25-year natural regeneration responses to varying levels of retention harvesting in a dry cool Montane forest of interior British Columbia

Abstract

This study was conducted to investigate the effects that different levels of overstory retention have on the density and species composition of natural regeneration in Douglas-fir forests, 25-years post harvesting, and whether retention silviculture systems could help enhance the establishment of shade-tolerant, late successional tree species. This was assessed by analyzing natural regeneration data that was collected in the fall of 2019 at the Mount 7 research trial near Golden, BC in the dry cool Montane Spruce (MSdk) biogeoclimatic zone. The Mount 7 research trial was harvested in 1996 with four treatments: (1) a clearcut treatment, (2) a light retention treatment (where ~25% of the overstory basal area was retained), (3) a heavy retention treatment (where ~50% of the overstory basal area was retained), and (4) a control site. The results followed the same trends found by Newsome et al (2010), who analyzed the natural regeneration effects 10-years post harvesting. The heavy retention treatment units had the highest density of natural regeneration, followed by the light retention sites, with only small amounts of natural regeneration found at the uncut control and clearcut sites. This was also the case for conifer and shade-tolerant species, but broadleaf and shade-intolerant species had their density peak at the light retention sites, followed by the heavy retention sites. The species composition and richness was also found to be affected by the level of retention; there was a more diverse mixture of species when overstory trees were retained, unless 100 percent of the overstory was retained. These results reveal that by retaining overstory trees, thus increasing the seed supply, the density of natural regeneration will increase, but harvesting, which increases the light availability, is also essential for the establishment of natural regeneration.