Appalachian mixed mesophytic forests

• Category: Forest Biomes.
• Geographic Location: North America.
• Summary: This deciduous forest community, with rich soils, is in the Appalachian Mountains of the southeastern United States.

The Appalachian mixed mesophytic forests are characterized as moist deciduous forests with nutrient-rich mesic (moderately moist) soils, found throughout the southeastern United States from Tennessee to Pennsylvania. These temperate forest communities have well-developed soil profiles and provide a home for a suite of endemic (adapted specifically for, and unique to, a particular habitat) species including birds, amphibians, and other freshwater-friendly organisms. The forest is composed largely of deciduous tree species; it provides a home for a wide variety of shrubs, ferns, and herbaceous vegetation.

While related to the Appalachian-Blue Ridge forests biome, this biome is spread along the slopes and valleys to the west of that zone—thereby receiving more precipitation from the prevailing west-to-east weather patterns—and exists at markedly lower elevations as well. The Appalachian mixed mesophytic forests are considered to be among the most diverse temperate forest ecosystems on the globe today. These biota-rich forests provide a glimpse into a forest cover type that previously dominated much of the Earth.

Geology and Climate

The Appalachian mixed mesophytic forests are located in two physiographic provinces: the Appalachian Plateau, Ridge, and Valley; and the Blue Ridge provinces. The forest sits upon sandstone, siltstone, shale, and coal, which are the parent material for the rich soils that support a great diversity of plants and animals. Soils here are characterized as mesic loams, with high pH, high nutrient-holding capacity, and rich organic layers. These soils drive the region's rich diversity in vegetation but are less common on the steep hillsides and the very mountainous terrain within this region.

The region is characterized by its warm summers and cool winters, and is classified as a temperature humid continental climate. This mild climate is another support for the biological diversity to thrive here.

Vegetation Community

These forests can be subdivided into lower-elevation and higher-elevation communities, both of which generally are composed of deciduous tree species. Furthermore, the Appalachian mixed mesophytic forests host a series of unique habitats, including relict populations from when the climate was much cooler, so species in these habitats are more similar to northeastern plants and animals than to their southeastern counterparts.

Within the lower-vegetation forests, the tree canopy is dominated by an oak-hickory forest type, which is composed of various oak species (Quercus spp.) and hickories (Carya spp.), with lower densities of walnuts (Juglans spp.), elms (Ulmus spp.), birches (Betula spp.), ashes (Fraxinus spp.), basswoods (Tilia spp.), maples (Acer spp.), locusts (Robinia spp.), and pines (Pinus spp.). A second very common forest type within the Appalachian mixed mesophytic lower-elevation forests is composed of tulip poplar (Liriodendron tulipifera), blackgum (Nyssa sylvatica), Eastern hemlock (Tsuga canadensis), black cherry (Prunus serotina), sweetgum (Liquidambar styraciflua), American beech (Fagus grandifolia), and yellow buckeye (Aesculusoctandra).

Before the decimation of the American chestnut (Castanea dentata) by the fungal pathogen chestnut blight (Cryphonectria parasitica), chestnuts were dominant features in this landscape. Similarly, the Eastern hemlock today is threatened by an introduced pest, the hemlock woolly adelgid, that has led to dramatic losses of hemlocks throughout this region and the entire eastern United States.

The higher-elevation forests are composed largely of yellow birch (Betula allegheniensis), mountain maple (Acer spicatum), sugar maple (Acer saccharum), American beech, and Eastern hemlock, with associated understory shrubs such as mountain laurel (Kalmia latifolia) and rhododendron (Rhododendron spp.).

Additionally, these forests are home to a variety of smaller ecosystems, including bogs dominated by cranberry, blueberry, and sphagnum moss, which provide important habitat for species (such as fisher martens, Northern goshawk, and parasitic plants) that thrive in the acidic environment. This type of bog is more typical of environments such as northern New England. Their presence in the Appalachians provides insights into a cooler climate regime. Other tree species in the mesophytic forests that are reminiscent of cooler past climates include the Canada yew (Taxus canadensis), Eastern larch (Larix laricina), red pine (Pinus resinosa), and balsam fir (Abies balsamea).

Important Fauna

The region is especially noted for its high diversity of freshwater organisms, especially fish such as the river trout, which not only provide recreational opportunities for fishing, but also perform important ecosystem services such as nutrient cycling within these freshwater communities. Stream macroinvertebrates are also important players within freshwater communities in the Appalachian mixed mesophytic forests. Many studies have researched the importance of these small organisms in nutrient cycling and fine woody debris decomposition. These organisms act as indicators of stream health (with the loss of pollution-sensitive invertebrates as stream quality declines). A variety of endemic fish species can also be found within the Appalachian mixed mesophytic forests, serving as further indicators of water quality and stream health.

The forests are home to mammals such as the big brown bat, martens, weasels, and black bears, as well as a variety of shrew species. They provide critical habitat for amphibians such as frogs, toads, and salamanders. The Appalachian mixed mesophytic forests are known as the global center of diversity for wood salamanders, which prefer the mild climate and moist conditions. A great diversity of land snails can also be found within these forests, which are considered to be a biodiversity center with claims for the highest endemism for land snails in localized environments.

