FL-SC Coastal Die off SurveyData on the size of die-offs and patch distribution within those dieoffs observed at marsh sites located along the southeastern US coastlineCoastal Survey.xlsxDieOff Recovery 2012-15Data on the area of marsh die offs on Sapelo Island Georgia in 2012 and in 2015, after 3 years of recovery. Data on the patch density within each die off is also includedMussel Addition Experiment: Ammonium Salinity Expansion DataData from a 2012 mussel addition experiment showing the concentration of ammonia and porewater salinity in cordgrass transplants either with mussels added or not added. In addition, data on the expansion (number of new tillers) observed after 8 months.Oakdale Ammonium Salinity Expansion Data.xlsxSalinity Well PairsSalinity data from pairs of wells either location inside mussel mounds or in the adjacent marsh without mussels; data were collected nearly weekly from May - August 2013Sapelo natural patch expansionData on the rate of expansion of 79 patches remaining wishing Sapelo Island marsh die offs after 6, 12 and 19 months.Soil water storageData on the grams of water stored per gram of soil from cores collected from inside and outside mussel mounds in a Sapelo Island, GA marsh

Droughts are increasing in severity and frequency, yet the mechanisms that strengthen ecosystem resilience to this stress remain poorly understood. Here, we test whether positive interactions in the form of a mutualism between mussels and dominant cordgrass in salt marshes enhance ecosystem resistance to and recovery from drought. Surveys spanning 250 km of southeastern US coastline reveal spatially dispersed mussel mounds increased cordgrass survival during severe drought by 5- to 25-times. Surveys and mussel addition experiments indicate this positive effect of mussels on cordgrass was due to mounds enhancing water storage and reducing soil salinity stress. Observations and models then demonstrate that surviving cordgrass patches associated with mussels function as nuclei for vegetative re-growth and, despite covering only 0.1–12% of die-offs, markedly shorten marsh recovery periods. These results indicate that mutualisms, in supporting stress-resistant patches, can play a disproportionately large, keystone role in enhancing ecosystem resilience to climatic extremes.