Ratings of white-tailed deer preferences for woody browse in Indiana.
Keywords: Browsing, herbivory, Odocoileus virginianus, shrubs, trees
White-tailed deer are generalist herbivores that feed on a variety of herbaceous and woody species of plants. Since their re-introduction to Indiana in the 1930s, deer have prospered in the state and have become overabundant in some areas, with devastating effects on vegetation and deer and human health (Ley et al. 1995; Swihart et al. 1998). When abundant, deer can play key roles in structuring forest ecosystems (Cote et al. 2004). In northeastern forests, shifts in community composition toward browse-tolerant species have been attributed to deer (Long et al. 2007), and regeneration of preferred browse species has been suppressed by deer (Russell et al. 2001; Rooney & Waller 2003; Casabon & Pothier 2008). Similarly, reduced species diversity of saplings has been attributed to deer browsing in the Great Smoky Mountains (Griggs et al. 2006).
White-tailed deer also can influence the success of woody plants in cultivated settings.
Nurseries and orchards can suffer substantial economic losses due to deer browsing (Lemieux et al. 2000). Plantation hardwoods can exhibit reduced growth when browsed, and timber quality may be reduced if apical buds are removed (Putman & Moore 1998; Morrissey et al. 2008). In suburban areas of the eastern U.S., deer often modify behavior to forage in close proximity to dwellings and include a greater variety of ornamental plants in their diets, thereby causing damage to expensive landscaping designs (Swihart et al. 1995; West & Parkhurst 2002). Overall, the economic costs of damage caused by deer in the U.S. exceeded $750 million over a decade ago (Conover 1997). Damage caused by feeding likely has been underestimated (Cote et al. 2004).
To predict better which woody species are at risk from deer browsing, many studies on diet selection have been done (reviewed by Russell et al. 2001). Unfortunately, nearly all of these studies have been local in scope, and few attempts have been made to generalize findings spatially or temporally. The problem of "scaling up" from small scales is common in field ecology because of the cost and logistical difficulties associated with data collection at landscape or regional scales. Recently, Swihart et al. (2007) demonstrated the value of surveys for testing ecological predictions at regional scales. The objective in the current study was to derive the first state-wide, quantitative ratings of native woody plants in terms of their preference to white-tailed deer. A survey of forestry and wildlife professionals across the state enabled acquisition of data on perceived preferences. Tests were then conducted for differences in preference ratings among species types, and for geographic variation in preference ratings.
Data collection.--A mail survey was sent in January 2008 to consulting foresters as well as district foresters and wildlife biologists employed by the Indiana Department of Natural Resources. The survey asked the 60 recipients to assign a score of 0 through 10 for each of a list of 22 species of trees (Table 1) and 13 species of shrubs (Table 2). The following rating system was used: 0 = never seen the species eaten; 1-4 = less than average preference; 5 = average preference; 6-9 = preferred; 10 = the most preferred food which is always eaten first by deer. Respondents were instructed not to assign a rating for species for which they did not have experience. Ratings were received from 31 individuals (52% response rate) and classified according to whether they worked in the southern or northern half of the state.
Statistical analyses.--For each species a statewide mean preference rating ([+ or -] standard error) was computed. A mean preference rating also was computed for each species after categorizing responses into northern and southern Indiana. A two-sample t test was used to compare mean preference ratings for: a) selected pairs of species in which there was particular interest because of their popularity or value, b) oaks versus all other tree species, and c) trees and shrubs.
Geographic differences in preference ratings were assessed using Poisson regression with location (north or south), species type (shrub or tree) and their interaction as predictors. Subsequent pair-wise nonparametric Mann-Whitney tests were conducted for each species using northern versus southern ratings.
