VEGETATION STATUS IN SELECTED WOODLOTS AT GETTYSBURG NATIONAL MILITARY PARK PRE AND POST WHITE-TAILED DEER MANAGEMENT

Technical Report NPS/NER/NRTR--2006/037

A. T. Niewinski¹, T. W. Bowersox¹, R. L. Laughlin<sup>2

1</sup>The Pennsylvania State University
College Of Agricultural Sciences
School of Forest Resources
University Park, PA 16802

²Elizabethtown College
Department of Biology
Elizabethtown, PA 17022

March 2006

U.S. Department of the Interior
National Park Service
Northeast Region
Philadelphia, Pennsylvania

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Executive Summary

Three types of plots (Demonstration, Long-term Monitoring, and Canopy Treatment) were established in Gettysburg National Military Park (NMP) woodlots between 1986 and 1992 to assess vegetation status over time in response to natural processes and experimental changes. The overall objective of these studies was to determine the regeneration and recruitment success of the mixed oak hardwood forests in the park. Specifically the studies were designed to 1) determine what impacts, if any, whitetailed deer were having on the presence of tree species, and 2) measure understory response to openings in the overstory canopy.

Long-term vegetation monitoring was started at Gettysburg NMP in 1986/87 with un-replicated Demonstration Plots in six woodlots (Biesecker, Bushman Hill, Cobean, Pitzer, Slyder, and Spangler). One pair of fenced and unfenced plots in each of the six woodlots was intended to demonstrate differences in the development of vegetation with and without the presence of deer. The Demonstration Plots were inventoried in 1986/87, 1989, 1991, 1994, 1996, and 2002. To describe the status of closed canopy woodlot plant community conditions and the effects of deer foraging on understory vegetation, sets of Long-term Monitoring Plots were established in 1990/92 in six woodlots (Biesecker, Bushman Hill, Herr Ridge, Pitzer, Powers Hill, and Spangler). In 1990, fifteen fenced and fifteen unfenced plots were established in Biesecker woodlot. In 1991, thirty unfenced plots were established in Pitzer woodlot. In 1992, fifteen unfenced plots were established in Bushman Hill, Herr Ridge, Powers Hill, and Spangler woodlots. In 1993, fifteen of the thirty plots already located in the Pitzer woodlot were randomly selected and a 3 x 3 m fence enclosure was constructed in the center of the plot. The Long-term Monitoring Plots were inventoried in 1990/92, 1996, and 2002. Finally, to evaluate understory response to openings in the canopy, replicated Canopy Treatment Plots were installed in three woodlots (Bushman Hill, Herr Ridge, and Powers Hill) in the summer of 1992. Three different overstory canopy treatments, closed canopy (control), partially open canopy (60% of the original overstory canopy remained), and open canopy (all overstory trees removed), were established within each of these woodlots. There were three replications of each canopy treatment within each woodlot for a total of 27 woodlot-treatment plots. The discussion includes baseline (pre-treatment) data results from 1992 and post-treatment data results from inventories in 1994, 1996, and 2002.

Management objectives for Gettysburg NMP include 1) the conservation of important features of the Gettysburg Battlefield Historic District which are necessary for maintaining the rural, agricultural, and memorial character of Gettysburg and conveying to visitors why, where, and how the battle occurred, 2) using the battlefield setting as the primary interpretive resource, and 3) managing the park as a memorial landscape which not only reflects the pre-battle 1863 rural agricultural environment but includes superimposed post-battle elements necessary for commemoration and visitor understanding of the battle (USDI NPS 1995). White-tailed deer and non-native plant species had altered the presence and appearance of understory vegetation thereby making preservation of historic woodlots and interpretation of the battle events difficult for National Park Service personnel.

