The Influence of Climate and Seed Dispersal on Restoration in the San Francisco Bay
- Author(s): Chapple, Dylan
- Advisor(s): Merenlender, Adina
- Suding, Katharine
- et al.
Ecosystem restoration is increasingly used as a tool to offset the contemporary loss of habitat or help address past ecosystem destruction. However, given the complex, interconnected nature of ecosystem processes, restoration outcomes are notoriously variable. In certain ecosystems, minimal intervention may be able to produce desired restoration outcomes, but in others even high levels of intervention may not achieve restoration goals. Significant uncertainties remain about restoration trajectories over time and space. In order to effectively deliver the benefits these projects purport to offer, a detailed scientific understanding of restoration trajectories is necessary to inform the practice of ecological restoration moving. To better understand the factors influencing restoration trajectories in California’s San Francisco Bay (SF Bay), I compare change in vegetation over time at an older restoration and reference site using field data, explore rates of change at a recently established wetland restoration site in the context of a historic drought using remote sensing of satellite imagery, and compare seed dispersal across three restoration and two reference sites. Addressing these questions will aid in the management of these projects and design of future project which will help improve conservation outcomes and address impacts associated with sea level rise and increased climate variability.
While ecosystem development can take decades if not centuries, most restoration projects are minimally monitored, and a five-year window for assessment is often the best-case funding scenario. This means that detailed temporal studies of restoration projects are rare, which is particularly problematic in variable areas where climate cycles proceed at decadal scales. To address this gap, I worked with a vegetation field data set collected most years between 1990 and 2005 at an established restoration site initiated in 1975 (Muzzi Marsh) and a historical reference wetland (China Camp) in Marin County, CA. To determine the factors influencing reference and restoration trajectories, I examined changes in plant community identity in relation to annual salinity levels in the SF Bay, annual rainfall, and tidal channel structure. Over the entire study period, both sites experienced significant directional change away from the 1990 community. Community change was accelerated following low salinity conditions that resulted from strong El Niño events in 1994–1995 and 1997–1998. Overall rates of change were greater at the restoration site and driven by a combination of dominant and sub-dominant species, while change at the reference site was driven by sub-dominant species. Sub-dominant species first appeared at the restoration site in 1996 and incrementally increased during each subsequent year, whereas sub-dominant species cover at the reference site peaked in 1999 and subsequently declined.
In addition to the later-stage restoration dynamics highlighted above, climate variability may also influence the early stages of restoration site development. At a developing restoration site in the SF Bay, I use object-based image analysis (OBIA) and change analysis of high-resolution IKONOS and WorldView-2 satellite imagery to explore whether mean annual rates of change from mudflat to vegetation are lower during drought years with higher salinity (2011-2015) compared to years with lower salinity (2009-2011). I found that vegetation increased at a mean rate of 1979 m2/year during California’s historic drought, 10.4 times slower than the rate of 20580 m2/year between 2009 and 2011 when the state was not in drought. Vegetation was significantly concentrated in areas in closer to channel edges, where salinity stress is ameliorated, and the magnitude of the channel effect increased in the 2015 image.
Seed dispersal is another critical but understudied mechanism driving restoration site development. Where and when seeds arrive at a restoration site can have major implications for how a restoration project proceeds. In my final chapter, I explore seed dispersal over a chronosequence of three restoration sites and two reference sites at Eden Landing Ecological Reserve (Hayward, CA), part of the South Bay Salt Pond Restoration Project. I find that seeds of wetland species in the restoration sites were significantly aggregated in areas with vegetation cover above 30%, and that many study plots were completely devoid of wetland seeds. Vegetation cover was significantly related to channel proximity and relative elevation at the sites. Reference sites contained significantly more seeds than restoration sites, but density was low overall at the reference sites. The oldest restoration site had statistically equivalent seed density compared to one of the reference sites. Across all sites, the pioneer dominant species Salicornia pacficia was the most common seed species, and sub-dominant species were only found in a single plot in the restoration sites and in overall low densities in the reference sites. These results highlight the fact that seeds or seedlings may need to be added to developing restoration sites, and that manipulating elevation and channel structure may be important for accelerating the rate of vegetation development.