Dynamics of regeneration gaps following harvest of aspen stands

Abstract

This study assessed the dynamics of gap development in postharvest regeneration in five stands in northwestern Alberta dominated by trembling aspen (Populus tremuloides Michx.). The pattern of gap development over time was determined from analysis of air photographs taken preharvest and 1, 4, 10, and 12 years postharvest. The area of each stand covered by gaps increased after harvest because of the addition of harvest-related gaps over and above those that had been present prior to harvest. The blocks we studied had a combined gap area of up to 29% of stand area 12 years postharvest. We measured regeneration characteristics, microsite, soil, light, and browse conditions in 30 aspen regeneration gaps (gaps in regeneration that were not gaps preharvest and were not due to obvious harvest-related disturbance) 14 years following harvest. Although deciduous trees within postharvest regeneration gaps were the same age as those outside (i.e., in a fully stocked matrix of newly established even-aged aspen stems), they were often suppressed, with significantly lower density and growth. Within the 14-year-old postharvest regenerating aspen stands, aspen height varied from 1 to 11 m; this substantial variability appeared to be largely due to the influence of browsing. There was little evidence of ongoing regeneration within postharvest regeneration gaps, indicating that these gaps will probably persist over time. This may impact future deciduous stocking and volume. It is unknown what may have initiated the formation of these gaps, although results suggest that they are not due to edaphic conditions or disease in the preharvest stands. There is evidence that bluejoint (Calamagrostis canadensis (Michx.) Beauv.) cover and browsing are important factors in the maintenance of postharvest regeneration gaps. The spatial heterogeneity resulting from gaps could be advantageous, however, either as part of ecosystem-based management emulating natural disturbance or as a template for mixedwood management, where white spruce (Picea glauca (Moench) Voss) are established in gaps.