High Allegheny Plateau Ecoregion

Description & Physiography

Because these areas are determined by natural terrain, ecology, and conservation priorities, many ecoregions do not adhere to state boundaries, and the High Allegheny Plateau (HAL) is no exception. The ecoregion straddles the New York (51.6%) and Pennsylvania (47.8%) border with the remaining 0.6% falling in the northwest corner of New Jersey. The area extends over 16.9 million acres, north from the Great Lakes Plains of Lake Ontario to the Ridge and Valley region of the Central Appalachians to the south, and from the Lake Erie Plain in the west to the southeastern Pennsylvania lowlands and the Hudson River Valley at its easternmost point.  Well known high elevations in the region include the Catskills, The Shawangunks, The Kittatinny Ridge, and The Pocono Mountains.

The face of HAL is influenced most by major rivers – the Delaware, Allegheny, and Susquehanna notably -- and hundreds of small streams that have, over time, cut a series of rounded, forested hills from an enormous plateau.  The northern and eastern parts of HAL are characterized by a full range of glaciated features including end moraine, eskers, drumlins, kame terraces, and kettleholes – all geologic forms associated with the terminus of the ice sheet advance and deposits associated with glacial meltwater flow. In the south-central and western portions of HAL, where glaciation did not occur, there are few natural lakes and ponds, and the land is made up of older eroded features and remnant bedrock exposes.



The climate of the ecoregion is characteristic of high elevation areas in the mid-Atlantic region with hot, humid summers and cold winters with moderate snowfall. Lake-effect snow off Lake Erie occasionally extends into the extreme western part of the ecoregion. There is usually a continuous cover of snow throughout the winter. Characteristic of the East, there are periodic droughts that occur principally in the summer and can have profound impacts on vegetation and aquatic systems. The hills do create some rain shadow effect with higher levels of rainfall in the western hills and the west slopes of the Catskills The growing season is shorter than in surrounding areas because of the general elevation effect on temperature – such a truncated time also influences species distribution.


Plants & Animals

The pre-colonial forest of HAL was vast and nearly continuous across the ecoregion with woodlands on dry ridgetops in the east and some open communities along major rivers with floods and ice scour. The low mountains in the west were all nearly consistently covered with a dense canopy.

Today, the High Allegheny Plateau has the highest percentage of natural cover (81%) of any Northeastern ecoregion besides the Northern Appalachian Ecoregion. Deciduous forest covers 52% of HAL; 21% is covered by Mixed forests: coniferous forests cover 6%; and only 0.7% of the ecoregion is covered by wetlands.  Agricultural uses account for 18% of HAL. Dairy farms are the principle agricultural use with row crops fields limited to floodplains. Only 1% of the ecoregion is covered by residential and urban development, industry, and transportation corridors.The dominant vegetation type of HAL is Beech-maple forest in lower elevation mesic sites and Appalachian oak on drier sites. Oak-hickory occupies many south-facing, dry slopes. In the eastern part of the ecoregion, pine barrens occur on rocky ridgetops and on the Pocono Plateau. Richer forests occur in the southwest part of the ecoregion with Liriodendron and Magnolia in more mesic sites. Spruce fir occurs at high elevation sites in the Catskills.

Most of the rare and significant animals that characterize HAL are associated with the major rivers. A high diversity of mussels, fish and dragonflies occur related to different drainages, including mid Atlantic coast and the Mississippi and to large remnant forest. Woodrats are scattered through steep rocky sections of the east with large talus slopes. Timber rattlesnakes are also common in these areas. Bog turtles are found at several locations in the southeast part of the ecoregion in remnant wetland complexes. Significant birds include Bicknell’s thrush in the Catskills, Cerulean warbler and Swainson’s thrush in floodplain corridors and grassland nesting birds in old fields and at sites owned by public agencies such as airports. Bear and bobcat are common over most of the ecoregion. Deer are abundant throughout.  Elk have been reintroduced to western Pennsylvania and are expanding in number. Five federally listed animal species occur in HAL: Peregrine falcon, Bald eagle, Dwarf wedgemussel, Bog turtle, and Indiana bat.


Humans & History

In the early 1700s, small European settlement farms were established in the narrow fertile valleys with expansion up hillsides nearest the Hudson River and spread slowly into nearby areas over the next 50 years. After the Revolutionary War and the reduction in hostilities from Indian, settlement expanded up the major river corridors beginning with the Delaware and extending south from the Mohawk.

The principal industry for most of the 19th century was logging to feed the expansion needs of major East Coast cities. Canals were constructed along river corridors to facilitate transport of raw materials. Later railroads were built linking the timber resources of the West to Eastern markets. Most of central and western part of the ecoregion in NY and the north-central part of Pennsylvania were eventually cleared and used for farming. The areas with poorest soils were soon abandoned. Most forests in these areas are now second or third growth.

Settlement of the western part of the ecoregion in Pennsylvania was much later and never as widespread, limited only to the narrow river valleys. In these areas, the forest was cleared, but quickly grew back. In the 1850s, oil was discovered along Oil Creek in the far western portion of HAL, beginning extensive exploitation of oil and gas deposits in the western sections of PA and NY. Many of these wells remain in operation today and have shaped the development of roads and pollution impacts on streams. Extensive sections of Western PA are recovering strip mines. Coal was also dug from many of the areas in Western PA, although the major coal fields lie just outside of HAL.



Population concerns will impact the High Allegheny Plateau in the near-term and future. In the highly-populated Northeast, HAL has a noticeable lack of big cites and associated suburbs. From Landsat imagery at 65,000 feet at night, the ecoregion is defined as being the dark area surrounded by intense development. The largest city is Binghamton, NY with 47,000 people. Other cities include Elmira, Corning, and Johnson City, NY, and Bradford and Warren, PA. A total of 1,773,000 people live in HAL, a density of 61 people per sq. mile on average, second only to Northern Appalachian ecoregion in low population density. That said, the population of the whole ecoregion is projected to increase by 38% over the next 50 years with most growth occurring in the southeast and eastern parts of the ecoregion and minor decreases occurring elsewhere. Extensive tracts of managed areas are scattered in all parts of HAL. Nearly 20% of the total acreage of the ecoregion is held by public agencies and private organizations with a conservation mission.*

* Much of the Humans & History and Today sections sourced from http://conserveonline.org/library/HAL_070130.pdf

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