Landscape patterns of hemlock decline in New England due to the introduced hemlock woolly adelgid

Abstract

Aim The hemlock woolly adelgid (HWA;Adelges tsugae Annand), a small, aphid‐like insect native to Japan, is currently migrating northward through eastern North America and threatens to eliminate eastern hemlock [Tsuga canadensis(L.) Carriere], one of the most abundant, long‐lived shade tolerant species, across its range. The major objectives of this study were: (1) to characterize the pre‐HWA distribution, composition, and structure of hemlock stands; (2) to characterize the spatial patterns of damage generated by HWA across southern New England since the time of its arrival in 1985; and (3) to examine environmental and stand factors that are associated with declines in crown vigour and mortality of hemlock. Location Our study was conducted in a 4900‐km 2 study region extending from Long Island Sound northward to the Massachusetts border and including the Connecticut River Valley in Connecticut, USA. Methods Aerial photographs and extensive field study were used to map and develop GIS overlays of 1000 hemlock stands in our study region. Intensive sampling of a random selection of 114 hemlock stands across the study area was used to document patterns of hemlock infestation, vigour, and mortality in relation to stand and site characteristics. Mantel tests were utilized to assess the relative importance of environmental and stand variables in controlling the intensity of HWA infestation and damage. Results Most stands were located along ridge tops, steep hillsides and narrow valleys. Hemlock importance values ranged from 22 to 96% and stand densities varied from 300 to 1450 stems ha–1. Adelgid presence and adelgid‐induced hemlock mortality were found in 88 and 74% of the sampled forests, respectively. Approximately 25% of stands were logged recently, ranging in intensity from partial hemlock cutting to large clearcuts. A geographical trend in reduced HWA infestation intensity and tree mortality and enhanced crown vigour of overstory and understory hemlock occurs from south to north, coincident with the temporal colonization pattern of HWA. Mantel analyses indicated that patterns of HWA infestation, hemlock mortality, and crown vigour were most strongly correlated with latitude. Mortality was also weakly related to aspect and stand size. Average mortality was highest on western aspects but exceeded 20% on most slopes. Remaining trees averaged over 50% foliar loss, with no significant difference among aspects. Main conclusions Results suggest that as HWA becomes abundant, stands on xeric aspects succumb rapidly, but that stand and landscape variables such as overstory composition and structure, slope, and elevation, exert little control over susceptibility or eventual mortality. Ultimately, duration of infestation controls the intensity of hemlock decline and mortality. Over 4290 ha of hemlock forest have been eliminated by logging or HWA just within the southern part of our transect since the mid‐1980s, and we predict continued HWA infestation will lead to unprecedented hemlock loss throughout the north‐eastern USA, regardless of site conditions or location.