Vegetation Development in a Tidal Marsh Restoration Project during a Historic Drought: A Remote Sensing Approach

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 important 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 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 1,979 m 2 /year during California's historic drought, 10.4 times slower than the rate of 20,580 m 2 /year between 2009 and 2011 when the state was not in drought. Vegetation was significantly concentrated in areas 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.

Turnover and change in plant species composition in a shielded salt marsh following variation in precipitation and temperature

“…The increased soil available N and available P might, however, partly be a result of the plants' decreased ability to acquire nutrients during a drought (Bista et al, 2018). Soil salinity levels are known to increase during a drought (Chapple & Dronova, 2017;Forbes & Dunton, 2006), as was also the case in this study, although not significantly (Table 1).…”

“…Shifts in rainfall regimes can cause the vegetation structure to shift between salt flats, mangroves and salt marshes (Osland et al, 2016). Further, droughts alter the soil chemistry and result in more saline conditions (Chapple & Dronova, 2017;Forbes & Dunton, 2006;Palomo, Meile, & Joye, 2013). Stressful conditions not only affect the physical environment but both salt and drought stress impact plant ability to utilize nutrients (Bista, Heckathorn, Jayawardena, Mishra, & Boldt, 2018;Hu & Schmidhalter, 2005).…”

“…We aimed at answering the following questions: (a) can precipitation and temperature variations between years change the edaphic conditions (moisture, salinity, nutrient levels) of a salt marsh? We expected a change in the abiotic environment following the drought toward more saline conditions (Chapple & Dronova, 2017;Forbes & Dunton, 2006;Palomo et al, 2013). As increased precipitation on salt marsh habitats influences plant species composition (Dunton et al, 2001), we expected species composition during a year of drought to be different from that of a wet…”

Assessing the Resilience of Coastal Wetlands to Extreme Hydrologic Events Using Vegetation Indices: A Review

“…Another study applied change analysis method using high-resolution IKONOS and WorldView-2 satellite imagery to identify the annual rates of change from mudflat to vegetation in a coastal wetland (Tidal march) restoration area. Not only the effects of wet years and drought, the trends of the vegetation in that tidal marsh area were likely influenced by a combination of other factors such as sedimentation rates [55].…”

Voluntary Restoration: Mitigation's Silent Partner in the Quest to Reverse Coastal Wetland Loss in the USA

“…As stated previously, not all voluntary wetland restoration is creating new wetlands, as several of the restoration actions reported (e.g., debris, pollutant, and invasive species removal) are not creating new wetlands. Further, successful wetland restoration projects can require upwards of a decade to vegetate (Zedler and Callaway, 1999;Zedler and Kercher, 2005;Kusler, 2012), resulting in a lag in the spectral change required for detection via the remote sensing approach (Chapple and Dronova, 2017) used by NOAA C-CAP to calculate wetland change (NOAA, 2018a). Thus, the potential lag in detectability of both compensatory and voluntary wetland restoration may result in an over or under estimation of wetland losses and gains for decades or even longer.…”