In 1987, the white-tailed deer density in the 2,862 ha Gettysburg study area was 264 deer/forested square mile (Storm et al. 1992). It increased to 333 deer/forested square mile in 1990 and to 325 deer/forested square mile in 1995 (Storm et al. 2004) (Appendix A). The deer density is for the spring season before new fawns are born. There is high potential for a conversion over time of tree species from oak and hickory to ash and black cherry in areas with high densities of white-tailed deer (Storm et al. 2002, Horsley et al. 2003). In the long term, selective feeding by deer could change the species composition and structure of the forest overstory (Storm et al. 2002). The White-tailed Deer (Odocoileus virginianus) Management Program (Deer Management Program) was started in late 1995 within Gettysburg NMP (Frost et al 1997) to reduce the deer population to the initial goal density of 25 deer/forested square mile and achieve the park¡¯s woodlot stocking level goals (USDI NPS 1995). Subsequently, the stocking level goals were reduced to 60% stocking level in order to encourage new recruitment to become established in the woodlots and maintain an uneven-aged tree diameter distribution. The current woodlot stocking level goals are 16,834 seedling tree stems/ha, 2,806 small sapling tree stems/ha, 935 large sapling tree stems/ha, and 306 overstory tree stems/ha (Bowersox 2002). Due to personnel limitations, time restrictions, effectiveness of the control action, movement of deer into the park, and deer productivity, it was expected to take three to four years to achieve the desired deer density goal. The deer population would then be maintained at the desired density goal of 25 deer/forested square mile until results of the woodlot monitoring determined if a change in the initial density goal was required to meet management objectives and woodlot stocking level goals (USDI NPS 1995). The initial reduction effort lowered the density to 158 deer/forested square mile in March 1996 and continued efforts reduced the density to 49 deer/forested square mile in March 2002 (Storm 2004). The initial deer density goal of 25 deer/forested square mile has not yet been achieved.

In both the fenced and unfenced Demonstration Plots, the trend over six woodlots was a decline in seedling tree density and an increase in seedling shrub density from 1986/87 to 2002. From 1986/87 to
1996 there had been a consistent trend of decreasing seedling tree densities in both the fenced and unfenced plots, with the unfenced plots always having fewer seedling tree stems than the fenced plots. However, in 2002 there were more seedling trees in the unfenced plots than in the fenced plots. The reason for the increase in seedling tree density in the unfenced plots between 1996 and 2002 was due to the reduced deer foraging after the implementation of the Deer Management Program in 1995. Seedling tree densities within the fenced Demonstration Plots continued to decrease from 1996 to 2002 because of competition with seedling and sapling shrub species and a build-up of leaf litter within the fenced area. Over all woodlots and both the fenced and unfenced treatments, seedling tree and shrub species diversity increased over time.

In the Long-term Monitoring Plots, there was a trend of decreasing tree species diversity in all size classes from 1990/92 to 1996. From 1996 to 2002, however, there were improvements in species diversity of seedling, sapling, and overstory tree species among the woodlots, with the exception of Herr Ridge woodlot. The increases in seedling and sapling tree species diversity was closely linked to the implementation of the Deer Management Program in 1995. Conversely, the diversity of seedling and sapling oak species and density of oak seedlings and saplings were low, and in the unfenced plots (Biesecker and Pitzer woodlots) the density of oak seedlings declined between 1990/92 and 2002. Over all woodlots, the diversity of seedling and sapling shrub species increased from 1996 to 2002. The invasion and subsequent growth of non-native plant species has increased in all six woodlots.

Among all Long-term Monitoring Plots, the density of large sapling trees decreased from 765 stems/ha in 1990/92 to 456 stems/ha in 2002, consistently lower than the 60% stocking level goal of 935 stems/ha. The lowest densities of large and small sapling trees were recorded in 2002. Over all woodlots, small sapling tree density decreased from 116 stems/ha in 1990/92 to 69 stems/ha in 2002, less than 3% of the 60% stocking level goal of 2,806 stems/ha. Large sapling shrub densities increased over time but were always lower than large sapling tree densities. From 1996 to 2002, the overall small sapling shrub density increased from 69 to 833 stems/ha.

