Layer: Condition Index Value for GCPO estuarine tidal marsh (ID: 0)

Name: Condition Index Value for GCPO estuarine tidal marsh

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Description: Description of condition index value scores for estuarine tidal marsh along and within 10 km of the GCPO LCC Gulf Coast subgeography. A series of raster calculations were used in a dichotomous decision-based framework to compile a per-pixel draft condition index value at a 10 m resolution for GCPO estuarine tidal marsh based on the number of configuration and condition endpoints met within each marsh pixel. Pixels not identified as a estuarine marsh but that were identified as having the potential to be marsh were given a score of 1, provided the pixels were not classified as developed. Potential estuarine tidal marsh pixels were derived from a combination of potential estuarine tidal marsh classes in the Landfire Biophysical Settings (BPS) and future marsh in the USGS Tidal Saline Wetland Migration (TSWM) layer. Developed areas were extracted from the 2011 National Land Cover Database (NLCD) and used as a mask to indicate that areas currently under development were not expected to be converted to marsh over time. We included the BPS class “Gulf and Atlantic Coastal Plain Tidal Marsh Systems” (BPS Code 149000), as part of the Saltmeadow Cordgrass-Saltgrass-Gulf Cordgrass grouping as one of the layers of potential estuarine tidal marsh in the assessment. Note that this BPS classification does not split out estuarine vs. palustrine tidal marsh, which may or may not be relevant as marshes should be dynamic and depend on salinity and water levels over time. Further, given that tidal marshes are expected to migrate inward in many areas along the GCPO Gulf Coast we used the USGS TSWM layer to incorporate future tidal saline marsh out to a 2060 time frame, using a moderate mean sea-level rise scenario of 0.5 m. We reclassified both these layers to a binary 1, 0 and used map algebra to calculate a combined layer of potential estuarine tidal marsh. When combined, these two layers identified where estuarine tidal marsh could potentially be on the landscape based on edaphic, geographic and local site conditions in combination with where it could potentially migrate to given moderate levels of sea-level rise between now and 2060. We then used map algebra to remove developed areas using 2011 NLCD classes for developed open space (class 21), and developed low, medium, and high intensity (classes 22-24), assuming that developed, impermeable areas would not be restorable in the foreseeable future. We also removed areas of existing estuarine tidal marsh (i.e., the marsh mask) to exclude current marsh from being quantified in the potential layer. This layer of “potential” estuarine tidal marsh was calculated at 10 m resolution, then reclassified to a binary 1 or 0. The product was a layer of potential and future marsh that is not currently in estuarine marsh state, and not currently developed. Pixels of potential marsh were given a score of 1 when included in the condition index calculations below. Pixels identified as current estuarine tidal marsh (from a combination of USGS Marsh Type Delineation Project saline, brackish and intermediate marsh classification and the 5240 salt marsh classification of the Florida Cooperative Land Cover map were given a score of 2, whereas pixels found in marsh patches >250 ac were given a score of 8, and pixels found in “unbroken” patches with >70% emergent vegetative cover and <20% open water cover were given a score of 4. Pixels meeting condition endpoints of edge density ≤652 m/ha, marsh type composition (≥5% each) as a surrogate for interdigitation, and submergent vegetative cover (15-30%) were given one additional point for each endpoint, totaling up to three points. This scoring system allowed for calculation of a condition index value (CIV) based on the decision tree outlined in the assessment of estuarine tidal marsh report. Under this scoring system, existing estuarine tidal marsh pixels that were found in patches <250 ac and considered broken (<70% emergent vegetative cover and >20% open water cover) scored a CIV from 2 - 5, depending on how many condition endpoints were met. Marsh pixels found patches <250 ac but that were considered unbroken (>70% emergent vegetative and <20% open water cover) scored a condition index value from 6 - 9, depending on the number of condition endpoints met, indicating small unbroken patches may be in better condition than small highly fragmented patches. Marsh pixels that were found in large (>250 ac) patches but considered broken or fragmented scored a CIV from 10-13. This scoring system assumes a large broken patch may be in better overall condition than a small unbroken patch, but this assumption needs to be validated with empirical research. Marsh pixels that were found in large (>250 ac) patches and considered unbroken scored a CIV from 14-17. An index value of 17 represents marsh pixels that are estimated to be in the desired ecological state, as determined by the suite of measurable condition endpoints. CIVs were developed in a series of ArcGIS raster calculator computations to classify each pixel in the GCPO landscape to a value from 0 to 17, with 0 representing pixels that were not nor had the potential to be estuarine tidal marsh, 1 representing pixels that were not presently estuarine tidal marsh but had the potential to be, and values from 2 to 17 representing the gradient of index values associated with pixels that were classified as estuarine tidal marsh.

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Copyright Text: Gulf Coastal Plains and Ozarks Landscape Conservation Cooperative

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