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Ecoregions of Pennsylvania

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About the Ecoregions of Pennsylvania

Pennsylvania spans many ecoregions, driven mostly by its diverse geology. Two key features are the northern limit of the recent Wisconsin glaciation, and the different layers of the Appalachians. These borders drive major changes both in natural vegetation and land use patterns, both due to changes in elevation and topography, and soil type. In addition to ecoregions, the state spans major watershed divides, with most of the state draining into the Chesapeake bay, mostly through the Susquehanna river, the east draining into the Delaware river, the northwest into lake Erie, and the rest of the west into the Ohio and then Mississippi rivers.

Most of the state has a humid continental climate, although the southern parts of the state, except at higher altitudes, are towards the cooler end of a humid subtropical climate. There is little seasonality of precipitation, except for the coldest areas which have slightly more precipitation in summer. The Appalachians are tall enough to produce changes in precipitation and temperature as a function of elevation, but not tall enough to completely redirect or block major weather systems as the Rocky Mountains do. Lake Erie influences the climate in the far northwest, and the southeast is subject to some weakened coastal storms, but most of the state's weather is driven by air masses moving across land.

Broadly, most of the state is contained within the Eastern Temperate Forests. The coldest, higher-elevation regions in the north and east of the state are considered part of the Northern Forests. Most of the state is covered in one of four forest types. Oak-hickory forest, including Appalachian oak forest, is the state's primary forest type, covering most of the lower two-thirds of the state. Northern hardwood forest dominates the northern third of the state, and extends farther south at high elevations. Great Lakes beech-maple forest is found in the northwest of the state, and mesophytic forest is found in a few of the southernmost parts of the state. In addition to these major forest types, there are numerous other plant communities existing on specific sites.

The temperate forests of the state are divided into three general sections: the Mixed Wood Plains in the northwest and northeast, the Appalachians, covering the largest area of the state, most of southwest and central PA, and the Southeastern USA Plains in the southeastern part of the state, specifically the Northern Piedmont. The Northern forests section is divided into the North Central Appalachians, which includes a separate portion for the Poconos, and the Northeastern Highlands, specifically the Reading Prong.

Much of the state lives within the Ridge and Valley system of the Appalachians. Entering the state from the south, this region is oriented more north-south, and then curves to be oriented more east-west as it extends through the rest of the state. This region has experienced significant folding and faulting, leading the different substrates to be fragmented, reflected in the patchwork distribution of regions on the map. The varying erodibility of substrates leads to changes in elevation, topography, soil type, and the presence of coal and other minerals. The geology of these regions explains much of regional differences in population, agriculture, and economic base. Farming and settlements are concentrated in the fertile Northern Limestone/Dolomite Valleys, and to a lesser extent the Northern Shale Valleys, and these valleys are separated by the more rugged Northern Sandstone Ridges and Northern Dissected Ridges and Knobs, where most of the land is forested.

The highest elevations in the state are reached in the Forested Hills and Mountains of the Central Appalachians, a region that reaches its northernmost end in PA, and is mostly located farther south and west. The highest point, Mount Davis, reaches 3,213ft (979m.)

Some regions are mostly or entirely contained within Pennsylvania, including the coal-rich and industrially-developed Anthracite Subregion, and the fertile farmlands of the Piedmont Limestone/Dolomite Lowlands. The Pittsburgh Low Plateau makes up much of western PA, and the Unglaciated High Allegheny Plateau covers a large area farther northeast. Although it extends well outside the state, the largest continugous area of the Trap Rock and Conglomerate Uplands are also located in PA, where they contain one of the largest unbroken pieces of forest in the region.

The four corners of the state each are home to regions that extend well outside the state. In the northwest, at the limit of glaciation, the Erie Drift Plain extends up until the narrow Erie Lake Plain. The northeastern part of the state contains part of the Northern Allegheny Plateau. The southwest corner has part of the Permian Hills and the surrounding Monongahela Transition Zone. And the southeast corner has various regions of the Northern Piedmont, and at the very edge of the state, the Delaware River Terraces and Uplands, which marks the beginning of the Mid-Atlantic Coastal Plain.

A few other important regions extend slightly into the state. In south-central PA, the Blue Ridge reaches its northern end near South Mountain. In the east-central part of the state, the glaciated portions of the Ridge and Valley system begin, extending into New Jersey. In the north, the Glaciated Allegheny Hills also extend slightly into PA.


1. Woods, A.J, Omernik, J.M., Brown, D.D. "Level III and IV Ecoregions of Delaware, Maryland, Pennsylvania, Virginia, and West Virginia", U.S. Environmental Protection Agency: National Health and Environmental Effects Research Laboratory, Corvallis, OR (1999) Web.