The density of seedling trees decreased among all the unfenced Long-term Monitoring Plots between the 1990/92 and 1996 inventories (20,917 and 14,665 stems/ha, respectively), then increased to 27,260 stems/ha in 2002. Herr Ridge woodlot had declining unfenced seedling tree densities from 1990/92 to 3 2002, which was probably due to the difficulty in reducing the deer density in that area of the park. Over all woodlots, unfenced seedling shrub densities increased substantially with a doubling of density between 1996 and 2002 (20,508 and 41,251 stems/ha, respectively). In Biesecker and Pitzer woodlots, the density of unfenced oak seedlings steadily decreased over the 1990/92, 1996, and 2002 inventories. By 1997, the Deer Management Program was successful in reducing the white-tailed deer population to less than 20% of the pre-1995 implementation levels in both the Biesecker and Pitzer woodlots (Storm 2004). Therefore, white-tailed deer browsing was not the only reason for oak species seedling decline in these two woodlots after 1996. Light restrictions due to dense overstory canopy, understory competition with shrub species, and small mammal predation were probably having negative impacts on the presence of oak species. Seedling and sapling tree species'densities should improve with continued reduction of the white-tailed deer herd, removal of non-native shrub species, and opening of the overstory tree canopy to the recommended 60% stocking level.

Canopy treatments were experimentally applied to unfenced 0.2 ha areas in Bushman Hill, Powers Hill, and Herr Ridge woodlots in 1992. Sapling plots and seedling subplots were established within the treatment areas. In general, in all canopy treatments and woodlots, sapling tree diversity and density improved between 1992 and 2002 with the greatest increases occurring from 1996 to 2002. Although sapling tree densities improved among all the canopy treatments between 1992 and 2002, sapling (large and small combined) tree densities among all three canopy treatments in 2002 in Bushman Hill (633 stems/ha), Herr Ridge (1,622 stems/ha), and Powers Hill (581 stems/ha) woodlots were substantially less than the 60% stocking level goal for total saplings (3,741 stems/ha) necessary for adequate recruitment. The reason for the lower sapling tree densities may be a combination of intense competition with native and non-native shrub species, poor growing conditions due to light restrictions, and continued browsing by white-tailed deer.

Sapling shrubs benefited from the open canopy treatments more than sapling trees. Over time, there were more sapling shrub species than tree species under all three, canopy treatments. The shrub species were opportunistic of the favorable conditions provided in the open canopy treatments and were able to exploit the resources and grow faster than the tree species. The abundance of sapling shrubs, especially in the open canopy treatments, may have had a deleterious effect on the recruitment of seedling tree and shrub species. The biggest change over time was in the abundance of non-native species, mainly Japanese honeysuckle, Japanese barberry, and multiflora rose. In 2002, non-native shrubs combined comprised 46% of the total sapling shrub density over all canopy treatments and woodlots. Spicebush was the dominant native sapling shrub (4,381 stems/ha) in 2002 comprising 39% of the total sapling shrub density over all woodlots and canopy treatments.

There were increases in native seedling tree species diversity and seedling tree height in the Canopy Treatment Plots since the implementation of the Deer Management Program. Over all canopy treatments in 2002, the seedling tree densities were above the 60% stocking level goal of 16,834 stems/ha in Bushman Hill (36,111 stems/ha) and Powers Hill (28,889 stems/ha) woodlots but not in Herr Ridge woodlot (6,111 stems/ha). Over all woodlots in 2002, seedling tree densities were adequate in the closed (27,500 stems/ha) and partially open (37,222 stems/ha) canopy treatments but not in the open canopy (6,389 stems/ha) treatments. The increase in seedling tree density in the partially open and closed canopy treatments was due to the combination of reduced white-tailed deer herbivory after 1995 and less competition with shrub species than in the open canopy treatments. Although seedling tree densities were lowest in the open canopy treatments in 2002, the more favorable growing conditions when the plots were first established in 1992 enabled the initial seedlings to achieve greater height growth and grow into saplings between 1994 and 1996. By 2002, over all woodlots, more seedling trees had grown into the sapling size category in the open canopy treatments than in either of the other two canopy treatments. However, seedling trees were not surviving or growing into the upper height classes and being recruited in sapling trees at densities sufficient to achieve the historic forest overstory as described in Fairweather and Cavanaugh (1990).

It is likely that the density of seedling shrubs influenced the presence and growth of seedling trees in the Canopy Treatment Plots. Among all woodlots and canopy treatments, the seedling shrub density increased from 37,834 stems/ha in 1992 to 64,444 stems/ha in 2002. Seedling shrub density was three times greater than seedling tree density (23,704 stems/ha) over all woodlots in 2002. Two native shrub species, Rubus and spicebush, were the dominant seedling shrub species with densities of 24,352 and 20,463 stems/ha, respectively, in 2002. Non-native seedling shrubs combined comprised 15% of the total seedling shrub density in 2002.

