Does partial harvesting promote old-growth attributes of boreal mixedwood trembling aspen (Populus tremuloides Michx.) stands?

Abstract

In the current context of forest ecosystem management, partial harvesting has been proposed as a silvicultural tool to augment forest variability on managed landscapes and to accelerate the development of structural and compositional attributes of old-growth/late successional stands. The aims of this paper were to (1) identify and characterize, based on the literature, the structural attributes of old-growth aspen-dominated stands in the North American boreal mixedwood forest, and (2) examine the short-term potential of partial harvesting in aspen-dominated stands to accelerate stand development toward these old-growth characteristics. Two stand types – pure aspen (93% aspen basal area) and mixed aspen (81% aspen basal area) – were monitored over a 12-year post-treatment period. The scientific literature suggests that compared to pure, even-aged premature or mature stands, old-growth aspen stands have lower merchantable stem densities and basal area, more large aspen stems, higher stem size variability, more than one cohort of trees, greater percentage area occupied by gaps, higher expanded gap area, and more and larger snags and downed wood. In addition, old-growth aspen mixedwoods characteristically have more shade-tolerant conifers in understory and overstory layers than younger, mature stands. Results of this study indicate that light thinning from below (33% basal area removal) applied in pure aspen stands successfully retained most of the structural attributes of mature aspen stands, but did not generally “accelerate succession” toward old-growth traits in the 12-year time interval since treatment. A dispersed free thinning (45% basal area removal in all merchantable size classes) applied in mixed aspen stands showed its potential to “accelerate succession” by creating canopy gaps similar to old-growth aspen stands and by promoting recruitment of both tolerant and intolerant tree species. Two high intensity partial harvesting treatments, a thinning from above of 61% basal area in pure aspen stands and 400m2 gap cuts (54% basal area removal) in mixed aspen stands may set back stand development by disproportionally favoring recruitment and growth of intolerant hardwood species.