Pennsylvania’s climate, rainfall, and soil fertility support forest growth throughout most of the state with the exception of areas that are too wet or too rocky. The major forest types are:
- Northern hardwood forest
- Oak-hickory forest
- Great Lakes beech-maple forest
- Mesophytic forest
Northern Hardwood Forest
The northern hardwood forest occupies the northern third of the state and extends south at high elevations along the Allegheny Front. It also occurs farther south on north-facing slopes and cool, moist ravines.
This forest type is characterized by a mixture of hardwoods and conifers and usually contains:
- Beech
- Birch
- Sugar maple
- Canadian hemlock
- White pine
Wild black cherry reaches its best development in this zone, especially in the northwestern part of the state.
Understory trees typically include:
- Moosewood
- Witch-hazel
- Mountain holly
- Shadbush
Oak-Hickory Forest
Oak forests dominate the southern two-thirds of the state.
Oak forests include red oak-mixed hardwood type on lower slopes where red and white oaks occur mixed with:
- Tuliptree
- Red maple
- Hickories
On drier upper slopes and ridge tops throughout the central Pennsylvania, oak forests dominated by white, black, and chestnut oak are common.
These forests often have a dense layer of shrubs such as mountain laurel and black huckleberry.
Before 1910, American chestnut was an important component of Pennsylvania’s dry oak forests, but the accidental introduction of chestnut blight in New York City in 1904 resulted in chestnut’s shift from widespread canopy dominance to minor status within just a few decades.
Great Lakes Beech-Maple Forest
The Great Lakes beech-sugar maple forest is represented at the western end of the state. The mixed mesophytic forests, which reach their greatest development in the Smoky Mountains, just reach southern Pennsylvania.
These forests contain:
- Tuliptree
- Sugar maple
- Beech
- Basswood
- Red oak
- Cucumber-tree
- Yellow buckeye
- Ohio buckeye
- White ash
- Black cherry
Understory trees include:
- Flowering dogwood
- Pawpaw
- Umbrella-tree
- Redbud
- >Witch-hazel
The herbaceous layer is very rich and diverse.
Mesophytic Forest
In the southeastern corner of the state, in a narrow sliver of the Atlantic Coastal Plain physiographic province that parallels the Delaware River, coastal plain forests contain:
- Sweetgum
- Willow oak
- Southern red oak
- Sweetbay magnolia
In the northeastern and northwestern corers of the state, in areas covered by ice during the most recent glaciation, peat deposits support forests with a northern character dominated by black spruce and tamarack.
Serpentinite rock, which occurs in a band of outcrops stretching across southern Delaware, Chester and Lancaster counties, supports forests of pitch pine or Virginia pine, coupled with:
- Red-cedar
- Scrub oak
- Blackjack oak
- Sassafras
Shale barrens and limestone barrens of the Ridge and Valley physiographic province contain drought-tolerant species including:
- Eastern red-cedar
- Virginia pine
- Table Mountain pine
- Yellow oak
- Post oak
- Hackberry
- Sumac
Riparian areas throughout the state, where periodic flooding is a limiting factor are characterized by:
- Sycamore
- Silver maple
- Box-elder
- American elm
- Slippery elm
- Black willow
- Green ash
- Black ash
- Black walnut
- Red maple
River birch is common along rivers and streams in the eastern part of the state but rare in the west. Swamp forests along Lake Erie are the only locations where pumpkin ash occurs.