Ailanthus altissima (Tree of Heaven), an invasive tree species native to China and East Asia, was first introduced into the US ca. 1784 by William Hamilton at his Philadelphia, PA estate. However, the means and temporal progression of spread from this and other early points of introduction are not clear. This species now occurs in >40 US states, primarily as an urban and roadside weed. The Northeast supports the highest densities of Ailanthus within the US, mainly in transportation corridors and urban areas, where it has become the dominant tree species. A recent, widespread increase in Ailanthus incidence in eastern hardwood forests, not unlike prior invasions along railways and roadsides, suggests that current conditions favor invasion in natural environments. To help elucidate the life history of Ailanthus in Pennsylvania and the northeastern US, as well as answer fundamental biological questions concerning this species, we conducted dendrochronological (tree-ring) studies and floristic surveys beginning in 2010. Although we studied population dynamics, age structure, and tree-ring characteristics of Ailanthus primarily in Pennsylvania, we supplemented our studies using trees from adjacent northeastern states. Floristic studies, conducted between 2010 and 2011, revealed Ailanthus to be present in 60 of 67 Pennsylvania counties, including nine previously unreported counties. Tree-ring studies of these trees, as well as of trees from adjacent states, indicated Ailanthus was a suitable species for tree-ring analyses, although false rings were observed in >20% of trees examined. Our results also revealed Ailanthus to be longer-lived than previously reported, reaching ages >100 years, at which time female trees still produced viable seed. However, extant Ailanthus did not exceed 120 years in age, and therefore could not be directly linked with any documented late 18th-century or early 19th-century plantings. Nevertheless, continuous colonization in southeastern Pennsylvania, spanning two centuries, by overlapping generations of Ailanthus was supported by observations at Bartram's garden in Philadelphia. Regression analysis revealed that tree diameter was a significant predictor (R2= 0.80) of Ailanthus age and could be used to estimate ages of historic trees whose diameters were reported in early literature, which revealed establishment of Ailanthus in six Pennsylvania counties 70–118 years earlier than previously reported. Tree-ring studies of extant trees revealed that Ailanthus had gone unreported in some counties for 50 years, during which subsequent invasions occurred. Lastly, we report that widespread invasion of forests by Ailanthus is a relatively recent phenomenon in Pennsylvania, with most invasions occurring after 1965, following major forest disturbances such as salvage logging in the aftermath of widespread insect defoliator outbreaks, especially Lymantria dispar (Gypsy Moth) defoliations. These findings are consistent with previously reported lag times between initial colonization and the onset of rapid range expansion for other invasive plant species. Our results emphasize the need for more thorough inventories of Ailanthus within forests prior to harvesting. Such inventories will afford forest managers opportunities to preemptively eradicate Ailanthus and mitigate future invasions by this species.
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