Influence of stand thinning on wildlife in managed second-growth forests: tree squirrels, small mustelids, and mammalian species richness

Context Clearcutting of forests reduces food, cover, and stand structure attributes for tree squirrels (Tamiasciurus hudsonicus, Glaucomys sabrinus), small mustelids (Mustela richardsonii, Neogale frenata), and other mammal species. Forest restoration is urgently needed and stand thinning of second-growth forests may help restore habitats for mammals.Aims We tested the hypotheses that in thinned stands of second-growth lodgepole pine (Pinus contorta var. latifolia) at 30-36 years post-thinning: (H1) tree dimensions would be comparable to, or higher than those in uncut old-growth forest; activity of tree squirrels, weasels, and mammalian species richness would be higher in (H2) heavily thinned than lightly thinned stands, and (H3) higher in uncut old-growth forest than in second-growth thinned stands.Methods We measured stand structure attributes and activity of tree squirrels, weasels, and mammalian species richness with camera traps from 2021 to 2023 in replicated (n = 3) treatments of three densities of thinned pine stands and uncut old-growth forest in south-central British Columbia (BC), Canada.Key results Thinned and old-growth stands had mean tree diameters ranging from 18 to 22 cm, but mean tree heights were highest in the old-growth stands. Low-density stands tended to have large diameter crop trees with substantial crowns. There was no evidence for a difference in mean activity of T. hudsonicus and G. sabrinus, other common mammal species, and overall species richness among stands. There were more weasel detections per camera-day in the low-density and old-growth stands than other thinned stands.Conclusions Comparison of stand structure attributes showed partial support for H1. There was insufficient evidence to detect biologically meaningful effects among stands in a mean index of activity for T. hudsonicus, G. sabrinus, other common mammal species, and overall species richness, and hence did not support H2 or H3. There was weak support for H2 and H3 with limited weasel activity.Implications Thinned second-growth stands <= 1000 stems/ha may provide habitat at least for the mammal species and post-treatment period outlined in this study. To our knowledge, this is the first convergence of managed coniferous stands (age 43 years) with old-growth forest (age 140-187 years) in terms of some components of stand structure and mammalian species richness.