Eighteen-year growth responses to thinning and fertilization of a height-repressed lodgepole pine stand in interior British Columbia

Challenges to the mid-term timber supply in interior British Columbia resulting from losses to insect outbreaks and wildfires have prompted interest in silvicultural interventions to hasten growth of regenerating forests, including high-density (overstocked), height-repressed stands of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.). A trial was established in a dense (4000 to 30 000 trees ha(-1)) 36-year-old wildfire-origin pine stand in the Cariboo-Chilcotin region of interior BC to assess the efficacy of thinning, fertilization and a combination of the two treatments for improving tree growth and stand development Height and diameter of the 36 crop trees in each plot was greater in fertilized plots then control plots for six years following fertilization, while the height and diameter responses to thinning and thinning + fertilization were sustained throughout the 18-year measurement period. Cumulative basal area and volume growth of crop trees and all stand trees over the 18-year period was significantly greater in thinned and thinned + fertilized treatments than control plots. Projections based on the TASS growth and yield model suggested that total stand volumes in thinned plots would surpass that of the control at age 74 and would surpass that of the fertilized treatment at 89 years. Unthinned stands (i.e., control and fertilized treatments), did not accumulate significant merchantable volume; in thinned stands (i.e., thinned and thinned + fertilized treatments), the minimum harvest criteria of 80 m(3) ha(-1) for pine-leading stands in this region was reached at age 102 years. Repeated fertilization would be needed to further hasten stand development. The potential long-term benefits of these silvicultural interventions for timber supply must be weighed against treatment costs and risks of mortality related to wildfire, insects and disease. These results highlight the value of long-term research trials for informing silviculture decisions.