Glossy buckthorn invasion across three land-use classes in northern Michigan1

Agricultural lands that are left to revert to forests are frequently colonized by nonnative shrubs, such as the invasive Frangula alnus (glossy buckthorn). To better understand this phenomenon, we assessed a glossy buckthorn invasion across three land-use classes on a 6-ha site in Houghton County, Michigan. The site included a remnant northern hardwood woodlot, a Pinus sylvestris (Scotch pine) plantation, and an abandoned field colonized by Fraxinus americana (white ash), which is now declining from Agrilus planipennis (emerald ash borer). We examined the height structure and minimum age of the invasion spatially, as well as factors such as overstory and understory composition, light availability, soil characteristics, and proximity to edges. We used nonmetric multidimensional scaling ordination and comparative analysis to examine the height structure of the invasion by land-use category. We observed differential invasion success across land-use histories, which may be associated with soil and vegetation legacies of land clearing and subsequent agricultural abandonment. We observed the greatest densities of glossy buckthorn on plots associated with past agricultural use and abandonment, which also contained the greatest range in size classes and on average the oldest individuals. Our findings suggest that the impact of land-use legacies on soils warrants further investigation as a potential mechanism underlying invasion success. An improved understanding of legacy effects could help land managers target control efforts in areas that may be predisposed to land invasion because of prior land use.