Eleven-year responses of a boreal mixedwood stand to partial harvesting: Light, vegetation, and regeneration dynamics

Abstract

Light, vegetation, and regeneration dynamics were monitored for 11 years as part of a partial harvesting experiment in an aspen-dominated boreal mixedwood stand in northeastern Ontario. The partial cut treatment removed 43% of the original basal area in the overstory. An outbreak of forest tent caterpillar between years 3 and 5 further reduced the original overstory (about 22%, mainly aspen) in the partial cut and uncut treatments. Clearcutting enhanced the density of hardwood regeneration but reduced the amount of conifer regeneration compared with the partial cut and uncut treatments. Reducing the overstory canopy increased the height growth of hardwood regeneration, density of speckled alder—the dominant tall shrub on site, and understory light levels but did not change the mean heights of conifer regeneration and shrubs. Harvesting generally did not affect the amounts of low understory vegetation, except mosses, which decreased significantly in cover in the clearcut. Although most of the conifer regeneration was still under hardwood regeneration and shrub layers, much of the advance conifer regeneration had reached over 4.0 m (large sapling) between years 5 and 11 in both partial cut and uncut treatments. From years 5 to 11, the residual overstory inhibited hardwood regeneration and understory shrubs even more, reducing density, height, and number of hardwood in-growth into large saplings. The dynamics of regeneration and understory vegetation that we observed during the 11 years following harvesting, compounded by a forest tent caterpillar infestation, suggest that partial harvesting can promote the establishment and growth of shade-tolerant conifers (mainly balsam fir and some black spruce) and inhibit the development of shade-intolerant hardwood regeneration and understory vegetation. Future stand structure will likely consist mainly of hardwoods in the clearcut treatment, and mixed conifers and hardwoods in the partial cut and uncut treatments, with more hardwood components in the partial. Further monitoring is required to verify this premise.