Potential Impacts of Hemlock Woolly Adelgid Infestation on Forest Overstory and Avian Assemblages in West Michigan, USA

Invasive species threaten the health and resiliency of native ecosystems wherever they are found. The damage caused by invasive species often leads to a cascade that degrades the ecosystem as a whole. In eastern North America, the invasive insect, hemlock woolly adelgid (Adelges tsugae; HWA), threatens the continued existence of eastern hemlock (Tsuga canadensis) and the species that are associated with it. Often considered a foundation species, eastern hemlock influences the structure and composition of eastern forests by lowering soil temperature, subsequently raising soil moisture content. In the absence of hemlock, many forests become dominated by hardwood species, which alters forest community composition. Several avian species, specifically, face displacement as eastern hemlock forests are decimated by HWA. To date, no study of HWA, eastern hemlock, or avian species associated with eastern hemlock has been published in Michigan, USA. Michigan has a unique eastern hemlock distribution and the Great Lakes freshwater coastal dune ecosystems in which many are found provide a novel habitat for studying these interactions. Similarly, locating eastern hemlock in Michigan has been notoriously difficult due to low densities and a lack of research into geographic information system (GIS) modelling of the species. We used avian point counts and forest vegetation surveys in three hemlock and three non-hemlock forests in west Michigan to determine if any differences existed in avian community composition and forest structure between forest types. We also analyzed the reproductive output of eastern hemlocks in the region. Results reveal that two unique avian communities existed between hemlock and non-hemlock forests (α = 0.1). We also found that eastern hemlock has a low reproductive output (~5 seedlings/hectare) and will likely be replaced by hardwood species. Finally, using multiple logistic regression, we show that the blue band in multi-spectral imagery could be the key to differentiating eastern hemlock from other coniferous species in aerial imagery. We show that HWA management and control in west Michigan must become a priority to protect the unique avian assemblages and eastern hemlock in the region. Similarly, our GIS model will assist land managers with targeted management by making eastern hemlock easier to locate