Low-marsh ecotypes of a dominant plant may not be better adapted to increased sea level

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Plant adaptation to sea level rise has important implications for conservation and shoreline protection. However, it has received much less attention than other global changes. Here I explored adaptation of one of the dominant plants in European saltmarshes to sea level rise and whether this adaptation is mediated by successional stages. Different successional stages have distinct characteristics such as nutrient availability. I collected Elytrigia atherica from high and low marsh at early, intermediate, and late successional stages. E. atherica samples were randomly assigned to two groups, one group was grown under optimal conditions, the other under permanent inundation, simulating an increased future sea level. Phenotypic traits, biomass, and chlorophyll fluorescence were measured. Also, genetic differentiation, genotype diversity, and its relationship with trait variation were tested. Under the optimal conditions, E. atherica originated from the low marsh increased root biomass, while reducing vegetative growth, and photosynthetic efficiency compared with that from the high marsh regardless of successional stages. These results indicate that E. atherica may adapt to low marsh. However, under the permanent inundation, no significant trait differentiation of E. atherica from the high and low marsh was found. No genetic differentiation was found, and genotype diversity was not correlated with trait variation. These results suggest that low-marsh ecotypes may exist, possibly due to adaptive maternal effects. However, they are not necessarily better adapted to sustained inundation. Therefore, increased sea level may impose a great threat to coastal vegetation.