This biome provides habitat for avian species including warblers (Dendroica spp.), vireos (Vireo spp.), Northern goshawks (Accipiter gentilis), and wood thrushes (Catharus spp.). Several reptiles of concern include the scarlet king snake (Lampropeltis elapsoides) and timber rattlesnake (Crotalus horridus), but a great diversity of reptiles exist and thrive throughout the ecoregion.

The Appalachian mixed mesophytic forest supports additional endemic species, including the Allegheny plum tree (Prunus alleghaniensis), the flattened musk turtle (Sternotherus depressus), and the Alabama map turtle (Graptemys pulchra). Endemic salamanders include the Black Mountain salamander (Desmognathus welten), Southern dusky salamander (Desmognathus auriculatus), Jordan's salamander (Plethodon jordani), and Cheat Mountain salamander (Plethodon nettingi).

Land Management

With urbanization occurring throughout the United States, a common policy of not burning forests and grasslands was enacted for forest managers: the so-called no-burn policy. Although this policy has many benefits for humans inhabiting the area, it can cause detrimental effects on the ecosystems that require fire to sustain their natural biologic rhythms. In the mixed mesophytic forests, this policy was especially harmful and disruptive because it reduced and even eliminated naturally occurring fires that, while frequent, usually occur only over a small portion of land and are not catastrophic to the entire ecosystem. Under a natural-burning fire regime, fuel loads do not build up. Conversely, no-burn leads to such buildup, which often allows forest fires to escape to the canopy or tree crowns and destroy virtually all the vegetation within a local community.

Frequent low-intensity fires provide numerous benefits to the ecosystem, including large inputs of nutrient from burned wood, which in turn jump-starts soil fungal and microbial communities to assimilate these new nutrients back into biologically available forms for new understory growth. Frequent low-intensity fires also self-regulate the density of vegetation within forests and can clear out competitors, leading to enhanced growth for the trees and plants that remain after the fires. The Appalachian mixed mesophytic forests experienced these types of low-intensity and frequent surface fires, which promoted the growth and establishment of oak species within this forest type, where oaks make up a large portion of the tree species.

As the United States was industrialized, these practices were switched to high-intensity, catastrophic, stand-replacing fires, which destroyed much of the forest. In more recent times, the no-burn policy has eliminated fire altogether, which, while initially helpful for forest regeneration, now encourages replacement of species such as oaks, which will in turn change the composition of these unique forest communities.

Threats to Community

A variety of forces are threatening the Appalachian mixed mesophytic forests, chief among them anthropogenic factors such as global climate change and pollution, and lesser but sometimes more direct forces such as plant invasions, and introduced pests and pathogens. Global climate change poses perhaps the greatest threat, as higher average temperatures, for example, add stress to key tree species by enabling the arrival and penetration of bark-boring pests and their accompanying fungal parasites. Studies have indicated that climate change will result in increases in precipitation and possibly longer periods between precipitation. This could effect the landscape as not all soils will be able to retain moisture in such intense rainfalls. Worries over flooding, storms, and increases in extreme weather events are also a concern.

Some forms of air and water pollution can greatly diminish the diversity of sensitive species such as stream macroinvertebrates and upset the balance of supportive fungal–plant associations like lichens. Habitat fragmentation is another threat; already it has dramatically reduced the connectivity of the landscape, affecting various species in unpredictable ways.

Regional threats to these forests include invasive animal pests such as the hemlock wooly adelgid; invasive plant species such as Japanese stiltgrass; and habitat fragmentation due to development, logging, mining, and such disruptive add-ons as pipeline construction. Although this forest community is a gem among forested habitats, these regional threats combined with global climate change can profoundly shape the trajectory of future forests, especially the majestic Appalachian mixed mesophytic forests.

Bibliography

Braun, E. Lucy. Deciduous Forests of Eastern North America. Caldwell, NJ: Blackburn Press, 1950.

Butler, Patricia R., et al. “Central Appalachians Forest Ecosystem Vulnerability Assessment and Synthesis: A Report from the Central Appalachians Climate Change Response Framework Project.” U.S. Forest Service, Feb. 2015, www.fs.usda.gov/nrs/pubs/gtr/gtr‗nrs146.pdf. Accessed 18 Dec. 2024.

Clipp, Hannah L., et al. "Distinct Forest Bird Communities Are Strongly Associated with Red Spruce-Northern Hardwood Ecosystems in Central Appalachia, USA." Ecological Indicators, vol. 135, Feb. 2022, doi.org/10.1016/j.ecolind.2022.108568. Accessed 18 Dec. 2024.

Eyre, F. H., ed. Forest Cover Types of the United States.Bethesda, MD: Society of American Foresters, 1980.

Muller, Robert N. “Vegetation Patterns in the Mixed Mesophytic Forest of Eastern Kentucky.” Ecology 63, no. 6 (1982).