Alien and native plant richness and abundance respond to different environmental drivers across multiple gravel floodplain ecosystems

Abstract Aim To assess whether native and alien plant cover and richness respond similarly to flow regime, propagule pressure, climate and floodplain characteristics in highly dynamic braided river ecosystems. Location Canterbury, New Zealand. Methods A regional, multiscale survey was conducted across 19 braided river floodplains in multiple catchments. We measured alien and native cover and richness across gradients of flow regime (flow magnitude, variability and high/low‐flow events), propagule pressure (inferred from land cover), climate and local‐scale floodplain substrate and topography. Boosted regression trees were used to determine the relative and absolute importance of these variables on plant cover and richness. Results The floodplain ecosystems were highly invaded with 154 alien species and only 31 natives. Alien cover was higher in rivers with larger maximum flows, in plots with fine substrate texture, and at higher local river bed elevations. Alien richness increased as the variability of winter flows increased and followed a hump‐shaped relationship with river bed elevation. In contrast, native species richness and cover were both shaped primarily by climate and by land cover, higher in cooler and wetter areas with more adjacent native vegetation. Main conclusions Alien and native richness and cover were shaped by different variables, so managing the ecosystem (e.g. flow regime) to mitigate aliens would not necessarily promote natives. Promotion of natives will require considerations of propagule supply and whether extant native species are suited to low‐elevation climatic conditions. Aliens were associated with predictors that approximate disturbance processes. Increased flow variability in winter could lead to an increase in the number of aliens; conversely, flow stabilization is likely to allow problematic invaders to increase in cover locally.