Mixed Mesophytic Forest

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System: Terrestrial
Subsystem: Forest
PA Ecological Group(s): South-Central Interior Mesophytic Forest

Global Rank:G4 rank interpretation
State Rank: S1S3

Mixed Mesophytic Forest, Pigeon Creek, Washington County
Mixed Mesophytic Forest, Pigeon Creek, Washington County, PNHP

General Description

This community is a rich community type that typically occurs on deep soils in protected concave coves or at a lower slope position. Dominant trees include tuliptree (Liriodendron tulipifera), sugar maple (Acer saccharum), American beech (Fagus grandifolia), basswood (Tilia americana), northern red oak (Quercus rubra), cucumber tree (Magnolia acuminata), wild black cherry (Prunus serotina), white ash (Fraxinus americana), black walnut (Juglans nigra), shagbark hickory (Carya ovata), and yellow buckeye (Aesculus flava). Eastern hemlock (Tsuga canadensis) may occur in these forests, but is not characteristically a dominant. Shrubs include pawpaw (Asimina triloba), bladdernut (Staphylea trifolia), redbud (Cercis canadensis), spicebush (Lindera benzoin), wild hydrangea (Hydrangea arborescens), and witch-hazel (Hamamelis virginiana). The herbaceous flora is extremely rich and includes such species as white trillium (Trillium grandiflorum), wake-robin (T. erectum), toadshade (T. sessile), yellow trout-lily (Erythronium americanum), wild blue phlox (Phlox divaricata), wood anemone (Anemone quinquefolia), squirrel corn (Dicentra canadensis), dutchman's-breeches (D. cucullaria), speckled wood-lily (Clintonia umbellulata), black snakeroot (Actaea racemosa), wood geranium (Geranium maculatum), blue cohosh (Caulophyllum thalictroides), foamflower (Tiarella cordifolia), liverleaf (Hepatica nobilis), maidenhair fern (Adiantum pedatum), wild leek (Allium tricoccum), bloodroot (Sanguinaria canadensis), yellow fumewort (Corydalis flavula), rattlesnake fern (Botrychium virginianum), spring-beauty (Claytonia virginica), cut-leaved toothwort (Cardamine concatenata), bishop's-cap (Mitella diphylla), goldenseal (Hydrastis canadensis), ginseng (Panax quinquefolius), twinleaf (Jeffersonia diphylla), and wild ginger (Asarum canadense). Most of these systems have a complete, or nearly complete, annual litter turnover.

The Mixed Mesophytic Forest is somewhat broadly defined in Pennsylvania. This broad type can be further divided into three sub-types following regional and physiographical factors such as soils, elevation, and landform, which drive variation in species composition. The most common sub-type of this forest is found in the southern western Pennsylvania south of the limit of the Wisconsin era glaciation and follows the general pattern of the Mixed Mesophytic Ecoregion described by Braun (Braun 1950; Muller 1982; Dyer 2006). A richer version of this forest is found in southern Greene County on steep mid to lower slopes of Ohio River basin tributaries such as Dunkard Creek, Wheeling Creek, and Enlow Fork. A third Mesophytic forest can be found within Pennsylvania’s Piedmont Ecoregion on rich mesic lower slopes. These divisions reflect association-level community descriptions in the National Vegetation Classification.

Rank Justification

The three subtypes/variations are tracked by the PA Natural Heritage Program separately; their SRanks are based on the range and number of occurrences, condition of occurrences, and threats.

Mixed Mesophytic Forests of the Western Allegheny Plateau are considered vulnerable due to a restricted range, relatively few occurrences (often 80 or fewer), recent and widespread declines, or other factors making it vulnerable to extirpation.

The very rich Mixed Mesophytic Forest, limited to mid-to lower slopes in Southwestern Pennsylvania (Greene and Washington Counties) and is considered critically imperiled because of extreme rarity (often 5 or fewer occurrences). There are two occurrences of this type in the PA Natural Heritage Program database attributed to CEGL008412. CEGL005222 is more common. Tributaries of Dunkard Creek and Wheeling Creek in Greene County should be investigated for more occurrences of this community type.

The Mixed Mesophytic Forest subtype occurring in the Piedmont Ecoregion is considered vulnerable due to a restricted range, relatively few occurrences (often 80 or fewer). There exists very little large patches of forest land within the range of the Mixed Mesophytic Forest community in the Piedmont of Pennsylvania and historic logging, surface mining, farming, and development has greatly reduced the historic extent of this type in the state. High quality examples of this community type are very rare.

