16 Deer and Ecosystem Management

Transcription

1 16 Deer and Ecosystem Management DAVID DECALESTA Within the last decade, managers of public and private forestlands have contemplated managing forest resources in ways that address the desired (future) condition (outcomes) of these resources (Society of American Foresters 1993), which include all plant and animal species, noncommercial as well as commercial. Such management necessitates dealing with ecosystems and dovetails with an emerging management concept identified as "ecosystem management" (Society of American Foresters 1993; Grumbine 1994; Salwasser 1994). According to Grumbine (1994), ecosystem management integrates knowledge of ecological relationships within a sociopolitical values framework. The goal of ecosystem management is to protect native ecosystem integrity over the long term. Grumbine (1994) identified a dominant theme of ecosystem management as that of managing to preserve diversity of communities (species and populations) and ecosystems as well as ecological patterns and processes. Such management will shape the future age, structure, species composition, and abundance of forest resources, which together constitute forest condition. Ecosystem diversity and ecological patterns and processes constitute ecosystem integrity. Ecosystem integrity and forest condition form the building blocks, or biological components, of ecosystem management as defined by Grumbine (1994). These biological components of ecosystem management are affected by weather and by patterns and composition of vegetation that occur within landscapes. Weather and composition and 267

2 268 D. S. DECALESTA pattern of vegetation change over time as does the spatial arrangement of vegetative patterns. Human sociological, economic, and political events influence, for example, harvest of timber and subsequent landscape patterns of vegetation. Forest health factors (insect and disease) and browsing by deer also affect the landscape pattern and composition of vegetation. Interactions among these factors, and the effects of these interactions on landscape patterns of vegetation, constitute ecosystem management contexts that influence the status of ecosystem components. Ecosystem management is defined as a strategy by which, in aggregate, the full array of forest values and functions is maintained at the landscape level over long periods of time (Society of American Foresters 1993; Irland et al. 1994), suggesting the importance of spatial and temporal scales. Definitions of ecosystem management do not include identification of plant or animal species that can affect ecosystem management components profoundly enough to be identified as keystone species, which by Hunter's (1990 : ) definition includes species upon which the integrity of whole ecosystems rely. In this context, McShea and Rappole (1992) identified the white-tailed deer as a keystone species. The premise of this chapter is that the effect of white-tailed deer, a keystone species, on vegetation can be of a magnitude sufficient to influence (1) ecosystem integrity, (2) future condition of forest resources, and, (3) the conduct and outcome of ecosystem management. CONCEPTUAL FRAMEWORK The interrelationships among ecosystem management components and contexts and deer impact may be expressed by a flow diagram (Figure 16.1). Ecosystem management contexts influence ecosystem management components as mediated by deer impact. Marquis et al. (1992) and Redding (1995) defined deer impact as a joint function of deer density and forage availability, suggesting that as forage availability increases, the impact of deer on forest resources decreases. Deer density is affected primarily by mortality associated with winter severity, hunting harvest, and deer vehicle collisions (Witmer and decalesta 1992). Forage availability and deer density are affected by landscape patterns of vegetation and deer harvest as they vary through space and time. Severe winter can limit the ability of deer to move to find forage under deep snow and can induce winter mortality. Natural (windstorm) and

3 Ecosystem Management for Deer 269 FIGURE Relationships among deer impact and ecosystem management components and contexts. human-induced (timber harvest) disturbance creates patterns and amounts of forest openings that produce deer forage. Mortality of trees from insect and disease organisms (forest health factors) can result in increased deer forage production by increasing the amount of light reaching the forest floor. Social and political values concerning use of natural resources affect timber and deer harvests. Factors that affect forage availability and deer density vary through time and space, creating stochasticities of forage availability and deer density that are integrated as deer impact. Deer impact, in turn varying through space and time, contributes to stochasticities of ecosystem integrity and forest condition. Managing for deer impact on ecosystem management requires an understanding of the relationships among deer impact and ecosystem management components and contexts. The following discussion of the effects of white-tailed deer on ecosystem management is based on research conducted in northern hardwood forest type in northwestern Pennsylvania at the Warren Laboratory, Northeastern Forest Experiment Station, U.S. Forest Service. The effect of varying white-tailed deer densities on regeneration of woody vegetation

