Abstract
Coastal marsh persistence depends on the ability of marshes to maintain elevation as sea levels rise, but plant species and intraspecific genetic variation may alter the processes that govern elevation gain. We used 15 years of Sediment Elevation Table (SET) measurements from the Smithsonian Environmental Research Center Global Change Research Wetland to compare elevation change between adjacent native- and introduced Phragmites australis-dominated areas and determine whether accretion rates varied with Phragmites genetic diversity. Phragmites-dominated areas gained elevation four times faster than adjacent native plant communities, averaging 3.16 +/- 0.25 mm yr-1 compared to 0.76 mm yr-1 in native areas. Consequently, Phragmites areas accumulated elevation at a rate of 82% of local sea-level rise, compared with only 20% in native areas. Areas containing multiple Phragmites genotypes accreted 37% faster than areas dominated by a single genotype and accumulated elevation at a rate of 94% of the local rate of sea-level rise. Differences among Phragmites areas were primarily associated with processes occurring below the native rooting zone, where deep-profile elevation gain was 27% greater in areas containing multiple genotypes. Genotypic composition was also significantly related to deep-profile accretion rates, suggesting that both genetic diversity and genetic identity influence belowground marsh-building processes. Our findings demonstrate that species shifts to Phragmites can substantially increase marsh elevation gain, and intraspecific genetic variation can further modify this response in this and other species. These results have important implications for predicting and managing coastal marsh resilience under accelerating sea-level rise and highlight an important biodiversity-resilience management tradeoff.