Salt marsh migration into salinized agricultural fields: a novel assembly of plant communities

QUESTIONS: Sea level rise and saltwater intrusion are changing low‐lying coastal landscapes, converting agricultural land and other upland habitats to tidal marsh. Abandoned, saline agricultural fields are affected by a unique combination of environmental filters, those traditionally found in tidal marsh — salinity and flooding — alongside those of cultivated lands — high nutrient availability and a history of disturbance. We asked how species composition and functional trait composition in saline fields compares to those in traditional old fields and natural ecotones, and whether trends in succession can be detected in saline fields during the first years post‐abandonment. LOCATION: Chesapeake Bay (Mid‐Atlantic, USA). METHODS: We surveyed plant communities assembling in saline fields and compared taxonomic and functional trait diversity to those in old‐field and marsh–forest ecotone communities. We also assessed changes in the saline fields after two and three years of abandonment to detect the direction of succession. RESULTS: Saline fields occupied an intermediate taxonomic and trait space between old fields and marsh ecotones. From old fields to saline fields to marsh, communities were less weedy, and more wetland, native, and perennial. Specific leaf area decreased across this transition, in concordance with expected changes in response to salinity. Over time, saline fields became less graminoid and less weedy, and more native, wetland, and woody. CONCLUSIONS: We conclude that marsh migration into abandoned farmland is producing a novel assembly of plant communities. Intermediate functional traits in the saline fields reflect the novel environmental filters imposed by saltwater intrusion and the cultivation legacy. These patterns suggest that abandoned, saline agricultural fields may develop somewhat differently than natural marsh boundaries, with more shrub dominance and greater resilience to Phragmites australis invasion. Importantly, these results suggest that saline fields will provide a facilitating route for marsh migration.