Responses of insect pollinators and understory plants to silviculture in northern hardwood forests

Communities of flower flies (Diptera: Syrphidae), bees (Hymenoptera: Apoidea), and flowering plants were compared between harvested and unharvested hardwood stands in Algonquin Provincial Park, Ontario, Canada. Group-selection silviculture (where groups of trees are removed from a forested matrix, rather than single trees), increased the abundance of pollinators and flowering stems, but only after leaf-out. Wild red raspberry (Rubus strigosus) and bees benefitted most from the creation of canopy gaps. The combination of increased light, warm, bare soils, and abundant nectar-rich raspberry flowers probably created ideal habitat for soil-nesting bees, factors which are relatively absent from unharvested stands. By contrast, before leaf-out, spring ephemerals and high light-levels were universal and pollinators were even across treatments. More pollinators were caught in canopy gaps than in forested areas, and the proportion of fertilized ovules of spring beauty (Claytonia caroliniana) was higher in gaps than in the forest, suggesting that pollinators prefer foraging in gaps, even in spring. Group-selection silviculture in hardwood forests proved beneficial to native pollinating insects, at least in the short-term.