Academic journal article The American Midland Naturalist

White-Tailed Deer (Odocoileus Virginianus) Alter Herbaceous Species Richness in the Hiawatha National Forest, Michigan, USA

Academic journal article The American Midland Naturalist

White-Tailed Deer (Odocoileus Virginianus) Alter Herbaceous Species Richness in the Hiawatha National Forest, Michigan, USA

Article excerpt

ABSTRACT.-

To assess the effects of deer browsing on understory composition, we examined hardwood forests with documented differences in browse intensity within two regions of the Hiawatha National Forest in Michigan's Upper Peninsula. A deciduous twig damage assessment confirmed that two sites within the southern region experienced high white-tailed deer (Odocoileus virginianus) browse intensity (44-49% of twigs browsed) and two sites within the northern region incurred low browse intensity (0.5% and 0.6% of twigs browsed). We used within-site sample means and site-level rarefaction curves to compare understory species richness in these two forests. The forest experiencing low browse intensity had higher woody species richness on a per plot basis than the heavily browsed forest, but the converse was observed for herbaceous species richness. Site-level evaluation with rarefaction curves confirmed the plot-scale results for herbaceous species, but different patterns in woody species richness emerged due to differences in the distributions of species across the four sites. When graphed as a function of cumulative number of plots, we again found that a low browse site had the highest woody plant species richness, but a high browse site had the highest richness values when richness was graphed as a function of cumulative number of stems sampled. This study demonstrates that although species richness is a useful tool for describing ecosystems, results for the same sites can differ based on how species are grouped and on the scale of analysis.

INTRODUCTION

White-tailed deer (Odocoileus virginianus) browsing is altering plant composition and regeneration in many northern hardwood/eastern deciduous forests. Several studies document the negative effects of herbivory on tree seedling regeneration (Frelich and Lorimer, 1985; Alverson et al, 1988; Waller et al, 1997; Rooney, 2001) and persistence of sensitive herbaceous species (Anderson, 1994; Balgooyen and Waller, 1995; Augustine and Frelich, 1998; Ruhren and Handell, 2000). Browsing further influences the structure of understory woody plant communities by changing the heights and densities of populations being consumed (Gill and Beardall, 2001). Intense browsing may result in the forest plant species' composition shifting towards nonpreferred species (Horsley et al., 2003) and, especially in small forest fragments, can lead to local extinctions of species favored by deer (Anderson, 1994). Thus, in areas where deer populations remain high, simplification of forest structure, altered patterns of secondary succession, and reduced woody plant species richness are likely to result (Rooney, 2001).

Given that a large proportion of a deer's spring and summer diet is composed of herbaceous plants, areas with high spring and summer deer densities may also exhibit strong changes in the species composition of herbaceous communities (Waller and Alverson, 1997). As with woody plants, browsing of herbaceous plants can have impacts ranging from the individual (plant heights and reproductive output) to community (species composition) level (Fletcher et al., 2001; Webster et al., 2001; Côté et al, 2004; Kraft et al, 2004). Grazing of preferred forest species, especially woody regeneration, may also allow populations of native and non-native invasives, ferns and grasses to populate habitats in which they are normally suppressed (Horsley et al, 2003). These browsing-induced changes in understory composition can then further reduce regeneration of typical forest species, potentially leading to alternate stable states in understory plant communities (Stromayer and Warren, 1997; Augustine et al, 1998).

To further refine the specific effects of deer browsing on understory plant communities, we examined the understory within two northern hardwood-dominated areas in northern Michigan where past work documented different intensities of browse on deciduous tree seedlings and saplings (Hall, 2002; in press; Kearns et al. …

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