Alnus maritima (Marsh.) Nutt. (seaside alder) is a rare North American species known to occur naturally in only three very small, disjunct populations in northwestern Georgia, the Delmarva Peninsula, and south-central Oklahoma. The most plausible explanation for this disjunction is the reduction of a once-larger range, but factors governing the size of the three existing populations have not been determined. This study examined the possible reproductive limitations of A. maritima caused by its autumnal fruit ripening and corresponding seed dispersal habit. We found that most (65.7%) of the seeds were dispersed during winter months, and a model simulating their fate showed a reduction in total germination percentage from 63.3% to 19.5% after exposure of seeds to winter temperatures typical of native habitats. The total germination percentage declined from 57.9% to 39.7% for seeds overwintered on trees. Based on this evidence, we propose that timing of seed dispersal reduces seed germinability, thereby limiting the natural distribution of A. maritima.