Hydrogeomorphology and Horizontal Movement of Juncus roemerianus

Abstract

Juncus roemerianus, black needlerush, is common in high marshes and occasionally in low marshes along the Mid-Atlantic and southern USA. Previous work found that J. roemerianus patches remained relatively stable in the absence of disturbance and under normal variations in flooding across a marsh. Disturbance will occur from storms through wrack (dead plant material) deposition and promote plant community shifts to reduce J. roemerianus patch size. I hypothesized that horizontal movement of J. roemerianus patch borders varies among hydrogeomorphic locations related to differences among those sites. A summary of the relationships between patch border dynamics, the condition of J. roemerianus, bordering communities, and environmental factors is shown in a conceptual model. The borders of patches of J. roemerianus within different areas of a salt marsh were tracked at Upper Philips Creek (UPC). UPC is located on the Delmarva Peninsula and is part of the Virginia Coast Reserve Long-Term Ecological Research (LTER) site. In 1990, eight 3 x 8 m permanent plots, which contained the interface between J. roemerianus and other species, were established throughout the UPC marsh. Two hundred squares within 1 x 2 m quadrats within the plots were assessed for ground cover. Every year from 1990 to 2014 ground cover was identified visually and non-destructively. Differences in horizontal movement of Juncus patch border were found among geomorphic locations within the marsh. Expansion occurred at high marsh locations both away from and near a creek with rapid rates of horizontal movement of Juncus outwards. Little to no expansion was observed at one low marsh site and a high marsh site bordering a hollow with slow rates of horizontal movement of Juncus outwards. Wrack reduced patch size at one low marsh site in 1994 without full recovery by 2014. This study helps better understand the geomorphic setting and context for this plant and helps track community structure and environmental factors associated with patches of J. roemerianus within the salt marsh. This is the first time that rates of horizontal movement of Juncus and community changes have been assessed in this way. It also helps in understanding ecosystem state changes associated with the long-term effect of sea-level rise versus wrack disturbance.