Degree Name

Master of Science (MS)

Semester of Degree Completion

2000

Thesis Director

Thomas A. Nelson

Abstract

High deer densities on state forests and nature preserves throughout Illinois have prompted concerns that deer are impacting the growth, survival, and reproduction of spring-flowering herbaceous plants. I investigated the impacts of white-tailed deer (Odocoileus virginianus) on the herbaceous vegetation, particularly spring wildflowers, at Beall Woods Nature Preserve (BWNP). BWNP was selected as the site for this study because it supports a diverse community of spring wildflowers and a large deer population. The objectives for this study were to: (1) quantify the extent of deer foraging on representative spring-flowering plant species at Beall Woods Nature Preserve, (2) investigate the effects of this foraging on the growth, survival and reproduction of these species, and (3) develop an inexpensive, practical method for Illinois Department of Natural Resources (IDNR) Natural Heritage biologists to monitor the impact of deer on these species on nature preserves throughout Illinois.

The effects of grazing by deer were monitored using a series of 90 1-m2 plots. Deer were excluded from grazing in 30 of these by 1-m3 exclosures constructed of PVC frames enclosed with 2.54 cm wire mesh. Weekly monitoring of the herbaceous vegetation in the 30 exclosures and the 60 open plots began in mid-February and ended in mid-July 1999. I monitored plant species richness and abundance, number of plants grazed, heights of grazed and ungrazed individuals, number flowering, and number producing seed in all plots.

A total of 63 plant species were recorded in the plots including 36 perennials, 6 annuals and 21 woody species. No federal or state endangered or threatened species were present; however, several species of concern such as Jack-in-the-pulpit (Arisaema triphyllum), green dragon (Arisaema dracontium), and goldenseal (Hydrastis canadensis) were found within the study plots. Species richness did not differ among exclosed, adjacent open and distant open plots. Spring beauty (Claytonia virginica) was the most widespread species. Toothwort (Dentaria laciniata) and red trillium (Trillium recurvatum) were also common.

Exclosures did not affect deer grazing in the treatment plots adjacent to them either by repelling or attracting deer. Some grazing on spring herbs was recorded in the open plots in every week from 22 February through 7 June. Grazing peaked during the first week of April then declined gradually through May with the emergence of foliage on woody plants.

Among all species, 49% were grazed to some extent. The intensity of grazing on individual species ranged from 63% on Jacob's ladder (Polemonium reptans) to 1% on Virginia knotweed (Polygonum virginianum). In addition to Jacob's ladder, other species that were grazed heavily included goldenseal, false Solomon's seal (Smilacina racemosa), red trillium, dwarf larkspur (Delphinium tricome), and spring cress (Cardamine bulbosa). Forty percent of the unprotected trillium were grazed. The 2% that flowered were grazed before reaching the seed production stage. All of the Jacob's ladder that flowered were grazed resulting in no seed production. In contrast, 38% of the protected Jacob's ladder flowered and produced seed. Generally, spring wildflowers did not recover from herbivory during the season they were grazed. No regrowth of reproductive organs or leaves occurred.

Not all wildflowers were grazed heavily. Those species that were affected minimally by deer included wild ginger (Asarum canadense), Dutchman's breeches (Dicentra cucullaria), green dragon, toothwort, and spring beauty. Many of these species emerged after the peak of grazing by deer and reproduced more successfully than species that were heavily grazed. Others, such as toothwort and spring beauty, were available during the peak of grazing but may have escaped heavy grazing by numerically swamping herbivores during the brief critical period of growth and reproduction. For example, only 4% of the toothwort and 10% of the spring beauty were grazed.

There were significant differences in height between the protected and unprotected plants of several species. Often larger, more vigorous plants were grazed leaving smaller, non-reproducing individuals. The mean height of ungrazed red trillium on open plots was 9.8 cm compared to 12.7 cm for those located in the exclosures. Jacob's ladder averaged 20.6 cm in height outside exclosures but 33.3 cm inside exclosures.

This study corroborates previous research showing that continual grazing by white-tailed deer has negative effects on forest herbs. Selective foraging results in species such as Jacob's ladder, trillium, and goldenseal being grazed heavily resulting in reduced leaf area, height, flowering, and seed production. The extent of grazing on many forest herbs warrants reducing the density of deer on BWNP to protect these species. The IDNR should consider a deer management program to reduce and maintain the population at a sufficiently low density to allow for the recovery of these sensitive herbaceous species.

Trillium, dwarf larkspur, false Solomon's seal, and Jacob's ladder appear to be useful indicator species of the impact of deer on forest herbaceous vegetation. These species are widely distributed in Illinois and are easy to identify. Furthermore, these species were heavily grazed in early spring when deer densities are high on BWNP and there are noticeable differences in flower production and stem height between protected and unprotected individuals of these species. Finally, this study suggests that small exclosures are a viable, inexpensive alternative to large, permanent exclosures for monitoring the impacts of deer on the forest herbaceous vegetation.

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