Effect of harvest intensity on development of natural regeneration and shrubs in an Ontario boreal mixedwood stand

Abstract

Resource managers in northern Ontario need guidelines for establishing productive species mixtures and maintaining a viable conifer component through natural regeneration on upland boreal mixedwood (BMW) sites. The hypothesis that partial cutting favours conifers by inhibiting development of hardwood reproduction and associated competitors was tested. Harvesting to nominal basal area (BA) reductions of 0, 36, 68, and 100% was conducted in a northeastern Ontario mixedwood stand dominated by trembling aspen ( Populus tremuloides Michx.) in the overstory and balsam fir ( Abies balsamea [L.] Mill.) and shrubs in the understory. A positive linear relationship existed between photosynthetically active radiation (PAR) transmittance and initial BA reduction at 1, 3, and 5 years after harvesting. However, the relationship weakened over the 4-year period. Although BA reduction level did not account for significant variation in height growth of conifer advance regeneration, it explained 31% of the variation in conifer sapling diameter growth, 61% of the variation in hardwood regeneration height growth, and 21% of the variation in shrub height growth. Based on post-harvest height comparisons among conifers, hardwoods, and shrubs, partial cutting to promote conifer advance regeneration on BMW sites cannot be recommended. Maintaining a viable understory of shade-tolerant conifers for the next rotation may be more feasible than promoting conifer–hardwood overstories in the current rotation. The increase in sapling growth with BA reduction level implies that the entire overstory should be removed if the site is adequately stocked with conifer saplings. Similarly, harvesting should remove the entire overstory if the objective is to maximize growth of hardwood regeneration. Clearcutting promoted growth of both hardwood crop trees and shrubs, but hardwoods remained dominant after 5 years. Height development patterns indicate that regeneration and competitors should be monitored long enough following harvesting to determine whether additional interventions may be necessary to achieve stand structure objectives. Further studies are required to investigate the effects of other factors on advance regeneration, such as pattern and season of harvest and degree of soil disturbance.