Pennsylvania sits squarely within the temperate deciduous forest biome, one of the most productive and species-rich terrestrial biomes on Earth. The state’s roughly 17 million acres of forest are dominated by broadleaf trees that shed their leaves each autumn, driven by a climate with warm summers, cold winters, and year-round precipitation. But calling Pennsylvania “temperate deciduous forest” only gets you so far. The state spans enough latitude, elevation, and geological variety that its forests, wildlife, and ecological dynamics differ substantially from one end to the other, and several forces, from deer overpopulation to invasive insects to a century of fire suppression, are reshaping what grows where.
The Temperate Deciduous Forest Biome
Temperate deciduous forests occur in regions with distinct warm and cold seasons, moderate to abundant rainfall spread throughout the year, and soils rich enough to support large broadleaf trees. Pennsylvania fits this description neatly. Average annual temperatures across the state range from the mid-40s°F in the northern highlands to the low 50s°F in the southeastern lowlands. Annual precipitation runs roughly 35 to 50 inches depending on location, with no prolonged dry season. Winters bring freezing temperatures and snow, especially in the Allegheny Plateau and Pocono regions, while summers are warm and humid. That combination of moisture and seasonal temperature swings is what defines the biome: trees invest heavily in broad leaves to capture summer sunlight, then drop them when cold and short days make photosynthesis unprofitable.
Within the broader temperate deciduous biome, ecologists recognize several distinct forest types, and Pennsylvania hosts more than one. The state sits at a transition zone where northern hardwood forests, rich in sugar maple and American beech, give way to oak-hickory forests that dominate the mid-Atlantic and southern Appalachian regions. Understanding that gradient is key to understanding Pennsylvania’s ecology.
Two Major Forest Types and Where They Meet
The most striking botanical division in Pennsylvania runs roughly east-west across the northern tier of the state. North of this line, on the cooler, more glaciated Appalachian Plateau, the presettlement forests were dominated by American beech and sugar maple growing on richer, finer-textured soils left behind by glaciers. South of the line, on the rugged, unglaciated plateaus, nutrient-poor and well-drained soils favored oaks and, historically, American chestnut.
Research on presettlement forest composition in northwestern Pennsylvania confirmed this pattern clearly. Beech-maple forests thrived on the glaciated landscape to the north, while fire-prone, leached soils to the south supported oaks and chestnut before the chestnut blight wiped out that species in the early twentieth century.1Canadian Journal of Forest Research. Physical and historical determinants of the pre- and post-settlement forests of northwestern Pennsylvania That division persists today, though chestnut’s role has been filled mainly by red maple, birch, and other opportunists.
In southeastern Pennsylvania, a survey of 82 forest stands found that about 36 percent were oak-hickory, led by white oak, black oak, scarlet oak, and several hickory species. Another 27 percent were mixed mesophytic stands where eastern hemlock, red oak, American beech, sweet birch, and sugar maple shared dominance, typically on sites with intermediate soil moisture.2Ecological Monographs. Distribution of Major Forest Species in Southeastern Pennsylvania Northeastern Pennsylvania tells a similar story, with both northern hardwood and oak-hickory types represented across the landscape.3Northern Journal of Applied Forestry. Adequacy of Natural Hardwood Regeneration on Forestlands in Northeastern Pennsylvania
What you see when you hike in Pennsylvania depends heavily on where you are. The northern and high-elevation forests feel closer to New England, with their maples, beeches, and birches. Head south or downslope and the canopy shifts to oaks and hickories, a community more typical of Virginia or the Carolinas. Pennsylvania straddles these two worlds, and that overlap is part of what makes its forests ecologically interesting.
Key Tree Species You Will Actually See
If you spend any time in Pennsylvania’s forests, a handful of tree species will dominate your view. In the northern hardwood zones, sugar maple is the signature tree, joined by American beech, yellow birch, sweet birch, and red maple. These species thrive in cool, moist conditions and on relatively fertile soils. In the oak-hickory zones farther south and at lower elevations, white oak, red oak, black oak, and shagbark hickory form the canopy. Red maple appears in both zones and has become one of the most common trees statewide, partly because it tolerates a wide range of soil and light conditions.
Eastern hemlock deserves special mention. This long-lived conifer is not a broadleaf deciduous tree, but it has historically been a critical component of Pennsylvania’s forests, especially along streams, ravines, and north-facing slopes. Hemlock stands create a distinctive microenvironment: deep shade, cool temperatures, and acidic soils that support their own community of plants and animals. As we will see, hemlock is under severe threat, and its decline is reshaping forest ecology across the state.