4 270 D. S. DECALESTA. and on songbird and herbaceous communities was evaluated during (Tilghman 1989; decalesta 1992, 1994; Jones et al. 1993). In the 10-year study, deer density was controlled within four 65-ha fenced enclosures and optimal timber harvest levels (10% clear-cutting, 30% thinning) were practiced. DEER AND ECOSYSTEM MANAGEMENT CONTEXTS Biological Contexts That Affect Deer Impact Presettlement forests of northwestern Pennsylvania were characterized by large areas of maturing timber and large and small openings created by natural disturbance (e.g., hurricanes, tornadoes, fire, and ice storms). These openings contained vegetation in various seres from regenerating to subclimax and climax communities (Braun 1964; Marquis 1975; de- Calesta 1989). Young timber stands approaching canopy closure supported understories of herbaceous and shrubby species, but even mature stands (characterized by uncut, virgin forests in the 1920s) were endowed with these plants (Lutz 1930; Bjorkbom and Larson 1977; Whitney 1984). Climax and subclimax forests probably dominated, but areas from less than an acre to thousands of acres contained early successional vegetation because of local and regional disturbances. This produced a "landscape of patches at different ages and stages of succession rather than a uniform climax forest" (Millers et al. 1989). These patches created a landscape of deer forage that increased and decreased in quality and quantity in relation to the amount of light reaching the forest floor. No quantitative data exist relative to associated pre- European settlement deer densities, but McCabe and McCabe (1984) estimated white-tailed deer densities of 3-4 deer/km = within the range of the species in the eastern United States. Alverson et al. (1988) suggested that white-tailed deer densities may have approximated 4 deer/km2 in undisturbed eastern hemlock (Tsuga canadensis) forests in Wisconsin. Deer density likely was affected by available forage in extensive tracts of mature forest and by predation (wolf [Canis spp.], mountain lion [Felis concolor], and Native Americans) (Marquis 1975; McCabe and McCabe 1984; Ellingwood and Caturano 1988; Witmer and decalesta 1992). By the end of the nineteenth century much of Pennsylvania's forests had been completely cut over, and the white-tailed deer herd nearly eliminated by unregulated harvest (Redding 1995). By the early twentieth century, large amounts of deer forage were produced by the extensive timber

5 Ecosystem Management for Deer 271 harvest, deer hunting was prohibited, and major deer predators were extirpated from Pennsylvania. The result was an irruption of the deer population from 1907 to 1939: deer density exceeded 20 deer/km2 in northwestern Pennsylvania (Clepper 1931; Redding 1995). The expanding deer herd depleted its food supply and, during a series of hard winters in the late 1930s, declined to around 4-6 deer/km' by the early 1940s. Coincidentally, timber harvest began anew in the 1950s as forests harvested years earlier approached commercial maturity. The deer population again increased, averaging over 12 deer/km2 in the ensuing 40 years, even after sustaining a population decline following a series of harsh winters in (Figure 16.2; Redding 1995). Hough (1965) and Whitney (1984) documented the loss of herbaceous and shrub species from unmanaged mature forests in northwestern Pennsylvania since the 1930s and attributed the declines to herbivore by deer. Composition of understory vegetation shifted from shrubs and herbs to grasses and ferns during this period (Marquis and Grisez 1978; Horsley and Marquis 1983), and species richness and abundance of seedlings of woody plants were reduced. Thus, biological contexts of ecosystem management have shifted qualitatively and quantitatively since pre-european settlement and most FIGURE Deer population and timber cutting trends on the Allegheny National Forest, Pennsylvania, (from Redding 1995; used with permission of author).

