COS 114-1 - Lessons from soil-based salt marsh restoration efforts in Elkhorn Slough, CA

Abstract: Salt marshes, globally prevalent ecosystems formed at marine-terrestrial interfaces, provide numerous valuable ecosystem services including carbon sequestration and nutrient remediation. However, they are also acutely threatened by anthropogenic risk factors such as coastal development, surface- and groundwater contamination, and accelerated sea level rise (SLR). In addition to extensive conversion to agricultural land over the preceding centuries, ongoing salt marsh dieback (often attributed to excess nutrient loads) and salt marsh drowning (driven by rising sea levels and reduced sediment budgets) have driven a steep decline in the global extent of salt marshes relative to pre-Industrial Revolution conditions - 50% by some estimates, with predicted salt marsh loss by 2100 conceivably reaching 90%. In recent decades, mounting need for effective management and restoration strategies to preserve, restore, and promote resilience of salt marshes has motivated restoration projects around the world. These efforts, which frequently involve sediment addition to elevate the marsh plain relative to SLR and revegetation to stabilize sediments and promote vertical accretion, have found varying degrees of success. The growing body of salt marsh restoration work indicates that success is highly context dependent, with the materials and mechanisms employed showing variable efficacy over different spatial and temporal scales. Furthermore, the complex, globally heterogeneous convergence of soil physicochemical properties, surface and subsurface hydrology, and uniquely adapted vegetation communities that forms salt marsh ecosystems challenges efforts to distill broadly applicable principles for restoration.

Here we present findings from an ongoing 119-acre salt marsh reconstruction/restoration project in Elkhorn Slough, California, that applied soil and sediment from multiple sources to restore a degraded marsh to elevations informed by SLR projections. Based on extensive soil and vegetation monitoring and experimental soil modification and revegetation, we share insights on i) impacts of soil properties and spatiotemporal environmental variation on restoration outcome trajectories, and ii) broader takeaways on improving restoration outcomes, from effectively synthesizing discipline-specific knowledge during planning, to dynamically adapting the restoration process.

Our findings demonstrate how sediment source and harvesting and placement methods, along with pre-placement marsh characteristics, significantly influence soil structure and hydrologic dynamics in the reconstructed salt marsh, which subsequently constrains vegetation habitat viability and the establishment of desired ecosystem structure and function. We offer a conceptual framework for drawing together diverse specialties (e.g. pedology, plant ecophysiology, hydrobiogeochemistry) to enhance project design and posit that successful integration of interdisciplinary knowledge and adaptive responses to ongoing monitoring are essential for optimizing restoration success.