Repetitive desiccation events weaken a salt marsh mutualism

1. Salt marshes suffered large-scale degradation in recent decades. Extreme events such as hot and dry spells contributed significantly to this, and are predicted to increase not only in intensity, but also in frequency under future climate scenarios. Such repetitive extreme events may generate cumulative effects on ecosystem resilience. It is therefore important to elucidate how marsh vegetation responds to repetitive stress, and whether changes in key species interactions can modulate vegetation resilience. 2. In this study, we investigated how moderate but repetitive desiccation events, caused by the combined effects of drought and high temperatures, affect cordgrass (Spartina alterniflora), the dominant habitat-forming grass in southeastern US salt marshes. In a 4-month field experiment, we simulated four consecutive desiccation events by periodically excluding tidal flooding and rainfall, while raising temperature. We crossed this desiccation treatment with the presence/absence of ribbed mussels (Geukensia demissa) - a mutualist of cordgrass known to enhance its desiccation resilience - and with grazing pressure by the marsh periwinkle (Littoraria irrorata) that is known to suppress cordgrass' desiccation resilience. 3. We found that each subsequent desiccation event deteriorated sediment porewater conditions, resulting in high salinity (53 ppt), low pH-levels (3.7) and increased porewater Al and Fe concentrations (approximate to 800 mu mol/L and approximate to 1,500 mu mol/L) upon rewetting. No effects on porewater chemistry were found as a result of snail grazing, while ribbed mussels strongly mitigated desiccation effects almost to control levels and increased cordgrass biomass by approximately 128%. Importantly, although cordgrass generally appeared healthy above-ground at the end of the experiment, we found clear negative responses of the repetitive desiccation treatment on cordgrass below-ground biomass, on proline (osmolyte) levels in shoots and on the number of tillers (-40%), regardless of mussel and/or snail presence. 4. Synthesis. Even though the mutualism with mussels strongly mitigated chemical effects in the sediment porewater throughout the experiment, mussels could not buffer the adverse ecophysiological effects observed in cordgrass tissue. Our results therefore suggest that although mussels may alleviate desiccation stress, the predicted increased frequency and intensity of hot dry spells may eventually affect saltmarsh resilience by stressing the mutualism beyond its buffering capacity.