6 272 D. S. DECALESTA appreciably during the twentieth century. Deer densities have been higher, and understory vegetation has shifted from a diverse assemblage of woody and herbaceous plants to a reduced number of species dominated by fern, grass, and woody vegetation not preferred by deer. Social and political contexts, in the form of values and economies assigned to resources, have been defined by the above biological contexts, again most appreciably during the twentieth century. Social and Political Contexts That Affect Deer Impact Deer are valued positively as an economic and recreational resource. Deer are valued negatively as an agent of significant damage to forest and agricultural interests and to private citizens in the form of deer vehicle collisions and depredation of urban suburban gardens and landscaping (Witmer and decalesta 1992). Over 400,000 deer were harvested annually in Pennsylvania over : average statewide posthunt deer density was 11 deer/km 2 ; average harvest level was 6 deer/km2 (W. Palmer, Pennsylvania Game Commission, personal communication). Posthunt deer density has been greater than 12 deer/km2 in northwestern Pennsylvania since the late 1920s (excepting two notable crashes; Redding 1995; Figure 16.2), and the last three generations of deer hunters have come to expect these deer densities and associated harvests as normal and sustainable. Hunters have numbered over one million in Pennsylvania for the last three decades (Witmer and decalesta 1992) and represent a significant lobbying force for maintaining these high deer densities and harvests. Income generated by deer hunting activities in Pennsylvania amounts to at least one billion dollars annually (Kosack 1991). Both business and hunting interests have benefited from the high deer densities and harvests experienced over the last 70 years; these interests exert considerable pressure to maintain posthunt deer densities in excess of 11 deer/km2 Since the second-growth forest in Pennsylvania became commercially mature in the late 1950s, there has been an increasing amount of timber harvested (Redding 1995; Figure 16.2). Expectations of the industry are that timber harvests can be sustained at a level comparable to that of the past decades. However, timber stands cannot be harvested if advance regeneration is inadequate to produce replacement trees. McWilliams et al, (1993) reported that only 4-20% of forested lands surveyed across Pennsylvania in 1990 had advance regeneration sufficient to provide adequate regeneration of diverse woody species under conditions ranging from un-

7 Ecosystem Management for Deer 273 favorable to favorable. Overabundant white-tailed deer populations were hypothesized as the primary cause for failure of advance regeneration. In 1994 only 33% of maturing forestland within the Allegheny National Forest carried sufficient advance regeneration to permit removal of overstory timber with reasonable expectation of successful regeneration (L. DeMarco, Allegheny National Forest, Warren, Pennsylvania, personal communication). Thus, management of deer and timber has been driven by social and political values and has resulted in creation of deer densities and forage production over the last 70 years that have exceeded levels created by natural disturbance and deer mortality. Changes in species composition of floral communities have been associated with these changes in deer density and forage production. DEER AND ECOSYSTEM MANAGEMENT COMPONENTS Deer Impact on Process: Regeneration When overstory forest trees are removed by natural processes (e.g., fire, windstorm, or insect and disease mortality) or by timber harvest, the succeeding forest is promulgated by seeds and vegetative reproduction from overstory trees. Seedlings constituting the advance regeneration are present at the time of overstory removal and form the nucleus of the emerging timber stand. Failure of advance regeneration (inadequate seedling abundance, height, and species richness) following timber harvest in the mid-1950s in northwestern Pennsylvania prompted research to determine causes of the failure (Tilghman 1989; Redding 1995). This 10-year study in northwestern Pennsylvania demonstrated that species richness, abundance (expressed as percent plots fully stocked), and height of saplings (small trees with diameter at breast height less than cm) declined significantly once deer density exceeded 7.9 deer/km2 (Tilghman 1989; decalesta 1992). At deer densities greater than 7.9 deer/ km 2, seedlings of six woody tree species were missing or prevented by deer browsing from becoming incorporated into the overstory (decalesta 1992). As deer densities exceeded 7.9 deer/km 2 clear-cut sites approached monocultures of black cherry (Prunus serotina); on uncut sites, American beech (Fagus grandifolia) and striped maple (Ater pensylvanicum) dominated (Tilghman 1989). Deer negatively affected the regeneration process and future overstory when populations exceeded estimated pre-european settlement deer density (Tilghman 1989; decalesta 1992).

