Early Growth Responses of Naturally Regenerated Eastern White Pine (Pinus strobus L.) to Partial Release from Juvenile Aspen and Pathological Pruning

Height, breast height diameter, and basal diameter growth responses of 7to 12-year-old naturally regenerated eastern white pine (Pinus strobus L.) to partial release from juvenile (15-year-old) trembling and bigtooth aspen (Populus tremuloides Michx. and Populus grandidentata Michx.) and pathological pruning were monitored over four growing seasons. Pathological pruning is the removal of disease-infected branches before the disease can reach the stem or the removal of lower branches that are most susceptible to infection. Results indicated that seedling growth responses to release and pruning depended strongly on the height stratum to which a seedling belonged. Whereas growth rates of small white pine up to 190 cm tall were significantly reduced by increasing aspen densities and by pruning, growth rates of white pine taller than 190 cm were not significantly affected by either aspen density or pruning. Effects of pruning on small white pine were restricted to the first 2 years after release, after which growth rates were similar between pruned and unpruned individuals. This likely was due to natural crown recession of unpruned white pine, which brought crown lengths and crown ratios closer to those of pruned individuals. Besides affecting natural crown recession rates and growth of small unpruned white pines, release intensity also affected upper stratum aspen, which responded vigorously to release. Findings of this study suggest that early release from upper stratum juvenile aspen should enable the conversion of an aspen cover type to a mixture of aspen and white pine, but may have to be followed up by repeated interventions into the upper canopy stratum. It is, however, questionable if the expense of pruning to prevent blister-rust infections under a partial canopy is warranted. North. J. Appl. For. 22(1):27–34.