Coastal freshwater wetlands are under threat of salinization due to sea level rise and increasing droughts which will have a profound impact on plant communities in these ecosystems. We ask, how will tree community composition change as a result of saltwater incursion on the coastal plain of North Carolina? We hypothesize that the magnitude of change will be predicted by an index of saltwater intrusion risk based on hydrology (SIVI), as well as elevation, soil nutrients and chloride concentrations. Both low elevation and high SIVI will induce shifts in community composition because of salt exposure to the vegetation. We also ask if nutrient availability protects species and stabilizes tree community composition under the impact of saltwater incursion? We predict that plant communities in areas with sufficient nutrients will be more resilient to salt stress, and therefore exhibit the least community change over time. Data including tree species and basal area was collected during the summer of 2016 from 29 plots on the coastal plain of North Carolina that had been previously surveyed (2003-2009) and we analyzed for community composition change over time. Environmental characteristics including soil nutrients and chloride concentrations were also measured at each site.
Tree community composition shifted within the study plots; most notably we observed declines in two commonly occurring species, Taxodium distichum (bald cypress) and Chamaecyparis thyoides (Atlantic white cedar), and a marked increase in Pinus taeda (loblolly pine). Overall our model predicting the amount of community composition change based on environmental characteristics explained 47% of the variability in the data and SIVI was the most significant predictor in the model. SIVI is a good predictor because it accounts for local hydrologic features which provide connectivity to coastal water and therefore make a site more vulnerable to saltwater incursion. The additional variables, nitrogen and phosphorus, were not significant predictors in the model, which was surprising based on our expectation that nutrients might buffer plants communities from the effects of salt water incursion. It is promising to see that SIVI is a good predictor of tree community composition shift, because of the applications for its use in coastal management. After 16 years, tree communities in coastal plain wetlands showed marked differences in species composition and SIVI is a reasonably good predictor of that change.