Substantial differences were evident among species (Fig. 1). For trees, northern red oak received the highest mean ([+ or -] standard error) preference rating (8.5 [+ or -] 0.3), whereas sycamore was rated as the least preferred of the tree species surveyed (2.6 [+ or -] 0.6). Oaks on average received greater preference ratings than other tree species (t = 4.62, n = 22, P = 0.002). Indeed, the oaks ranked as 11 of the 12 most preferred tree species surveyed (Fig. 1). For shrubs, black chokeberry received the highest rating (7.0 [+ or -] 0.6), whereas pawpaw received the lowest rating (1.1 [+ or -] 0.3). With the exception of black chokeberry and common chokecherry, ratings for the shrubs fell into the "average" or "less than average" preference categories (Fig. 1). Trees on average received higher preference ratings than shrubs (t = 2.42, n = 35, P = 0.025). American hazelnut and ninebark were the most variable species in terms of preference ratings, with ranges of 0-10 and 0-9, respectively.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
A comparison of hardwood species highly valued for their timber revealed the following ordering according to preference ratings (species sharing a common superscript did not differ at the 0.05 level of significance): northern red [oak.sup.a] > white [oak.sup.b] [less than or equal to] sugar [maple.sup.bc] [less than or equal to] black [walnut.sup.cd] = black [cherry.sup.d]. Statistically, sugar maple had a marginally (P = 0.12) lower preference rating than white oak and a marginally (P = 0.11) greater preference rating than black walnut.
Geographic differences in preference ratings also were evident. A Poisson regression fitting ratings to species type (shrub or tree), location (north or south portion of state), and their interaction indicated lower overall preference ratings in northern than southern Indiana (Wald [chi square] = 2.92, df = 1, P = 0.09). Subsequent pair-wise nonparametric comparisons for each species revealed that geographic differences were evident for black, bur, and chestnut oak, as well as red pine, sycamore, and sweetgum. Ratings of deer preference were lower in northern Indiana for all of these species (Fig. 2).
Plant species exhibit a range of susceptibility to herbivory (Swihart & Bryant 2001), and the species considered in the survey were no exception. Forestry and wildlife professionals recognized considerable variation among browse species in terms of perceived preferences of deer. Statewide, northern red oak received the highest preference rating, and oaks generally were classified as "preferred". Negative effects of deer on oaks have been documented 1 in other studies (Healy 1997; Rooney & Waller 2003) and suggest that oaks are relatively poorly defended. In contrast, several species, such as sycamore, pawpaw, and button bush, appear to be unpalatable to deer and thus likely are resistant to the potential negative effects of herbivory. In species with moderate preference ratings (such as sugar maple, tulip poplar, and black cherry), other studies have demonstrated negative effects attributable to deer (reviewed by Russell et al. 2001). Whether deer negatively affect growth and survival of these species in Indiana will depend on factors such as deer density and availability of other food plants (reviewed by Cote et al. 2004).
The survey revealed some differences from a study of browse preferences done in central Illinois (Strole & Anderson 1992). Of the species included in the current survey, Strole & Anderson (1992) listed chokecherry, gray dogwood, white oak, and black cherry as preferred, and sugar maple and hazelnut as low use. Indiana respondents rated white oak (mean = 7.2) and chokecherry (6.7) as preferred. Gray dogwood was above average preference (5.7) and variable, with ratings ranging from 2-8. Black cherry was rated as about average preference in Indiana (4.6), but it also was quite variable (range = 1-9). In contrast to Illinois, sugar maple (mean = 6.2) and American hazelnut (mean = 5.6) are considered above average in preference to deer in Indiana.
In temperate North America, conspecific and congeneric variation in palatability of woody plants to mammalian herbivores tends to exhibit latitudinal patterns, with southern genotypes more palatable than their northern counterparts (Swihart & Bryant 2001). Survey results from the current study are consistent with this trend, as northern populations in Indiana received lower preference ratings than southern populations for all six species in which significant differences occurred. Historical gradients in browsing pressure have been proposed as a factor driving these patterns, an explanation supported by selection for plant defenses on islands to which herbivores have only recently been introduced (Vourc'h et al. 2001). Effects of deer on woody plants is predicted to be greater in landscapes fragmented by human activity (Reimoser 2003), due in part to seasonal concentration of animals in forest remnants and to resource supplementation in agricultural areas that enhances carrying capacity beyond levels supported by native vegetation. The northern, glaciated portion of Indiana has been fragmented by agriculture for 175 years and consisted of a confluence of native eco-regions before European settlement. Thus, historical browsing pressure may well have been greater in northern Indiana.
