Climate change is posing a significant threat to America's forests as rising temperatures and decreasing moisture levels create unfavorable conditions for trees. A recent study conducted by researchers from UC Santa Barbara and the University of Utah has shed light on the potential fate of these woodlands in the near future. By combining mathematical models with data collected by the U.S. Forest Service and plant physiologists, the study aimed to assess the vulnerability of forests to drought and their capacity to adapt to changing conditions.

The findings, published in Global Change Biology, indicate that while most forests have the potential to adapt to hotter and drier conditions, their adaptation is not occurring rapidly enough to avert the impending stress. The study serves as a benchmark for future forest research and offers guidance for conservation and management efforts.

Greg Quetin, the first author of the study and an assistant project scientist in the UCSB Department of Geography, expressed concern over the forests' slow rate of change in response to climate change-induced water stress. However, he emphasized that there is hope, as many forests in the continental U.S. possess sufficient functional diversity to enhance their drought tolerance through shifts in species composition.

The study identified several mechanisms through which forests can adapt to drier conditions. Individual trees can modify their activity, physiology, and gene expression to suit the new environmental conditions. Drought-tolerant species already present in the ecosystem can also become dominant, and the composition of the forest can change as hardier species migrate in while vulnerable species decline. However, the study concluded that evolutionary changes through natural selection would have a negligible effect on long-lived organisms like trees over the next century.

To determine whether the existing traits and species in American forests are adequate for acclimation to future climate change without widespread mortality, Quetin and his colleagues utilized data from the Forest Inventory and Analysis program, a comprehensive database maintained by the U.S. Forest Service since 2000. This database includes information on forest inventory plots, such as location, species, size, density, health, growth, mortality, and harvesting. By cross-referencing this data with the Xylem Functional Traits Database, which provides measurements of tree physiology and hydraulic traits, the researchers developed a model to simulate a forest's response to increased water stress. The model predicted key processes such as photosynthesis, respiration, growth, and plant stress, while also incorporating an optimization technique to evaluate how changes in leaf area could mitigate the stress caused by changing environmental conditions.

Co-author Lee Anderegg, an assistant professor in the Department of Ecology, Evolution, and Marine Biology, highlighted the significant role of leaf area in managing water stress, stating that it is the primary mechanism through which individual trees can adapt. Forests in drier regions tend to have sparser canopies, while those in wetter areas can sustain denser foliage.

The study revealed that approximately 88% of forests across the continental U.S. possess the necessary trait and species diversity to acclimate to climate change. However, most forests were not adapting as quickly as the model predicted was required to avoid increased water stress and subsequent mortality. Despite this concern, co-author Anna Trugman, an assistant professor in the Department of Geography, emphasized the potential of biodiversity to buffer the impact of climate change on forests, offering a glimmer of hope.

Anderegg humorously referred to the slow movement of trees, comparing it to the pace of Ents, the ancient tree-like beings in "The Lord of the Rings," indicating that trees are naturally slow to adapt.

The team's calculations were complicated by higher concentrations of carbon dioxide in the atmosphere, which affect plant water loss. While increased CO2 can enable plants to decrease the size of their leaf pores and still acquire the necessary carbon for photosynthesis, the drying effect of a warming climate results in greater water loss. The intricate nature of these interactions requires nuanced models to untangle. The team had previously discovered the significant energy expenditure involved in water transportation within trees.

The researchers are currently collecting data on changes in tree physiology following climate-driven fires in Sequoia National Park, aiming to empirically verify the extent to which trees can adjust their physiology. They are also investigating whether trees can entirely avoid future water stress through changes in leaf area and examining whether maximizing carbon gain or stress avoidance is more limiting.

Forests are already undergoing changes, with sparser canopies and a shift in species composition expected as the atmosphere becomes drier. These factors have implications for forest carbon storage, which currently accounts for approximately 30% of anthropogenic emissions. However, the research team recently found that this contribution is likely to decrease under climate change.

The study emphasizes the critical importance of implementing management strategies that promote forest adaptation. Forests should not be seen as static entities but rather as dynamic and resilient systems that require change to keep pace with the changing climate. Gradual changes facilitated by management efforts can help prevent abrupt and catastrophic alterations, such as wildfires and die-offs, which have detrimental effects on forests, wildlife, and nearby communities.

The authors recommend various strategies, including planting more drought-tolerant species and conducting prescribed burns to foster healthy woodlands. However, they stress that the most crucial action is to mitigate climate change itself.

Quetin highlights that climate adaptation is as challenging as climate mitigation, and the extent of adaptation required is directly linked to the trajectory of greenhouse gas emissions. Reducing emissions will not only lessen the need for adaptation but also contribute to securing a sustainable future for forests and society as a whole.