Hemlock (white pine) Forest

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System: Terrestrial
Subsystem: Forest
PA Ecological Group(s): Appalachian – Northeast Mesic Forest

Global Rank:G4G5 rank interpretation
State Rank: S4

General Description

The Hemlock (white pine) Forest is characterized by over 75% combined relative cover of eastern hemlock (Tsuga canadensis) and white pine (Pinus strobus). Hardwoods may also be present up to 25% relative cover; common species include American beech (Fagus grandifolia), red maple (Acer rubrum), yellow birch (Betula allegheniensis), and black cherry (Prunus serotina). Shrubs such as rosebay (Rhododendron maximum) or witch-hazel (Hamamelis virginiana) can be present. The herb layer is typically sparse; common species are mountain wood sorrel (Oxalis montana), intermediate wood fern (Dryopteris intermedia), starflower (Trientalis borealis), Canada mayflower (Maianthemum canadense), teaberry (Gaultheria procumbens), and partridge berry (Mitchella repens). Ground pine is also common (Lycopodium obscurum, L. dendroideum, L. hickeyi).

Rank Justification

The Hemlock (white pine) Forest is widespread throughout Pennsylvania but tends to be a smaller patch within a larger hemlock or white pine mixed forest. Over 20,000 acres are mapped on public land; still only making up less than 1%. While the largest occurrence on public land is about 230 acres, the average size is less than 20 acres. This type is threatened by hemlock woolly adelgid (HWA; Adelges tsugae) and may transition to a separate forest type if infestation is severe.

Identification

  • Shrub and groundcover layers are typically sparse
  • Canopy dominated by hemlock and white pine with hardwood associates

* limited to sites with higher soil calcium
Vascular plant nomenclature follows Rhoads and Block (2007). Bryophyte nomenclature follows Crum and Anderson (1981).

International Vegetation Classification Associations:

USNVC Crosswalk:

Central Appalachian-Northeast Hemlock – Northern Hardwood Forest (A3302)

Representative Community Types:

Lower New England-Northern Piedmont White Pine - Hemlock Dry-Mesic Forest (CEGL006328)
Central Appalachian White Pine - Eastern Hemlock Forest (CEGL006019)

NatureServe Ecological Systems:

Appalachian (Hemlock) - Northern Hardwood Forest (CES202.593)

NatureServe Group Level:

Appalachian-Northeast Mesic Forest (G742)

Origin of Concept

Fike, J. 1999. Terrestrial and palustrine plant communities of Pennsylvania. Pennsylvania Natural Diversity Inventory. Harrisburg, PA. 86 pp.

Fleming, G. P. and Coulling, P. P. 2008. Central Appalachian White Pine - Eastern Hemlock Forest (CEGL006019). NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. (Accessed: February 22, 2022).

Neid, S. L. and Gawler, S. C. 2006. Lower New England-Northern Piedmont White Pine - Hemlock Dry-Mesic Forest (CEGL006328). NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. (Accessed: February 22, 2022).

Pennsylvania Community Code*

FF : Hemlock (white pine) Forest

*(DCNR 1999, Stone 2006)

Similar Ecological Communities

This type can be very similar to the Hemlock (white pine) - Northern Hardwood Forest type, which typically has more of a hardwood component (maple, birch, and beech with 25-75% relative cover) and less hemlock and white pine (reduced to 25-50% relative cover). The Hemlock (white pine) – Red Oak – Mixed Hardwood Forest is also similar but has more red oak and other hardwood species (25-75% relative cover) in the canopy and subcanopy, and often supports mountain laurel (Kalmia latifolia) in the shrub layer.

Fike Crosswalk

Hemlock (white pine) Forest

Conservation Value

Very few state-listed plants are found in Hemlock (white pine) Forests. However, a number of breeding birds prefer hemlock or white pine-dominated forests, including black-capped chickadee (Poecile atricapillus), blue-headed vireo (Vireo solitaries), black-throated green warbler* (Setophaga virens), blackburnian warbler* (Setophaga fusca), brown creeper* (Certhia americana), Canada warbler* (Cardellina canadensis), dark-eyed junco (Junco hyemalis), hermit thrush (Catharus guttatus), magnolia warbler (Setophaga magnolia), northern flicker (Colaptes auratus), Swainson’s thrush* (Catharus ustulatus), winter wren* (Troglodytes hiemalis), and wood thrush* (Hylocichla mustelina; Sargent et al., 2017). *SGCN species

Threats

Eastern hemlock forms a significant component of the Hemlock (white pine) Forests in Pennsylvania. Hemlock wooly adelgid (HWA; Adelges tsugae) has resulted in significant hemlock mortality, and continued loss of hemlock could drastically alter the distribution and abundance of Hemlock (white pine) Forests in Pennsylvania. Eastern hemlocks are also afflicted by the elongate hemlock scale (Fiorinia externa). This exotic pest tends to co-occur with HWA and has been found to cause tree death within 10 years (McClure, n.d.). Additionally, white pine is affected by white pine blister rust (Cronartium ribicola), which causes stem and branch cankers that result in tree mortality (Iverson et al., 2008). It is currently spread throughout Pennsylvania. Widespread hemlock mortality could result in replacement by white pine in some forests, or a transition to mixed deciduous-conifer forest. Hemlock (white pine) Forests could experience significant changes to the composition or distribution in a changing climate. Some authors have suggested that species that are currently dominant in our northern hardwood forests spectrum, such as eastern hemlock, sugar maple, red maple, or black cherry, could become less common in both high and low emissions scenarios (Iverson et al., 2008). With component conifer and hardwood species potentially moving north or being eliminated by invasive pests, Hemlock (white pine) Forests could become less common in a changing climate. Invasive plant species that threaten Hemlock (white pine) Forests include garlic mustard (Alliaria petiolata) and Japanese barberry (Berberis thunbergii), among others. Throughout much of its range, Hemlock (white pine) Forests have experienced forest fragmentation from roads, shallow gas development, and habitat conversion. Furthermore, part of the range of the Hemlock (white pine) Forest is found in the shale gas region in Pennsylvania. Development of the shale gas region may result in additional fragmentation throughout the range in Pennsylvania and could negatively impact forest interior birds who depend on this conifer-dominated community type.

