Influence of variable retention harvests on forest ecosystems: Plant and mammal responses up to 8 years post-harvest

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Green-tree retention systems are an important management component of variable retention harvests in temperate zone coniferous forests. Residual live trees (“legacy trees”) provide mature forest habitat, increase structural diversity, and provide continuity in the regenerating stand. This study was designed to test the hypotheses that, at up to 8 years after harvest, abundance and species diversity of communities of (i) understory plants and (ii) forest-floor small mammals, and (iii) relative habitat use by mule deer ( Odocoileus hemionus), will decline with decreasing levels of tree retention. Communities of plants and forest floor small mammals were sampled in replicated clearcut, single seed-tree, group seed-tree, patch cut, and uncut forest sites in mixed Douglas-fir ( Pseudotsuga menziesii)—lodgepole pine ( Pinus contorta) forest in southern British Columbia, Canada from 2000 to 2003 (5–8 years post-harvest). Habitat use by mule deer was measured during summer and winter periods each year from 1999 to 2003 in these same sites. Mean total abundance (crown volume index) of herbs, shrubs, mosses, and lichens was similar among sites. Mean species richness of herbs, shrubs, and total plants was similar among sites, but total species diversity and structural diversity were significantly lower in the patch cut and uncut forest sites than in the other harvesting treatments. Thus, hypothesis (i) was not supported. Mean total abundance, species richness, and species diversity of small mammals were similar among sites, contrary to hypothesis (ii). However, the southern red-backed vole ( Clethrionomys gapperi) declined in abundance as conditions became intolerable for this microtine to persist at numbers > 1/ha in the clearcut and seed-tree sites. The early successional and mycophagist northwestern chipmunk ( Tamias amoenus) occurred at 2.3–4.4 times higher abundance on the seed-tree sites than the other sites. Relative habitat use by mule deer was highest in the seed-tree sites during summer periods and highest in the group seed-tree, patch cut and uncut sites in winter periods. The responses to our treatments were species specific, and hence a range of different harvesting systems should be used to maintain plant and mammal diversity across forest landscapes.