Salt marsh vegetation change during a half-century of experimental nutrient addition and climate-driven controls in Great Sippewissett Marsh

• Published in: The Science of the total environment Vol. 867; p. 161546
• Main Authors: Valiela, I., Chenoweth, K., Lloret, J., Teal, J., Howes, B., Goehringer Toner, D.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.04.2023
• Subjects: Climate; Coastal wetland; Distichlis spicata; dominant species; Ecophenotypes; Ecosystem; ecosystem engineers; environment; greenhouses; Long-term experiment; Massachusetts; Nitrogen supply; Plant ecological engineers; Poaceae - physiology; salt marshes; sea level; Sea level rise; Spartina alterniflora; species; vegetation; Water; Wetlands; Massachusetts; Long-term experiment; Plant ecological engineers; Nitrogen supply; Ecophenotypes; Sea level rise; Coastal wetland
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2023.161546

Vegetative cover was mapped annually, 1976–2022, in experimental plots in Great Sippewissett Marsh, Cape Cod, USA, chronically fertilized at different doses, and subject to changes in sea level and other climate-related variables. Dominant species within areas of higher elevation in the plots followed different decadal trajectories: rise in sea level diminished cover of Spartina patens; higher N supplies increased cover of Distichlis spicata. The opportunistic growth response of D. spicata to high N supplies unexpectedly fostered increased platform accretion, a feature that persisted for succeeding decades and led to further changes in vegetation: D. spicata functioned as an effective ecosystem engineer with long-term ecological consequences. Shrubs usually found in upper marsh margins expanded into areas where D. spicata had stimulated accretion, then shaded and excluded D. spicata, but subsequently lost cover as sea level rise continued. Increased N supply converted stands of Spartina alterniflora, the dominant low marsh species, from short to taller ecophenotypes; sea level rise had minor effects on S. alterniflora, but during 2019–2022 appeared to reach a tipping point that fostered taller S. alterniflora and bare space even in un-fertilized control plots, and in Great Sippewissett Marsh in general. Model results anticipate that—in spite of potential accretion enhanced by vegetation and ecosystem engineer effects—there will be loss of high marsh, transient increases of low marsh, followed by loss of low marsh, and eventual conversion to shallow open water by the end of the century. Dire local projections match those of the plurality of recent reports from salt marshes around the world. Proposed management strategies may only delay unfortunate outcomes rather than maintain wetlands. Concerted reductions of warming from greenhouse gases, and lower N loads seem necessary to address the coming crises in wetlands—and many other environmental threats. [Display omitted] •Detailed maps of salt marsh vegetation from 1971 to 2022 found major shifts in species.•S. patens decreased with sea level rise (SLR); D. spicata increased with N supply.•High N supply and SLR converted S. alterniflora from its short to tall ecophenotype.•Regardless of N supply, SLR will replace high marsh species with low marsh species.•GSM, as well as >90 % of world's salt marshes, may be submerged by 2100.