In What Ways Can Deer Affect Wildflowers?

For wildflowers and many shrubs, however, height escape is not possible. Thus, deer can change the abundance, size, and flowering of understory plants, which in turn affects the plants’ ability to reproduce. The preferences of deer can change the species composition and diversity of understory communities. The plants that deer like to eat decrease in abundance or disappear, while the plants that deer do not like or that are able to bounce back from deer browsing increase as a proportion of the understory. This affects food and cover for other wildlife species, and affects the visual variety that we see as we hike or hunt in the woods.

Many scientists have noticed the effect of deer on spring wildflowers. Several species of trillium, a wildflower that can form a carpet in eastern forests in the spring, have been studied, but the most work has been done on white trillium, or Trillium grandiflorum. Scientists have studied this plant in Illinois², Minnesota³, Pennsylvania⁴, and other locations. Deer eat both the flowers and the fruit of this plant, and where deer impact is severe, the flowers disappear. Ben Moyer, a hunter and outdoor writer from Pennsylvania, tells of a steep slope on which he and his grandfather hunted turkey together in the spring. When he hunted with his grandfather as a child, trillium flowers covered the opposite slope of the valley during turkey season. One spring when he revisited the valley to hunt turkey as an adult, he was struck by the fact that the opposing hillside had only a few remaining trillium blossoms – and as he watched, a doe walked across the hillside eating each and every remaining blossom.

In Pennsylvania, scientists have used tall boulders as a way to assess deer impact on wildflower communities, comparing the diversity and population characteristics of plants on top of boulders too tall for deer to climb with the population on the forest floor or on low rocks whose tops are easily accessible to deer. Rooney⁵ focused on Maianthemum canadense (Canada mayflower) and Oxalis acetosella (wood sorrel). Canada mayflower is a preferred browse species and wood sorrel is not. Rooney found that on high boulders, there were more Canada mayflower shoots per unit surface area, and their leaves – the vital surface for photosynthesis – were larger. There were no such differences for wood sorrel. Maybe even more important was the fact that only 1 of the Canada mayflower plants he found on low, deer-accessible, boulders was flowering, and therefore able to produce seed, compared to nearly 20% of the Canada mayflowers on high, deer-inaccessible, boulders.

In 2005, Comiskey and others also examined wildflowers on high and low boulders in the same region of northwestern Pennsylvania. They found 1300 reproducing wildflowers of ten different species on the tops of boulders more than 1.5 meters tall. On an equivalent area of low boulders (0.5 meters or less) and forest floor, they found six reproducing wildflowers!

The region of northwestern Pennsylvania where Rooney, Comiskey and others⁶ worked had high deer impact levels through most of the twentieth century, making it a region where deer impacts are particularly vivid and where long-term change can be studied. In 2001, Ristau⁷ revisited plots in an old-growth area known as Hearts Content where Lutz^6,8 sampled wildflowers in 1928. Using the same methods as Lutz used, Ristau found that 24 species found on the plots in 1928 were rarer in 2000, with seven not found at all. In particular, Lutz found Viburnum alnifolium, or witch hobble, on half the plots in 1928 but Ristau did not find it on any of the plots in 2000. Ferns that are not preferred by deer increased in abundance from 3 to 21 percent, on average. There were also new species found near the trails through the area.

¹ Photos in Figure 1 and 3 by © 2005 Louis-M. Landry. Photo in Figure 2 by Thomas G. Barnes @ USDA-NRCS PLANTS Database / Barnes, T.G. & S.W. Francis. 2004. Wildflowers and ferns of Kentucky. University Press of Kentucky.
² Anderson, R. C. 1994. Height of white-flowered trillium (Trillium grandiflorum) as an index of deer browsing intensity. Ecological Applications 4: 104-109.
Anderson, R. C., and A. J. Katz. 1993. Recovery of browse-sensitive tree species following release from white-tailed deer Odocoileus virginianus Zimmerman browsing pressure. Biological Conservation 63: 203-208.
³ Augustine, David J.; Frehlick, Lee E. 1998. Effects of white-tailed deer on populations of an understory forb in fragmented deciduous forests. Conservation Biology 12: 995-1004.
⁴ Kirschbaum, C.D.; Anacker, B.l. 2005. The utility of Trillium and Maianthemum as phyto-indicators of deer impact in northwestern Pennsylvania, USA. Forest Ecology and Management 217: 54-66.
⁵ Rooney, Thomas P. 1997. Escaping herbivory: refuge effects on the morphology and shoot demography of the clonal forest herb Maianthemum canadense. Journal of the Torrey Botanical Society 124(4): 280-285.
⁶ Comiskey, Lauren, Alejandro A. Royo, Walter P. Carson. 2005. Deer browsing creates rock refugia gardens on large boulders in the Allegheny National Forest, Pennsylvania. American Midland Naturalist 154:201-206.
⁷ Ristau, Todd E. 2001. Seventy-two years of change in the herbaceous vegetation layer of Hearts Content Scenic Area, Warren County, PA. in Abstracts, Keeping All the Parts: Preserving, restoring, and sustaining complex ecosystems. The Ecological Society of America 86th Annual Meeting, Monona Terrace, Madison, WI: 5-10 August, 2001. p. 190.
⁸ Lutz, H.J. 1930. the vegetation of Heart’s Content: A virgin forest in northwestern Pennsylvania. Ecology 11: 1-29.