High aqueous salinity does not preclude germination of invasive Iris pseudacorus from estuarine populations

Authors: GILLARD, MORGANE - University Of California, Davis; CASTILLO, JESUS - University Of Seville; MESGARAN, MOHSEN - University Of California, Davis; Futrell, Caryn; Grewell, Brenda

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2021
Publication Date: 5/11/2021
Citation: Gillard, M.B., Castillo, J.M., Mesgaran, M.B., Futrell, C.J., Grewell, B.J. 2021. High aqueous salinity does not preclude germination of invasive Iris pseudacorus from estuarine populations. Ecosphere. 12(5). Article e03486. https://doi.org/10.1002/ecs2.3486.
DOI: https://doi.org/10.1002/ecs2.3486

Interpretive Summary: Coastal ecosystems are threatened by biological invasions and climate change. Among global changes, sea-level rise is broadly impacting tidal wetlands, through alteration of estuarine salinity and inundation regimes. Estuaries are characterized by a gradient of tidal conditions ranging from highest salinity and tidal inundation ranges at the coast, to freshwater tidal conditions with reduced tidal range at the inland extent of ocean influence. Within tidal wetlands, glycophytes are plant species that maintain low sodium salt levels in their leaf tissue and have evolved in ecosystems with low soil sodium levels. In contrast, halophytes can sequester significant sodium in plant tissues, and are adapted to more saline environments. Changes to gradients of aqueous salinity concentrations can alter the germination of tidal wetland plant species, depending on their adaptations to these environmental stresses. In addition, the potential for invasive glycophytes (e.g. Iris pseudacorus, yellow flag iris) to colonize tidal wetlands with higher salinity raises concerns about maintenance of native biodiversity and ecosystem services. The successful establishment of these invasive plants will be dependent on acclimation or adaptation to salinity at the germination life stage. In this study, we explored germination responses of seeds from two invasive yellow flag iris populations from freshwater and brackish tidal sites in California’s San Francisco Bay-Delta estuary. We tested germination dynamics under salinity levels ranging from freshwater to seawater (0, 12.5, 25, 45 dS·m-1), and under two hydrological conditions (moist substrate; flooded substrate). Results show germination of seeds from both populations was inhibited at mid-brackish to marine salinity levels >12.5 dS·m-1. These results are consistent with our documented observations of viviparism, which is germination of seeds within capsules prior to dispersal to the environment from the mother plant. Experimental germination results were also consistent with data on seedling emergence recorded at field sites. However, seeds exposed to seawater for 55 days germinated once exposed to freshwater. Germination velocity (the speed or rate of germination over time), and seed buoyancy (the capacity of seeds to float) differed between populations. Saline soil water a strong selective force. Differences in trait measurements of seeds between study populations indicate these germination responses are likely due to thicker seed coats produced in the maternal environment with higher salinity. Our results highlight the evolutionary divergence of germination traits of yellow flag iris in response to differences in natural selection pressures at extreme ends of an estuarine gradient. This is a significant scientific finding of the study. There are also important applications of this work that are relevant for improved invasive plant management. Our results suggest invasive populations of I. pseudacorus can colonize new sites following potentially long-distance dispersal of buoyant seeds with tidal currents. Prolonged exposure to seawater will not impede the germination capacity of their seeds. These findings can improve risk assessments and management strategies for yellow flag iris in estuarine ecosystems impacted by rising sea-level. Global climate change has also been implicated with increased soil salinization in irrigated agricultural lands, that can reduce the growth and productivity of glycophytes (e.g. rice), which are the majority of crop species in agricultural production. Global food security requires development of salt tolerant crops and understanding of the evolutionary capacity of glycophytes in response to rising environmental salinity. Our experimental approach with yellow flag iris serves as a model for assessment of glycophyte trait responses to increasing salinity at the import

Technical Abstract: 1. Estuarine ecosystems are threatened by biological invasions and climate change. Among global changes, sea-level rise is broadly impacting tidal wetlands, through alteration of salinity and inundation regimes. Changes to gradients of aqueous salinity concentrations can alter the germination of tidal wetland plant species. In addition, the potential for invasive glycophytes (e.g. Iris pseudacorus) to colonize tidal wetlands with higher salinity raises concerns about maintenance of native biodiversity and ecosystem services. The successful establishment of these invasive plants will be dependent on acclimation or adaptation to salinity at the germination life stage. 2. We explored germination responses of seeds from two invasive I. pseudacorus populations from freshwater and brackish tidal sites in California’s San Francisco Bay-Delta estuary. We tested germination dynamics under salinity levels ranging from freshwater to seawater (0, 12.5, 25, 45 dS·m-1) and two hydrological conditions (moist; flooded). 3. Germination of seeds from both populations was inhibited at salinity levels >12.5 dS·m-1, consistent with viviparism and seedling emergence recorded at field sites. However, seeds exposed to seawater for 55 days germinated once exposed to freshwater. 4. Germination velocity and seed buoyancy differed between populations, likely due to thicker seed coats produced in the maternal environment with higher salinity. Results highlight divergence of germination traits at extreme ends of an estuarine gradient. 4. Synthesis and applications: Our results suggest invasive populations of I. pseudacorus can colonize new sites following potentially long-distance dispersal of buoyant seeds with tidal currents. Prolonged exposure to seawater will not impede the germination capacity of their seeds. These findings inform risk assessments of invasive glycophytes in estuarine ecosystems impacted by rising sea-level.