Effects of co-occurring non-native invasive plant species on old-field succession

Old fields are diverse forb-dominated ecosystems transitioning into woody-dominated forested ecosystems. However, the susceptibility of old fields to high levels of plant invasion can lead to the co-occurrence of multiple non-native invasive plant species at the fine-scale of plant neighborhoods, which can alter native species co-occurrence patterns and successional trajectories into forest communities. Community disassembly by invasive species occurs when the presence of one or more invaders shifts co-occurrence patterns of native species from structured to random. Disassembly can imply a loss of existing co-evolved interactions among native species, which has ramifications for community dynamics and trajectories of invaded ecosystems. Here, we quantify relationships among multiple invasive plant species and two indicators of community succession in old-field plant communities in East Tennessee: co-occurrence patterns of native and non-native species and successional trajectories. First, we examine how biotic and abiotic factors shape the abundance of invasive species, as well as native and invasive functional groups across old fields. Second, we ask whether invasive species influence co-occurrence patterns among native species and whether invasive species are associated with altered herbaceous:woody foliar cover ratios. We found that biotic and abiotic predictors associated with invasive species were not consistent in identity or direction of association, indicating that predicting which sites or suite of biotic and abiotic variables are associated with invasive species will be challenging. Importantly, as the number of invasive plants increased in 1-m2 plots, native species co-occurrence patterns shifted from structured to random, whereas invasive species co-occurrence patterns remained random irrespective of the number of invasive species. Plots containing three or more invaders had significant changes in native and invasive herbaceous:woody foliar cover ratios. The herbaceous:woody foliar cover ratio of native species was 4× lower and of invasive species was 2× greater compared to plots with one or two invaders, and this shift was in part explained by an increase in foliar cover of non-native woody species. Our data suggest that increased number of invasive species in old fields alters both native species interactions and the trajectories of old-field communities, which could influence the developing understory community as old fields transition into forests. We recommend that management of fields during succession should focus on decreasing the total number of invasive species to restore species co-occurrence patterns and prevent altered successional trajectories, including accelerated succession of non-native woody species.