TRENDS IN VEGETATION AND HEIGHT OF THE TOPOGRAPHIC SURFACE IN A TIDAL FRESHWATER SWAMP EXPERIENCING ROOTING ZONE SALTWATER INTRUSION

The ground surface height of coastal freshwater wetlands is decreasing worldwide due to a number of factors including salinification in the rooting zone. This loss of surface height can lead to the loss of peat, carbon, vegetation stability, ecosystem function, and biodiversity. This work identifies indicators of impending vegetation transition in freshwater swamps by examining long-term changes in the salinity levels of the rooting zone. In a tidal Taxodium distichum swamp in the Hickory Point State Forest, Pocomoke River, Maryland, the topographic surface height (~elevation) decreased by 25.6 ± 2.2 to 50.8 ± 3.8 cm at two Surface Elevation Tables. As a result of the change in rooting zone salinity, the peat became unconsolidated following hurricanes and offshore storms from 2015 to 2021. For example, after Hurricane Melissa, root biomass and surface height decreased after rooting zone salinity exceeded 5 ppt for more than 25% of the time, with a maximum salinity level of 12.5 ppt. Sixty percent of the mature T. distichum trees died, and growth of surviving trees was very slow. In addition, no T. distichum seedlings survived. A vegetation shift toward more salt tolerant Typha x glauca was apparent by 2021. This work captures a multi-year trend of decreasing ground surface height, tree growth, and health following storm driven salinification in the rooting zone of tidal forested wetlands of the Chesapeake Bay.