8 274 D. S. DECALESTA Deer Impact on Community SONGBIRDS Deer browsing can affect vegetation that son g birds use for foraging surfaces, escape from predators, and nesting. By significantly reducing the height of woody vegetation, white-tailed deer affected habitat for songbirds that fed, nested, and sought cover in the m height interval (decalesta 1994). The bird community that uses this intermediate foliage canopy exhibited a significant reduction in species richness and abundance when deer density exceeded 7.9 deer/km 2 and five songbird species were no longer observed on study sites. Some songbird species that remained at higher deer densities did not exhibit reduced abundance; they increased in dominance as abundance of other species declined. HERBS AND SHRUBS Scientists noted that species richness and abundance of herbaceous and shrubby vegetation began to decline in northwestern Pennsylvania in the 1920s (Hough 1965; Whitney 1984), when white-tailed deer density exceeded approximately 7.9 deer/km 2 (Redding 1995). Species richness and abundance (expressed as percent ground cover) of shrubs and herbaceous plants (excepting ferns, grasses, and mosses) were negatively affected when deer density exceeded 3.7 deer/ km 2 ( Jones et al. 1993). Two shrub species and one herbaceous species disappeared when deer density exceeded 7.9 deer/km 2. Species composition and abundance of plants in the herb shrub layer shifted when deer densities exceeded 7.9 deer/km 2 : percent ground cover of ferns, grasses, and mosses increased while that of shrubs and other herbs decreased (decalesta 1992). Many herb and shrub species noted in earlier (pre-1950; Jennings 1953) surveys were not found (decalesta 1992), having been eliminated by 70 years of browsing pressure by deer populations in excess of 7.9 deer/km 2. Species richness and abundance of plants in the herb and shrub communities, like that of songbirds, declined when deer population density exceeded estimated pre-european settlement density. Deer Effects on Biodiversity Most definitions of biodiversity include variety of genetic material within species, variety of species, and variety of communities over time and space (Thorne et al. 1995). Reduction of abundance of individuals within a species and reduction of number of species when deer density exceeds 7.9 deer/km 2, as documented in the studies cited above, results in reduc-

9 Ecosystem Management for Deer 275 tion and elimination of genetic material. Three communities (songbird, herb and shrub, and tree) were affected by deer, exhibiting changes in species composition, dominance, and abundance within communities. The toss of some species (herbs and shrubs) occurred over approximately 70 years, reflecting a temporal aspect of biodiversity. Deer Effects on Forest Condition The effect of deer on forest condition is largely a function of how deer influence biodiversity the abundance and richness of the various species composing the many forest communities, floral and faunal over time and space. Deer impact is a long-term influence: species richness and abundance of tree seedlings determine the composition and structure of stands that develop following natural or human disturbances and that will persist for decades or longer. Characteristics of understory vegetation (species richness and abundance and vertical habitat structure) and associated wildlife communities are likewise affected by deer and by the nature of the overstory vegetation as influenced by deer. Unmanaged mature forests in northwestern Pennsylvania consist primarily of eastern hemlock, American beech, and sugar maple (Acer sacch arum) (Braun 1964; Marquis 1975). Retention of these forests depends on continual seed supply and regeneration success of trees. Deer can prevent successful regeneration of eastern hemlock by eliminating seedlings in the understory (Aiverson et al. 1988). Beech bark disease, an introduced scale insect and fungus complex, can eliminate American beech from the overstory (Houston 1975) and remove it as a reliable seed supplier. Sugar maple decline syndrome is associated with a general failure of sugar maple regeneration in parts of northwestern Pennsylvania (R. White, Allegheny National Forest, personal communication). It is possible that deer can eliminate eastern hemlock regeneration and that overstory American beech and sugar maple trees may become unreliable seedling producers as a result of disease and parasitic organisms. This scenario could result in no replacement of eastern hemlock, American beech, or sugar maple trees to provide the seed source for future forests. The resulting forest might then be populated by species resistant to deer browsing (black cherry and striped maple) or others not affected by the disease and parasitic organisms. Composition, structure, and age of such forests, as affected by deer, may be quite different from past and present forests. Wildlife communities associated with these deer-affected forests may also differ. Second-growth Allegheny hardwood forests in northwestern Pennsyl-

