Long-Term Impacts of Commercial Thinning and Nitrogen Fertilization on Lodgepole Pine: A Two-Decade Analysis of Growth and Temporal Dynamics

Abstract

Commercial thinning and fertilization are potential silvicultural tools to enhance Alberta’s Forest productivity in an imminent timber supply gap. In 2000, an experiment was initiated in a 68-year-old lodgepole pine stand located in Alberta’s upper foothills, using a 2x6 factorial design. Commercial thinning, removing 50% basal area was carried out at two levels (thinned and unthinned), while nitrogen fertilization was applied at six levels (No fertilizer, 200 kg/ha N Urea + Boron, 200 kg/ha N + blend, 400 kg/ha N + Boron, 400 kg/ha N + blend, and 400 kg/ha N Ammonium nitrate + Boron). Two studies were conducted, utilizing 20 years of post-thinning data, to analyze: a) the individual and combined effects of thinning and fertilization on growth and mortality and b) the temporal growth response to commercial thinning using tree cores. Key findings of the first study were that commercial thinning led to an increase in the cumulative merchantable volume (the volume removed at the time of thinning + the final standing volume) and the proportion of trees suitable for good sawlogs (>20 cm DBH) by 20%. Individual tree DBH was enhanced through thinning and fertilization treatments, both individually and in combination. This indicates that trees subject to both thinning and high fertilization (at 400 kg/ha levels) exhibited the greatest individual growth. From the second study we found the size of individual trees at the time of thinning significantly predicted their subsequent growth for both studies. Smaller trees (10.1-12.5 cm) exhibited a delayed response to thinning within the first 5 years, followed by a slow growth increment. Medium-sized trees (12.5-17.5 cm) displayed an immediate growth response to thinning but declined after 15 years. The largest trees (17.6-20 cm) exhibited a slower growth response, which persisted beyond 15 years. Consequently, commercial thinning and fertilization can serve as a viable management tool to enhance the growth of natural lodgepole pine stands in Alberta. Commercial thinning and fertilization are potential silvicultural tools to enhance Alberta’s Forest productivity in an imminent timber supply gap. In 2000, an experiment was initiated in a 68-year-old lodgepole pine stand located in Alberta’s upper foothills, using a 2x6 factorial design. Commercial thinning, removing 50% basal area was carried out at two levels (thinned and unthinned), while nitrogen fertilization was applied at six levels (No fertilizer, 200 kg/ha N Urea + Boron, 200 kg/ha N + blend, 400 kg/ha N + Boron, 400 kg/ha N + blend, and 400 kg/ha N Ammonium nitrate + Boron). Two studies were conducted, utilizing 20 years of post-thinning data, to analyze: a) the individual and combined effects of thinning and fertilization on growth and mortality and b) the temporal growth response to commercial thinning using tree cores. Key findings of the first study were that commercial thinning led to an increase in the cumulative merchantable volume (the volume removed at the time of thinning + the final standing volume) and the proportion of trees suitable for good sawlogs (>20 cm DBH) by 20%. Individual tree DBH was enhanced through thinning and fertilization treatments, both individually and in combination. This indicates that trees subject to both thinning and high fertilization (at 400 kg/ha levels) exhibited the greatest individual growth. From the second study we found the size of individual trees at the time of thinning significantly predicted their subsequent growth for both studies. Smaller trees (10.1-12.5 cm) exhibited a delayed response to thinning within the first 5 years, followed by a slow growth increment. Medium-sized trees (12.5-17.5 cm) displayed an immediate growth response to thinning but declined after 15 years. The largest trees (17.6-20 cm) exhibited a slower growth response, which persisted beyond 15 years. Consequently, commercial thinning and fertilization can serve as a viable management tool to enhance the growth of natural lodgepole pine stands in Alberta.