Effects of the invasive leafy spurge (Euphorbia esula L.) on plant community structure are altered by management history

Invasive species threaten biodiversity and ecosystem functioning, often causing changes in plant community composition and, thus, the functional traits of that community. Quantifying changes in traits can help us understand invasive species impacts on communities; however, both the invader and the plant community may be responding to the same environmental drivers. In North America, leafy spurge (Euphorbia esula L.) is a problematic invader that reduces plant diversity and forage production for livestock. Its documented effects on plant communities differ amongst studies, however, potentially due to differences in productivity or land management. To identify the potential effects of leafy spurge on plant communities, we quantified leafy spurge abundance, plant species richness, forage production, functional group composition and community weighted mean traits, intensively at a single site and extensively across ten sites differing in management. We then tested how leafy spurge abundance related to these variables as a function of site management activities. Leafy spurge abundance was consistently associated with fewer plant species, reduced forage production and more invasive grass. Community-weighted specific root length also consistently increased with leafy spurge abundance, suggesting that belowground competition may be important in determining co-existence with leafy spurge. Other changes were dependent on management. Native forbs were excluded as leafy spurge became more abundant, but only in grazed sites as these species were already absent from ungrazed sites. Taller plants better persisted in dense leafy spurge patches, but only in grazed sites, consistent with either facilitation of taller species via associational defences or competitive exclusion of shorter species in ungrazed sites and dense leafy spurge patches. These results show that, despite some emergent properties of invasion, management context can alter invasion impacts by causing changes in the plant community and its interactions with the invader.