COS 199-2 - Native diversity buffers against severity of non-native tree invasions globally

Abstract: Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies. By leveraging global tree databases, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and subsequent invasion severity. We consider three hypotheses: greater native phylogenetic diversity reduces non-native invasion (H₁); High levels of environmental filtering in extreme environmental conditions will lead to similarity of non-natives with the surrounding natives, and moderate conditions will be associated with greater levels of niche differentiation and dissimilarity (H₂), and finally; human drivers, specifically proximity to ports and areas of high human population density, will mediate and potentially override these ecological relationships (H₃). To test how hypothesized human and environmental drivers affected the probability a forest plot was invaded or the invasion severity within invaded plots, we built generalized linear models and random forest models using either phylogenetic or functional metrics as predictor variables, accounting for human drivers (distance to shipping ports and population density) and several additional soil chemical and climate variables. Next, to test whether non-native species invade by being phylogenetically or functionally similar or dissimilar to the native community, we built models predicting non-native similarity along with the same environmental and human impact variables. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as the strongest predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, these patterns are obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence the non-native tree presence, but that native phylogenetic and functional diversity play a critical role in the establishment and spread of subsequent invasions. Ultimately, this emerging understanding of global introduction and invasion mechanisms has important implications for forest management practices to limit the spread and impacts of non-native tree introductions worldwide.