Green-tree retention and relative habitat use by mule deer (Odocoileus hemionus) 20 years after harvest of Douglas-fir (Pseudotsuga menziesii) – Lodgepole pine (Pinus contorta) forest

Abstract

Mule deer (Odocoileus hemionus) are a major mammal species in many interior Douglas-fir (Pseudotsuga menziesii) forest ecosystems in western North America. During winter in areas of relatively high snowpacks, mule deer seem to require older trees with large crowns for snow interception, but these forests are often targeted for harvest. Green-tree retention (GTR) may offer a solution to this conundrum. We tested three hypotheses (H) that, at 20–22 years after harvest, (H1) abundance (e.g., basal area and density of residual trees and understory stand structure) of conifers, (H2) abundance, species richness, and diversity of understory vegetation, and (H3) relative habitat use by mule deer, will decline with lower levels of tree retention. Fecal pellet-groups of deer were counted in summer and winter periods for three years in replicated stands of dispersed retention, aggregated retention, patch-cut, group selection, and uncut forest in mixed Douglas-fir - lodgepole pine (Pinus contorta) forest in southern British Columbia, Canada.Mean basal area, density, and canopy closure of overstory Douglas-fir trees were generally greater in the patch-cut, group selection, and uncut forest sites than dispersed and aggregated retention sites, and hence supported the overstory part of H1. Mean density of understory conifers did not support H1 as the majority of these trees were in the dispersed and aggregated retention stands. There was no difference in mean abundance nor species richness of understory vegetation among treatment sites. Mean species diversity tended to be 1.4–1.8 times higher in the patch-cut and group selection than the other sites, and hence provided weak support for H2. Patch-cut and group selection sites were dominated by Douglas-fir trees in both the overstory and understory, along with potential forage shrub species, and were used by deer more than the dispersed and aggregated retention stands in both summer and winter. Relative habitat use by deer in uncut forest was greater than the retention stands and generally similar to the patch-cut and group selection sites in both seasons. These results suggest that partial harvesting of these mixed Douglas-fir – lodgepole pine stands across a range of opening sizes (e.g., 0.1 to 2–5 ha) should provide suitable habitat for mule deer over time.