The Demography of a Sand Dune Annual: Vulpia Fasciculata: III. The Dispersal of Seeds

(1) On dispersal the caryopses of the winter annual grass, Vulpiafasciculata, remain within the spikelet, forming a seed-dispersal unit. The dispersal of these units, which was studied on two dune systems in North Wales, has two distinct phases. Firstly, dispersal units fall onto the soil surface from the infructescences (Phase I), and, secondly, they may be moved along the ground surface by various inorganic or organic agencies (Phase II). (2) The release of seed-dispersal units began immediately after they had ripened at the end of June and continued over 3 months, although 94% were dispersed in July. The rapid shedding of seed maximizes the time available for Phase II dispersal. (3) The distance of Phase I and Phase II dispersal was in general restricted to a few centimetres. The mean distance of Phase I dispersal was density-dependent and strongly correlated with the height of the infructescence, and the distance of Phase II dispersal was strongly dependent upon the percentage cover of the vegetation. The greatest proportion of dispersal units moved in the direction of the local prevailing winds. Few units were buried on the fixed dunes, but most were buried on the one yellow dune site studied. (4) The distance of dispersal is discussed in relation to the population dynamics of V. fasciculata, and in relation to the colonization of new areas of habitat. The curves relating the logarithm of seed density to the logarithm of distance from seed source indicate that the species will spread more as isolated colonists from tall infructescences and as an advancing front from short ones. On the ground the species will spread as a colonizing front where the vegetation cover is less than 50%, and more as isolated colonists where it is greater than this.