The following research explores how abiotic and biotic processes interact to shape the distributions of the marsh-upland ecotone, a characteristic high marsh plant community in Pacific coast salt marshes that forms the transition zone between vegetated marsh plain and upland habitats. Understanding how abiotic and biotic ecological features interact to structure the marsh upland ecotone is necessary for predicting how the boundaries and distributions of this plant community will respond to disturbance, both human and natural, and for the design of effective strategies to restore and conserve degraded habitats. The abrupt boundaries, relatively simple community composition, and rapidly transitioning abiotic gradient of the marsh-upland ecotone make both observational and manipulative approaches feasible for addressing these research goals. Here both approaches are used to quantify the abiotic and biotic factors responsible for setting species distributions, to test how the relative influence of these factors changes across the underlying abiotic gradient resulting from variable tidal influence, and to design an effective restoration strategy for habitats degraded by disruption of the natural abiotic regime. These results challenge the applicability of a classic theoretical framework commonly applied to describe the structure of the marsh-upland ecotone, increase our understanding of the ecological processes, both biotic and abiotic, structuring the plant community of the marsh-upland ecotone, and optimize a time- and cost-effective restoration strategy to restore degraded ecotone habitats. This body of research significantly enhances our understanding of the complex abiotic and biotic processes structuring the marsh and also contributes to the understanding of how these processes structure species distributions in general.