Augustine, D.J. 1997. Grazing patterns and impacts of white-tailed deer in a fragmented forest ecosystem. M.S. Thesis, University of Minnesota. 169 pp.
The conversion of deciduous forests in the upper midwestern United States to agricultural and residential land uses has changed the relationship between white-tailed deer populations and remaining patches of deciduous forest. This thesis examines the grazing patterns and consequent impacts of white tailed deer on understory forbs in old growth Big Woods forest remnants in southeastern Minnesota. Surveys of all understory species at four study sites in 1995, and of a select list of species at 11 study sites in 1996 documented relatively low grazing intensity on spring ephemeral species, high grazing intensity on a few preferred forb species in early and late summer, and a wide range of grazing intensities among sites in all seasons.
To examine deer grazing impacts on the understory plant community, two approaches were used. First, deer impacts on the most intensively grazed early summer forbs, Trillium spp., were studied using individual plant exclosure experiments, between site comparison of grazing intensity and population structure, and transplant experiments. Secondly, impacts on the overall understory herb community were examined using 10 m2 deer exclosures constructed at high and low deer density study sites.
No effects of deer herbivory were detected at low deer density sites. Trillium experiments showed severe impacts of grazing on growth, reproduction, and population structure at sites with high deer density. At the one high deer density site where spring ephemerals were present, a significantly greater increase in Erythronium density occurred inside the 10 m2 exclosures compared to grazed control plots. In late summer, the forb community within exclosures at one high deer study site diverged dramatically from the community in grazed control plots, primarily due to increased abundance and flowering rates of Laportea canadensis and Circaea lutetiana inside exclosures, and increased abundance and flowering rates of unpalatable Eupatorium virginiana and Hackelia virginiana outside exclosures. However, significant deer effects were not observed at second high deer density site where Laportea canadensis is abundant. Results from the exclosure experiments and the pattern of among-site variation in grazing intensity indicates that impacts on the late-summer understory community will be most severe when local deer densities are high and palatable forb species are rare.
Forest surveys showed that grazing intensity varies widely among Big Woods remnants in southeastern Minnesota. The predictability of grazing intensity based on winter deer density, landscape composition surrounding forest fragments, and characteristics of forb population within a stand was examined using the 1996 surveys of 11 study sites. Regression analyses showed that alfalfa availability is an important determinant of grazing intensity in early summer, and the availability of row crops, alfalfa, and fields is an important determinant in late summer. In early summer, the effects of landscape composition also depended on winter deer density (high vs. low) and the flowering rate of palatable forbs within the forest stand (high vs. low). In late summer, among-site variation in grazing explained by landscape composition could be equally well explained by winter deer density and forb abundance within the stand. Results indicate that agricultural practices surrounding and the current abundance of forb populations within parks designed to protect fragmented forest communities should be important considerations in addition to deer density when managing local deer populations.