Patterns and potential drivers of exotic plant diversity in managed northern temperate and boreal forests

Although exotic (i.e., non-native) plant species are among the main factors contributing to ecosystem change globally, investigations of northern temperate and boreal forest ecosystems in North America in the context of invasion science are scarce. Multiple hypotheses, including the ‘biotic resistance’, ‘intermediate disturbance’ and ‘disturbance-mediated’ hypotheses, can serve as a framework to assess invasibility. Here we used the NEBIE (natural disturbances and extensive, basic, intensive, and elite silviculture intensities) plot network, consisting of field sites across different forest types that originated from a range of silviculture systems to assess the role of silviculture intensity on exotic plant diversity. Botanical surveys were conducted pre- and two-, five-, and ten years post-forest harvesting. Thirty exotic plant species were observed across all studied forests, of which 20 belonged to the Asteraceae, Fabaceae, and Poaceae families. Exotic species richness was highly correlated with native species richness and peaked as a result of intermediate linked-press (i.e., climate, soils, historic disturbance regimes, and silviculture systems) stresses/disturbances and compounded pulse (i.e., harvest intensity, site preparation, and herbicide use) disturbances for all post-harvesting measurement years. Observed exotic species are considered to have traits consistent with those of ruderal species. In this context, our results support the intermediate disturbance and disturbance-mediated hypotheses, but not the biotic resistance hypothesis. To reduce the richness of exotic plant species in northern temperate and boreal forests we recommend minimizing the use of intermediate compounded pulse disturbances.