Eastern hemlock (tsuga canadensis) regeneration in the presence of hemlock woolly adelgid (adelges tsugae) and elongate hemlock scale (fiorinia externa)

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The hemlock woolly adelgid (Adelges tsugae Annand) is an invasive hemipteran that poses a major threat to eastern hemlock (Tsuga canadensis (L.) Carrière) forests in the United States. We conducted three surveys over a five-year period that assessed the density of hemlock woolly adelgid (HWA) and a second invasive pest, the elongate hemlock scale (EHS; Fiorinia externa Ferris), overstory hemlock mortality, and hemlock regeneration in ~140 hemlock stands (mean size, 44 ha; range, 7-305 ha) within a 7500 km2 north-south transect of southern New England (USA). In each stand, we rated HWA and EHS density on 50 hemlock trees using a 0-3 scale (0, none; 1, 1-10 organisms/m branch; 2, 11-100 organisms/ m branch; 3, >100 organisms/m branch). Data on the presence or absence of regeneration were taken in 2005; in 2007 and 2009, we quantitatively assessed regeneration by counting the number of hemlock seedlings in three 16 m2 plots per stand. In 2005, 81% of sampled stands had HWA, 72% had EHS, and 66% had hemlock regeneration. In 2007, 86% of sampled stands had HWA, 79% had EHS, and 46% had hemlock regeneration. In 2009, 91% of stands had HWA, 87% had EHS, and 37% had hemlock regeneration. The proportion of stands with hemlock regeneration declined 46% between 2005 and 2009, and hemlock seedling density declined 71% between 2007 and 2009. A best-fit model selection algorithm found that this decrease was inversely correlated with stand-level adelgid density. There was no correlation between the change in seedling density and stand-level density of the elongate hemlock scale. The apparent decline in regeneration suggests that the ecosystem-level changes currently occurring in southern New England may be difficult to reverse.

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Canadian Journal of Forest Research