Invasive plant infestations in forest communities impacted by emerald ash borer (Agrilus planipennis)

Background/Question/Methods Emerald ash borer (EAB, Agrilus planipennis) is an invasive wood-boring beetle that is expected to eliminate white, green, and black ash trees from eastern North America, leaving canopies open to colonization by invasive plants. We initiated studies in forests along the progressing EAB invasion front, in Michigan and Ohio, to study effects of ash mortality and associated gap formation on forest community structure, composition, and succession, as well as changes in the presence, abundance, and spread of invasive plant species. Forest structure was characterized as per USDA Forest Service protocol: species composition, frequency, density, and basal area of all overstory trees were quantified within18-m-radius main-plots. Species composition, frequency, and density of tree saplings and native woody understory plants were determined within 8-m radius sub-plots. All known invasive plant species were identified, counted, and percent cover determined.

Results/Conclusions Ash decline and mortality progressed rapidly in all stands regardless of basal area, density, species composition, or other stand variables. Invasive plant density was low in most (70%) plots, ranging from 0 to 1% cover, while a small number of plots had over 20% cover of invasives. The invasive plant species present were mostly woody shrubs, including various Lonicera (bush honeysuckles) and Rhamnus (buckthorn) species, Berberis thunbergii (Japanese barberry), Eleagnus umbellata (autumn olive), Euonymus alatus (winged burning bush), Ligustrum vulgare (border privet), and Rosa multiflora (multiflora rose). The number of invasive species in a single 0.25-acre plot ranged from 0 to 6. Soil seedbank samples revealed a high level of variation in plant species composition, as expected, a very low presence of propagules of any woody invasives, and virtually no ash seeds. We encountered significant infestations of woody invasives along access roads and trails leading toward the sample sites, and often in the woods beyond the sample areas. In addition, we observed small sapling of invasive species in the shade of the forest canopy that might respond quickly to EAB-induced canopy gaps. These data will provide baseline information for accessing change in forest community structure and invasive plant colonization over time.