The emerald ash borer (EAB; Agrilus planipennis Fairmaire), an invasive insect from Asia, threatens trees in the ash (Fraxinus) genus across North America. Of particular concern are black ash (Fraxinus nigra) trees, as they often dominate inundated areas where few other tree species can thrive. To study the potential effects of dominant species loss in black ash swamps, EAB infestation was simulated using girdle and ash cut treatments applied in 2013 in the Ottawa National Forest of Northern Michigan. Vegetative responses in the first three years after treatment included some unexpected results such as little residual overstory tree growth and unhindered small woody stem regeneration. Some of these responses might be explained by initial treatment shock, but how is the vegetative community continuing to respond six years after treatment? How might other site characteristics impact vegetation post EAB simulated infestation? We revisited the 108 permanent vegetation monitoring plots to assess any new vegetation changes and determine what impact microsite characteristics might have on herbaceous community composition and woody regeneration. Hummock and Hollow cover classes were used to quantify microtopography variation at each subplot, and herbaceous, seedling, sapling, and overstory data were collected to compare to previous years’ data.
Results/Conclusions
Results show that more seedlings and a higher species diversity of seedlings can be found in vegetation plots that fall into the “driest” hollow cover classes. A Kruskal-Wallis test returned a p-value of 0.018 when grouped hollow classes were compared for total amount of seedlings. Pairwise Wilcoxon rank tests showed differences between plots which were almost entirely classified as being in a hollow with plots that had the lowest amounts of hollow (p = 0.011). A similar trend followed with number of seedling species in each plot when compared to hollow cover classes (p = 0.036). These results indicate that plots with more hummock area support woody regeneration by protecting seedlings from inundation, and will have both more woody regeneration and a higher diversity of regenerating woody species. Therefore, black ash swamps with higher areas of “dry” microtopography may have a higher likelihood of remaining forested post EAB infestation.