2025-03-07 オックスフォード大学
＜関連情報＞
- https://www.ox.ac.uk/news/2025-03-07-tropical-forests-americas-are-struggling-keep-pace-climate-change
- https://www.science.org/doi/10.1126/science.adl5414
アメリカ大陸の熱帯林は、気候変動を追跡するには変化が遅すぎる Tropical forests in the Americas are changing too slowly to track climate change
Jesús Aguirre-Gutiérrez, Sandra Díaz, Sami W. Rifai, Jose Javier Corral-Rivas, […], and Yadvinder Malhi
Science Published:7 Mar 2025
Editor’s summary
Species are expected to shift their ranges as the climate changes, but shifts may not occur fast enough, especially for immobile species such as plants. Two papers in this issue assess the degree to which plant species are tracking climate change in the American tropics, where data availability has constrained inference. Ramírez-Barahona et al. show that in Mesoamerican cloud forests, climate change and deforestation together have led to a mean upward shift in species ranges since 1979, mainly due to contracting lower range edges. In tropical forests across the Americas, Aguirre-Gutiérrez et al. found that tree traits are not shifting fast enough to track climate change based on trait-climate relationships, with smaller shifts in montane forests. —Bianca Lopez
Structured Abstract
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.
Mechanisms driving changes in community trait composition and climate tracking.
Changes in climatic conditions significantly influence tropical forest tree community dynamics, including survival (survivor assemblages), recruitment (recruit assemblages), and mortality (fatality assemblages). Quantifying these community dynamics is crucial for understanding how tropical forests adapt to and track a changing climate. Survivor assemblages consist of trees with traits potentially suited to existing climatic conditions, aiding in incremental climate tracking. Conversely, recruit assemblages are composed of individuals with traits potentially better adapted to emerging climatic conditions, enhancing their ability to thrive under new environmental conditions. By contrast, fatality assemblages represent individuals with less resilient traits, leading to their inability to cope with climatic shifts. Our analysis reveals that survivor assemblages are tracking climate changes at <8% of the expected rate given current climate shifts. Recruit assemblages demonstrate a higher tracking rate, ~22% of the expectation. However, fatality assemblages by definition do not track climate because they consist of individuals that have already perished, likely due to their inadequate adaptation to changing climatic conditions.
Abstract
Understanding the capacity of forests to adapt to climate change is of pivotal importance for conservation science, yet this is still widely unknown. This knowledge gap is particularly acute in high-biodiversity tropical forests. Here, we examined how tropical forests of the Americas have shifted community trait composition in recent decades as a response to changes in climate. Based on historical trait-climate relationships, we found that, overall, the studied functional traits show shifts of less than 8% of what would be expected given the observed changes in climate. However, the recruit assemblage shows shifts of 21% relative to climate change expectation. The most diverse forests on Earth are changing in functional trait composition but at a rate that is fundamentally insufficient to track climate change.