Identification

  • This type can be found in the southern Western Allegheny Plateau, Allegheny Mountains, and Piedmont regions of Pennsylvania.
  • Very rich examples are limited to mid-to lower slopes in Southwestern Pennsylvania, and in protected calcium-rich coves in the Laurel Highlands.
  • Described in Braun’s Mixed Mesophytic Forest Region (Braun 1950; Dyer 2006).
  • High diversity of overstory tree species. Yellow buckeye (Aesculus flava) is a canopy associate.
  • High cover and diversity of herbaceous layer, especially vernal flora. American ginseng (Panax quinquefolius), goldenseal (Hydrastis canadensis), twinleaf (Jeffersonia diphylla) and ramps (Allium tricoccum, A. burdickii), are associated with this forest type
  • Soils are generally deep loamy calcareous colluvium - with a pH greater than 6.0. In the richest examples of this type, soils range in pH from 7.5-8.0.
  • This community type is dominated by mesophytic hardwood tree species. Forests greater than 25% cover of eastern hemlock (Tsuga canadensis), are classified as the Rich Hemlock – Mesic Hardwoods Forest.

* 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:

None

Representative Community Types:

Northern Mixed Mesophytic Forest (CEGL005222)
Central Appalachian-Piedmont Rich Cove / Mesic Slope Forest (Twinleaf - Blue Cohosh Type) (CEGL008412)
Upper Piedmont-Northern Blue Ridge Basic Mesic Hardwood Forest (CEGL006816)
Central Appalachian Rich Cove Forest (CEGL006237)
Central Appalachian Rich Cove Forest (Tuliptree - Northern Red Oak - Cucumber-tree Type) (CEGL008510)

NatureServe Ecological Systems:

South-Central Interior Mesophytic Forest (CES202.887)

NatureServe Group Level:

Appalachian - South-central Interior Mesic Forest (G020)

Origin of Concept

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

Pennsylvania Community Code*

MM : Mixed mesophytic forest

*(DCNR 1999, Stone 2006)

Similar Ecological Communities

There is considerable overlap between the Mixed Mesophytic Forest associations and the more commonly occurring Northern Hardwood Forest, Sugar maple – Basswood Forest, and Tuliptree – Beech – Maple Forest. The Mixed Mesophytic Forest differs from these and other hardwood forest types in the diversity of the canopy and richness of the spring flora, many of which have an affinity to calcareous geology. The abundance of yellow buckeye and pawpaw is often a key indicator, along with the species-rich vernal flora. There is considerable overlap between the Mixed Mesophytic Forest and Rich Hemlock – Mesic Hardwood Forest, which also is found on lower slopes and coves of creeks and rivers in Southwestern Pennsylvania. The Mixed Mesophytic Forest community type is dominated by hardwood tree species. Forests greater than 25% cover of eastern hemlock (Tsuga canadensis), are classified as the Rich Hemlock – Mesic Hardwoods Forest. The Sugar Maple – Basswood Forest is less species-rich than this type, has a much broader range, and soils are considerably more acidic (pH 5.5-6.5). Outside of the Western Appalachian Plateau, forests with prevalence of tuliptree, American beech, white ash, sugar maple, and basswood occurring may be more correctly typed as Sugar Maple – Basswood Forest, particularly belonging to the USNVC’s CEGL006237. Less diverse forest communities with less than 25% cover of eastern hemlock most likely belong to the Northern Hardwood Forest community type.

Fike Crosswalk

Mixed Mesophytic Forest

Conservation Value

Occurring from the western portion of the Mid-Atlantic south to Tennessee, this community is more common outside Pennsylvania – where it is limited to southern Western Pennsylvania (Allegheny, Fayette, Washington, Beaver, Butler, Indiana, and Greene Counties). The richest, most diverse examples of this type occur in protected concave topographic positions. This community type supports a number of rare and vulnerable plant species and the community itself is also an element of conservation concern (S1). There are only two occurrences of this type in the PA Natural Heritage Program database. Neither site is considered “old growth,” but similar to sites in West Virginia, it has recovered well and supports a diversity of large overstory trees due to high site productivity. Other sites in Pennsylvania may support this community. The forests of Greene and Washington Counties are a mosaic of low-density residential development, mine-lands, and agriculture, many small patches of lower slope cove forest in a somewhat precarious position as they are somewhat disconnected from other patches. This is also true for Mixed Mesophytic Forests of Pennsylvania’s Piedmont. This forest type is much more common to the south, and forest patches exist in a fragmented landscape of farmland and development.

Threats

There exists very little protected land within the range of the Mixed Mesophytic Forest community in Pennsylvania and much of the remaining acreage which is formally protected is not of high quality. Most stands represent second-growth forests in some stage of recovery and a majority of the higher quality sites remain threatened by future timber harvests. Timber harvest activities disturb the rich forest floor, disrupt the canopy, and introduce invasive plants. Invasive plant species, including garlic mustard (Alliaria petiolata) and tree of heaven (Ailanthus altissima), and others, are the most serious threat to this forest type. Climate change may also impact Mixed Mesophytic Forests. Several species that are currently present in the overstory, such as sugar maple, eastern hemlock, or black cherry, could become less common in future climate scenarios (Peters et al. 2020, Shortle et. al 2015 ). Forest pests/pathogens have already impacted tree species in this community type, including emerald ash borer (EAB; Agrilus planipennis), hemlock wooly adelgid (HWA; Adelges tsugae), and beech bark disease. The Asian long-horned beetle (Anoplophora glabripennis), while not known from this region, has been known to infest many of the overstory trees of this forest type and may decimate the hardwood forests of this region.

