Patterns and predictors of survival in Tsuga canadensis populations infested by the exotic pest Adelges tsugae: 20 years of monitoring

Abstract

Infestations of the exotic pest, hemlock woolly adelgid (HWA; Adelges tsugae Annand), have resulted in the widespread decline and mortality of eastern hemlock (Tsuga canadensis (L.) Carrière) throughout much of the eastern United States. As HWA continues to spread across the range of eastern hemlock, forest managers need a better understanding of the projected rates of hemlock mortality and improved quantification of the site and environmental factors that influence this rate. Our main objectives in this study, encompassing 1993–2012, were: (1) to document the long-term patterns of hemlock tree mortality following HWA infestation and (2) to assess the importance of tree, site, and weather factors in influencing mortality patterns in hemlock trees infested with HWA. In addition, to provide forest managers with a means of assessing the risk of tree mortality in infested hemlock stands, we evaluated the use of crown condition rating data to predict hemlock mortality at various time scales.Our results suggest that HWA-induced mortality can be a slower process than has previously been reported. Ten-year survivorship at our study sites ranged from 70% to 94%. From 1993 to 2012, 65% of the studied hemlock trees survived, with survival by site ranging from 39% to 82%. When calculated across all sites, survivorship after ten years of HWA infestation was 73%. Our findings indicate that inaccurate dating of HWA arrival and interaction with weather patterns may contribute to reports of elevated mortality rates. Our analysis found no support for inclusion of tree and site factors in models of hemlock mortality. However, winter temperature and summer drought explained a significant proportion of the variation in reported time to mortality of HWA infested trees. In addition, our analysis suggests that the use of crown condition indices can provide a reliable means of estimating near-term risk of hemlock mortality in HWA infested stands. Models based on foliar transparency and crown dieback were able to predict hemlock mortality with excellent discrimination at one, three, and five years following measurement. These indices can be used to provide information about impending hemlock mortality on a time scale that is relevant to many management decisions.