Abstract
Salt marshes exist in shallow-gradient landscapes where microtopographic changes on the order of centimeters to decimeters can drastically alter plant species composition. Thus, salt marshes are highly vulnerable to the effects of relative sea level rise and altered sediment supply. Of increasing importance within these systems is understanding the precise elevation and topographic gradient at which ecotonal transitions occur between lower-intermediate salt marsh platforms and upland wooded ecological communities. These narrow ecotones exist where subtle environmental changes result in dramatic changes in plant species composition and dominance. Thus, they potentially may be used to quantify and predict marsh transgression. The goal of this research was to use field surveys to quantify the elevation at which marsh-upland ecotone transitions occur within Mississippi’s coastal salt marshes. Elevation and slope was measured using parallel line transects (n = 33) extending from the intermediate marsh platform through marsh-upland ecotones sampled at ~1 m intervals using high precision integrated GNSS and traditional surveying methods at 12 sites located among 5 coastal preserves on the Mississippi Gulf Coast. Ecotones occurred in narrow elevation ranges across all combined sites (mean = 0.475 m, interquartile range (IQR) = .099 m) and individual coastal preserve sites Biloxi (mean = 0.424 m, IQR = .124 m), Grand Bay (mean = 0.544 m, IQR = .098 m), Hancock (mean = 0.468 m, IQR = .075 m), Pascagoula (mean = 0.357 m, IQR = .025 m), and Wolf (mean = 0.475 m, IQR = .082 m) respectively. Initial analysis suggests local maxima along first-derivative topographic profiles and rates of change (second-derivative) may prove useful in identifying and modeling ecotones. Defining the highly precise elevation thresholds at which marsh-upland ecotones occurs will assist in predicting marsh sustainability and upland transgression under continued relative sea level rise.
