Relations of Small Mammal Populations to Even-Aged Shelterwood Systems in Sub-Boreal Spruce Forest

Abstract

Natural regeneration of cutover forest lands and integration of nontimber resources into forest management have generated investigation of alternative silvicultural systems to clearcutting. An overstory canopy, or shelterwood, may reduce the frequent frost problems observed in interior Douglas-fir (Pseudotsuga menziesii var. glauca) plantations and improve the success of natural regeneration, if seed predation by small mammals does not threaten the survival of the seed supply. We tested the hypothesis that a shelterwood silvicultural system would adversely affect dynamics of small mammal populations in 30% and 50% basal area (BA) removal stands, and we evaluated natural seed production, predation, and germination under a shelterwood canopy in the southcentral interior of British Columbia. Deer mice (Peromyscus maniculatus) and southern red-backed voles (Clethrionomys gapperi) were the most common small mammal species prior to harvest, and both species continued to be dominant in the first and second year postharvest. Abundance (P = 0.007) and recruitment (P = 0.015) of red-backed voles was 50% higher in shelterwood (treatment) than unharvested (control) stands. The shelterwood harvest did not affect abundance (P = 0.42) or recruitment (P = 0.47) of deer mice. There were no differences in average survival of deer mice and red-backed voles between control and treatment stands (P > 0.05). Mean body mass of adult male deer mice was similar in control and treatment stands (P = 0.27) but was lower for red-backed voles in the 50% BA removal than control stands (P = 0.05). Seedfall crops in the second year postharvest were 7.4-29.6 times larger than in the first year on treatment and control units, but there was no difference between units in either year (1992: P = 0.24; 1993: P = 0.93). Seed predation seemed to fluctuate with seasonal levels of small mammal populations (lower in spring and higher in fall) but not with the amount of seed crop available. Germination occurred in 62% of the seeds that survived overwinter on the seedbeds created during harvesting. Thus, regeneration success, as measured by seed survival and establishment, did not seem affected by the small mammal communities in 30% and 50% BA removal stands.