0947-B3
William H. McWilliams, Todd W. Bowersox, Patrick H. Brose, Daniel A. Devlin, James C. Finley, Steve Horsley, Kurt W. Gottschalk, Susan L. King, Tonya W. Lister, Larry H. McCormick, Gary W Miller, Charles T. Scott, Kim C. Steiner, Susan L. Stout, James A. Westfall and Robert L. White 1
It is well known that overpopulation of ungulates can devastate forest regenerative capacity; however, few studies have quantified this impact over large areas. Data from the USDA Forest Service, Northeastern Forest Inventory and Analysis Unit's inventory of Pennsylvania are being used to monitor forest understory communities. The Pennsylvania Regeneration Study addresses a fundamental need for new indicators of regenerative capacity for managed and unmanaged native broadleaf forests. Understory vegetation of this region often is dominated by species that are not preferred as deer food or are highly tolerant of browsing. The region's forest composition and dynamics, and ambient high populations of deer make it an excellent case study on indicators of regenerative capacity. This paper describes an approach that compares the composition of the existing regeneration component to thresholds representing "sustainable levels." The proposed indicators address issues of natural regeneration of native forests that are not currently included in the Montreal Process Criteria and Indicators. Applying stocking thresholds to the sample data provides estimates of the proportion of forest that does or does not meet accepted silvicultural guidelines for advance tree-seedling stocking. One-half to two-thirds of the region's forests would require remedial treatment to ensure adequate regeneration following harvest if replacing the existing canopy is the management objective. These results are controversial because solutions to regeneration problems can require radical and expensive forest management treatments that often conflict with other resource values. General constructs of this approach could be adapted to compare existing regeneration to sustainable levels in any forest setting where guidelines exist. One advantage over the long term is that results will include data sufficient to distinguish between advance and post-disturbance tree seedling stocking levels.
It is well known that overpopulation of ungulates can devastate forest regenerative capacity; however, few studies have quantified this impact over large areas. Forest inventory data are being used to monitor forest understory communities as part of the data generated by the USDA Forest Service, Northeastern Forest Inventory and Analysis (NE-FIA) unit's ongoing inventory of Pennsylvania (McWilliams and others 2002). The long-term Pennsylvania Regeneration Study (PRS) is addressing a fundamental need for new indicators of regenerative capacity for managed and unmanaged native broadleaf forests of the eastern United States. The PRS is part of a larger research initiative by cooperating institutions to develop site- and species-specific stocking guidelines and management criteria. A particularly difficult aspect of the research problem is a ubiquitous and longstanding overpopulation of white-tailed deer (Odocoileus virginianus Zimmermann) across most of the study region whose understory vegetation often is dominated by species that are not preferred as deer food or are highly tolerant of browsing. The region's forest composition and dynamics and ambient, high populations of deer make it an excellent case study on indicators of regenerative capacity.
In this paper, an approach is presented for developing indicators of regenerative capacity that compares the composition of the existing regeneration component to thresholds representing "sustainable levels," defined as the number and size of understory trees needed to replace existing canopy dominants or other desired forest cover. The approach is flexible, can be applied to evaluate large geographic areas, and uses the most current scientific data available. Although designed for evaluation of broadleaf deciduous forests in the East, this method can be applied to any forest setting where guidelines exist.
The Pennsylvania study region lies mostly between 39 and 41 degrees north latitude, is characterized by temperate climatic conditions, averages about 100 centimeters of rainfall annually, and encompasses a diverse mix of ecoregions and associated forest types (Figs. 1a, b). Total forest land is nearly 70,000 square kilometers, all but 5 percent of which is broadleaf deciduous forest. The region is divided by the Central Appalachian Broadleaf Forest- Coniferous Forest-Meadow Province (Allegheny Mountains) that is characterized by steep ridge and valley topography. A prominent feature of the ecoregion boundaries is the line of glaciation that weaves along the region's northern tier and bisects the Allegheny Plateau. The most common forest associations are Quercus spp. (mixed oak) and Acer-Fagus-Betula (Northern hardwoods). Ownership of the region's forests includes federal and state government, but is dominated by private owners. These owners, who control about 80 percent of the forest land, range from forest industry and corporate investment firms to farmers and sport and recreation clubs.
