Wetlands: Phragmites australis
 The Center for Coastal Resources Management is working in collaboration with the W&M Keck Environmental Field Laboratory to study the expansion of Phragmites into wetlands. This project focuses on developing a model of Phragmites invasiveness at upland/wetland interfaces by determining the mechanism of invasion, competition and spread of Phragmites . With respect to weed control, one of our applied hypotheses is that CRP sites enhance N removal prior to groundwater discharge to the wetland-upland interface, and that best management practices like buffer strips are environmentally sound forms of controlling Phragmites invasion and spread.
High nutrient concentrations in wetlands are known to shift the competitive hierarchy among wetland plants. Specifically, Levine et al. (1998) and Emery et al. (2001) have demonstrated how zonation of wetland plants is controlled by belowground competition for nutrients. An increase in nutrient supply (i.e., nutrients are no longer limiting) can shift the competition for nutrients belowground to a competition for light aboveground (Emery et al. 2001). Under these conditions, plants that allocate more biomass to leaves may outcompete other species that were superior competitors under nutrient limiting conditions (Bertness et al. 2002).
In the Chesapeake Bay region, the absence of a buffer strip between agricultural fields and adjacent aquatic environments could lead to a stimulation of invasion and expansion by the weedy species Phragmites australis in tidal wetlands. Tobias et al. (2001a) have demonstrated that tidal wetlands fringing uplands receive nitrogen-rich groundwater inputs that stimulate both denitrification and internal N recycling. Likewise, the presence of a buffer strip may serve as a barrier to the invasion and expansion of the weedy species Phragmites in tidal wetlands. Maintenance of a low flux of nitrogen to wetlands may keep Phragmites nutrient-limited and thus inhibit invasion and expansion (Bertness et al. 2002).
We propose to determine whether local increases in nitrogen availability shift the competitive balance between Phragmites and other native species of tidal wetland vegetation and spur the invasion and expansion of this weedy species. Our approach is to first examine the current distribution of the invasive stands of Phragmites along upland/brackish wetland borders of the Virginia portion of the Chesapeake Bay estuary. If the hypothesis proposed by Bertness is correct, then we'll see more boundary covered by Phragmites, and greater expansion into wetlands adjacent to agricultural fields without buffer strips.
Change in percent phragmites over time. |
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