Assessment of landscape correlates of Eastern hemlock decline due to hemlock woolly adelgid

Eastern hemlock (Tsuga canadensis) is in decline
throughout its range in the eastern US due to infestation by
an exotic insect pest, the hemlock woolly adelgid (Adelges
tsugae). In Shenandoah National Park, the hemlock woolly
adelgid (HWA) rapidly killed many stands of hemlock after
first appearing in the late-1980’s, while having only minor
impact in other stands. At present, few investigators have
examined the mechanisms that produce this discontinuous
impact, although landscape factors are predicted to play a
major role (Orwig and Foster, 1998: Perry 1988). In an effort
to address possible landscape correlates to hemlock
decline, we conducted a preliminary analysis of 5 years of
hemlock health estimates in comparison to measures of
terrain, stand isolation, and potential dispersal corridors at
the stand level. We found that elevation, slope, light conditions, and distance to streams all exhibited relatively
strong correlation with hemlock decline, although the
relationship varied by year. In addition, there appears to be
some evidence of spatial autocorrelation in decline,
suggesting that similar environmental conditions are either
controlling the adelgid or making hemlock stands more
susceptible to HWA. We are using the results of this
preliminary analysis to guide more detailed efforts aimed at
modeling hemlock stand vulnerability as a result of site,
landscape, and regional factors.