Invasive plants are species introduced from another region (non-native) that have established self-sustaining populations and are spreading, often with substantial negative consequences (Lockwood et al., 2007). Invasive species are a prominent component of global change (Vitousek et al., 1997, 1996), and have been identified as one of five major threats to ecosystems by the Millennium Ecosystem Assessment (along with, for example, climate change; MA, 2003). Invasive plants are transforming terrestrial ecosystems (Mack et al., 2000; Vitousek et al., 1996) and significantly reducing native plant and animal abundance and diversity (Vilà et al., 2011). Invasive plants have numerous detrimental effects on forest ecosystems (Webster et al., 2006). Several forest understory invasive plants, such as oriental bittersweet (Celastrus orbiculatus; Fike and Niering, 1999), autumn olive (Elaeagnus umbellata;
Munger 2003), and honeysuckle (Lonicera spp.; Miller and Gorchov, 2004) outcompete or reduce growth of native vegetation. For example, glossy buckthorn (Frangula alnus) grows in dense thickets that shade out native tree saplings and reduce their overall survival by up to 90% (Fagan and Peart, 2004). Invasive plants also threaten forest regeneration by altering soil chemistry. For example, garlic mustard (Alliaria petiolata) releases allelopathic chemicals that kill soil mycorrhizae and inhibit the establishment of native tree seedlings (Wolfe et al., 2008). As a result of their vigorous growth, invasive plants are often able to
dominate ecosystems following disturbance and impede forest succession (Webster et al., 2006).
Invasive plants affect not only forest regeneration, but also mature forest trees. For example, a nine-year study in the Northeast showed that presence of invasive vines inhibited tree growth to only half the rate of trees in uninvaded forest (Ladwig and Meiners, 2009). Similarly, invasive vines in Mid-Atlantic National parks showed that vines were most detrimental for larger trees, reducing overall growth rates in large trees by up to 2 mm/year and doubling mortality rates (Matthews et al., 2016). A dendrochronology study of mature tree growth rates in Eastern Temperate Forest found that trees in invaded sites had a basal
area growth rate 16% lower than trees in uninvaded sites (Hartman and McCarthy, 2007). The magnitude of these impacts attests to the importance of managing invasive plants to reduce both ecological and economic impacts in Northeast forests. Given the large number of invasive plants already present in forested ecosystems, it is critical to prioritize treatment and control efforts (Webster et al., 2006). An important tool used to identify and prioritize potential future invasions in the Northeast and elsewhere in the US is an invasive plant risk assessment (Koop et al., 2012; Pheloung et al., 1999), which identifies likely invasive plants from the broad array of non-native plants being introduced daily to the US. While several plant physiological traits (e.g., rapid growth rate) are useful for predicting invasiveness, physiological traits alone cannot predict all invasions. Underexplored 'spatial traits' could provide new insight into what makes some plants more likely to become invasive. Currently, the only spatial trait used in invasive plant risk assessments is the size of the native range (with larger ranges more likely to be
invasive; Pheloung et al., 1999). But, many other characteristics of the habitats in which species evolve could also increase the risk that they will become invasive, including disturbance history, resource availability and competition. The spatial patterns of invasive plants' native ranges have been understudied in large part because we have incomplete and poor knowledge about which species are invasive. Before we can leverage spatial distributions of species to understand correlations to invasion risk,we first need to know which species are invasive. This research will produce a comprehensive global list of invasive plant species through a review of all ISI indexed invasive plant literature. Using this novel database, we will evaluate whether the spatial distributions of invasive plants in their native range can be used to identify elevated invasion risk. This analysis will provide important information about invasive plant 'traits'
that can be used to inform risk assessments and management prioritization for newly-arrived species in Northeast forests and surrounding ecoregions.
