Potential plant community shifts in eastern hemlock-dominated ravines on the front of the hemlock woolly adelgid invasion in Ohio

Background/Question/Methods Across the central and southern Appalachian Mountains and unglaciated Allegheny Plateau, eastern hemlock (Tsuga canadensis L.; hereafter hemlock) dominates ravine and riparian forests. In these ecosystems, hemlock is a foundation species that regulates important ecosystem processes. Thus, the loss of hemlock due to the invasive non-native pest, the hemlock woolly adelgid (HWA), will result in shifts in vegetation composition and structure that will alter ecosystem function. For this study, we examined hemlock-dominated riparian forests in the unglaciated Allegheny Plateau of southeastern Ohio, which is currently on the front of the HWA invasion. These areas are important for natural history and native diversity, as well as recreational activities. A greater understanding of the composition and function of these hemlock-dominated riparian forests should provide Ohio land managers to develop planning options prior to widespread mortality of eastern hemlock. Results/Conclusions In southeastern Ohio, hemlock is particularly dominant in the overstory immediately adjacent to streams, with few other species in the overstory and understory layers. Non-metric dimensional scaling (NMDS) ordination analyses indicate distinct associations of deciduous hardwood species along environmental gradients within riparian forests. Specifically, lower slope positions are characterized by interspersed sweet birch (Betula lenta L.) and tulip-poplar (Liriodendron tulipifera L.). At upper slope positions, overstory species richness is higher, with increasing abundance of maples (Acer rubrum L., A. saccharum Marsh) and oaks (Quercus alba L, Q.rubra L. and Q. prinus). This may indicate that moving upslope, communities similar to adjacent upland ecosystems may develop should widespread hemlock mortality occur. At the same time, greater shifts in composition will occur at positions immediately adjacent to streams. We believe it is therefore reasonable to expect that in these systems, ecosystem function across riparian and headwater stream areas will experience the greatest change. Managers may want to prioritize HWA strategies in headwater riparian areas to mitigate impacts from hemlock mortality including nitrate leaching, loss of habitat for migratory birds and cold water fishes, and the spread of invasive plant species in large hemlock gaps.