As climate change continues to reshape ecosystems across the globe, forests – key players in the Earth’s carbon cycle – are also undergoing significant transformations. A study conducted by ecologists at Cary Institute of Ecosystem Studies and published in the February 2025 issue of the journal Ecosystems explores how climate and disturbance (such as logging or pest outbreaks) drive forest community composition, and what this means for future forest biodiversity and carbon storage.
The authors focused on forests in the eastern region of the United States (from Minnesota south to Missouri, east to Maryland, and north to Maine), which constitute about 60 percent of the nation’s forest carbon sink. Using data from the U.S. Forest Service’s Forest Inventory and Analysis program, they tracked changes in tree regeneration – specifically the abundance of seedlings and saplings (the next developmental step in the growth process after seedlings) of the 99 most common tree species in the eastern United States – from 2003 to 2021. These early life stages can serve as indicators of future forest composition, although regeneration rates do not necessarily translate into changes in mature forest composition, because seedlings and saplings may fail to become part of the overstory. To examine whether changes in seedling and sapling regeneration impacted mature forest composition, the researchers assessed changes in species richness for adult, sapling, and seedling communities over time. The researchers also used a simulation model, SORTIE-ND, to project how today’s regeneration patterns might shape future biomass and forest structure.
Their findings reveal a complex picture. Overall, seedling densities declined by 51.1 seedlings per hectare per year across the region (which represents an annual decrease of between 0.5 percent and 1.0 percent, depending on the forest type), with sugar maple (Acer saccharum) exhibiting the steepest declines. While the overall trend across the region was for declining seedling densities, some species showed modest increases in seedling density, led by yellow birch (Betula alleghaniensis), which increased by 2.7 seedlings per hectare per year. The data also revealed that disturbances promoted seedling regeneration, particularly for tree species adapted to warmer climates. This suggests that disturbances may be helping forests “thermophilize,” or shift toward species better suited to a warmer future. However, sapling numbers often declined in disturbed areas, and sapling recruitment to the overstory did not compensate for losses due to disturbance.
The researchers also found that forest disturbance accelerates reassembly toward warmer-adapted communities but can reduce overall biodiversity. Seedling communities in disturbed forests became more diverse, possibly reflecting an influx of new, warm-adapted species. However, adult tree communities became less diverse following disturbance. This trade-off suggests that while disturbances may make forests more resilient to climate change by promoting warm-adapted species, they may also result in less diverse forests with potentially lower ecological stability and productivity.
Future projections using the SORTIE-ND model showed that sapling densities, not seedling densities, are better predictors of future forest biomass. In simulations, it took an average of 37 years for saplings to grow into mature trees and significantly affect forest carbon stocks. However, because disturbance often reduced sapling numbers, disturbed forests are expected to store less biomass in the long run relative to undisturbed forests, despite being more climate-adapted.
In short, this research underscores a fundamental tension: disturbances such as harvesting or pest outbreaks can help forests adapt to warming conditions by favoring warm-adapted species. But this may come at the cost of biodiversity and long-term biomass growth. The findings point to a need for thoughtful forest management that balances climate adaptation, biodiversity conservation, and carbon storage. Moderate disturbances may be especially important, as they create openings for regeneration without overly degrading forest integrity, structure, or diversity.
Ultimately, the study highlights the importance of managing forests not just for their short-term productivity or carbon sequestration, but for their long-term resilience and ecological integrity in a changing climate.