The combined impact of exotic forest insect pests and nonnative invasive plants may drastically alter the trajectory of forest succession. Emerald ash borer (EAB; Agrilus planipennis) has already killed thousands of ash (Fraxinus spp) trees throughout the eastern United States, resulting in canopy gaps and altered microenvironments. Furthermore, many nonnative invasive plant species have been accidentally or intentionally introduced and are already present within the affected forests. There is limited information available to help predict how invasive plants will respond to EAB-induced forest gaps and whether they will respond differently than native species. We are quantifying the changes in forest community composition in relation to canopy openness in sites with near 100% ash mortality. Plots were established in seven state or metro parks within the Huron River watershed in southeast Michigan that have extensive public forests. Since 2008, we have characterized the frequency and abundance of understory native and invasive woody plant species. In addition, we have established native-invasive plant pairs to monitor and compare their individual relative growth rates. Hemispherical photographs are also being taken in each plot to quantify light levels in relation to canopy gaps and correlate with plant data.
In quadrat surveys (four 4-m2 quadrats per plot) of the seedling layer, we detected at least one invasive plant species in 71 out of the total 129 plots. Fraxinus was by far the most abundant species encountered, while Rhamnus cathartica, Celastrus orbiculatus, and R. frangula were the most abundant invasive species. In shrub layer surveys, Lindera benzoin, Hamamelis virginiana and Zanthoxylum americanum were the most abundant species overall. The most abundant invasive shrub species were Elaeagnus umbellata, C. orbiculatus, and Lonicera maackii. The most abundant understory tree species were Fraxinus spp, Carpinus caroliniana and Acer saccharum. In 2008, we evaluated 480 native-invasive pairs throughout all seven parks; after one growing season, we saw no clear trends in relative growth rate of invasive plants compared to paired native plants. An additional 150 pairs were established in 2009, and data will be collected on all pairs for two more growing seasons. Canopy openness values from hemispherical photographs taken at plot centers in 2008 ranged between 3.4% and 25.2%; canopy openness ranges also varied according to the hydrological classification of the site (hydric, mesic, or xeric). Hemispherical photographs taken in 2009 are currently being analyzed.