Adaptation Actions for Mixedwood Forests

Mixedwood forests are already being affected by climate change and this creates challenges for sustaining the ecological, economic, and cultural benefits that the forests provide. There are many adaptation management actions that can address these key challenges. Adaptation actions are informed by site-specific conditions, including historical land-use legacies, and local manager knowledge and expertise.

A group selection harvest in a mixedwood stand on the Green Mountain National Forest.

Climate Change and Mixedwood (Spruce-Fir-Northern Hardwood) Forest Ecoystems

Spruce-fir-northern hardwood mixedwood forests are typically located below 2,400 feet on moderate to well-drained, shallow soils on lower mountain slopes and flats, often in the transition zone between northern hardwood and montane spruce-fir forests. The canopy is typically dominated by red spruce and yellow birch, with components of beech, sugar maple, and white ash at nutrient-rich sites. Balsam fir, red maple, and paper birch are sometimes present. Nearly all of these forests have been harvested in the past. In highly degraded sites where spruce was selectively removed, the resulting canopy is dominated by northern hardwood species with spruce and fir more common in the understory. Mixedwood forests occur across a variety of sites with a range of past management regimes, which means that climate change will affect forests in different ways. Information from this section is summarized from a vulnerability assessment for regional forests.

Climate Change Impacts

Moderate-High Vulnerability

• Mixedwood forests have limited tolerance to changes in precipitation and hydrologic conditions.
• Sites with a higher risk of drought stress could impair tree regeneration or shift tree species composition toward the few hardwood species found in these areas.
• Many of the dominant conifer species are expected to decline by the end of the century, particularly balsam fir.
• Warmer temperatures may allow balsam and hemlock woolly adelgid to increase while dampening the effects of the eastern spruce budworm. Spruce budworm may be a threat to regeneration, especially if it coincides with other stressors. Beech bark and beech leaf disease may spread and cause more damage in climate-stressed forests.
• Changes in herbivore populations may also have substantial effects on forest growth and composition.

Adaptive Capacity of Mixedwood Forests

Adaptive capacity is defined as the range of potential climate impacts a forest is exposed to and how well it can cope with these potential impacts.

These factors may increase adaptive capacity in mixedwood forests:

• These forests are dominated by a mix of hardwood and softwood species which increases overall tree species diversity and reduces vulnerability to spruce budworm and other climate stressors.
• Impacts from climate change are projected to be less severe at higher latitudes.

These factors may decrease adaptive capacity in mixedwood forests:

• These forests tend to have a very limited suite of tree species physiologically adapted to the moisture and nutrient conditions found in these areas, including a limited number of different conifer species, making them particularly vulnerable to the loss of a given species due to climate change and associated impacts.
• Past management has degraded many mixedwood stands, resulting in a canopy dominated by hardwood species over historically dominant conifers, reducing species diversity and options of recovery pathways in the face of climate change.

Site-level Considerations for Mixedwood Forests

Site-level factors could make a mixedwood stand more or less vulnerable to climate change, and the considerations below include some site-level factors that could increase or reduce risk.