Landscape Response to Hemlock Woolly Adelgid in Southern New England

• Investigators: David Foster, David Mausel, David Orwig
• Contact: David Orwig
• Start date: 1997-06-01
• End date: ongoing
• Location: Central Connecticut and Central Massachusetts
• Latitude: +41.28 to +42.73
• Longitude: -72.73 to -72.25
• Elevation:
• Taxa: Adelges tsugae (hemlock woolly adelgid), Betula lenta (black birch), Tsuga canadensis (eastern hemlock)
• Keywords: disturbance, hemlock woolly adelgid, insects, landscape patterns, logging, mortality
• Release date: 2003
• EML version: knb-lter-hfr.81.2
• Revisions: data updated 2006-02-28
• Abstract:

  The hemlock woolly adelgid (Adelges tsugae Annand; HWA), 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 vigor and mortality of hemlock. Aerial photographs and extensive field study were used to map and develop GIS overlays of 1000 hemlock stands in a 4900 km2 study region extending from Long Island Sound northward to the Massachusetts border and including the Connecticut River Valley in Connecticut, USA. Intensive sampling of a random selection of 114 hemlock stands across the study area was used to document patterns of hemlock infestation, vigor, 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. 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 vigor 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 vigor 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.

  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 4,290 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 northeastern U.S., regardless of site conditions or location. The same techniques were then applied to a large study area in central Massachusetts to continue these investigations at the northern extent of the HWA infestation range.

• Methods:

  Aerial photographs and extensive field study were used to map and develop GIS overlays of 5000 hemlock stands in a 4060 km2 study region extending from Long Island Sound northward to the Massachusetts border and including the Connecticut River Valley in Connecticut, USA. During the summers of 2002 - 2004, vegetation was sampled in one fixed-area (20 x 20 m) plot and 5 to 10 variable-radius plots located every 30 to 50 m along a transect oriented through the long dimension of the forest in each stand. In fixed-area plots, all trees (stems at least 8 cm diameter breast height (dbh)) were tallied by species and dbh, and assigned a canopy position based on a visual estimation of the amount of intercepted light received by the tree crown. Hemlocks that died within the previous 2 to 4 years, which were identified by extensive retention of fine twigs in the crown, were tallied to determine species composition prior to HWA infestation. Crown vigor classes were assigned to each hemlock tree based on the amount of retained foliage: 1 = 76 - 100%; 2 = 51 - 75%; 3 = 26 - 50%; 4 = 1 - 25%; 5 = dead. Presence and intensity of infestation by HWA, Elongate hemlock scale, and hemlock borer were estimated from the number of egg sacs present on branches from several trees in each plot and categorized as: 0 = absent; 1 = low density; 2 = moderate density; or 3 = heavy density, or egg sacs at the base of almost all the needles.

  All saplings (less than 8 cm dbh and over 1.4 m tall) were tallied by species. Overstory species composition, degree of hemlock mortality, and basal area were also assessed within variable radius cruise plots using the Bitterlich method with a 5 or 10 basal area factor gauge. Slope, aspect, elevation, and depth of the soil organic horizon to the nearest 0.5 cm were recorded at each sampling point. A relative importance value was calculated for each overstory species in every stand by summing the relative basal area derived from the variable radius sampling and the relative density derived from the fixed-area plots. Values for hemlock included both live and recently dead trees to represent "pre-HWA" importance.

• Use:

  This dataset is released to the public and may be freely downloaded. Please keep the designated Contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement. For more information on LTER Network data access and use policies, please see: http://www.lternet.edu/data/netpolicy.html.

• Related datasets: HF021