The hot, dry forests of southern BC’s Interior Douglas-fir biogeoclimatic zone are difficult to regenerate after partial retention harvesting. Now research shows that the conventional wisdom of seedlings needing shade to survive is misleading. The more open the overstory, the better conifers are at surviving and growing.

When overhead trees block all but 45% of the sunlight, more than 40% of planted lodgepole pine and ponderosa pine die within six years. Douglas-fir seedlings are somewhat more tolerate of shade, with 40% mortality exceeded at less than 35% of full sunlight. Under light levels above 55%, survival rates rise to over 70%, and both stem diameter and height gains increase. Of the three species, lodgepole pine responds most markedly to reduced shading.

This translates into these dry Douglas-fir forests needing an overstory tree basal area below 15 m² for more than 60% of tree seedlings to survive. At less than 10 m² basal area, young conifers grow vigourously. Instead of the predominant dispersed retention method of harvesting, an aggregated retention would create more open sites, enabling better tree regeneration.