Pennsylvania’s understory and forest floor host a range of wildflowers, ferns, and shrubs. Spring ephemerals like trillium, bloodroot, and mayapple take advantage of the brief window of sunlight before the canopy leafs out. Mountain laurel, the state flower, forms dense thickets on acidic soils, especially in the Appalachian ridges. Rhododendron fills a similar role in wetter ravines. Blueberry and huckleberry shrubs are common in areas with acidic, nutrient-poor soils.
Pennsylvania’s Wildlife
The temperate deciduous forest biome supports a rich animal community, and Pennsylvania is no exception. The state’s forests, grasslands, wetlands, and streams collectively support hundreds of vertebrate species. According to a statewide conservation assessment, the highest concentrations of mammal and bird species occur in areas with large expanses of intact forest, particularly on public lands in the north-central part of the state.4BioOne (Northeastern Naturalist). Terrestrial Vertebrates in Pennsylvania: Status and Conservation in a Changing Landscape
White-tailed deer are by far the most visible large mammal, and their population density has profound effects on the rest of the ecosystem (more on that shortly). Black bears are common in the mountainous and heavily forested regions, with Pennsylvania hosting one of the densest black bear populations in the eastern United States. Smaller mammals include gray squirrels, chipmunks, raccoons, opossums, striped skunks, and a variety of bats, several of which are threatened by white-nose syndrome.
The state’s bird life is especially notable during migration. Hawk Mountain Sanctuary, located on the Kittatinny Ridge in eastern Pennsylvania, is one of the most famous raptor migration watchpoints in the world. The ridge, the southernmost in the Appalachian chain in that region, funnels large numbers of migrating hawks, eagles, and falcons along its length each autumn.5Ecology. A long‐term database on raptor migration at Hawk Mountain Sanctuary, northeastern United States Broad-winged hawks pass through in enormous kettles in September, while red-tailed hawks and golden eagles follow later in the season. Breeding songbirds in Pennsylvania’s forests include wood thrushes, scarlet tanagers, ovenbirds, and several warbler species, many of which depend on large unbroken tracts of mature forest.
In the state’s waterways, brook trout are the only native stream-dwelling trout species. They prefer cold, clean headwater streams, exactly the kind of habitat that hemlock-shaded ravines provide. A study of 78 trout streams in northwestern Pennsylvania found that brook trout and nonnative brown trout sort themselves predictably along a stream-size gradient, with brown trout becoming dominant in larger waterways. Streams were 12 times more likely to hold brook trout rather than brown trout when a downstream barrier blocked brown trout from migrating upstream, suggesting that physical barriers serve as critical refugia for the native species.6Oxford Academic. Evaluating the Trade‐Offs between Invasion and Isolation for Native Brook Trout and Nonnative Brown Trout in Pennsylvania Streams
The Coyote as a Newcomer
One of the more recent additions to Pennsylvania’s wildlife is the eastern coyote. Coyotes were historically absent from the state, but they expanded eastward during the twentieth century as wolves were eliminated and forests regrew. A genome-wide study of nearly 1,200 coyotes from the northeastern United States found subtle but significant population structure within the state, with two genetic clusters separated by a weak transition zone. That partitioning aligned with patterns of human population density, and the researchers identified Pennsylvania as a recent contact zone where two separate waves of coyote expansion, one moving south along the Great Lakes and another moving through the northern lakes toward New England, met and mixed.7PubMed Central. Signatures of the Anthropocene: Population Genomic Structure Detected in Pennsylvania Coyotes
Today, coyotes are found in every county. They are generalists that eat everything from rabbits and voles to deer fawns to fruit and garbage, and they have adapted readily to suburban and even urban edges. Their arrival has filled part of the ecological role left vacant by the extirpation of wolves and mountain lions, though coyotes are much smaller and tend to prey on smaller animals.
How Deer Are Reshaping the Forest
White-tailed deer are the single most influential herbivore in Pennsylvania’s forests, and their impact on vegetation is hard to overstate. Deer densities in much of the state far exceed historical norms, and decades of heavy browsing have fundamentally altered which plants can survive in the understory.
