Overstory Tree Composition of Eastern Hemlock Stands Threatened by the Hemlock Woolly Adelgid at Delaware Water Gap National Recreation Area

Overstory Tree Composition of Eastern Hemlock Stands Threatened by the Hemlock Woolly Adelgid at Delaware Water Gap National Recreation Area

Mahan, Carolyn


We described overstory tree age and composition among hemlock stands that varied in physiographic structure at Delaware Water Gap National Recreation Area located in northeastern Pennsylvania and northwestern New Jersey. Hemlock stands were comprised primarily of eastern hemlock (Tsuga canadensis), chestnut oak (Quercus prinus), black birch (Betula lento), Eastern white pine (Pinus strobus), and white oak (Quercus alba). Forest stands characterized by steep slopes and high gradients supported the highest basal area of hemlock. Trees within hemlock stands ranged in age from 68.0-148.0 years. These data are valuable because as eastern hemlocks decline throughout the mid-Atlantic region of the United States due to infestation of the exotic insect pest, the hemlock woolly adelgid (Adelges tsugae), hardwood species will likely become more prominent in these stands in the future. The documentation and description of tree composition of a threatened ecosystem prior to its decline is important for future restoration efforts and potential tree replacement in forest stands.


The eastern hemlock [Tsuga canadensis (L.) Carr] is a slow-growing, shade-tolerant late-successional conifer that provides a unique cover type in the eastern forest of the United States (Rogers 1978, Burns and Honkala 1990, Orwig and Foster 1998). The largest of these trees can reach nearly 200 cm in diameter, exceed fifty meters in height, and live up to 800 years (Burns and Honkala 1990). At its northern boundaries, the range of eastern hemlock extends from northern Wisconsin and Michigan through Nova Scotia. Hemlock occurs throughout New England, New York, Pennsylvania, and extends south through the Appalachian Mountains into northern Alabama and Georgia (Burns and Honkala 1990). Hemlock is a major component of four forest types recognized by the society of American foresters: white pine hemlock, eastern hemlock, hemlock-yellow birch, and yellow poplar eastern hemlock (Burns and Honkala 1990). It is also a common associate in seven forest types and a minor species in eighteen types (Burns and Honkala 1990). Historically, hemlock was widely used by the tanning industry for the tannins in its bark. Now the species is important to the pulp and paper industry and for the unique habitat that dense hemlock stands can provide for wildlife (Burns and Honkala 1990).

Eastern hemlock stands are valued because of their distinctive aesthetic, recreational, and ecological qualities. However, hemlock stands throughout the mid-Atlantic and Northeast are threatened by the hemlock woolly adelgid (Adelges tsugae Annand), an exotic insect pest that can cause extensive mortality of eastern hemlock trees (McClure 1991, Orwig and Foster 1998). Although the history of exotic species introductions is often well documented, the response of temperate forest ecosystems to the selective decimation of a dominant tree species is poorly understood (Lawton 1994, Castle et al. 1995, Orwig and Foster 1998). As hemlocks decline throughout the mid-Atlantic region of the United States, forest composition and ecological components will change. When a dominant coniferous species, such as eastern hemlock, is removed from forest stands, opportunistic hardwood species will become established in the resulting forest gaps (Partial 1995, Hannah 1999). Therefore, the documentation and description of tree composition of a threatened ecosystem prior to its decline is important for future restoration efforts and potential tree replacement in forest stands.

Protected public lands, like the Delaware Water Gap National Recreation Area (DEWA), provide an important opportunity for long-term monitoring and research of ecosystems affected by disturbances, such as the hemlock woolly adelgid. This insect pest has been present at DEWA since the mid-1980s and since then, hemlocks have declined throughout the recreation area due to infestation by this exotic species (Evans 1995).

The objective of our project was to describe overstory tree age and composition within eastern hemlock stands at DEWA and compare overstory forest composition among hemlock stands that varied in physiographic conditions. Physiographic characteristics and land-use history have important influences on how these forest stands will change as hemlocks decline. An understanding of the components of hemlock forest stands will provide baseline data for assessing the effects of hemlock woolly adelgid infestation and may help the development of predictions as to how forest structure will change with a concurrent decline of hemlocks in the mid-Atlantic region of the United States.


We conducted our study at DEWA, which is located in northeastern Pennsylvania and northwestern New Jersey. The national recreation area encompasses 27,742 ha of forested hills, ravines, and bottomlands bordering the Delaware River. Approximately 21,885 ha of DEWA is forested; of this total, 18,575 ha is hardwood forest, 1,295 ha is coniferous forest, and 2,015 ha is mixed evergreen-deciduous forest (Young et al. 2002). DEWA is located within the glaciated Pecan low plateau physiographic province of Pennsylvania. Elevation within DEWA ranges from 84 to 490 meters (Snyder et al. 1998). Our study focused on 14 hemlock forest stands that varied in their physiographic characteristics. The boundaries of hemlock stands were designated by vegetation mapping and confirmed by site visits (Myers and Irish 1981). All hemlock stands examined in our study contained either 1st or 2nd order streams.


