A foundation tree at the precipice:<i>Tsuga canadensis</i>health after the arrival of<i>Adelges tsugae</i>in central New England

Abstract

Hemlock ( Tsuga canadensis ) plays a unique role in Eastern forests, producing distinctive biogeochemical, habitat, and microclimatic conditions and yet has begun a potentially irreversible decline due to the invasive hemlock woolly adelgid ( Adelges tsugae ; HWA) that causes foliar damage, crown loss, and mortality of host trees. Understanding the regional, landscape, site, and stand factors influencing HWA spread and impact is critical for predicting future landscape dynamics and directing effective management. Using aerial photographs, we documented hemlock distribution throughout central Massachusetts and subsampled 123 stands to examine the spatial pattern of HWA and its impact on tree vigor and mortality since its arrival in 1989. In the study region, over 86,000 ha of hemlock forest were mapped in 5,127 stands. White pine ( Pinus strobus ), red oak ( Quercus rubra ), red maple ( Acer rubrum ), and black birch ( Betula lenta ) were common overstory associates. Hemlock abundance increased from south to north, commonly on western and northwestern slopes. Average stand size was 55 ha, overstory basal area ranged from 23 to 55 m 2 ha −1 and overstory stem densities averaged 993 ha −1 . By 2004, 40% of sampled stands were infested, but most stands remained in good health overall; only 8 stands contained high HWA densities and only two had lost &gt;50% overstory hemlock. Out of fifteen stand and landscape predictor variables examined, only latitude and winter climate variables were related to HWA density. Cold temperatures appear to be slowing the spread and impact of HWA at its northern extent as HWA infestation intensity and hemlock mortality and vigor were significantly correlated with average minimum winter temperature. Contrary to predictions, there was no regional increase in hemlock harvesting. The results suggest that regional HWA‐hemlock dynamics are currently being shaped more by climate than by a combination of landscape and social factors. The persistence and migration of HWA continues to pose a significant threat regionally, especially in the northern portion of the study area, where hemlock dominates many forests.