Functional trait responses of emergent and free-floating Alternanthera philoxeroides to increasing salinity with sea level rise: Stress tolerance, avoidance and escape strategies

Authors: GREWELL, BRENDA - Former ARS Employee; GALLEGO-TÉVAR, BLANCA - University Of Seville; CASTILLO, JESUS - University Of Seville; Futrell, Caryn; DRENOVSKY, REBECCA - John Carroll University; HARMS, NATHAN - Us Army Engineer Research And Dvelopment Center; PRATT, PAUL - Former ARS Employee

Submitted to: NeoBiota
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2025
Publication Date: 10/7/2025
Citation: Grewell, B., Gallego-Tévar, B., Castillo, J.M., Futrell, C.J., Drenovsky, R.E., Harms, N.E., Pratt, P., Mueller, R.C. 2025. Functional trait responses of emergent and free-floating Alternanthera philoxeroides to increasing salinity with sea level rise: stress tolerance, avoidance and escape strategies. NeoBiota. 10:9-36. https://doi.org/10.3897/neobiota.102.150325.
DOI: https://doi.org/10.3897/neobiota.102.150325

Interpretive Summary: America, long recognized as one of the world’s worst freshwater aquatic weeds, recently invaded tidal wetlands in California’s San Francisco Bay-Delta Estuary. Observations suggested the invasive macrophyte may have some degree of salinity tolerance, though it was generally considered to be limited to freshwater. Sea level rise is increasing environmental stress gradients in estuaries, yet the salinity tolerances and capacity of A. philoxeroides to spread with increased salinity intrusion were unknown. In two full factorial greenhouse experiments, we assessed responses of emergent (soil-rooted) and free-floating growth forms of A. philoxeroides to four salinity concentrations (freshwater to seawater) at the whole plant level (growth, biomass production and allocation, fitness), and for physiological and biochemical functional trait responses that can contribute to salinity tolerance. While sensitivity of A. philoxeroides to increasing salinity was documented, the survival of both growth forms in the full range of salinity treatments was notable and unexpected. Next, we conducted a third experiment to explore the potential for recovery of free-floating A. philoxeroides in freshwater, following extended exposure in three brackish to marine aqueous salinity concentrations. Our results indicate A. philoxeroides is a facultative halophyte well-adapted to freshwater through moderately brackish (12 ppt) salinity levels. Results also revealed the invasive weed’s multiple strategies to survive salinity-induced physiological stress supporting its survival even at elevated brackish (18ppt) to marine (36 ppt) salinity. The macrophyte expressed functional trait responses revealing stress tolerance, avoidance and escape strategies which can contribute to its invasiveness as estuarine salinity intrusion increases with sea level rise.

Technical Abstract: Alternanthera philoxeroides Mart. (Griseb.) (alligator weed) native to South America, long recognized as one of the world’s worst freshwater aquatic weeds, recently invaded tidal wetlands in California’s San Francisco Bay-Delta Estuary. Observations suggested the invasive macrophyte may have some degree of salinity tolerance, though it was generally considered a freshwater-limited glycophyte. Sea level rise is increasing abiotic stress gradients in estuaries, yet the salinity tolerances and capacity of A. philoxeroides to spread with increased salinity intrusion were unknown. In two full factorial greenhouse experiments, we assessed responses of emergent (soil-rooted) and free-floating growth forms of A. philoxeroides to four salinity concentrations (freshwater to euhaline) at the whole plant level (growth, biomass production and allocation, fitness), and for physiological and biochemical functional trait responses that can contribute to salinity tolerance. While sensitivity of A. philoxeroides to increasing salinity was documented, the survival of both growth forms in the full range of salinity treatments was notable and unexpected. Next, we conducted a third experiment to explore the potential for recovery of free-floating A. philoxeroides in freshwater, following extended exposure to mesohaline to euhaline aqueous salinity. Our results indicate A. philoxeroides is a facultative halophyte well adapted to oligohaline - mesohaline salinity levels. Results also revealed the invasive weed’s multiple strategies to survive salinity-induced physiological stress supporting its survival even at elevated polyhaline to euhaline conditions. The macrophyte expressed functional trait responses revealing stress tolerance, avoidance and escape strategies which can contribute to its invasiveness as estuarine salinity intrusion increases with sea level rise.