Figure 1. Ecoregions of the United States (a), study region (b), systematic interpenetrating sample design (c), sample location layout (d), Pennsylvania, USA.
The PRS uses the NE-FIA systematic sample design laid across Pennsylvania (Fig. 1c). Each sample location occupies a 2400-hectare hexagon. The sample is measured over a five-year period. Twenty percent of the sample locations are measured annually in an "interpenetrating" fashion to obtain a representative sample of the state each year. Regeneration measurements are taken on a subset of sample locations during the leaf-on season. The sample location consists of four 7.3-meter fixed-radius circular subplots spaced 36.5 meters apart (Fig. 1d). Prior to the study, a pilot test was conducted to determine the appropriate number of 2.0-meter radius circular microplots needed to capture the variability in tree-seedling abundance (McWilliams and others in press). Modeling the coefficient of variance for a range from one to four microplots per subplot, indicated that one microplot per subplot was adequate. At each microplot, all fully established seedlings (less than 2.5 centimeters in diameter) are tallied by species, source, and height class.
The diversity of canopy-dominant species common in eastern broadleaf forests complicates evaluation of regenerative capacity. Deer browsing often creates a mismatch between overstory and understory composition. Although the principal objective is to evaluate the capacity for establishing future canopy dominants, another concern is prospective conversion from high-value species, e.g., oak, to associated species, e.g. red maple (Acer rubrum L.).
Long-term silvicultural research has documented the complex silvics of the region's forests (Hough 1965; Marquis and Bjorkbom 1982) and similar systems in the Central United States (Sander and Clark 1971; Sander 1972). The resulting stocking guidelines were the basis for the indicators. Indicators used to analyze advance-tree seedlings were developed to represent stocking requirements for moderate deer pressure (Gingrich 1967; Sander and others 1976) and the high deer pressure that is common in Pennsylvania (Marquis 1994). Moderate and high-deer thresholds are: 25 and 100 seedlings per 2-meter circular microplot, and respectively represent the number of seedlings required to establish at least one canopy dominant per microplot. The size of the microplot is roughly equivalent to the ground area occupied by a single stem when it reaches the canopy. Seedling height is a critical factor in determining the probability of survival under high deer pressure. Seedlings are weighted by height class before the thresholds are applied. Both guidelines are applied to the sample data to compare impact levels.
Assessing species composition is another important factor due to the potential for species conversion. Results are partitioned by species groupings (preferred, commercial, and woody) that reflect a range of management objectives. The preferred group represents the current mix of canopy dominants found across the region, the commercial group includes preferred and other species that can form high canopy; the woody group includes the preferred and commercial species as well as those that develop low-canopy forest cover, e.g., striped maple (Acer pensylvanicum L.) and hawthorn (Crataegus spp.).
It has been well reported by Marquis and Brenneman (1981), Trumbull and others (1989), Healy (1997), and DeCalesta and Stout (1997) that white-tailed deer have controlling impacts on understory communities--their distribution, composition, structure, and function--and thus the quality and abundance of tree regeneration. Studies have shown that advance tree-seedling regeneration often is lacking and that oak regeneration is rare (Lorimer 1993; McWilliams and others; 1995a, b). These studies document the need for continued monitoring of regenerative capacity. One advantage over the long term is that future results will include data sufficient to distinguish between advance and post-disturbance tree seedling stocking levels.
Table 1. Percentage of samples that met regeneration indicator thresholds by species group, stocking guideline, and ecological province (Bailey 1995), Pennsylvania, 2001.
| 1 EBF-W=Eastern Broadleaf Forest (Oceanic) Province-West; PLAT=Laurentian Mixed Forest Province (Plateau); C APP=Central Appalachian Broadleaf-Coniferous Forest-Meadow Province; EBF-E=Eastern Broadleaf Forest (Oceanic) Province-East. 2 Differences between moderate and high deer guidelines were not significant for H0: PMD>PHD at the 95% confidence level. |
Applying the stocking thresholds to the sample data provides estimates of the proportion of forest that did or did not meet accepted silvicultural guidelines for advance tree-seedling stocking (Table 1). Before the thresholds were applied, sample data were filtered to exclude closed-canopy stands for which silvicultural findings indicate inhibition of seedling development due to low levels of sunlight. The findings indicate that one-half to two-thirds of the region's forests would require remedial treatment to ensure adequate regeneration following harvest or other significant disturbance, assuming replacement of the existing canopy composition is the management objective. As expected, the results were somewhat better for the commercial species group, i.e., one-third to one-half would need treatment. The woody-species results are only slightly better than the preferred and commercial combined. Estimates for the indicators were lower for the Laurentian Mixed Forest Province and Eastern Broadleaf Forest (Oceanic) Province (East) than for the Central Appalachian Broadleaf - Coniferous Forest - Meadow Province and Eastern Broadleaf Forest (Oceanic) Province (West). The available evidence shows that tree-seedling regeneration is poor across vast areas of the study region rather than specific to a particular landowner or forest type.
