Stable Coastal Wetlands (Southeast Blueprint Indicator)

Reason for Selection Threats like sea-level rise, erosion, and development are driving widespread salt marsh loss. Tidal marshes protect coastlines from storms, filter pollution to improve water quality, and provide important habitat for birds, fish, and shellfish. Studies show that the unvegetated-vegetated ratio (UVVR) of tidal marshes serves as a good surrogate for marsh degradation processes like sediment loss and conversion to open water (Ganju 2017, Wasson et al. 2019). In general, “...marshes with high vegetative cover and lower open-water area tend to trap and retain sediment, while marshes that are losing plant cover will further lose sediment and convert to open water” (Wasson et al. 2019). Examining snapshots of the UVVR and how it changes over time can differentiate between relatively stable and more vulnerable marshes. This can help land managers determine which marshes are the best candidates for restoration to help them persist in the face of changing conditions (USGS 2017). Recent research also shows that UVVR correlates with elevation measurements that detect vertical changes due to accretion, subsidence, or compaction—meaning that stable marshes with high vegetative cover tend to better keep pace with sea-level rise. This suggests that marshes with low vegetative cover and an unstable UVVR tend to be “both horizontally and vertically vulnerable”. It also shows that “horizontal integrity is a prerequisite for vertical stability: a marsh can only maintain elevation if the plain is intact with minimal unvegetated area” (Ganju et al. 2023). Input Data * [Base Blueprint 2022 extent] * [Southeast Blueprint 2023 extent] * [National Wetlands Inventory] * [An unvegetated to vegetated ratio (UVVR) for coastal wetlands] This dataset computes the UVVR for coastal wetlands using Landsat data from 2014-2018. The average and standard deviation of the UVVR across that five-year period can be used to characterize both a marsh’s overall stability and its variability from year to year. The figure on the following page, from [Ganju et al. 2022], identifies the threshold used in this indicator to define stable, low uncertainty tidal marshes (<0.15 average UVVR and <0.15 standard deviations from the average UVVR). Mapping Steps * Calculate the mean and standard deviation of the UVVR 2014-2018 imagery. * Extract areas with mean and standard deviation values <0.15, which represent stable vegetated areas. Reclassify the resulting raster to assign stable coastal wetlands a value of 2. * Extract fresh and saline tidal wetlands from NWI (note: because of the complex way the NWI is classified, the query to extract these classes is too long to include here, so consult the code for more specifics). Buffer by 30 m and covert to raster. Clip the stable vegetated areas produced in the previous step to the NWI tidal wetlands. * Reclassify the standard deviation rasters produced in the first step to assign a value of 0 to the study area. Zero values are intended to help users better understand the extent of this indicator and make it perform better in online tools. * Cip the study area raster to the NWI raster to create a coastal wetland extent raster and reclassify to assign it a value of 1. * Use the Cell Statistics “MAX” function to combine the rasters produced above. This produces a new raster with the indicator values seen below, where each pixel is assigned the highest value it received in any of the previous steps. * Clip to the spatial extent of Base Blueprint 2022. * As a final step, clip to the spatial extent of Southeast Blueprint 2023. Note: For more details on the mapping steps, code used to create this layer is available in the [Southeast Blueprint Data Download] under > 6_Code. Final indicator values Indicator values are assigned as follows: 2 = Stable coastal wetlands 1 = Other coastal wetlands 0 = Not identified as coastal wetlands Known Issues * The age of the NWI data is inconsistent across the Gulf and Atlantic coast. * Newly created wetlands, such as the Wax Lake Delta in Louisiana, are missing from this indicator. Disclaimer: Comparing with Older Indicator Versions There are numerous problems with using Southeast Blueprint indicators for change analysis. Please consult Blueprint staff if you would like to do this (email [hilary_morris@fws.gov]). Literature Cited Couvillion, B.R., Ganju, N.K., and Defne, Z., 2021, An Unvegetated to Vegetated Ratio (UVVR) for coastal wetlands of the Conterminous United States (2014-2018): U.S. Geological Survey data release. [[https://doi.org/10.5066/P97DQXZP]]. Ganju, N., Defne, Z., Kirwan, M. et al. Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes. Nat Commun 8, 14156 (2017). [[https://doi.org/10.1038/ncomms14156]]. Ganju, Neil. May 19, 2022. Developing standardized geospatial metrics for salt marsh management and restoration. Webinar hosted by the Southeast Conservation Adaptation Strategy. [[https://www.youtube.com/watch?v=9ULO2ZZNVxc]]. Ganju, N. K., Defne, Z., & Fagherazzi, S.. (2020). Are elevation and open-water conversion of salt marshes connected?. Geophysical Research Letters, 47, e2019GL086703. [[https://doi.org/10.1029/2019GL086703]]. Ganju, N.K., Couvillion, B.R., Defne, Z. et al. Development and Application of Landsat-Based Wetland Vegetation Cover and Unvegetated-Vegetated Marsh Ratio (UVVR) for the Conterminous United States. Estuaries and Coasts (2022). [[https://doi.org/10.1007/s12237-022-01081-x]]. Ganju, N.K., Defne, Z., Schwab, C. et al. Horizontal Integrity a Prerequisite for Vertical Stability: Comparison of Elevation Change and the Unvegetated-Vegetated Marsh Ratio Across Southeastern USA Coastal Wetlands. Estuaries and Coasts (2023). [[https://doi.org/10.1007/s12237-023-01221-x]]. U. S. Fish and Wildlife Service. October 2020. National Wetlands Inventory. U.S. Department of the Interior, Fish and Wildlife Service, Washington, D.C. [[https://www.fws.gov/program/national-wetlands-inventory/data-download]]. U.S. Geological Survey. January 24, 2017. “New technique quickly predicts salt marsh vulnerability.” [[https://www.usgs.gov/programs/cmhrp/news/new-technique-quickly-predicts-salt-marsh-vulnerability]]. Wasson, Kerstin, Neil K Ganju, Zafer Defne, Charlie Endris, Tracy Elsey-Quirk, Karen M Thorne, Chase M Freeman, Glenn Guntenspergen, Daniel J Nowacki and Kenneth B Raposa. (2019). Understanding tidal marsh trajectories: evaluation of multiple indicators of marsh persistence. Environmental Research Letters 14 124073. [[https://iopscience.iop.org/article/10.1088/1748-9326/ab5a94/]].