Comments: Coastal sand dune systems around the world are threatened by the introduction and establishment of Ammophila arenaria. First, it is able to outcompete native dune plants. Second, it interferes with the natural dynamics of dune systems. In northern California, A. arenaria changes the geomorphology of the foredune community from a gentle slope to a vertical wall which prevents adequate sand movement from beach to interior dunes (Barbour and Johnson 1977). In Oregon, it has severely reduced the sand supply from beach to large inland dunes. Along the mid-Atlantic coast of the United States it is known to greatly alter beach profiles and subsequently change the impact and effect of storms on the coastline (Dolan et al. 1973).
The Northern Foredune Grassland Community described by Holland (1986) has been most severely threatened by the invasion of A. arenaria. This community is restricted to foredunes and is dominated by Elymus mollis. Only two undisturbed examples of this community remain in California, one of which occurs on the North Spit of Humboldt Bay (Holland 1986). The most pristine remaining occurrence is at the Lanphere-Christensen Dunes Preserve. In 1963, A. arenaria existed as one small clump 1 km north of the preserve boundary and as several clumps 4 km to the south. By 1984, it occupied 2.2 acres.