How Might Vermont Forests Look in 200 Years Under a Changing Climate?

Global change is the greatest challenge facing forest managers today. The uncertain impacts of changing climate, increasing disturbances, such as extreme weather events and pest outbreaks, land use and ownership changes, and fluctuating markets and demands for forest goods and services have led resource managers to look for alternative ways to maintain healthy, productive forests into the future.

Climate change is shifting global and regional temperature, precipitation, and disturbance patterns beyond the historical trends of variability observed over the last two centuries. In the northeastern United States, climate trends over the last century show rising average annual temperature, increased precipitation, and a greater prevalence of extreme weather events.

These trends are projected to continue and intensify in the next century. A warming climate and changes in occurrence, timing, frequency, duration, extent, and intensity of disturbances, such as wind or ice storms, droughts, or pest outbreaks, can stress forest ecosystems and cause tree mortality, resulting in significant changes to forest ecosystems and suitable habitat ranges for tree species.

Scientists remain unsure about the degree to which tree species or forest community types will actually shift their range in response to these changes. Some research shows tree species’ ranges moving northward as temperatures increase.

Researchers and managers are also still learning how forest change might be influenced by adaptive management strategies designed to address climate impacts. Forest managers may increasingly include adaptive tactics, such as planting of climate suitable tree species, into management plans.

In a new study, researchers at the University of Vermont Rubenstein School of Environment and Natural Resources used a forest landscape simulation model, called LANDIS-II, to evaluate impacts and interactions of climate, timber harvesting, and adaptive forest management techniques on future forest composition of a landscape in southeastern Vermont. The team looked at three climate scenarios and three forest management approaches.

The 140,000-acre study area in Windsor County encompasses predominantly northern hardwood forest and small areas of agricultural and residential land. Private ownership is primarily family forest owners while public and federal ownership includes the Marsh-Billings-Rockefeller National Historical Park and the Appalachian Trail corridor. This mosaic of private, public, and federal ownerships provided a unique study area to evaluate long-term impacts of multiple management strategies within a single landscape.

In addition to the current forest management being used on properties in the study area, the researchers considered two alternative adaptive forest management practices. For all treatments, researchers used input from local managers and other partners to design scenarios representing current and potential future approaches to sustaining forest ecosystems in the region.

The first alternative method would promote an increased diversity of tree species, age classes, and adaptive responses among tree species to projected climate changes. This treatment included group and patch cuts with large openings to allow for a wider range of shade tolerant species to become established.

The second alternative management scenario would increase diversity of tree species, age classes, and adaptive responses within forest stands and assist in transition of more future-adapted, climate resilient species. This treatment used uneven-aged management and less harvest intensity for species projected to maintain suitable habitat under future climate scenarios.

The researchers modeled 26 of the most abundant tree species on the landscape, simulated the landscape dynamics for 100 and 200 years into the future, and included current climate conditions and two climate change projection scenarios: extreme warming and moderate warming.

Projected Changes in Tree Species Composition

The researchers found that climate change is likely to cause shifts in forest composition, or the relative biomass or abundance of each tree species, in the northeastern United States over the next 200 years. The extreme climate warming projection resulted in more significant species shifts and reductions in landscape-scale tree biomass at the end of the 200-year simulation when compared to the more moderate warming projection and current climate conditions.

Sugar maple, American beech, and eastern hemlock are currently the three most abundant tree species within the landscape. Although sugar maple is projected to increase in relative biomass over the next century under all climate scenarios, the model shows a decline in the species over the next two centuries under current climate conditions which allow competing tree species to perform better. Under the two climate change scenarios over the next two centuries, sugar maple is expected to remain a prominent component of the forest composition.

American beech is projected to increase in relative biomass over the next 100 years. The model shows a decline by the end of the next two centuries under current climate conditions, but an increase under the two climate change scenarios.

Eastern hemlock would be expected to increase slightly under both changing climate scenarios but increase rapidly by the end of the next two centuries under current climate conditions if the pest, hemlock woolly adelgid, is held in check. Red spruce is projected to increase some in relative biomass under current climate conditions, but a changing climate could limit the abundance of this species in the future.

Alternative adaptive management practices did not have significant impacts on the most dominant tree species on the landscape, but some of the less common species, including red maple, bigtooth aspen, and black or sweet birch, were projected to increase in relative biomass as a result of alternative management. Also in the model, yellow birch is expected to respond well to both alternative treatments, and red oak is expected to increase under one adaptive treatment by the final year. Planting and tree retention in harvest treatments as part of one adaptive management scenario is expected to increase red oak and hickory abundance.

In the absence of large-scale disturbance or dramatic alterations to forest management regimes, current forest composition remains largely unaffected for the next 100 years. In contrast, over the next 200 years, climate is likely to play a much more important role in shaping the species composition and health of northeastern and Vermont forests. Unfortunately, these shifts could generate forests that are less diverse if climate-sensitive northern temperate species experience major reductions in relative biomass.

These findings highlight expected lag effects of a changing climate, which make it challenging for forest managers to sustain critical ecosystem services in the region. Because of the limited effects observed from the adaptive management scenarios simulated in this study, the authors suggest that the scale of forest management intervention may need to be greater than current applications of adaptive management tactics.