Partial and Clear-Cut Harvesting of High-Elevation Spruce-fir Forests: Implications for Small Mammal Communities
Authors: Walt Klenner and Thomas P. Sullivan
By Matt Dolhoff (2004), revised by Ruth Yanai in 2005.
Small mammals are an integral part of forest ecosystems. They disperse plants and fungi, and they are prey for larger mammals, birds and reptiles (Dickson 2001). Small mammals prey on some insects that have the potential to drastically alter forest ecosystems (Carey and Johnson 1995). Though they may seem to be only a small part of the puzzle, small mammals can serve as indicators of forest sustainability (Sullivan and Sullivan 2001). The abundance of some small mammal populations can serve as a measure of the impact associated with different forest management practices on ecosystem function (Klenner and Sullivan 2003). Small mammals can affect the regeneration of a stand after harvest, for example because the mammals prefer spruce seeds to fir seeds (Hart et al. 1968). High elevation spruce-fir forests are of particular interest because they are expected to be a major source of timber in the future (Klenner and Sullivan 2003). As a result, there have been a number of studies performed in the area, but few of them have focused on aspects of ecosystem components unrelated to timber production.
Summary of Study
The objective of this particular study was to describe the effect of several types of forest management on red-backed voles, long-tailed voles, northwestern chipmunks and deer mice. The authors hypothesized that reproduction, abundance and survival of red-backed voles would decline as levels of tree retention decreased, while long-tailed voles and chipmunks would increase, and the deer mice would remain stable through most management practices. To test this hypothesis, they sampled 18 areas in a forest of subalpine spruce, fir, and Engelmann spruce near Sicamous, British Columbia. The treatments were a control that had no cutting, a single tree selection that cut every fifth tree, a 0.1 ha patch cut, 1 ha patch cuts, 10 ha clear-cuts, and an edge habitat bordering the 10 ha clear-cut. They live trapped the small mammals at 3-week intervals from July to September in 1994, 1995, and 1996, from June to September in 1997, and from June to August in 1998, with the harvests occurring after the captures in 1994.
They found that red-backed voles, which are associated with closed-canopy forests, decreased in only the clear-cut portions of the study, where they almost became non-existent, and remained consistent through the other treatments. The long-tailed vole and the chipmunks, which are associated with early successional habitats, were non-existent in the uncut stands, highest in the 10 ha clear-cuts, and intermediate through the remaining treatments. The deer mice, which are habitat generalists, were found in about the same numbers in all 6 of the treatments, as expected.
This study provided new information in that the population dynamics of small mammals had not previously been evaluated as a function of harvesting intensity in this forest type. The animals studied responded almost exactly as predicted; the purpose of the study was not to provide new information about the habitat requirements for these species. Over many such studies, however, new patterns may be observed, such as the success of red-backed voles in clearcuts in eastern North America, in contrast to western North America.
I was concerned that not all of the species of mammals in the test site would be equally willing to enter the live traps. This would skew the total numbers of individuals in each site. The authors acknowledged this problem and used Jolly-Seber population estimates to correct for it. Second, all of the measurements were taken in a single experimental forest in British Columbia. This makes it hard to apply the information to another part of the country or continent, since factors affecting population dynamics, other than those associated with forest management, could vary geographically. If the study could be replicated in one or more other areas, we would know if the results were more widely applicable. A strength of the study was the replication of the treatments. We can be confident, for this site, at least, that the differences reported are in fact due to treatments, and not to other factors not controlled for in the experimental design.
I was intrigued by the suggestion that the reason red-backed voles increased after clearcutting in eastern North America (in 15 of 21 studies reviewed by Kirkland, 1990) was that over 300 years of this forest practice in that region, the voles have adapted by changing their diets (Schloyer 1977; Gliwicz and Glowacka 2000). The same species in western North America declines after clearcutting. That this difference is due to behavioral adaptation would seem to be a testable hypothesis. If 300 years of experience is required for these voles to learn to eat seeds, leaves, and berries instead of fungi and lichens, then we should see a difference in the success of eastern and western members of the species in a reciprocal transplant experiment (bring western voles to an eastern clearcut and vice versa). The dietary preferences of the eastern and western voles could presumably be studied directly in captivity. I am skeptical about this hypothesis, and so I expect that additional research would show it to be unfounded. Other differences between the eastern and western environments might prove to be more important than differences in the dietary preferences of the voles.
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