Abstract
Tidal wetland restoration efforts can be challenging to monitor in the field due to unstable local conditions and poor site access. However, understanding how restored systems evolve over time is essential for future management of their ecological benefits, many of which are related to vegetation dynamics. Physical attributes such as elevation and distance to channel play crucial roles in governing vegetation expansion in developing tidal wetlands. However, in Mediterranean ecosystems, drought years, wet years and their resulting influence on salinity levels may also play a crucial role in determining the trajectory of restoration projects, but the influence of weather variability on restoration outcomes is not well understood. Here, we use object-based image analysis (OBIA) and change analysis of the 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) at a developing restoration site in California’s San Francisco Bay. We 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 effect increased in the 2015 image. In our image analysis, we found that different distributions of water, mud and algae between years led to different segmentation settings for each set of images, highlighting the need for more robust and reproducible OBIA strategies in complex wetlands. Our results demonstrate that adaptive monitoring efforts in variable climates should take into account the influence of weather on tidal wetland ecosystems, and that high-resolution remote sensing can be an effective means of assessing these dynamics.
Highlights
Tidal wetland ecosystems worldwide are threatened by a range of human activities (Zedler and Kercher, 2005; Erwin, 2009; Klemas, 2013) and have been in steady decline for the last 150 years in California (Goals Project, 2015)
Our results show that salinity was significantly higher during California’s historic drought, and the magnitude of mean annual vegetation change was 10.4 times slower during this period compared to the lower salinity period that preceded it (Figures 2, 3)
Our results demonstrate that drought may impact vegetation change rates in Mediterranean-type tidal wetland restoration projects, leading to non-linear recovery patterns
Summary
Tidal wetland ecosystems worldwide are threatened by a range of human activities (Zedler and Kercher, 2005; Erwin, 2009; Klemas, 2013) and have been in steady decline for the last 150 years in California (Goals Project, 2015). To effectively implement and plan restoration efforts, detailed understandings of system dynamics are necessary for driving adaptive management approaches (Spencer et al, 2016). Due to a variety of interacting factors, restoration projects may not proceed in a simple linear manner over time (Holmgren and Scheffer, 2001; Peters et al, 2004; Holmgren et al, 2006; Scheffer et al, 2009; Sitters et al, 2012; Chapple et al, 2017). Rates of restoration change over time and the factors that influence these transitions are critical yet understudied aspects of the restoration process. Since restoration projects increasingly use iterative, data-driven adaptive management strategies to plan projects, an improved understanding of how systems change over longer time periods is necessary
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