Wetlands in the coastal plain of the southeastern United States are biodiversity hotspots that are vulnerable to ecosystem changes driven by increasing salinization due to more frequent storm surges and drought events. Salinization of freshwater wetlands is further exacerbated by human alteration of hydrologic flow. Our work aims to document the changes in herbaceous vegetation structure in response to salinization in an effort to determine if presence or absence of taxa can be used as indicators of saltwater intrusion. First, to assess the primary drivers of community composition we conducted two large-scale surveys to sample vegetation abundance and soil chemistry in 669 plots on the Albemarle-Pamlico Peninsula in eastern North Carolina. Then we used Threshold Indicator Taxa ANalysis (TITAN) to (1) identify thresholds of maximum change in abundance across all taxa in response to saltwater exposure (i.e. community thresholds), and (2) categorize taxa as negative- or positive-responders to increased soil salinity. For a subset of the plots for which we had comparable data from 2004, we identified plots where salinity crossed the threshold level identified by TITAN to assess whether tolerant and sensitive taxa changed abundance in the predicted direction.
Results/Conclusions
We found that sodium, calcium and magnesium explained 65% of the variation in community composition across all plots. Although elevation was a driver of composition in upland (mean=0.96m) survey plots, it was not a reliable predictor peninsula-wide (mean=0.32m). Of the 54 taxa included in the analysis, TITAN categorized 81% of taxa as negative responders, and 19% of taxa as positive responders to increasing salinity. TITAN also identified community thresholds where the majority of sensitive taxa declined (265 ug Na+/g) and tolerant species increased (3,842 ug Na+/g) as salinity increased. All of the plots that crossed a salinity threshold from 2004-2016 saw a rise in dead vegetation and/or bare ground abundance. In plots where we found major changes in abundance, taxa were often identified accurately by TITAN as sensitive or tolerant taxa, but this was not consistent across all sites exceeding a threshold. Historical data validated our approach of using ordinations to select drivers of salinization coupled with indicator taxa analysis. Based on our results, assessments of herbaceous vegetation change over time is a promising approach for predicting proximity to thresholds where plant communities undergo taxonomic changes in coastal wetlands due to salinization.