Cakile edentula var. lacustris, a summer annual of the family Brassicaceae, is abundant between the first dune ridge and shoreline of Lake Huron. Populations are most dense in the middle of their distribution (driftline) and least dense at the landward and lakeward ends of the gradient. Survivorship and fruit production were highest on the lakeward end of the gradient and decreased landward. A majority ot the truits produced on plants close to the shoreline were dispersed landward by high waves. Several environmental and biotic factors, such as sand accretion, exposure of roots by sand erosion, desiccation, insect damage, disease infection and human trampling may be directly, or indirectly, responsible for seedling mortality. INTRODUCTION Cakile edentula var. lacustris, a member of the family Brassicaceae, is one of the most abundant summer annuals along the Lake Huron shoreline. The species grows on an environmental gradient from the base of the first dune ridge to the shoreline. In early spring there are dense populations of seedlings on the driftline (the farthest inland reach of waves) and scattered seedlings both below and above the driftline. The plants produce dimorphic fruit segments (upper and lower). The upper fruit segments are deciduous and disperse long distances by water waves and currents and short distances by rolling caused by strong winds (Barbour, 1972). The lower fruit segments usually remain attached to the parent, which may be uprooted by high waves and then transported by wind to new locations (Payne and Maun, 1981). Studies by Baldwin and Maun (1983), Valk van der (1974) and Oosting and Billings (1942) indicate that several microenvironmental factors vary along the gradient because of differences in nutrient availability (Pemadasa and Lovell, 1974), shifting of sand (Oosting and Billings, 1942), sand accretion or erosion (Maun and Riach, 19815, sand temperatures (van der Valk, 1974), moisture conditions (Baldwin and Maun, 1983) and inundation of plants by high waves (Payne and Maun, 1981). The variable microenvironmental conditions at different positions on the gradient would probably influence the population dynamics of Cakile edentula by differential alteration of plant characters such as reproduction, survivorship, immigration and emigration. In an earlier report, Payne and Maun (1981) examined the immigration and emigration mechanisms of C. edentula and showed that the fruits produced by the plants on the lakeward end (closest to the shoreline) of the gradient are dispersed vertically by high waves and cast on the driftline during autumn and winter months. The objectives of these investigations were: (a) to examine the reproduction and survivorship of seedlings of Cakile edentula var. lacustris on the Lake Huron shoreline and (b) to identify the physical and biotic factors affecting reproduction and survivorship of plants. MATERIALS AND METHODS The studies were conducted on a strip of beach (ca. 1 km) along the Lake Huron shoreline at Pinery Provincial Park, Canada (for details on location, see Maun, 1981). The width of the beach from the base of first dune ridge to the edge of the water ranges between 20-30 m depending on the lake level. Plant establishment and mortality. In May 1978, two permanent plots (8 x 10 m) were established on the gradient from the base of first dune to the shoreline with the long