Management

Hemlock trees infested with HWA can be treated, but treatment is expensive (Forest Health Fact Sheet: Hemlock Woolly Adelgid, n.d.). As with any terrestrial forest management, forest conditions at a given location may require a specific plan to achieve management goals. Private landowners can consult with service foresters at the Pennsylvania Bureau of Forestry for more information on sustainable forest management practices that are best suited for their property. Management of invasive species is encouraged. Also, management of white-tailed deer could improve hardwood regeneration in forests experiencing high levels of herbivory.

Research Needs

Potential range shifts (north) for hemlock, sugar maple, birch, and beech, all of which are components of the Hemlock (white pine) Forest, could result in a change in distribution and abundance of this community type in a changing climate (Bauer et al., 2016; Shortle et al., 2015). It is possible that oaks could replace some hardwood species in a changing climate and the role of oaks in Hemlock (white pine) Forests should be assessed in contemporary examples of this community type.

Trends

Hemlock (white pine) Forests account for less than 1% of mapped forested acreage on public lands and the majority of this acreage is found in the North Central Appalachians. There are some historical data on forest composition that have been compiled from witness trees/surveys. Lutz (1930; see also Marquis, 1975) noted that eastern hemlock had a strong presence in Allegheny National Forest around the time of European settlement. Marquis (1975) suggests white pine forests (which would be similar to the Hemlock (white pine) Forest classification) occurred in small patches across the landscape, and were less common than hemlock-beech forests (similar to our Hemlock (white pine) – Northern Hardwood Forest classification). Other authors (Day, 1953; Lobeck, 1927) have suggested that white pine-dominated forests were most common in abandoned clearings and in river terraces where sandy soil was common, and it is possible that the Hemlock (white pine) Forest was a climax community; because it rarely established with periodic fire or other disturbances.

Range Map

range map

Pennsylvania Range

The Hemlock (white pine) Forest is found in the following USEPA Level III (Level IV) Ecoregions: Northern Allegheny Plateau (60), Erie Drift Plain (61), North Central Appalachians (62), Blue Ridge (66), Ridge and Valley (67), Central Appalachians (69) and Western Allegheny Plateau (70).

Global Distribution

Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Ohio

Bauer, A., Farrell, R., & Goldblum, D. (2016). The geography of forest diversity and community changes under future climate conditions in the eastern United States. Ecoscience, 23, 41-53.

Day, G. M. (1953). The Indian as an ecological factor in the northeastern forest. Ecology, 34, 327-346.

Forest Health Fact Sheet: Hemlock Woolly Adelgid. (n.d.). Pennsylvania Department of Conservation & Natural Resources. https://elibrary.dcnr.pa.gov/GetDocument?docId=1738100&DocName=sf-FrstHlthFactSheet-HWA.pdf

Iverson, L. R., Prasad, A. M., Matthews, S. N., & Peters, M. (2008). Estimating potential habitat for 134 eastern US tree species under six climate scenarios. Forest Ecology and Management, 254(3), 390-406.

Lobeck, A. K. (1927). A popular guide to the geology and physiography of Allegany State Park. New York State Museum Handbook.

Lutz, H. J. (1930). Original forest composition in northwestern Pennsylvania as indicated by early land survey notes. Journal of Forestry, 28(8), 1098-1103.

Marquis, D. A. (1975). The Allegheny hardwood forests of Pennsylvania. Gen. Tech. Rep. NE-15. Upper Darby, PA: US Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 32 p., 15.

McClure, M. S. (n.d.). Elongate Hemlock Scale (NA-PR-01-02; Pest Alert). United States Department of Agriculture Forest Service. https://www.forestpests.org/acrobat/EHScale.pdf

Sargent, S., Yeany III, D., Michel, N., & Zimmerman, E. (2017). Forest Interior Bird Habitat Relationships in the Pennsylvania Wilds, Final Report for WRCP-14507. Audubon Pennsylvania, National Audubon Society.

Shortle, J., Abler, D., Blumsack, S., Britson, A., Fang, K., Kemanian, A., Knight, P., McDill, M., Najjar, R., Nassry, M., Ready, R., Ross, A., Rydzik, M., Shen, C., Wardrop, D., & Yetter, S. (2015). Pennsylvania Climate Impacts Assessment Update. The Pennsylvania State University, University Park.

Cite as:
Braund, J., E. Zimmerman, A. Hnatkovich, and J. McPherson. 2022. Pennsylvania Natural Heritage Program. Hemlock (white pine) Forest Factsheet. Available from: https://naturalheritage.state.pa.us/Community.aspx?=16066 Date Accessed: April 24, 2024

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