Possible implications of hemlock woolly adelgid on forest composition and structure in southeastern Ohio hemlock riparian forests

Background/Question/Methods

Hemlock woolly adelgid (HWA) is an invasive, exotic insect causing widespread mortality in eastern hemlock (Tsuga canadensis (L.) Carr) forests of the eastern United States.  Eastern hemlock is thought to be a foundation species, regulating local ecosystem structure and function (e.g., microclimate, nutrient cycling).  Across the central and southern Appalachians, hemlock tends to dominate ravine and riparian forests, indicating that the loss of this foundation species may have dramatic effects across riparian and stream ecosystems. In the northeastern United States, where hemlock is more widespread, data indicate that it will likely be replaced by sweet birch (Betula lenta L.). However, the future composition and structure of ravine and riparian forests is not as clear for the central and portions of hemlock’s range.  Prior to any possible invasions by HWA, we are sampling the vegetation composition and structure of hemlock forests of the unglaciated Allegheny Plateau region of Ohio.  This data will provide a baseline for planning and prevention, which may be particularly valuable to a region with a significant tourism and recreation investment in hemlock-dominated ravine systems.

Results/Conclusions

Initial analyses indicate that hemlock forest ecosystems in southeastern Ohio may respond to large-scale disturbance associated with HWA differently than models developed for New England.  In southeast Ohio, eastern hemlock is associated with short, steep slopes or cliffs.  Hemlock distribution does not seem to be limited by aspect, as it occurs on both sides of east-west and north-south oriented ravines.  This indicates its distribution may be driven by processes such as cold air drainage.  Hemlock is particularly dominant at lower slope positions adjacent to streambeds, where few other woody species are found in either the overstory or sapling layers.  At midslope or upper slope positions, overstory species richness increases, with associates such as red maple (Acer rubrum L.), chestnut oak (Q. prinus L.),  northern red oak (Q. rubrum L.), and several hickory species (Carya spp.).  Overall stand basal area remains relatively constant across the riparian areas; however, hemlock basal area is lower at middle and upper slope positions than at lower slope positions.  Unlike New England and some areas of the southern Appalachians, sweet birch is not a significant forest component in the overstory or sapling layers.  Rather, species such as red maple and Amercian beech (Fagus grandifolia Ehrh.) present in the sapling layer may be more likely to replace hemlock if HWA reaches these forest stands.