The results are controversial because solutions to regeneration problems can entail radical and expensive forest management treatments that often conflict with other resource values. Abrams (1992), Brose and others (2001), Gottschalk (1993), Kolb and others (1990), and Marquis (1994) documented the need to manage deer herds and implement techniques that boost tree-seedling stocking. The primary recommendation for deer management is to significantly reduce the herd to below the level known to interfere with establishment of regeneration, or 8 deer per square kilometer of forest. The current estimate for Pennsylvania is 14 deer per square kilometer of forest. Forest management recommendations include installing and maintaining deer fencing, applying herbicides, and adopting other control measures. Prescribed fire has been demonstrated to be a useful tool for ensuring regeneration in areas in which species such as oak are desired future stand dominants (Brose and Van Lear 1998). In addition to the loss in value resulting from shifts in species composition, the loss of oak and other heavy seeded species reduces or eliminates food sources for the many faunal species that depend on hard mast (Healy 1997). High populations of deer in the Mid-Atlantic, Great Lake, and Southern United States suggest that regenerative capacity may be less than optimal across much of the East. The findings should provide insight into the indicators of regenerative capacity for researchers studying temperate mixed-oak and northern-hardwood forest systems. General constructs of this approach could be adapted to gauge existing regeneration.
Complicated social, economic, and political forces play a role in forging future resource sustainability. Timber species, such as oak, maple (Acer spp.), black cherry (Prunus serotina Ehrh.), and ash (Fraxinus spp.) command high prices and support a large domestic and international market. Recreation competes on nearly equal footing with timber; hunting is popular, as is camping, fishing, hiking, and other recreational activities. In recent decades, human populations have created a forest-urban mosaic which overlays forested areas in heavily populated areas, exacerbating conflicts between forest managers and expectations of the public. The proposed indicators address issues of natural regeneration of native forests that are not currently included in the Montreal Process Criteria and Indicators. The salient Montreal indicators are the "area and growingstock in plantations" and "annual removals for products versus sustainable volume," which are vastly different from that needed to address sustainability in diverse broadleaf deciduous forests of the eastern United States. Understory condition is an accepted indicator of future composition and structure, and hence resource sustainability. In the East, about 80 percent of the forest land is occupied by natural broadleaf deciduous forests. As such, additional indicators could address managed and unmanaged systems; and be used to evaluate advance- and post-disturbance regeneration for broadleaf systems. While the prototype indicators presented here are specific to the eastern United States, a broad indicator under the Maintenance of Productive Capacity of Forest Ecosystems (Criteria Two) seems appropriate, i.e., following the example of comparing removals for timber products to sustainable volume. An indicator that compares existing regeneration to sustainable levels would be especially germane to international discussions of long-term sustainability.
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William H. McWilliams; Bowersox, Todd W.; Brose, Patrick H.; Devlin, Daniel A.; Finley, James C.; Gottschalk, Kurt W.; Horsley, Steve; King, Susan L.; LaPoint, Brian M.; Lister, Tonya W.; McCormick, Larry H.; Miller, Gary W.; Scott, Charles T.; Steele, Harry; Steiner, Kim C.; Stout, Susan L.; Westfall, James A.; White Robert L. In: Ream's, G.V.; McRoberts, R. E.; Van Deusen, P. C., eds. Measuring tree seedlings and associated understory vegetation in Pennsylvania's forests: Proceedings of the third annual Forest Inventory and Analysis symposium; 2002 November 19-21; New Orleans, LA. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station.
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USDA Forest Service, Northeastern Research Station, Newtown Square, PA, USA. [email protected]