The Deal Island Marsh & Community Project
Marsh vegetation on the Deal Island Peninsula: Indicators of ecosystem health
Andy Baldwin and Diane Leason, Department of Environmental Science and Technology, University of Maryland
Wetland plants play a number of vital roles in wetland ecosystems, providing food for habitat, improving water quality, and influencing the flow of water as they lessen the impact of flood events, stabilize shorelines, and regulate the water cycle. The tidal marshes of the Deal Island Peninsula are home to a variety of plants. These specialized plants each have adaptations, ecological tolerances, and life history strategies.
Plants are often used as indicators of wetland health since plant growth is easy to measure and growth occurs in direct response to environmental conditions. In coastal wetlands, two of the most important environmental variables are hydrology (water level over time) and the salinity. Incoming tides bring in nourishing sediments and nutrients, but too much flooding can cause wetland plants to die back. Similarly, the salinity of water in marshes affects how well species can grow, so if salinity changes so can the species that live there and the services they provide. Most of the Deal Island Peninsula's marshes have been ditched in an effort to control mosquito populations. Ditching the marshes changed the natural hydrologic and probably salinity regimes, and may have altered plant community composition and in turn the services they provide. Efforts to restore the pre-ditching hydrology by plugging ditches may or may not be successful because the ecosystem may have changed over them many years they were ditched.
Researchers sampling vegetation at the unditched EA Vaughn study site.
Vegetation studies were conducted to understand general marsh vegetation conditions and the effects of plugging of ditches on marsh plants. Vegetation studies were conducted at three pairs of ditched and unditched marshes (two pairs on the Deal Island Peninsula and one pair at EA Vaughan). We inventoried vegetation species, estimated the amount of visual cover,
measured height, and measured the amount of ditches at the three ditch-drained marsh sites in an attempt to restore a pre-ditching hydrological regime. We compared ditches and unditched sites for each location. We expected to see a shift in the plant composition to species that were more flood-tolerant, under the assumption that ditches were draining the wetlands. We also anticipated that there would be changes in above ground and below ground biomass.
Our results were mixed. We observed no clear relationship between plant covers or diversity and ditch status (before and after ditch plug installation). On the Deal Island ditched site, above ground biomass decreased after plugging; however we also saw a decrease at the paired unditched site, indicating that the decrease was due to natural variation rather than plugging. At EA Vaughn, above ground biomass decreased after the plug was installed at the ditch site, but increased at the paired unditched site, again making it difficult to associate cause and effect in before and after comparisons. Root production did respond to the change in hydrology; however, the response varied by root fraction (e.g. rhizome vs. course roots) and site. Total below ground production decreased at sites after plugging, but it also decreased at the unditched sites, meaning that the decrease may have ben due to the ditching. Coarse root and rhizomes increased after plugging, but again the increased occurred at both the ditched and unditched sites.
Our control site, Monie Bay, saw no major shifts over time.
There are several reasons that might explain the lack of a clear response to plugging. First, it may be that the ditch plugging had little effect on hydrology. Second, while the vegetation found at our sites are sensitive to water levels, water levels remained in the tolerance range for most of the species found and insufficient time had gone by for there to be detectable changes attributable to the plugging. Third, disturbances at the sites may have introduced variability greater than that caused by ditch plugging. For example, there was a high storm tide at one site that deposited wrack and disturbed vegetation, complicating the interpretation of results. In light of these explanations, it is possible to say that there was not a strong, short-term effect of ditching that eclipsed the patterns of natural spatial and temporal variability in these marshes.
For a complete list of the vegetation found at our study sites click here.
Dr. Patricia Delgado holding an excavated root in growth core. Cores were installed and left underground for one year each year of the study period. Cores were used to measure below ground growth.