Forest [FAR1]

Red spruce and balsam fir (the keystone species in this ecosystem) are limited to the Allegheny Mountains and the Northern Ridge and Valley sections, and models project suitable habitat and growth potential to decline dramatically for both species under both climate scenarios. Balsam fir has the lowest adaptive capacity of all the species in this ecosystem, largely due to its fire- and drought-intolerance and susceptibility to balsam woolly adelgid and other insect pests.

In the cooler and moister sites, hemlock is a keystone species that has been declining and is projected to decline further. For these forests, the loss of hemlock is likely to change the species assemblage dramatically, with fast-growing generalists like red maple or a variety of invasive species likely to overtake the newly vacated niche.

Increased flashiness followed by dry periods could cause amplification of the current hydrologic cycle, potentially increasing the spread and establishment of current and newly introduced invasive species.

Many riverine species in this forest type were modeled only by the Tree Atlas; thus evidence is somewhat limited regarding dominant species. Suitable habitat is projected to remain stable or increase for sycamore, river birch, black walnut, and boxelder. Silver maple is projected to decrease under low climate scenarios and increase under high climate scenarios. Hemlock and red maple were modeled by all three models.

Wildfire, currently episodic and human-caused, could increase under drier conditions, although the extent would be limited by the fragmented nature of small stream ecosystems. Mortality and damage from drought or storms may result in increased coarse woody debris, contributing to wildfire fuels.

Potential changes to the precipitation regime could intensify peak stream flow and shift the timing to earlier in the spring. Reduced precipitation in the summer and fall would result in drier conditions, increasing the potential for late-summer drought. Moisture deficits have the potential to create dry vegetation conditions late in the growing season, stressing hydrophilic seedlings and supporting wildfire conditions. Drought-stressed trees may be more susceptible to diseases such as thousand cankers and elm yellows, and insect pests such as hemlock woolly adelgid.

Episodic precipitation is likely to result in more intense storms, and subsequent flooding events. Although this ecosystem is highly dependent on disturbance and a regular influx of seeds, nutrients, and water during periodic flooding, increases in flood intensity or more frequent drought may not be tolerated by many species, especially in the early growth stages. Many riverine species in this forest type depend on high moisture availability, and are especially threatened by discrete periods of dry soil conditions.

Heavy deer browsing is also limiting seedling establishment and growth, and protection from herbivory will be critical in establishing regeneration, now and under future climate conditions.

Invasive plants such as princesstree, silktree, ailanthus, and glossy buckthorn compete directly with understory plants and native tree regeneration and these invasives are likely to take advantage of increased temperatures and disturbance.

Beech bark disease, emerald ash borer, hemlock woolly adelgid, anthracnose disease, and a variety of other pests and pathogens currently affect this ecosystem. The emerald ash borer has already reduced the white ash component in parts of the assessment area.