Forests of eastern North American are losing Tsuga candensis (eastern hemlock) ecosystems throughout an expanding portion of its range due to the invasive pest insect Adelges tsugae (hemlock woolly adelgid: HWA). Tsuga canadensis represents a small portion of the landscape, particularly in the central and southern Appalachians where it is largely restricted to cove and riparian areas; however, it is a foundation species that defines a unique forest ecosystem. Consequently, loss of T. canadensis will result in a re-organization of ecosystem structure and function as alternate communities develop. A greater understanding this transition will advance ecological theory, but is also directly applicable to management and restoration planning. While county-level patterns of detection are readily available, less is known about the process of decline and compositional shifts. We identified riparian T. canadensis forests along thirty headwater streams across West Virginia and Virginia, representing an invasion chrononosequence of nine to thirty-two years. Sites encompassed a range in elevation, slope, and aspect, intended to identify patterns of T. canadensis dominance and decline. At each site, we sampled the overstory and sapling vegetation and recorded the crown health of each T. canadensis in transects at 10, 30, and 50 meters from the bank-full stream channel. Although in severe decline, T. canadensis continued to dominate both the overstory and sapling layers across riparian-upland transects across the central Appalachians. Structural equation modeling (SEM) indicated decline was moderated by higher elevations and landscape positions that received less incident radiation due to aspect and slope, but duration of Adelges tsugae invasion was the most influential factor in the decline of crown health for the overstory. SEM also identified decline in the overstory as the most influential factor in the decline of the sapling layer. Future forest function depends on composition, which will likely vary depending on the presence of Rhododendron maximum, an aggressive native shrub. Most importantly, functional processes that respond rapidly to T. canadensis decline are likely to experience continuing fluctuation as the composition shifts more gradually.
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