Fossil pollen records forecast response of forests to hemlock woolly adelgid invasion

Forecasting the impact of invasive species can provide valuable insights to land managers and scientists. In this study we examined the threat posed to hemlock Tsuga canadensis forests by an invasive insect, the hemlock woolly adelgid Adelges tsugae (HWA). Our main objective was to determine if paleo-ecological pollen records, showing the response of tree genera to a previous Tsuga decline, could predict contemporary responses to the HWA. To assess this, we matched fossil pollen sites from the Global Pollen Database to sites in the northeastern US that were invaded by the HWA approximately two decades ago. Our analyses showed that several of the same genera responded positively to Tsuga declines at both fossil pollen and contemporary sites. We then applied this methodology to an area currently threatened by the HWA, Great Smoky Mountains National Park (GSMNP), where invasions began just seven years ago. Our analysis suggests that the canopy genera mostly likely to respond positively to declining hemlock abundances in GSMNP are Acer, Betula, Fagus, and Quercus. Our findings provide land managers in GSMNP with the first predictions of likely shifts in canopy composition. We expect that this methodology will have application at other areas threatened by the HWA. Finally, we believe that studies on the impacts of contemporary invasions have underutilized relevant fossil data and that greater efforts should be extended to doing so in the future.