By all indicators, the fate of all North American native ash (Fraxinus spp.) in the wake of emerald ash borer (EAB) is bleak. This study is concerned with creating a better understanding of the potential replacements of ash in native stands as the ash dies out. We present the current distribution and abundance of four species of ash and co-occurring species in the eastern United States, according to US Forest Service Forest Inventory Data. With this information, along with 38 environmental variables, we create suitable habitat models for the species using the RandomForest statistical modeling tools. We then model the suitable habitat for each species under various scenarios of climate change for ~2040, 2070, and 2100. The model outputs are intended to give some indication of potential changes in species composition under climate change. Coupled with this effort is a field assessment of current co-occurring species within ash ecosystems in Ohio and Michigan. A total of 182 plots in Ohio and 93 in Michigan were evaluated prior to, or in some cases, during, EAB invasion for seedlings, saplings, and overstory trees to assess potential species mixes in the next forest.
Results/Conclusions
The climate change suitable habitat models, even when excluding the direct impacts of EAB, provide evidence that black ash (F. nigra) would lose much of its suitable habitat over the eastern US by 2100. White ash (F. americana) would also generally lose habitat. Though these models are of lesser reliability, green (F. pennslyvanica) or blue (F. quadrangulata) ash could potentially gain some suitable habitat. Of course the ash models are mute for long-term climate change in the face of EAB, so we focus on co-occurring species and potential migrants and how their habitats may change. Major co-occurring tree species in ash-dominated plots included primarily elms (Ulmus spp.) and maples (Acer rubrum, A. saccharinum, A. saccharum) in Ohio, and maples, oaks (Quercus spp.), and basswood (Tilia sp.) in Michigan. By coupling this plot-level information with the overall trends in suitable habitat for co-occurring species, we provide new analyses on the possible future composition of these ecosystems. For example, the future northward expansion of the potential range of some non-ash tree species into Ohio or Michigan may allow them to move into areas where they currently do not co-occur with ash.