Growth and form of Douglas-fir adjacent to persistent vine maple gaps in southwestern British Columbia

Abstract

Vine maple is distinct in its ability to prevent conifer establishment and create persistent gaps. The goal of our research was to determine the influence of persistent vine maple gaps on the morphology, basal growth, and the productivity of Douglas-fir within a temperate rainforest. On 10 paired plots, gap size, Douglas-fir crown and bole size, site chronologies and site productivity were examined in a 75-year-old, second-growth stand. Median gap size was 80 m 2. Douglas-fir on the gap edge had 61% greater growing space, 182% larger crowns and 46% larger boles than Douglas-fir in the forest matrix. Crown expansion into the gap and the long-term presence of the canopy gap has resulted in crowns on the side of the tree adjacent to the gap that were 62% wider and 36% deeper than closed canopy crowns, resulting in crown volumes of Douglas-fir adjacent to gaps that were 182% greater than for trees in the closed canopy. Annual basal area increments (BAI) for Douglas-fir on gap plots exceeded BAI for Douglas-fir in the closed canopy by as much as 70%. Site index was significantly higher on gap plots when compared to closed canopy plots, indicating vine maple may play an important role in the long-term fertility of forest ecosystems where it is a dominant component in the understory. Site BA and recent BA productivity did not significantly differ between gap and canopy plots. Persistent gaps in Douglas-fir stands offer an opportunity for silviculturists to incorporate biodiversity and stand structural objectives into silvicultural systems without significant trade-offs in timber production.