In the Sub-Boreal Spruce dry warm (SBSdw) biogeoclimatic subzone, on the Interior Plateau of British Columbia, frost is a limiting factor for the establishment and growth of Douglas-fir (Pseudotsuga menziesii). A research trial, using a uniform shelterwood silvicultural system, was harvested in 1991, then again in 2001, to test how residual basal area retention affected regeneration establishment, growth, and condition. Microclimate stations were installed in two of the residual basal area treatments (15 m²/ha and 20m²/ha) to measure frost events. From 2001 to 2008, near-ground air and soil temperatures were monitored on two pairs of adjacent 20m²/ha and 15 m²/ha treatments along with one additional replicate of the 20m²/ha treatment, and a clearcut.
Minimum air temperatures and total duration (minutes) of air temperatures below 0°C were compared between treatments during the bud flush (15 May-31 July) and bud set (15 August-30 September) seasons. The clearcut treatment had a much longer duration of subfreezing minutes than the forested treatments for both bud flush and bud set periods, as well as a greater number of frosts and lower extreme minimum temperatures. Differences between the 15 and 20m2/ha treatments were not as great; however, both 15 m²/ha treatments had longer duration of subfreezing minutes and increased numbers of frosts compared to the 20m2/ha treatments. The sky view factor increased as basal area decreased, and was positively correlated with the duration of subfreezing minutes. There were few significant frost events during the June to mid-August period in the forested treatments over the 7-year study. The data suggest that residual basal areas of 15 m2/ha or greater provide adequate frost protection for regeneration.
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Updated March 17, 2010