Research at timber harvest sites in Pennsylvania found that deer browsing in the first decade after a harvest did not necessarily reduce the total number of woody seedlings, but it considerably reduced seedling height, overall vegetation structure, and the composition of the understory. Unfenced sites had shorter saplings and a greater prevalence of less palatable species like blueberry, huckleberry, ferns, and certain forbs, while the tree species that foresters want to see reach the canopy struggled to grow tall enough to escape browsing pressure.8Forest Ecology and Management. Evaluating the impacts of white-tailed deer (Odocoileus virginianus) browsing on vegetation in fenced and unfenced timber harvests
The relationship between deer and invasive plants adds another layer of trouble. A long-term experiment in a Pennsylvania forest compared plots that deer could access freely with plots fenced to exclude them. In the fenced plots, native wildflowers like large-flowered trillium thrived and species abundances were more evenly distributed. In the unfenced plots, most native species were rare or absent, and the invasive herb garlic mustard had become dominant. The contrast was stark: in one survey year, only six individual native plants flowered across three deer-access plots, compared to 145 flowering individuals in the exclosures.9BioOne Complete (Natural Areas Journal). Deer Facilitate Invasive Plant Success in a Pennsylvania Forest Understory Deer selectively eat native plants while ignoring garlic mustard, effectively clearing the field for invaders.
The Hemlock Crisis
Eastern hemlock is being killed across Pennsylvania by the hemlock woolly adelgid, a tiny insect native to East Asia that feeds on hemlock sap. The adelgid has spread throughout the northeastern United States, causing large-scale dieback that is transforming entire stream corridors and forest stands.
The ecological consequences extend well beyond the trees themselves. A study of breeding birds at Fort Indiantown Gap in central Pennsylvania found that hemlock decline affected bird communities in complex ways. Of nine bird species more abundant in hemlock than in other forest types, only two, the Acadian flycatcher and the black-throated green warbler, were positively associated with living hemlocks and stood to lose habitat as trees died. Meanwhile, species like the eastern wood-pewee, great crested flycatcher, and wood thrush appeared to benefit from the dead trees and canopy gaps the adelgid created.10Northeastern Naturalist. Effects of Hemlock Woolly Adelgid on Breeding Birds at Fort Indiantown Gap, Pennsylvania The picture is one of winners and losers: hemlock specialists decline while opportunistic species move in.
Below the canopy, hemlock die-off alters the chemistry of forest soils and streams. Research at five hemlock-dominated stands in southern Pennsylvania, spanning a range from no mortality to complete adelgid-induced death, found that soil nitrogen cycling rates were significantly higher at sites with heavy mortality. Stream water nitrate concentrations were lowest at the healthiest site and highest at sites with intermediate mortality, suggesting a pulse of nitrogen pollution as dead trees release stored nutrients. Over time, regenerating birch seedlings may begin absorbing some of that excess nitrogen, but for several years after an infestation, headwater streams face elevated nutrient loads that can alter aquatic communities downstream.11Castanea. Influences of Hemlock Woolly Adelgid–Induced Stand-Level Mortality on Nitrogen Cycling and Stream Water Nitrogen Concentrations in Southern Pennsylvania
The loss of hemlock shade along streams also raises water temperatures, which threatens cold-water species like brook trout. The connections are layered: an invasive insect kills a tree, which changes shade cover, soil chemistry, water temperature, and nutrient flow, which reshapes communities of fish, birds, insects, and plants. This is the kind of cascading disturbance that defines modern forest ecology in Pennsylvania.
Fire Suppression and the Shift Toward Shade-Tolerant Forests
Before European settlement, fire played a major role in maintaining Pennsylvania’s oak-dominated forests. Native Americans set fires to manage landscapes, and lightning-ignited fires were common on dry ridgetops. Oaks are well adapted to periodic burning: they have thick bark, resprout vigorously, and their seedlings tolerate the open, sunny conditions that follow a fire. But beginning in the 1920s, aggressive fire suppression policies effectively removed fire from eastern forests.
