Effect of Microtopography on the Survival and Reproduction of Medicago Lupulina

(1) The density of Medicago lupulina, a short-lived perennial herb, decreased from the top (ridge) to the bottom (hollow) of shallow depressions (0-5-1-m deep) in an abandoned pasture in southern Ontario, Canada. To explain this density gradient we compared the survival and reproduction of plants on ridges with those in hollows. (2) Plants growing on moss-covered ridges had a greater life expectancy and were more likely to produce seeds before dying than were plants growing on moss-free ridges or in hollows. (3) Shading, by grasses and herbs, was probably responsible for the relatively poor survival and reproduction of Medicago lupulina in hollows. Shading was less intense on ridges and, where a ground cover of mosses maintained damper conditions, both the life expectancy and reproduction of Medicago lupulina increased. Grazing and trampling increased the life expectancy of plants in hollows, probably by improving the light regime. However, animals had no significant effect on the expected life span of plants on ridges. (4) The number of seedlings recruited from the seed bank did not differ significantly between study plots on ridges and in hollows, so it is more likely that the difference in adult density resulted from differential survival of seedlings.