American beachgrass (Ammophila breviligulata) is the major stabilizing plant of the north and mid-Atlantic coastal dunes of the United States. This foredune plant is critical to sand dune formation and stabilization, and therefore to coastline protection. When health of the vegetation is compromised, vulnerability of the coast to erosion is increased. A dieout of American beachgrass on mid-Atlantic dunes stimulated an investigation as to its cause. Effects of biotic and abiotic factors on American beachgrass were studied, including acid rain, soil nutrients, and soil-borne pathogens. In greenhouse studies, acid rain was ruled out as the cause and American beachgrass was shown to be quite tolerant to acid rain and low soil pH. Further field investigation determined pathogenic nematodes to be the cause of the dieout; nine species were identified. Management strategies to alleviate the problem were tested in the field. Macronutrient addition, lime application, and increased sand accretion were determined to be measures that could be used to control the dieout and the loss of vegetation vital to dune stabilization. Specifically, macronutrients increased vigor and bunch size of the plants; lime increased survival of newly planted plants; and enhanced sand accretion increased vegetative spread. Another dune stabilization threat exists along the mid-Atlantic coast, on Assateague Island, where feral horses graze the beachgrass. With the increase in the horse population from 21 in 1965 to 170 today, destabilization of the dunes due to overgrazing and the subsequent erosion has become a concern. To assess grazing along a 13 mile stretch of foredunes, 18 exclosure plot pairs, i.e. fenced and unfenced plots, were established. Plant cover and vegetative spread were significantly greater in the exclosures than in the unfenced plots, as was the percent of culms flowering.

Key words: American beachgrass, Ammophila breviligulata, dune stabilization, feral horses, grazing, nematodes