Academic journal article The American Midland Naturalist

Effects of Blowdown on Small Mammal Populations

Academic journal article The American Midland Naturalist

Effects of Blowdown on Small Mammal Populations

Article excerpt

ABSTRACT.-

Over 150,000 ha of standing forest was altered as a result of a large-scale blowdown in the Boundary Waters Canoe Area Wilderness, Minnesota in 1999. We collected data in summers 2000 and 2001 to assess the effects of windthrow perturbation on small mammal communities in northern coniferous forests. Small mammal diversity, as well as density of the two most common species, red-backed voles (Clethrionomys gapperi) and woodland jumping mice (Napaeozapus insignis), were determined in three different treatments with varying proportions of blowdown (<33%, 33-66% and >66% blowdown). Diversity of small mammals increased from 2000 to 2001 and was highest in forest stands with <33% blowdown. The density of the two most abundant species of small mammals also differed among blowdown categories. Red-backed voles predominated at all sites, but exhibited peak densities (>25 individuals/ha) at sites most affected by blowdown. In contrast, density of woodland jumping mice exhibited an inverse relationship with red-backed voles, attaining peak densities (8 individuals/ha) in stands with <33% blowdown. Age ratios (juveniles: adults) were not influenced by year for either woodland jumping mice or red-backed vole populations, but differed for vole populations among blowdown categories. Juvenile red-backed voles predominated at sites with 33-66% (2.2: 1) and <33% blowdown (1.2: 1). Red-backed voles selected for blowdown and appeared to displace other small mammal species from this habitat. Because red-backed voles feed on coniferous seedlings, are primary dispersers of mycorrhizae and are prey for many predators, their selection of blowdown habitat could significantly influence community assemblages and forest succession following blowdowns.

INTRODUCTION

Small mammals are critical members of conifer-northern hardwood forests because of the multiple ecological roles they fulfill as prey (Hayward and Phillipson, 1979) predators (Maxson and Oring, 1978) and dispersers of seeds and of spores of mycorrhizae (Maser et al., 1978; Terwilliger and Pastor, 1999). Small mammal populations are particularly influential in northern forests following large-scale disturbances by altering subsequent successional processes (Sullivan and Sullivan, 2001; Howe and Lane, 2004). Understanding how small mammal communities adjust to major alterations in their habitat enhances our understanding of the ecology of forest systems (Sousa, 1984) and provides a basis for predicting long-term floral and faunal responses to large-scale disturbances.

Three forces that dramatically alter extensive tracts of forest habitat are clearcuts, fires and wind (Powell and Brooks, 1981). Changes in vegetation and small mammal populations are well documented for clearcuts (Noble et al, 1977; Kirkland, 1990; Sullivan et al., 1999) and burns (Ahlgren, 1966; Krefting and Ahlgren, 1974), but few studies have quantified the effects of windthrow on forest structure or mammal populations. Past observations suggest that the vegetational effects of blowdowns do not closely parallel those reported after fire or logging (Powell and Brooks, 1981). Unlike fire and clearcuts, which destroy much of the vegetation, blowdowns leave affected vegetation intact on the forest floor. Different effects on vegetation might cause distinctive perturbations of animal communities. However, only limited data were collected previously concerning mammalian responses to blowdown, even though blowdown events are common in coniferous forests (Dunn et al., 1983; Canham and Loucks, 1984).

Indeed, only two studies have addressed the effects of blowdown on small mammal communities (Powell, 1972; Powell and Brooks, 1981). These studies were limited to just three species [red-backed vole (Clethrionomys gapperi), deer mouse (Peromyscits maniculatus) and masked shrew (Sorex cinereus)], which were investigated by means of kill trapping. In both studies, overall small mammal abundance appeared to increase in response to blowdown, but red-backed voles seemed to benefit most from blowdown, exhibiting the most pronounced increases in density. …

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