The National Park Service's initiative in reducing the white-tailed deer herd has enabled the establishment of a sustainable understory. The lower number of white-tailed deer in some woodlots has resulted in greater seedling tree presence and growth. In Herr Ridge woodlot, where the whitetailed deer herd has not been reduced as much as in other areas of the park due to proximity of residential housing and a golf course, the density of seedling tree species was much lower than in the other woodlots. The partially open canopy conditions created by removing individual trees were effective in increasing seedling tree species diversity and density. Multiple-tree openings in the canopy were effective in providing the conditions for seedlings to grow into saplings. Overall, in 2002 there was good potential for tree species to regenerate the woodlots at Gettysburg NMP. Although these studies were not designed to evaluate the effects of non-native invasive plants on the understory tree species, there were strong implications that these plants were poised to expand their presence. The woodlots exhibited the potential to reproduce a great diversity of seedling tree species at densities great enough to regenerate healthy woodlots. Overall, seedling densities increased from 1996 to 2002, exceeding the 60% stocking level goals. Although sapling tree densities generally improved after 1996, the average total sapling (small and large combined) densities over all three types of study plots ranged from 450 to 945 stems/ha, only 12 to 25% of the 60% stocking level goal (3,741 stems/ha) in 2002.

Continuation of the Deer Management Program is vital for an increase in native tree species diversity and height of tree stems. The initial deer density goal of 25 deer/forested square mile has yet to be achieved park-wide. Plant data collected in the summer of 1996 were considered to be representative of conditions prior to the deer population reduction. Data from the 2002 inventory were representative of reduced white-tailed deer populations but not indicative of the effect that would have been achieved had the deer density been at the goal for the previous four to five years. As stated in the final environmental impact statement, the decision to make any adjustments to the deer density goal can not be made until vegetation is resurveyed after the deer density has been maintained at 25 deer/forested square mile for a period of four to five years (USDI NPS 1995). Any effects of a change in level of deer browsing on the presence and growth of tree seedlings would not be known for at least four years, since it takes approximately that long to determine if tree seedlings have become successfully established. A poor seed crop, severe drought, or other environmental factors could affect the establishment of seedlings in the woodlots and extend the length of that period (Todd Bowersox, Professor of Silviculture, The Pennsylvania State University, personal communication). The Pennsylvania Department of Environmental Protection declared Adams County under drought emergency status in 1999 and 2002, and under a drought watch in 2001
(http://www.dep.state.pa.us/dep/subject/hotopics/drought.news.htm).

Therefore, to sustain a healthy diversity and density of native seedling, sapling, and overstory tree and shrub species that will perpetuate the appearance and composition of the historic 1863 woodlots (Fairweather and Cavanaugh 1990) in Gettysburg National Military Park, it is recommended to:

1. Continue to implement the Deer Management Program to reach the goal of 25 deer/forested square mile (USDI NPS 1995), periodically evaluate the effectiveness of this density, and adjust the goal accordingly, if necessary.

2. Continue executing the Exotic Plant Management Program within the park to control the nonnative, invasive plant species.

3. Fully implement the park-wide plan for increasing the sustainability of woodlots (Bowersox 2002), where the removal of individual non-sustainable overstory trees will create openings in the canopy to improve the health, vigor, and density of native seedling tree species. Woodlot stocking should be maintained between the 60% and 80% level of full stocking.

4. Develop a plan to recruit seedling tree stems into sapling stems and then into the overstory to replace the aging trees where there is a mixture of vigorous native tree species seedlings that have a competitive advantage over the herbaceous and shrub plants. Although canopy openings of 0.2 ha (radius 21 m) were tested, it is believed that similar opportunities for seedlings to become canopy tree replacements can be achieved with a combination of singletree and multiple-tree openings (0.05 to 0.1 ha) to reduce the stock to a 45-55% level.

5. Develop a long-term vegetation monitoring program for all wooded areas that are greater than or equal to 2 ha. The program should be designed to document changes in plant diversity and density for all sizes of tree and shrub species, and forb, grass, grass-like, and vine species groups. When choosing plot locations, take into consideration the effect of moisture on regeneration. Inventory frequency need not be greater than once every five years, but not less than once every ten years.

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