Abstract:
Over the last 20 years, hemlock woolly adelgid (HWA; Adelges tsugae), an introduced aphid-like insect from Asia, has expanded its range from southern Connecticut to northern Massachusetts, causing widespread mortality and decline of eastern hemlock (Tsuga canadensis). Building on similar analyses in Connecticut, we have mapped the distribution of all hemlock stands (> 3 ha) prior to HWA infestation in a 4000 km2 transect through Massachusetts to characterize the temporal and spatial patterns of damage generated by HWA since the time of its arrival into the area in 1989. Over 5000 stands with > 10% hemlock were mapped, representing over 86,000 ha, or 21% of the study area. Over a three-year sampling effort, we collected data on forest structure, composition, and crown vigor, site and edaphic characteristics, potential replacement species, and the spatial pattern of HWA and associated canopy damage from 123 stands. Eighty-two stands contained > 50% hemlock while 41 stands contained < 50% hemlock. Overstory hemlock importance ranged from 26 – 90% and total stand densities varied from 400 - 2125 ha-1. Stand size varied from 7 to 317 ha in size and over 60% of stands were found on western, northwestern, or northern aspects with slopes of 20 - 30%. HWA was found in close proximity (a few km) to VT, suggesting it is continuing to migrate unimpeded at the northern extent of its current range.
HWA was present in 40% of the sampled stands, although only 7% of stands had high HWA densities and overstory hemlock mortality levels are still very low in most stands. Only 5 stands had greater than 20% overstory hemlock mortality and 2 forests located along the southern border had overstory losses > 50%. Almost half of the stands visited had experienced some level of hemlock logging in the last 10 years, although very few were harvested due to HWA-induced mortality. This information has been incorporated into a GIS analysis of the landscape-level, biological, edaphic, and historical factors associated with the patterns of HWA-induced hemlock damage. Potential replacement species already present in the canopy of many hemlock forests include red oak (Quercus rubra), white pine (Pinus strobus ), black birch (Betula lenta), and several maple (Acer) species. The original sampling effort from 2002-2004 served as the baseline data for a re-sampling effort in 2005 (See Preisser et al. abstract) and will serve as important information as we continue to re-examine HWA and stand dynamics over time.
