Effects of partial harvesting on species and structural diversity in aspen-dominated boreal mixedwood stands

Partial harvesting is thought to result in more structurally diverse forest stands, providing a promising alternative to clearcutting and helping balance timber production and biodiversity conservation. Evidence for this hypothesis is based mostly on understory vegetation communities, with little information available on how partial harvesting affects tree species and structural diversity. In this study, we examined overstory and understory tree species and structural diversity of boreal mixedwoods over 20 years in stands that were unharvested, partially harvested with 40% canopy removal, and clearcut. As expected, clearcutting reduced overstory tree species and structural diversity relative to that of unharvested stands; partial harvesting, however, did not affect neighbourhood (area of 4 trees including one subject tree and 3 neighbours) or stand diversity until after a forest tent caterpillar outbreak that occurred during years 3-5 post-harvesting. The insect-induced trembling aspen (Populus tremuloides Michx.) mortality was higher in the partially harvested than in the unharvested areas decreasing structural diversity at both neighbourhood and stand scales. In comparison, within-stand variations (horizontal complexity or spatial heterogeneity) in neighbourhood diversity values (Shannon species diversity, Shannon structural diversity by 2-m height class, and structural diversity by height variation) increased following the insect outbreak. The post-harvesting temporal dynamics of overstory diversity appeared time dependent and related to harvest removal, forest tent caterpillar defoliation, and growth of released balsam fir (Abies balsamea (L.) Mill.) and aspen regeneration. In all treatments, understory diversity was decreased by clearcutting, which stimulated profuse aspen suckering, and abundant balsam fir regeneration. We conclude that partial harvesting with 40% canopy removal did not significantly change tree species or structural diversity of boreal mixedwood stands in northeastern Ontario, Canada. Changes in overstory diversity following a forest tent caterpillar outbreak, however, suggest that partial harvesting that removes more of the canopy and reduces individual tree dominance may increase species diversity and within-stand structural variation but reduce structural diversity in the overstory.