Management and productivity of cedar-hemlock-salal scrub forests on the north coast of British Columbia

Abstract

The outer north coast of British Columbia has extensive areas (approx. 235,000ha) of old-growth forests characterized by western redcedar (Thuja plicata), yellow-cedar (Chamaecyparis nootkatensis) and salal (Gaultheria shallon) on gentle to moderate slopes with imperfectly-drained soils and deep forest floor accumulations. Uncertainty surrounding the long-term sustained yield of these low productivity forests led to the establishment of an operational trial in 2001 on Porcher Island, British Columbia, to address management issues, including the risk of paludification with tree removal, the uncertainty in site potential estimates, and the possible extent of nutrient deficiency amelioration with soil raking, mounding or phosphorus fertilization. Each site treatment was randomly replicated four times in an 18ha cutblock. In the second year following logging, the aerobic depth of the soil (as determined by the extent of rust on steel pins) averaged 16.9cm and did not differ between unharvested and harvested plots, suggesting losses in canopy interception and evapotranspiration did not alter the site hydrological balance. Western redcedar survival was 95% with planting and browse protection, and site index estimates of the plantation by age 12 averaged 19.5m at 50years (range 7.6–25.9), in stark contrast to measurements of 3.9m obtained from old-growth stands. Cedar foliar N concentrations increased significantly (range 0.9–1.3%) with the mineral content of soil mounds, demonstrating the benefit of mixing subsoils into the rooting zone, but the wide range in forest floor depths resulted in mounds being overall no more effective than light raking of the forest floor in improving productivity. Phosphorus fertilization (75kgha−1) also provided no gains in soil fertility or cedar nutrition, and initial increases in inorganic P were not detected in soils after 10years. Both site treatments stimulated the natural establishment of red alder (Alnus rubra), averaging 3950 stems per ha, which may both facilitate and compete with the cedar plantation over time. Overall we found few obstacles to sustainable yields from cedar-hemlock-salal scrub forests, albeit with the recognition of longer rotation lengths and smaller volume increments than current operational sites on the north coast of British Columbia.