Salinity and inundation effects on Iris pseudacorus: implications for tidal wetland invasion with sea level rise

Aims Sea level rise and increases in species invasions are impacting estuarine ecosystems, but physiological tolerances of invasive plants are poorly understood. We investigated the responses of invasive Iris pseudacorus to increasing salinity, inundation and their interaction. We hypothesized that I. pseudacorus would show poor tolerance to salinity and high tolerance to inundation, and we expected deleterious effects in response to their interaction. Methods We analyzed 34 traits categorized into five functional groups (biomass production and allocation, leaf morphology, leaf chemistry, below-ground storage and gas exchange), and their responses to three salinity levels (0.5, 17 and 35 ppt), two inundation depths (-5 and -45 cm below water surface) and their combinations. These traits were recorded in a greenhouse experiment for pre-reproductive plants of three populations invading intertidal wetlands in the San Francisco Bay-Delta Estuary. Results Experimental outcomes indicate I. pseudacorus is highly vulnerable to increasing salinity during the pre-reproductive life stage. Even a mid-range brackish salinity concentration (17 ppt) was sufficient to induce maximum negative response effects on I. pseudacorus compared to freshwater conditions. While the species showed tolerance to inundation, increasing salinity limited its capacity to acclimate to greater inundation. Conclusions Pre-reproductive plants of I. pseudacorus are vulnerable to salinity and tidal range increases that accompany sea level rise. Efforts to eradicate and control expansion of the invasive populations in estuaries should focus on rapid response to manage newly colonizing populations in freshwater tidal locations, since the successful growth of I. pseudacorus is best supported in these benign environments.