Overharvesting of American ginseng, goldenseal, and ramps has been cited as a threat to Mixed Mesophytic Forest patches on rich lower slopes. Soil disturbance and introduction of invasive pests, such as jumping worms (Amynthas agrestis) from harvesting wild plants may further impact the quality of Mixed Mesophytic Forest occurrences.

Management

The steep lower slopes and coves that support this forest type are often protected by the relative difficulty of access. However, timber harvesting activities should be limited in forest stands of this type because of the steepness of the slope and the potential for further spreading invasive plants. Management of invasive plant species is encouraged. Also, management of white-tailed deer will limit over-browsing.

Research Needs

This forest type is at the northern and eastern end of its range; it is considerably more abundant in West Virginia, Ohio, Kentucky, Maryland, Virginia, and Tennessee. More inventory needs to take place to identify and map occurrences of this type in Greene County, Pennsylvania, as there are most likely more than the two occurrences in Washington and Greene Counties. More sites in these counties, along with Allegheny and Fayette Counties should be investigated; however, this type has long been a target of the PA Natural Heritage Program’s County Inventory and targeted surveys have failed to find this type in other southwestern counties. Forests on State Game Lands (SGL 179, 233, 138, 238, and 302) mapped as Mixed Mesophytic Forest should be investigated to confirm the presence of high-quality examples of this type on state-managed land in the region; it is currently not recognized as occurring on State Forest land. Tuliptree – Beech – Maple and Sugar Maple – Basswood Forests in the Forbes State Forest and William Penn State Forests should be surveyed to determine if these stands may be re-typed as Mixed Mesophytic.

Trends

The historic extent of the Mixed Mesophytic Forest in Pennsylvania is unclear. However, the community was most likely more common than it is today, having endured considerable logging of forest stands and conversion of the more moderate slopes to agriculture, pasture, and development. Coal mining likely would have had a considerable impact to this type as large areas. Higher quality examples of this type are limited to the steep coves and lower slopes where clearing was limited and where site conditions were rich enough for the forest to easily recover from logging. Much of the forests on State Game Lands in Greene County are mapped as Mixed Mesophytic; however, much of these forests are of poor quality due to the poor quality of the canopy and presence of invasive plants. There are no stands of Mixed Mesophytic Forest situated on lands managed by DCNR Bureau of State Parks or Bureau of Forestry in the region. Tuliptree – Beech – Maple and Sugar Maple – Basswood Forests in the Forbes State Forest and William Penn State Forests should be surveyed to determine if these stands may be re-typed as Mixed Mesophytic.

Range Map

range map

Pennsylvania Range

Mixed Mesophytic Forests of the Western Allegheny Plateau (CEGL005222) are found in the USEPA’s Western Allegheny Plateau Ecoregion (USEPA Level III Ecoregion 70)

The very rich Mixed Mesophytic Forest (CEGL008412), limited to mid-to lower slopes in Southwestern Pennsylvania (Greene and Washington Counties) in USEPA’s Western Allegheny Plateau Ecoregion (USEPA Level III Ecoregion 70) (PNHP Data)

The Mixed Mesophytic Forest subtype occurring in the Piedmont Ecoregion (CEGL008510) is limited to USEPA’s Northern Piedmont Ecoregion (USEPA Level III Ecoregion 70)

Global Distribution

This type is found primarily in the Western Allegheny Plateau, Cumberland Plateau, and Northern Piedmont regions of the United States, ranging from southern Pennsylvania and eastern Ohio south to West Virginia and Tennessee, with outliers in Indiana (USNVC 2019).

Braun EL. 1950. Deciduous forests of eastern North America. Blakiston, the University of California.

Dyer JM. 2006. Revisiting the Deciduous Forests of Eastern North America. BioScience 56:341.

Fike J. 1999. Terrestrial & palustrine plant communities of Pennsylvania. Available from http://www.naturalheritage.state.pa.us/Uplands.aspx (accessed April 16, 2018).

Muller RN. 1982. Vegetation Patterns in the Mixed Mesophytic Forest of Eastern Kentucky. Ecology 63:1901.

Peters MP, Prasad AM, Matthews SN, Iverson LR. 2020. Climate change tree atlas, Version 4. U.S. Forest Service, Northern Research Station and Northern Institute of Applied Climate Science, Delaware, OH. Available from https://www.fs.fed.us/nrs/atlas/tree/.

USNVC. 2019. United States National Vegetation Classification Database, V2.03. Federal Geographic Data Committee, Vegetation Subcommittee, Washington DC. Available from http://usnvc.org/.

Cite as:
Braund, J., E. Zimmerman, A. Hnatkovich, and J. McPherson. 2022. Pennsylvania Natural Heritage Program. Mixed Mesophytic Forest Factsheet. Available from: https://www.naturalheritage.state.pa.us/Community.aspx?=16057 Date Accessed: May 22, 2024

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