Background/Question/Methods Across the southeastern US, the recovery of vegetated coastal ecosystems including salt marshes and sand dunes hinges upon the clonal expansion of patches of foundational grasses able to survive disturbance events. Despite widespread acknowledgement that patch dynamics govern the resilience of these systems, relatively little is known about the mechanisms controlling patch survival and expansion rate, or whether such mechanisms shift in relative importance across ubiquitous environmental stress gradients. This presentation will summarize the findings of two field experiments, one focused on resolving the relative importance of mutualistic partners in mediating cordgrass patch survival and expansion in salt marshes suffering from massive drought-generated die-back events and a second focused on resolving the relative importance of intraspecific facilitation versus competition in controlling coastal dune recovery after hurricanes. Results/Conclusions Field experiments reveal that ribbed mussels function as ‘keystone’ mutualists in drought-stricken salt marshes as they both promote cordgrass survival as well as fuel cordgrass patch expansion. Numerical simulations highlight that, by forming stress-resistance nuclei for cordgrass clonal expansion within die off areas, mussel dramatically accelerate the recovery of this high-value ecosystem from drought. Likewise, we discovered that coastal dune recovery hinges on density-dependent intraspecific facilitation under high stress conditions observed at lower beach elevations but that competitive interactions govern patch performance under lower stress conditions at upper beach elevations. Together these findings identify that the rate of patch-driven recovery of coastal systems can be powerfully controlled by mutualism and density-dependent interactions that flip in directionality across physical stress gradients.