10 276 D. S. DECALESTA vania, characterized by black cherry, American beech, eastern hemlock, red maple (Ater rubrum), white ash (Fraxinus americana), cucumbertree (Magnolia acuminata), yellow poplar (Liriodendron tulipifera), and black (Betula lenta) and yellow (Betula alleghaniensis) birch overstories, could regenerate to near monocultures of black cherry with remnants of red maple, American beech, and striped maple if deer browsing were sufficient to prevent establishment of tree species preferentially browsed by deer. Understories in old- and second-growth stands could be composed primarily of ferns, grasses, mosses, and seedlings of American beech, striped maple, and black cherry. CONCLUSIONS The ecosystem management scenarios presented in this chapter were developed within the context of deer density and forage availability found in northwestern Pennsylvania where forage production occurs primarily as a function of logging and natural disturbance. Other places in the Northeast, notably national parks and other deer refugia (Storm et al. 1989; Palmer et al., Chapter 10), have considerably higher deer densities (25 62 deer/km2 a ) and greater forage production resulting from interspersion of old and current agricultural and forest lands. Deer densities higher than those of pre-european settlement may be required to produce negative effects on ecosystem management components because of the countereffect of greater deer forage provided by agricultural fields. Unfortunately, studies that relate deer impact to ecosystem management components within agricultural forested landscapes are lacking. Providing for sustained ecosystem integrity and future forest condition will require understanding and proactive management of deer impact. Studies in northwestern Pennsylvania provide evidence of the pervasive effect of deer on ecological integrity and forest condition under ecosystem management contexts that are typical of heavily forested ecosystems. This chapter demonstrates how deer impact, as influenced by ecosystem management contexts, can affect ecosystem integrity and forest condition. Passive, reactive deer management may well result in continued reductions in species richness and abundance and shifts in composition of plant and animal communities, effectively thwarting the main goal of ecosystem management (preservation of ecosystem integrity). Future conduct of ecosystem management must incorporate proactive management activities that produce benign rather than negative deer impact, either by

11 Ecosystem Management for Deer 277 reducing deer density, increasing forage availability, or integrating both activities. If increasing forage availability for deer is chosen as an ecosystem management activity, it must be conducted in a manner that does not jeopardize other elements of ecosystem integrity across temporal and spatial scales (e.g., increasing timber harvest or agricultural development must not result in forest fragmentation or proceed at a rate that precludes sustained yields of timber and other forest resources). Consideration should be given to adjusting deer densities, possibly in concert with increasing forage availability, to levels compatible with sustaining ecosystem integrity and to retaining options for producing a variety of desired forest conditions. Practitioners of ecosystem management, where it incorporates activities designed to reduce deer impact, must be prepared to address and counter political and social pressures that will resist such change. Responsible management of deer and forest resources will require that deer impact be investigated and quantified over a range of ecosystem management contexts. REFERENCES CITED Alverson, W. W., D. M. Waller, and S. L. Solheim Forests too deer: Edge effects in northern Wisconsin. Conservation Biology 2: Bjorkbom, J. C., and R. G. Larson The Tionesta scenic and research natural areas. U.S. Forest Service General Technical Report NE-31. Braun, E. L Deciduous Forests of Eastern North America. Hafner Publishing Co., New York. Clepper, H. F The deer problem in the forests of Pennsylvania. Pennsylvania Department of Forestry and Waters Bulletin 50, Harrisburg, PA. decalesta, D. S Even-aged forest management and wildlife populations. Pages in Symposium on Effects of Forest Management on Wildlife. ( R. H. Yahner and M. Brittingham, eds.) Pennsylvania State University Press, University Park. decalesta, D. S Impact of deer on species diversity of Allegheny hardwood stands. Proceedings of the Northeastern Weed Science Society Abstracts 46: 135 decalesta, D. S Impact of white-tailed deer on songbirds within managed forests in Pennsylvania. Journal of Wildlife Management 58 : Ellingwood, M. R., and S. L. Caturano An Evaluation of Deer Management Options. Connecticut Department of Environmental Protection, Wildlife Bureau, Hartford.