In order to describe overstory forest structure and composition within hemlock stands, we selected hemlock forest stands at DEWA based on a sampling design model developed by researchers at the Leetown Science Center (Young et al. 2002) of the United States Geological Survey’s Biological Resources Division (USGS BRD). Young et al. (2002) used a geographic information system (GIS) to assess and summarize landscape variation by forest stand; they also employed statistical analysis to explore this landscape information by stratifying terrain conditions and classifying physiographically similar hemlock stands into 3 physiographic types. The 3 physiographic types (termed bench, ravine, or mid-slope) were based on the stand’s landscape attributes of elevation, slope, aspect, shape, and shade (Young et al. 2002). Bench stands were relatively low gradient sites; ravine stands were steep sites with a highly variable and often step-like, gradient; and mid-slope stands had a steep, but less variable, slope with a convex or only slightly concave shape in cross-section. Using this analysis, 14 hemlock stands (4 bench, 6 ravine, 4 mid-slope) were selected as study sites. Therefore, we were able to compare overstory tree composition in hemlock forest stands among three different physiographic characteristics in the recreation area.

During summer 1997, data on overstory tree composition were collected within the 14 hemlock forest stands along 3 transects established perpendicular to the stream in each stand. If the stand contained a 1st order stream, 2 of the 3 transects were established 80 m apart with the third transect established 160 m upstream. If the stand contained a 2nd order stream, 2 of the 3 transects were established 160 m apart with the third transect established 320 m upstream (Figure 1). We used point-sample timber cruising to inventory overstory vegetation at fixed sampling points along each transect (Burkhart et al. 1984). Along each transect, one sampling point was established 15 m from the stream edge on each side of the stream. Additional sampling points then were established along the transects at 30-m intervals to a maximum distance of 75 m away from 1st order streams or 135 m from 2nd order streams; a transect was terminated if it reached the border of a watershed. A ten-factor metric basal area prism was used to identify trees counted at each sampling point (Burkhart et al. 1984). Number and species of all trees identified with the prism were recorded.

Because age since establishment of a forest stand is critical in determining what overstory species dominate a stand, we determined stand age using an increment borer for core samples (Phipps 1985) of hemlock trees with the largest dbh along the transects located at the beginning and end of the study reach (Stokes and Smiley 1996). Although large dbh does not necessarily indicate age since stand establishment, resource managers at the park were interested in determining the age of these large hemlocks. In each stand, at least one tree was cored. Trees with signs of rot or damage were not sampled. Selected trees were cored at approximately breast height (1.37 m above ground level). If the tree occurred on a slope, the core was taken perpendicular to the direction of the grade to avoid reaction or compression wood. Because tree cores were taken at breast height and because our sample sizes were small, the ages of cored trees only can be considered estimates. We cored a total of 64 trees in all stands combined.

Cores were allowed to dry at room temperature for at least 24 hours. Cores then were mounted on wooden core holders using wood glue and sanded with a random orbital sander using 200 grit sandpaper or by hand (if core was broken) with higher grit sandpaper (usually 320 grit sandpaper) to more clearly define the annual rings. A dissecting microscope and magnifying glass were used to count all annual rings within each core from the pith to bark. In some instances, the pith was missed during coring, and occasionally a fragment of the core was lost during sanding. Extremely fragmented cores, cores without pith, and cores with missing pieces (n = 7) were discarded.

We calculated relative frequency of each tree species within (percentage of sampling points containing a particular species) and among (percentage of stands in which each species occurred) forest stands at DEWA. Therefore, we were able to compare the relative frequency of a tree species within a forest stand to its distribution throughout the recreation area.

In order to compare stand composition among physiographic clusters, we compared percent basal area per sampling points of overstory trees among clusters using single-factor analysis-of-variance (ANOVA) (Sokal and Rohlf 1995). If significant differences among the 3 physiographic clusters were found, we used a Hsu’s multiple comparison test to determine rank differences among groups a posteriori (Minitab version 12.0, 1998). We only compared the basal area of a species of tree among clusters if that species occupied > or =3 percent of the total basal area. We also conducted single-classification analysis-of-variance to determine whether presence of hemlock varied among physiographic types (bench, ravine, mid-slope).


Twenty-four species of overstory trees were found within the hemlock stands (Table 1). The mean percent basal area per sampling point of eastern hemlock was 54.2 for all stands combined and ranged from 32.0-73.4 (Table 2). After eastern hemlock, chestnut oak (Quercus prinus L.), black birch (Betula lenta L.), white pine (Pinus strobes L.), and white oak (Quercus alba L.) comprised the highest percent basal area per sampling point in forest stands (Table 1). Eastern hemlock had the highest relative frequency of occurrence within the forest stands, followed by chestnut oak, black birch, northern red oak (Quercus rubra L.), and northern white oak (Quercus alba L.) (Table 3). Those five species also were present within the greatest number of hemlock stands, indicating that they are locally as well as regionally abundant within hemlock stands at DEWA (Table 3). Snags were present at 21.6 percent of hemlock sampling points and 85.7 percent of hemlock stands.