The ecological consequences have been profound. Without fire, open forests and woodlands converted to closed-canopy stands. A process researchers call “mesophication” took hold: as canopies closed and the forest floor became cooler, damper, and more shaded, conditions improved for shade-tolerant, fire-sensitive species like red maple, blackgum, and American beech, and deteriorated for sun-loving, fire-tolerant species like oaks and hickories. The feedback is self-reinforcing because the leaf litter of mesophytic species decomposes quickly and does not burn well, further reducing fire potential and favoring the very species that created the conditions.12BioScience. The Demise of Fire and “Mesophication” of Forests in the Eastern United States
In the Appalachian mixed-oak forests that cover much of Pennsylvania’s ridge-and-valley region, modern fire protection created what researchers describe as a “no-fire” regime. The forests recovered from heavy nineteenth-century logging, but the recovery favored mesophytic species that began replacing oaks in the understory.13Journal of Forestry. Bringing Fire Back: The Changing Regimes of the Appalachian Mixed-Oak Forests Today, many Pennsylvania forests have aging oak canopies with very few young oaks coming up beneath them. If the current trajectory continues, the next generation of forest in many areas will be dominated by maples and beeches rather than oaks, a transformation that would change everything from acorn availability for wildlife to fall foliage color.
Habitat Loss and Threatened Species
Pennsylvania’s total forest cover has remained relatively stable since the 1970s, which sounds like good news. But the picture is more complicated. While forest acreage has held steady, the extent of early successional habitats (young forests and shrublands), grasslands, and wetland and riparian areas has been declining. These trends, combined with direct human disturbance and water quality degradation, have contributed to the endangered or threatened status of 31 vertebrate species in the state.4BioOne (Northeastern Naturalist). Terrestrial Vertebrates in Pennsylvania: Status and Conservation in a Changing Landscape
Species that depend on grasslands, wetlands, or young regenerating forest are losing ground even as mature forest species may be holding steady. Eastern meadowlarks, American woodcock, and various grassland sparrows all require open habitats that are shrinking. Meanwhile, forest fragmentation in the southeastern part of the state, where suburban development is most intense, creates edges that favor nest predators and brown-headed cowbirds at the expense of interior forest birds. The conservation challenge in Pennsylvania is not simply about having enough forest; it is about maintaining the full range of habitat types that the state’s wildlife evolved to use.
Serpentine Barrens and Other Micro-Habitats
Pennsylvania’s biome classification as temperate deciduous forest applies broadly, but the state harbors pockets of habitat that look nothing like typical eastern forest. Among the most unusual are the serpentine barrens of southeastern Pennsylvania. These are open, grassy areas underlain by serpentine rock, which produces soils with unusual chemistry: high in magnesium and heavy metals, low in calcium, and generally hostile to most plants.
The result is a landscape that looks more like a prairie or savanna than a forest. Serpentine barrens support a distinctive plant community including species found almost nowhere else. Research on these outcrops found that native plants restricted to serpentine coexist with weedy invaders through contrasting survival strategies. The weedy species grow faster, produce seed earlier, and suffer less seedling mortality on normal soils. But the serpentine specialists persist because they tolerate the harsh soil chemistry that excludes most competitors.14Ecology. The Coexistence of Weeds and Restricted Native Plants on Serpentine Barrens in Southeastern Pennsylvania These barrens also support rare butterflies, grassland birds, and other species that have lost habitat elsewhere.
Other unusual habitats in Pennsylvania include bogs and fens in the glaciated northeast, where sphagnum moss, sundews, and other boreal plants persist as relics from the last ice age. Shale barrens on dry south-facing slopes in the ridge-and-valley region support drought-adapted plants more typical of the Great Plains. And the Lake Erie shoreline in the state’s northwestern corner hosts sand plain communities with their own distinct flora. These micro-habitats are tiny in total area but outsized in ecological significance, supporting species that cannot survive in the surrounding matrix of closed-canopy forest.
How Climate Change May Shift the Picture
The temperate deciduous forest biome depends on a specific climate envelope, and that envelope is shifting. Modeling of projected tree species distributions across protected areas in the eastern United States found that forest composition and species ranges could change substantially under all greenhouse gas emissions scenarios. The models showed stronger agreement when projecting where species would lose suitable habitat than where they might gain it, which means the losses are more certain than the potential expansions.15Ecosystems. Projected Tree Species Redistribution Under Climate Change: Implications for Ecosystem Vulnerability Across Protected Areas in the Eastern United States
For Pennsylvania specifically, warming temperatures could push the transition between northern hardwood and oak-hickory forests northward, shrinking the range of species like sugar maple and yellow birch within the state while expanding conditions favorable to oaks and hickories, as well as species currently centered farther south like tulip poplar and sweetgum. Hemlock, already struggling with the woolly adelgid, faces a double threat from warming temperatures that favor the insect and stress the tree. Whether the forests of northern Pennsylvania look more like Virginia’s in a century depends on how quickly the climate shifts and how well tree species can migrate or adapt, a process that unfolds over generations, not years.