Central U.S. hardwood forests
Central US hardwood forests represent a significant ecological region characterized by temperate broadleaf and mixed broadleaf-coniferous stands. Spanning over 100 million acres, these forests are primarily located along the floodplains of the Ohio-Mississippi River confluence and extend into the rugged Ozark Plateau. Historically, the area was known for its diverse ecosystems, including oak savannas, which have dramatically declined due to human activities, with over 95% of the original habitats altered for agriculture and urbanization.
The forests host a rich variety of flora, including oak and hickory species, along with diverse fauna such as deer, squirrels, and various bird species. However, the ecosystem faces serious challenges, including climate change, invasive species, and habitat fragmentation, which threaten its biodiversity. Fire suppression practices have further altered the forest composition, allowing shade-tolerant species to dominate where oaks once thrived. Conservation efforts are ongoing, with several reserves established to protect remaining habitats and species, yet significant portions of these forests remain under threat.
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Central US hardwood forests
Category: Forest Biomes.
Geographic Location: North America.
Summary: These forests are biologically rich, even after two centuries of continually expanding habitat fragmentation.
The central hardwood forests of the United States are characterized by temperate broadleaf (angiosperms) and mixed broadleaf-coniferous (angiosperm-gymnosperm) stands. They cover more than 100 million acres (40 million hectares) in all. The boundaries of the ecoregion are rather ambiguous, but the commonly accepted range occurs along transition zones or ecotones, with several other bioregions, with the northern hardwood-conifer forest forming a northern border, the eastern and southern boundaries covered by the southeastern-pine forest, and the tallgrass prairie region bordering the west.
First referenced by ecologist E. Lucy Braun in 1950, the definition of Central Hardwood Forest has developed over time and pertains to the forest communities of deciduous and coniferous trees existing mainly on the floodplains of the Ohio-Mississippi River confluence area in the east-central United States, and extending into the more rugged topographic regions of the Ozark Plateau and some hill-and-valley regions in the western parts of Tennessee and Kentucky.
The region today consists of some of the most diverse floral habitats in North America, and the climate is temperate. The Central US Hardwood Forests biome covers approximately 116,000 square miles (300,000 square kilometers).
Biodiversity
Before European contact, the regions consisted of both open oak savannas and forests dominated by oak and hickory species. Oaks in the intact remnants here include red (Quercus rubra), white (Q. alba), and black oak (Q. velutina); prevalent hickories are shagbark (Carya ovata) and bitternut hickory (C. cordiformis).
Most of these native forests and historic oak savannas have been disturbed, with more than 95 percent ecosystem alteration in some regions, and great swaths have been replaced by agriculture, urbanized areas, or introduced species. The oak savanna regions, of which only about 0.2 percent remains, are fragmented and heavily interspersed with other native vegetation, led by flowering dogwood (Cornus florida), sassafras (Sassafras albidum), and hop-hornbeam or ironwood (Ostrya virginiana). The ecoregion toward its northern boundary contains patches of open prairie. The riparian zones, bottomlands, and marsh areas feature tulip (Liriodendron tulipfera), sweetgum (Liquidambar styraciflua), and American elm (Ulmus americana) as the main tree types.
Along with native flora, the fauna that exist in hardwood forests in the east-central United States are quite diverse and include eastern gray squirrels, opossums, chipmunks, raccoons, deer, and American black bear among major mammals. An array of bird species includes warblers, vireos, wood thrush, sapsuckers and woodpeckers, tanagers, hawks, eagles, owls, and black-billed cuckoo, among many others. Also present are large populations of moths and butterflies, snakes and tortoises, and salamanders and frogs.
Policies of fire suppression implemented in the 1930s precipitated a major decline event in this biome, altering the composition of flora and fauna. With fire no longer keeping savannas in an open state, closed-canopy forests were allowed to develop in many regions. Shade-tolerant species such as maples and yellow poplar now dominate the formerly shade-intolerant composition of oaks; thus, decreases in oak regeneration and establishment have been noted. Invasive vegetation—such as kudzu, garlic mustard, and privet—threaten forest understory species throughout the Central Hardwood Forests biome.
While most forest stands of this region are predominantly angiosperms, softwood conifers are found throughout. A common conifer across the central hardwoods is the red cedar (Juniperus virginiana), which has adapted to disturbed sites and does well in drier conditions. Other common conifers are more often found in xeric, or low-moisture, locations in the Ozark Mountains of southern Missouri, and in some of the hillside zones on the eastern fringes of the biome; these include shortleaf and Virginia pine (Pinus echinata and P. virginiana).
Threats
Much damage has already been done to this biome. Forest-clearing for pasture, cropland, monoculture plantation forests, urban and suburban development, and transportation infrastructure have combined to destroy the majority of the original habitat area. Native species in the fragmented stands that remain must contend with continued counter-cyclical fire suppression regimes, human overuse for recreation, climate change that is leading to a shift in habitat by latitude and elevation, and the persistent onslaught of invasive species. Insects and disease also exact a toll more readily than in the past, as some parts of the complex web of defenses have vanished along with the flora and fauna that once provided them. According to a 2016 study published in the journal Ecological Modelling, climate change, fire threats, and population movement could reduce the forest area by about 60 percent over the next 70 years.
There are some generally small reserve areas throughout the biome. Among them: Flat Rock Cedar Glades and Barrens, in central Tennessee, with 846 acres (342 hectares) protecting such species as leafy prairie-clover (Dalea foliosa), Boykin’s milkwort (Polygala boykinii), and wavy-leaf purple coneflower (Echinacea simulata); Hoosier National Forest, in Indiana, with 202,000 acres (82,000 hectares) protecting oak and hickory stands, along with pileated woodpecker, wild turkey, white-tailed deer, and fox; and Shawnee Hills National Forest, in Illinois, with 280,000 acres (113,000 hectares) protecting cherrybark oak (Quercus pagoda), cerulean warbler (Setophaga cerulea), northern copperbelly watersnake (Nerodia erythrogaster neglecta), and bigclaw crayfish (Orconectes placidus).
Bibliography
Fralish, James S. The Central Hardwood Forest: Its Boundaries and Physiographic Provinces. St. Paul, MN: U.S. Forest Service, 2003.
Illinois Endangered Species Protection Board (IESPB). Checklist of Endangered and Threatened Animals and Plants of Illinois. Springfield, IL: IESPB, 1999.
Ma, Wu, et al. “Fundamental Shifts of Central Hardwood Forests Under Climate Change.” Ecological Modelling, vol. 332, 24 July 2016, pp. 28–41, doi.org/10.1016/j.ecolmodel.2016.03.021. Accessed 29 Aug. 2022.
Shawnee National Forest. Biological Evaluation of Regional Forester Sensitive Species. Washington, DC: Shawnee National Forest, U.S. Forest Service, 2004.