Climate sensitivity and restoration trajectories: Insights from tidal marsh restoration in Elkhorn Slough, California

Abstract

Understanding restoration trajectories and their sensitivity to climate is critical for designing effective adaptation strategies for restoration projects. Tidal marsh restoration often involves initial bare earth conditions that may be stressful to colonizing plants, especially on high elevation marsh platforms built to be resilient to sea-level rise. Under these circumstances, stressors such as soil salinity may increase over time, but can be mitigated by strong rainfall. At Hester Marsh, a large tidal marsh restoration site in Elkhorn Slough, California, we evaluated passive restoration success, tracking colonization by plants whose seeds arrived naturally on tides, and active restoration success, monitoring greenhouse-grown transplants. Our investigation revealed nonlinear restoration trajectories with high climate sensitivity, at the scale of the entire landscape and of individual plants. We found strong effects of drought on marsh restoration success indicators. Plant colonization rate decreased dramatically over time in the first area to be completed, which experienced more drought conditions following construction. In contrast, it declined more slowly in the second area, which experienced more rainy years following construction. Both passive and active restoration showed strong differences across these areas and across dry and rainy years. Facilitation can sometimes improve conditions for later-arriving plants, but we found higher mortality of seedlings under existing vegetation than in bare areas. Thus, plant colonization may slow over time both due to increasing abiotic stress and through competition by early colonizers. Our findings lead to recommendations for climate adaptation strategies for tidal marsh restoration. Since we found that the first year following construction appeared to have the least stressful conditions, we recommend managers invest especially heavily in supporting plant colonization during this early window of opportunity. We also found plant size and species affected drought tolerance and recommend larger plant sizes and hardy species be incorporated into active tidal marsh restoration. Furthermore, we recommend planning for phased completion of restoration projects to generate a mosaic of areas with different trajectories and increase the probability that some areas will be completed during optimal climate conditions. We thus illustrate how an understanding of climate sensitivity of restoration trajectories can enhance restoration success.