Tree Harvesting Needs Changing for Seedling Survival

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 percent of the sunlight, more than 40 percent of planted lodgepole pine and ponderosa pine die within six years. Douglas-fir seedlings are somewhat more tolerate of shade, with 40 percent mortality exceeded at less than 35 percent of full sunlight. Under light levels above 55 percent, survival rates rise to over 70 percent, 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 percent 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.