Do tree-level monocultures develop following Canadian boreal silviculture? Tree-level diversity tested using a new method

Concern about forestry practices creating tree-level monoculture plantations exists. Our study investigates tree diversity responses for six early seral boreal forest plantations in Ontario, Canada, representing three conifer species; black spruce (Picea mariana), white spruce (P. glauca), and jack pine (Pinus banksiana), 14 release treatments, and 94 experimental units. Dominance-diversity curves and Simpson’s indices of diversity and evenness indicate tree alpha diversity. We propose a new method for assessing diversity, using percentage of theoretical species maximum (%TSM) which is determined by comparing post-disturbance richness (S) with a theoretical species maximum (TSM). Our results support the hypothesis that alternative vegetation release treatments generally do not reduce tree species diversity levels (%TSM) relative to untreated plots. The only %TSM (P ≤ 0.05) comparison that produced less diversity than in control plots was repeated annual treatments of Vision herbicide at one of the black spruce study sites. Our results generally support the hypothesis that tree monocultures do not develop after vegetation release. Only one out of 94 experimental units developed into a tree layer monoculture (Simpson’s reciprocal diversity index = 1). Again this was one of the repeated annual treatments of Vision herbicide at one of the black spruce study sites—a treatment which is atypical of Canadian forest management.