The abundance of yellow birch in the Great-Lakes St. Lawrence hardwood forest region declined significantly during the early 20th century following over-harvesting. Recovery has been inhibited by wide-spread use of single tree selection silviculture (STS) methods since the 1970s, which favours growth of more shade tolerant tree species. Shelterwood harvesting can improve yellow birch recruitment, but incurs higher silvicultural costs, and produces an even-aged structure that may alter ecological function. Group selection (GS) harvesting provides an alternative method with benefits of STS (mixed-age structure, greater biodiversity, yet produces larger gaps that improve light availability and tree species diversity. GS has shown promise in early establishment of yellow birch in parts of the USA, but longer-term results vary and differences in soils, climate, and competition dynamics in Ontario may influence responses. We evaluated the efficacy of GS on recruitment of yellow birch in an Ontario tolerant hardwood forest by comparing change in abundance over ten years in harvested GS gaps relative to STS and unharvested forest. Some gaps were harvested in late summer, others in winter. Vegetation was assessed as percent ground cover (stems 0.01-0.5 m height) or large stem counts (>0.5 m tall, <2.5 cm DBH).
Results/Conclusions
Summer harvested gaps established 2-4x more yellow birch cover than STS and winter harvested gaps, and recruited 7-10x more large stems by year ten; values remained unchanged in unharvested controls. Mineral soil exposure/mixing and seed tree proximity were correlated to early establishment of yellow birch, which in turn predicted 10-year recruitment rates. Winter harvesting favoured establishment of sugar maple as ground cover within the first post-harvesting year, which predicted exponentially lower recruitment of yellow birch by year ten. Together, these indicate that summer harvesting in gaps optimised conditions for germination and growth of yellow birch, i.e. light availability, mineral soil exposure from disturbance, reduced competitor abundance, and seed dispersal timing. While these results remain preliminary, they suggest that GS harvesting in summer can be used as a tool to increase recruitment of yellow birch in the understorey of tolerant hardwood stands, and should be considered where shelterwood harvesting is not feasible.