Establishment and early growth of Douglas-fir seedlings under low shelter in southern Sweden

Diversifying tree species is increasingly important for enhancing forest resilience under climate change in Sweden. Douglas-fir (Pseudotsuga menziesii) is a promising alternative to native conifers but seedlings are sensitive to frost damage during establishment on exposed sites. This study evaluated the establishment and early growth of Douglas-fir seedlings planted under low shelters of different compositions. A field experiment in southern Sweden tested five shelter treatments: two densities of naturally regenerated birch (Betula spp.), Norway spruce (Picea abies L. Karst.) shelter, mixed birch-spruce shelter, and a clearcut control. Seedling survival, frost damage, height growth, top shoot elongation, and morphological quality were monitored over five years. Survival was high across all treatments and unaffected by shelter treatment. Frost damage was significantly reduced under all shelter treatments compared with the control, with the strongest protection provided by the densest birch shelter. Shelter also helped preventing multiple leaders, indicating improved seedling quality. No significant negative effects of low shelter on early height or shoot growth were detected. The results demonstrated that a low shelter can effectively mitigate frost risk during Douglas-fir establishment without compromising early seedling height growth, supporting its use as a practical silvicultural tool for climate-adapted forestry in Sweden.