The composition of overstory trees in hemlock stands differed among physiographic types (Table 4). Bench stands contained significantly less eastern hemlock (F^sub [2, 242]^ = 25.44, p

The average age of hemlocks cored in hemlock stands ranged from 68 to 148 years (Table 5). Age of trees cored in hemlock stands differed significantly among physiographic cluster (F^sub [2, 65]^ = 3.24, p = 0.044), with significantly younger trees located at bench sites (x¯ = 112.7, SE = 26.22) than at ravine (x¯ = 130.9, SE = 34.5) or midslope (x¯ = 136.1, SE = 28.5) sites (Hsu’s multiple comparison test, p


Forest stands were composed mostly of eastern hemlock, which is a late-successional species. Stands also were composed of moderately shade-tolerant species, e.g., chestnut oak and white oak (Foster 1992, Orwig and Foster 1998). These overstory tree species can survive in the dense shade that prevails in the understory of hemlock stands (Keever 1972, Foster 1992). Black birch was a major component in hemlock stands as well, indicating areas of localized recent disturbance (Burns and Honkala 1990).

Site factors, such as slope position and soil drainage also play a role in determining forest characteristics (Motzkin et al. 1996). Keever (1972) found that eastern hemlock was present on steep slopes where litter was removed by erosion and where soils were moist, thereby creating suitable conditions for growth and survival of hemlock seedlings. At DEWA, the steep slopes of ravine and mid-slope sites facilitate hemlock survival with shallow leaf litter, well-drained soils, and moist microclimate (Keever 1972).

Although site factors influenced the presence of hemlock at DEWA, there were differences in the amount of hemlock among stands with similar physiographic characteristics. These differences occurred because forest characteristics are strongly controlled by stand age and historical factors of land use, including stand age, logging history, agriculture, and timing of site abandonment (Foster et al. 1992). In addition, natural disturbances (e.g., windstorms, fire, insects, and pathogens) possibly influenced forest composition (Foster 1988).

As eastern hemlocks continue to decline throughout the mid-Atlantic region of the United States, hardwood species, such as oak, black birch, maple, and yellow poplar will likely establish themselves in resulting forest gaps (Partial 1995, Hannah 1999). Our research indicates that this pattern of hardwood recruitment may differ depending on physiographic conditions of the site. A replacement of hemlocks by hardwood species could negatively affect hemlock-dependent wildlife species, such as black-throated green warbler (Dendroica virens), blackburnian warbler (Dendroica fusca), and Louisiana waterthrush (Seiurus motacilla) (Benzinger 1994, Ross et al. 2002).

Foster (1992) found that hemlock forests in Massachusetts were restricted to areas of permanent woodlots. Other species, such as American chestnut [Castanea dentata (Marsh.) Borkh.] and white pine, were confined to areas of old pastures. Using information derived from our study in conjunction with knowledge of prior land use and specific forest management techniques, it may be possible to manage for an increase in hemlock in some of the existing hardwood stands at DEWA. Efforts are currently under way at DEWA to model factors that influence hemlock vulnerability to hemlock woolly adelgid. Once those factors are identified, stands with a low risk of hemlock woolly adelgid-induced mortality may be identified and managed to increase the hemlock component, thereby mitigating hemlock loss occurring in the more vulnerable stands.


This research was funded by the United States Department of Interior National Park Service. We thank Craig Snyder, John Young, David Smith, and David Lemarie of the Aquatic Ecology Laboratory, Biological Resources Division of the United States Geological Survey for their assistance in sampling design and stand selection. Robert Ross and Randy Bennett of the Research and Development Laboratory, Biological Resources Division of the U.S. Geological Survey also provided valuable field assistance. We also thank John Karish (Chief Scientist, National Park Service), and Richard Evans, Al Ambler, Jeff Shreiner, Keith High, and Beth Johnson (Delaware Water Gap National Recreation Area, National Park Service) for their support and collaboration during all phases of this study. We also thank Victoria Hesser for assistance with manuscript preparation.


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Received March 19, 2002; Accepted April 15, 2003.


1 Department of Biology, Penn State Altoona, Altoona, Pennsylvania 16601-3760;

2 Department of Natural Resources, Cornell University, Ithaca, New York 14853-0001;

3 School of Forest Resources, The Pennsylvania State University, University Park, Pennsylvania 16801-1014;

4 Frost Entomological Museum, Department of Entomology, Center for BioDiversity Research, The Pennsylvania State University, University Park, Pennsylvania 16801-1014;

5 School of Forest Resources, The Pennsylvania State University, University Park, Pennsylvania 16801-1014

* email address: cgm2@psu.edu

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