Factors influencing tree species diversity and Betula alleghaniensis establishment in silvicultural openings

We examined the short-term effects of group-selection harvesting with seed tree retention on the diversity, abundance and establishment of tree seedlings in a northern hardwood forest in the Upper Peninsula of Michigan (49 openings, 20 closed canopy reference sites). Three opening sizes were examined - opening radius 0.5 × canopy height (267 ± 62 m², n = 16), 0.75 × canopy height (642 ± 85 m², n = 17) and 1.0 × canopy height (1192 ± 155 m², n = 16) (canopy height = 22 m). A single yellow birch (Betula alleghaniensis Britt.) seed tree was retained in the centre of each opening. Tree seedling density was significantly higher in the largest group-selection opening than at the closed canopy reference sites (P < 0.05), the main factor for this was the increased proportion of yellow birch, red maple (Acer rubrum L.) and other minor species. Nevertheless, yellow birch was still a minor component of the developing gap cohort, comprising 5.9 per cent of the seedlings and 1.1 per cent of the saplings. Within openings, microsite variables, such as per cent covers of bare soil and coarse woody debris, were the best predictors of yellow birch occurrence and density. Our results suggest that microsite limitations and competing vegetation may greatly reduce the efficacy of openings for ensuring the maintenance of mid-tolerants.