Influence of variable retention harvests on forest ecosystems. II. Diversity and population dynamics of small mammals.
Abstract
Variable retention harvests in temperate coniferous forests provide various intensities and sizes of disturbance across a gradient, from clear felling to single-tree harvesting. These 'green-tree retention systems' leave large live trees after harvest (i.e. residual trees) to increase structural diversity of the regenerating stand. It is unclear what effect these harvesting patterns will have on wildlife in general, and small mammals in particular. This study was designed to test the hypotheses that: (i) the abundance and diversity of forest-floor small mammals will decline with lower levels of tree retention; (ii) the abundance, reproduction and survival of Microtus spp. and southern red-backed vole Clethrionomys gapperi populations will decline and increase, respectively, with the basal area and density of residual trees after harvest; and (iii) habitat heterogeneity generated by variable retention harvesting will limit population size of Microtus, thereby reducing feeding damage to tree seedlings planted on harvested areas. Small mammals were intensively live-trapped from 1996 to 1999 in replicated clear-cut, single seed tree, group seed tree, patch-cut and uncut forest sites in mixed natural forests of Douglas fir (Pseudotsuga menziesii)-lodgepole pine (Pinus contorta) in southern British Columbia, Canada. The seed tree harvesting system leaves a few windfirm seed trees standing singly, or in groups, to provide seed to regenerate an area naturally. The patch-cut system harvests timber from small (<1-ha) units dispersed over a given area of forest. Contrary to hypothesis i, the mean total abundance of small mammals was similar among sites, and mean species richness and diversity of small mammals was lowest in the uncut forest but similar across the other four treatments. The overall mean abundance (ha-1) of Microtus spp. was higher on clear-cut (11.3) sites than single seed tree (6.0), group seed tree (3.9), patch-cut (1.8) or uncut forest (0.1) sites. Overall mean abundance (ha-1) of C. gapperi was similar in uncut forest (16.2) and group seed tree (10.0) sites, which were both higher than patch-cut (6.4), clear-cut (4.2) or single seed tree (2.7) sites. The recruitment of new Microtus spp. differed significantly among sites, declining from clear-cut to uncut forest. The recruitment of C. gapperi exhibited the opposite trend. The mean number of successful pregnancies was similar among treatments for Microtus spp. but for C. gapperi was highest in uncut forest, group seed tree and patch-cut sites. The survival of each vole species was similar across treatment sites. Hypothesis ii was supported. The mean abundance of Microtus spp. was inversely, and that of C. gapperi positively, related to mean basal area and density of residual trees after harvest. The mean abundance of Microtus spp. was also inversely related to percentage cover and crown volume index of residual trees, and positively related to number of lodgepole pine cones (seeds) in logging debris. 8. In terms of hypothesis iii, vole feeding damage to planted lodgepole pine seedlings appeared highest in the patch-cut sites during the 1997-98 and 1998-99 overwinter periods, and appeared to have been caused mainly by C. gapperi rather than Microtus spp. The group seed tree and patch-cut systems achieved both aims of C. gapperi persistence and prevention of Microtus outbreaks. These harvesting methods are as good for overall small mammal abundance, species richness and diversity as any other.