12 278 D. S. DECALESTA Grumbine, R. E What is ecosystem management? Conservation Biology 8 : Horsley, S. B., and D. A. Marquis Interference by weeds and deer with Allegheny hardwood reproduction. Canadian Journal of Forest Research 13: Hough, A. F A twenty-year record of understory vegetational change in a virgin Pennsylvania forest. Ecology 46 : Houston, D. R Beech bark disease. Journal of Forestry 73 : Hunter, M. L Wildlife, Forests, and Forestry. Regents-Prentice Hall, Englewood Cliffs, NJ. Irland, L. C Getting from here to there: Implementing ecosystem management on the ground. Journal of Forestry 92: Jennings, 0. E Wildflowers of Western Pennsylvania and the Upper Ohio Basin. University of Pittsburgh Press, Pittsburgh, PA. Jones, S. B., D. S. decalesta, and S. B. Chunko Whitetails are changing our woodlands. American Forests 99:20-25, Kosack, J Hunting is a booming business. Pennsylvania Game News 62: Lutz, H. J The vegetation of Heart's Content, a virgin forest in northwest ern Pennsylvania. Ecology 11 :1-29. Marquis, D. A The Allegheny hardwood forests of Pennsylvania. U.S. Forest Service General Technical Report NE-15. Marquis, D. A., R. L. Ernst, and S. L. Stout Prescribing siivicultural treatments in hardwood stands of the Alleghenies (revised). U.S. Forest Service General Technical Report NE-96. Marquis, D. A., and T. J. Grisez The effect of deer exclosures on the recovery of vegetation in failed clearcuts on the Allegheny Plateau. U.S. Forest Service Research Note NE-270. McCabe, R. E., and T. R. McCabe Of slings and arrows: An historical perspective. Pages in White-Tailed Deer: Ecology and Management. (L. K. Halls, ed.) Stackpole Books, Harrisburg, PA. McShea, W. J., and J. H. Rappole White-tailed deer as keystone species within forest habitats of Virginia. Virginia Journal of Science 43: McWilliams, W., S. L. Stout, T. Bowersox, and L. McCormick Can we regenerate Pennsylvania's hardwood forests? Pages in Penn's Woods- Change and Challenge. ( J. C. Finley and S. B. Jones, eds.) Proceedings of the Forest Resource Issues Conference, Pennsylvania State University, University Park. Millers, I., D. S. Shriner, and D. Rizzo History of hardwood decline in the eastern United States. U.S. Forest Service General Technical Report NE-126. Redding, J. A History of deer population trends and forest cutting on the Allegheny National Forest. Pages in Proceedings of the 10th Northcentral Hardwoods Conference. U.S. Forest Service Technical Re p ort NE-197.

13 Ecosystem Management for Deer 279 Salwasser. H Ecosystem management: Can it sustain diversity and productivity? Journal of Forestry 92:6-11. Society of American Foresters Task Force Report on Sustaining Long-Term Forest Health and Productivity. Society of American Foresters, Bethesda, MD. Storm. G. L., R. H. Yahner, D. F. Cottam, and G. M. Vecellio Population status, movements, habitat use, and impact of white-tailed deer at Gettysburg National Military Park and Eisenhower National Historic Site, Pennsylvania. U.S. National Park Service Technical Report NPS/MAR/NRT R-89/043. Thorne, S. G., K. C. Kim, K. C. Steiner, and B. J. McGuinness A Heritage for the 21st Century: Conserving Pennsyivania's Native Biological Diversity. Pennsylvania Fish and Boat Commission, Harrisburg. Tilghman, N. G Impacts of white-tailed deer on forest regeneration in northwestern Pennsylvania. Journal of Wildlife Management 53 : Whitney, G. C Fifty years of change in the arboreal vegetation of Heart's Content, an old-growth hemlock white pine northern hardwood stand. Ecology 65: Witmer, G. W., and D. S. decalesta The need and difficulty of bringing the Pennsylvania deer herd under control. Proceedings of the Eastern Wildlife Damage Control Conference 5 : :.