Effects of warming and altered precipitation on plant and nutrient dynamics of New England salt marshes

Salt marshes are highly productive ecosystems, valuable for their contributions in exporting biomass and nutrients, and storing carbon. Global climate models predict substantial increases in average global temperatures and associated regional shifts in precipitation patterns over the next century. Salt marsh structure and function, and consequently ability to continue to provide benefits, may be affected by these changes. Plots were established in a brackish marsh in Massachusetts in two vegetation types: Spartina patens/Distichlis spicata and short form Spartina alterniflora. Experimental manipulations consisted of a factorial design of warming via passive open-topped chambers, increased precipitation (double normal rainfall), and decreased precipitation via rainout shelters. Warming increased total biomass production of S. alterniflora (22.4%), but not S. patens or D. spicata. Decreasing precipitation also increased biomass of S. alterniflora (51.4%) and S. patens (69.3%), perhaps by alleviating waterlogging of sediments. Warming increased stem heights of S. alterniflora (9.3%), S. patens (10.9%), and D. spicata (15.9%). Decomposition was accelerated by increased precipitation and slowed by decreased precipitation in both vegetation types, but warming had no effect. Porewater salinity, sulfide, ammonium, and phosphate concentrations showed no treatment effects in either vegetation type. Phenological measurements of flowering also showed no effects. These results suggest that salt marshes may be fairly resilient to future changes in temperature and precipitation. Increased decomposition and exportation of sediment from the marsh also has implications for the ability of marshes to adapt to sea level rise. Increases in biomass and stem heights suggest marshes may be able to increase their carbon storage capability by increasing plant growth under future climate conditions.