The shift in aerial/subterranean fruit ratio in Amphicarpum purshii: causes and significance.

Amphicarpum purshii Kunth, an annual grass, produces both small aerial and larger subterranean seeds, and previous research has indicated that the ratio of the number of viable aerial seeds to the number of viable subterranean seeds decreases with secondary succession and/or the lack of frequent disturbance. The objective of this research was to determine if increasing density and/or greater seed depth could produce this shift in reproductive allocation. Plants arising from aerial and subterranean seeds were grown in pure and mixed cultures at varying densities in a greenhouse to note the effects of intraspecific competition on biomass allocation. In addition, subterranean seeds were sown at varying depths to note the effects of seed depth. Results showed that the growth of plants from aerial seeds was severely depressed in mixtures of plants from the two types of seeds. Increasing density in both pure and mixed cultures led to drastic decreases in allocation to aerial seeds, but the percentage allocation to subterranean seeds was not significantly reduced. Greater seed depth led to decreased emergence rates and increased percentage allocation to subterrancean seeds. It is suggested that as secondary succession progresses, A. purshii and other plants increase in abundance, and the increasing density and the deeper burial of subterrranean seeds result in plants producing mostly subterranean seeds which accumulate in the soil seed bank. These strongly indurate propagules remain viable following the disappearance of Amphicarpum plants in secondary succession and can give rise to "instant populations" upon subsequent vegetation removal and/or soil disturbance.