Understory competition effect on tree growth and biomass allocation on a coastal old-growth forest cutover site in British Columbia

We studied the effect of salal (Gaultheria shallon Pursh.) competition on height, diameter and biomass growth and biomass partitioning in coniferous trees planted to a recent clearcut site of old growth western red cedar-western hemlock (CH) forest on northern Vancouver Island, British Columbia. Tree species used were western red cedar (Thuja plicata Donn ex D. Don), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Sitka spruce (Picea sitchensis (Bong.) Carr). Salal removal treatment was initiated at the time of planting in spring 1987. Plots were fertilized with 200 kg N ha−1 in spring 1991 and destructively sampled in fall 1992. Height growth from planted to 1989, in 1992 and total height growth were significantly greater in treated plots (salal removed) than in the control plots (salal remaining). Salal removal had a rather uniform impact on height growth for the three species tested. Total root collar diameter was 38% (P < 0.1), 88% (P < 0.05), and 65% (P < 0.05) greater in the treated plots than in the control plots, for red cedar, hemlock and spruce, respectively. Exclusion of understory vegetation had resulted in biomass increases of all the components (namely in the 1-year and 2-year foliage and branches and the older than 3-year components and various sized roots) we studied. Improved tree growth in the treated plots was attributed to the reduced uptake and immobilization of N and other nutrients by the competing understory. Below-ground understory was found to be quite persistent to surviving even after a prolonged period (6 years) of above-ground understory vegetation removal