"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) in clearcuts. A replicated research trial, using a uniform shelterwood silvicultural system, involved harvesting in three passes over a 20-year period to test how residual basal area retention affected establishment, growth, and condition of natural regeneration."-- page iii.

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) in clearcuts. A replicated research trial, using a uniform shelterwood silvicultural system, involved harvesting in three passes over a 20-year period to test how residual basal area retention affected establishment, growth, and condition of natural regeneration. Starting with even-aged stands of mature Douglas-fir averaging 61 m2/ha, a preparatory cut in 1991 resulted in 40 m2/ha retention, which was followed by a regeneration cut in 2001 that reduced the basal area to 20 m2/ha. Final harvesting took place in 2011, which reduced the basal area of mature trees to 0 m2/ha in the previously uncut control and shelterwood treatments. Microclimate monitoring stations were installed at three research sites in 2012 and were monitored for 5 years to compare vertical air temperature profiles in the former 60 m2/ha uncut controls (60) and in the former 20 m2/ha shelterwoods (20) at each site. In the 20 treatment units, the percent cover and modal height of the tall (> 2 m) shrub layer, primarily Douglas-fir and hybrid white spruce (Picea glauca × engelmanni) regeneration, was much greater (27−53%, 3.5−5.7 m) than in the 60 treatment units (< 10%, 3.1−3.6 m) that had no previous harvesting history. Air temperature profile data at 15, 40, 75, and 150 cm above ground showed that strong temperature inversions frequently occurred in the 60 treatment units on clear, calm nights, while well-mixed, neutral (isothermal) profiles were observed in the 20 treatment units (2012–2016). This led to more frequent frosts, greater duration of subfreezing minutes, and lower extreme air temperatures during the growing season at the lower measurement heights in the 60 treatment units. We assessed frost damage on planted Douglas-fir seedlings in two of the 60 treatment units. Nighttime minimum air temperatures were as low as –8.1°C on the night of June 4–5, 2014 for the 15-cm measurement height in the 60 treatment units, which caused frost damage on 64% of the seedlings. In contrast, tall regeneration in the 20 treatment units showed no apparent frost damage. Over the 25-year study, air temperature data were collected during three time periods to compare various treatments (uncut, clearcut, and shelterwood). When combined for one of the research sites (Beedy Creek), they clearly showed the relationship between overstorey forest cover and frost frequency. Frost occurred frequently (up to 12 occurrences during the May 15−July 31 period) following harvesting of the 60 treatment units in 2011, compared with less than one occurrence per season in the same location prior to harvesting (1993−1995). The frequency of frost occurrences increased from less than one per season (1993–95) to about seven per season (2012–16) in the same 20 treatment unit after basal area was reduced to 0 m2/ha in 2011. However, the moderate frost frequency that occurred in the shelterwood 20 treatment units following final harvesting in 2011 was of little consequence to the tall, well-established Douglas-fir regeneration. A shelterwood system that uses a regeneration cut (or in combination with a preparatory cut) can lower the frequency and duration of frost events, as well as moderate extreme temperatures during the establishment phase, iv and after the final cut, can leave a well-developed regeneration layer that can moderate and survive frost.