Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise

Abstract

The distribution of New England salt marsh communities is intrinsically linked to the magnitude, frequency, and duration of tidal inundation. Cordgrass (Spartina alterniflora) exclusively inhabits the frequently flooded lower elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) typically dominate higher elevations. Monitoring plant zonal boundaries in two New England salt marshes revealed that low-marsh cordgrass rapidly moved landward at the expense of higher-marsh species between 1995 and 1998. Plant macrofossils from sediment cores across modern plant community boundaries provided a 2,500-year record of marsh community composition and documented the migration of cordgrass into the high marsh. Isotopic dating revealed that the initiation of cordgrass migration occurred in the late 19th century and continued through the 20th century. The timing of the initiation of cordgrass migration is coincident with an acceleration in the rate of sea-level rise recorded by the New York tide gauge. These results suggest that increased flooding associated with accelerating rates of sea-level rise has stressed high-marsh communities and promoted landward migration of cordgrass. If current rates of sea-level rise continue or increase slightly over the next century, New England salt marshes will be dominated by cordgrass. If climate warming causes sea-level rise rates to increase significantly over the next century, these cordgrass-dominated marshes will likely drown, resulting in extensive losses of coastal wetlands. R ecent studies indicate that both climate warming (1, 2) and increases in the rate of sea-level rise (SLR) in New England (3) over the last 150 years are unprecedented in at least the last 1,000 years. The possibility that emission of greenhouse gases is influencing and will continue to influence global climate and potentially SLR has prompted considerable research into the possible implications for plant and animal communities (4). The distribution of salt-marsh communities is mechanistically linked to the duration of tidal inundation. As a result, coastal wetlands may be particularly sensitive to changes in sea level (5). Wetland loss has been documented in areas of the Mississippi River Delta (6) and Chesapeake Bay (7), where rates of local SLR exceed marsh accretion.In New England salt marshes, cordgrass (Spartina alterniflora) exclusively dominates daily f looded low-marsh elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) dominates higher marsh elevations (8). Lower species borders are controlled by physical stress tolerance to flooding and soil anoxia, whereas upper species borders are controlled by interspecific plant competition (9, 10). Marsh hay and spike grass are excluded from the low marsh by low substrate oxygen levels. The ability of cordgrass to oxygenate substrates (11) allows this species to dominate frequently flooded lower-marsh elevations, where...

Does vegetation prevent wave erosion of salt marsh edges?

“…Indeed, coastal plants can build coastal landforms through a coupled process of ecological succession and sedimentary accretion (13,22). We conclude that coastal vegetation is best suited to modify and control sedimentary dynamics in response to gradual phenomena like sea-level rise or tidal forces (23,24), but is less well-suited to resist punctuated disturbances (25,26) at the seaward margin of salt marshes, specifically breaking waves. In addition, the soil type (27) and geographical setting (28) are the most important factors to consider when comparing erosion rates among sites.…”

Coupling instrumental and geological records of sea‐level change: Evidence from southern New England of an increase in the rate of sea‐level rise in the late 19th century

“…The NYC tide-gauge data further support the late 19th century timing of the SLR increase. Linear regression of segments of the NYC tide-gauge data indicate an increase in the rate of SLR from about 1.0 mm/year between 1856 and 1878 to 2.4 mm/year between 1893 and 1921 A.D. [Donnelly and Bertness, 2001]. An increase in the rate of SLR at this time is also supported by the onset of transgressive migration of low-marsh cordgrass (Spartina alterniflora) into the high marsh in the region [Donnelly and Bertness, 2001].…”

Spatial response of coastal marshes to increased atmospheric CO 2

“…The model describes the evolution of a marsh transect perpendicular to a tidal channel, which delivers inorganic sediment, by focusing on accretion−submersion processes rather than on marsh lateral erosion or expansion (33,34). Changes in the marsh surface elevation, z, at a position x along the marsh transect are dictated by the sum of the rate of inorganic soil deposition, Q s (x), the rate of sediment trapping by where B a (x) and B b (x) are aboveground and belowground biomass (linear functions of ΔCO 2 ), and R is the RRSLR (Table S3).…”