Growth response of young lodgepole pine to thinning and repeated fertilization treatments: 10-year results

Abstract

This study was designed to test the hypothesis that large-scale pre-commercial thinning (PCT) to various stand densities, at ages 12–14 years, combined with repeated fertilization, would, over a 10-year treatment period, enhance productivity of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) crop trees. Study areas were located near Summerland, Kelowna and Williams Lake in south-central British Columbia, Canada. Each study area had nine treatments: four pairs of stands thinned to densities of ∼250 (very low), ∼500 (low), ∼1000 (medium) and ∼2000 (high) stems ha−1, with one stand of each pair fertilized five times at 2-year intervals, and an unthinned stand. The very low, low- and medium-density stands were also pruned to a 3-m lift 5 years after thinning. At the tree level, fertilization treatments significantly increased diameter at breast height (DBH), basal area (BA) and volume growth and heavy PCT significantly increased DBH and BA growth. Pruning may mitigate some of the negative stem form and wood quality attributes associated with fast-growing trees without adversely affecting stem growth. At the stand level, PCT to very low and low densities significantly decreased the volume growth compared with high-density stands. The potential beneficial impacts that PCT and repeated fertilization treatments have for mitigating timber supply shortfalls, as well as potentially minimizing crop tree losses due to mountain pine beetle (Dendroctonus ponderosae Hopk.), are also discussed.