Pre-settlement habitat stability and post-settlement burial of a tussock sedge (Carex stricta) wetland in a Maryland Piedmont river valley

Background/Question/Methods

Ancient, buried sedge-dominated wetlands in dark, organic-rich deposits have been discovered at the base of incised stream banks in the Piedmont of Maryland and southeastern Pennsylvania, although it is not known how long these wetlands persisted at any one site. In this study, a paleoecological analysis of macrofossil seeds and charcoal from a core extracted from an organic-rich river bank stratum 0.54 m thick, combined with geomorphic data, radiocarbon dates, and a record of historical land use, provide a 5000-yr history of a valley-bottom wetland site in north-central Maryland.  A comparative analysis of seeds in surface samples and hydrology within a local modern tussock sedge wetland provided an analog for seed dispersal, species composition, controlling hydrology and wetland stability in the paleo-wetland. Analysis of seeds from surface samples along transects reveal that 97% of seeds are deposited within a 5 m radius of the source plants, while an analysis of Sorenson’s Index of Similarity between samples reveal that indices > 0.40 provide a measure of habitat similarity. Hence, a seed assemblage in a sediment sample from a core represents dominant species within an 80 square meter quadrat (A= Π r²  r=5 m), and habitat stability through time is established when indices of sequential core samples are > 0.40.

Results/Conclusions

Core results show that a triad of dominant Carex species (C. stricta, C. stipata, and C. scoparia) occurred from 4300 ybp to ~300 ybp, indicating a tussock sedge assemblage persisted in a continually saturated substrate spring fed from the valley wall, despite major storm events, droughts, regional beaver activity, fire and anthropogenic disturbance within the watershed. Sequential core samples produced a running Sorenson’s average of 0.58 (range = 0.40 – 0.82), thus validating the stability of the tussock sedge wetland throughout the 4000-yr period. Burial of the wetland by lacustrine sediment occurred rapidly in the late 18^th and early 19^th centuries when sedimentation from land clearance accumulated in a mill pond behind a downstream dam.  Eventual breaching of the mill dam created an incised, high-banked meandering channel which exposed the post-settlement lacustrine sediment, paleo-wetland organic layer, periglacial basal gravels, and valley bedrock. These results have important implications for the decline and local extinction of other tussock sedge wetlands as well as the endangered bog turtle (Clemmys muhlenbergii).