The demographic role of soil seed banks. I. Spatial and temporal comparisons of below- and above-ground populations of the desert mustard Lesquerella fendleri

Abstract

1. Although seed banks may often affect the colonization, succession and structure of surface plant communities, few studies have investigated the demographic relationship between seeds in the soil and above-ground plant populations over space and time. We examined this relationship in the perennial mustard Lesquerella fendleri between 1991 and 1994 within a New Mexico desert ecosystem characterized by open patches of soil (intershrub areas) interspersed with dominant creosote (Larrea tridentata) shrubs (subshrub areas). 2. For the first 2 years of the study, Lesquerella soil seed, surface plant and seed production densities were greater in subshrub vs. intershrub areas. Within the subshrub areas, there were more Lesquerella soil seeds and surface plants in the northern and central microsites, compared with the southern and perimeter microsites, respectively. The mean density of the subshrub seed bank did not increase following large inputs of new seeds (2400 seeds m-2) in the summer of 1992. In contrast, virtually all of the relatively modest Lesquerella intershrub seed production at this time (169 seeds m-2) appeared to survive, so that for the remainder of the study most soil seeds were found in the intershrub sites. This spatial reversal of seed bank densities preceded a similar switch in the relative densities of the Lesquerella sub- and intershrub surface plant populations in the last 2 years of the study. 3. This study supports the hypothesis that desert seed bank patchiness contributes to surface plant patchiness, but does not support the idea that seed banks are comprised primarily of seeds produced during favourable reproductive periods. Because there was a closer spatial match between Lesquerella soil seeds and surface plants than between soil seeds and seed production, the above-ground population may be limited to sites favourable for soil seed persistence.