in more community traits than montane forests. across forests and for the survivor assemblages, the abundance of deciduous species is increasing, with accompanying increases in leaf photosynthetic capacity and decreases in leaf area and leaf thickness, perhaps as an adaptation to a warmer and dryer climate. however, the recruiting communities in the lowland forests have, on average, exhibited decreases in their abundance of deciduous species, in leaf carbon and nitrogen content, and in wood density. crucially, most of these traits are changing at only a fraction of the rate required to maintain equilibrium with climate in the full tree community and survivor assemblages. the recruiting communities show the best tracking of a changing climate. Conclusion : Our analysis demonstrates that tree community composition is shifting to track climate change, but tree species composition and functional properties of tropical american forests (and possibly all tropical forests) are increasingly out of equilibrium with local climate. such disequilibrium likely increases vulnerability to climate change.