The lower preference ratings for shrubs relative to trees also may be explained by evolved defenses against herbivory. Shrubs are exposed to herbivory throughout their lives, whereas trees typically are susceptible only while within reach of deer during the juvenile stage of growth. A meta-analysis of 37 tree species demonstrated that palatability to mammalian herbivores is much greater in the mature (out-of-reach) stage than the juvenile (within-reach) stage of ontogeny (Swihart & Bryant 2001).
From a practical perspective, the findings from the survey provide a basis for selecting planting stock when planning a tree planting or landscaping project. In areas accessible to many deer, selection of species with high preference ratings is likely to result in browse damage. In fairly large plantings, selecting a mixture of species with average or below average preference ratings could reduce deer visitation and browsing at a site. If all else is equal, landowners should use shrubs in areas prone to deer traffic. For oaks, regeneration in the presence of deer likely will depend on control in the form of, e.g., fencing or hunting.
The preference ratings provided here are rooted in taxonomic categorizations of woody plants. Of course, browse selection by deer should focus on factors that maximize fitness, which may have little to do with taxonomy. Thus, future work should consider how deer preferences for woody browse are related to plant morphological, chemical, and life history traits, in conjunction with energetic, nutritional, and other constraints affecting foraging deer.
We thank the forestry and wildlife professionals who graciously donated their time in completing the survey. J.O. Whitaker, Jr., and an anonymous reviewer provided constructive comments on the manuscript.
Manuscript received 12 January 2009, revised 17 March 2009.
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Bruce Wakeland: Wakeland Forestry Consultants, 10560 East State Road 8, Culver, Indiana 46511 USA
Robert K. Swihart: Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana 47907 USA
Correspondence. Robert K. Swihart, Department of Forestry and Natural Resources, Purdue University, 715 West State Street, West Lafayette, IN 47907.
Table 1.--Species of trees for which preference ratings were requested in a mail survey sent to consulting foresters, district foresters, and state wildlife biologists. The number of respondents providing a preference rating for a species is given in the column denoted by N. Common name Scientific name N Black oak Quercus velutina 28 Bur oak Q. macrocarpa 27 Chestnut oak Q. prinus 8 Chinkapin oak Q. muehlenbergii 23 Northern red oak Q. rubra 31 Pin oak Q. palustris 20 Scarlet oak Q. coccinea 13 Shumard oak Q. shumardii 20 Swamp chestnut oak Q. michauxii 22 Swamp white oak Q. bicolor 26 White oak Q. alba 29 Black cherry Prunus serotina 27 Black walnut Juglans nigra 31 Pecan Carya illinoinensis 14 Persimmon Dispyros virginiana 17 Red pine Pinus resinosa 22 Sugar maple Acer saccharum 22 Silver maple Acer saccharinum 17 Sycamore Platanus occidentalis 18 Sweetgum Liguidambar styraciflua 18 Tulip poplar Liriodendron tulipifera 28 White pine Pinus strobus 31 Table 2.--Species of shrubs for which preference ratings were requested in a mail survey sent to consulting foresters, district foresters, and state wildlife biologists. The number of respondents providing a preference rating for a species is given in the column denoted by N. Common name Scientific name N American elderberry Sambucus canadensis 15 American hazelnut Corylus americana 16 American plum Prunus americana 28 Black chokeberry Aronia melanocarpa 10 Buttonbush Cephalanthus occidentalis 27 Common Prunus virginiana 10 chokecherry Eastern redbud Cercis canadensis 15 Flowering dogwood Cornus florida 16 Gray dogwood Cornus racemosa 14 Ninebark Physocarpus opulifolius 10 Pawpaw Asimina triloba 20 Silky dogwood Cornus amomum 19 Washington Crataegus phaenopyrum 16 hawthorn
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|Author:||Wakeland, Bruce; Swihart, Robert K.|
|Publication:||Proceedings of the Indiana Academy of Science|
|Date:||Jul 9, 2009|
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