The pollen production (number of pollen grains per catkin) ofAlnus incanais studied along two vertical transects across the altitudinal limit of the species. Annual variation in pollen quantity, and a considerable reduction in the number of pollen are found with increasing altitude. Normal pollen is virtually not produced at the ecotone. It is suggested that parameters such as the temperature of the preceding summer, length of the growing season, and soil factors are limiting factors. A test study shows no germination of seed from the ecotone, and reduced fertility is suggested.