Using Dendrochronology to Understand the Response of Eastern Hemlock to Past Stresses and its Current Status in Southern Maine

Eastern hemlock (Tsuga canadensis (L.) Carr.) is an ecologically important species that is threatened across its range by the non-native hemlock woolly adelgid (HWA, Adelges tsugae (Annand) (Hemiptera: Adelgidae)). In order to understand potential impacts of HWA in southern Maine, we conducted a treering study of 36 sites in southern Maine (York and Cumberland counties) to evaluate how stress events affected eastern hemlock increments, especially after the 1999-2002 drought. The primary objectives were to: 1) create a master chronology and identify reduced growth events 2) model abiotic factors associated with differences in drought response among plots and 3) evaluate the impact of the 1999-2002 drought on growth trends across the southern Maine hemlock population. Reduced growth events coincided with records of drought and defoliation by hemlock looper (Lambdina fiscellaria (GUEN.) and gypsy moth Lymantria dispar (L.). Hemlock growth for the master chronology was also associated with 1) previous year’s June and July temperatures, 2) both previous and current summer precipitation, and 3) both maximum and minimum February temperatures. A predictive model for the 2003 drought year showed that plots with higher expressed population signals (and adjustment of interseries correlation for sample size), shallower O soil horizons, lower average February maximum temperatures, higher B soil horizon aluminum saturation, and greater stand densities had the largest decreases in mean growth, as expressed by the percent growth change of increments over a 3 year period. The plots with the most negative percent growth change in 2003 had the most positive percent growth change in 2005. The 2005 recovery was positively related to average plot age and calcium in the B soil horizon. The low number of trees (59 of 503) experiencing decreased growth in 2005 supports the observation from the master chronology that hemlock trees in this study are typically healthy and able to quickly recover following drought stress. These findings support the consideration of management strategies that increase hemlock vigor leading to greater growth potential and enhanced ability of trees to recover from stres