Spatial pattern analysis of seeds of an arable soil seed bank and its relationship with above-ground vegetation in an arid region

The composition and spatial pattern of weeds was analysed in an arable-soil seed bank after the harvest of wheat. Four hundred cores were collected from a 10×10 m grid in April 1999. Three indices of pattern detection (variance/mean ratio, Lloydʹs index of mean crowding, and Morisitaʹs index of aggregation) and Moranʹs I statistic of spatial autocorrelation were computed. The above-ground vegetation of the corresponding area was sampled by 100 randomly placed quadrats in March and September 1999 and variance/mean ratio of all the species was calculated. The seed bank of the field was dominated by annuals including Convovulus arvensis, Amaranthus viridis, Anagallis arvensis, Chenopodium album and Conyza canadensis. In all 27 species were recorded from the seed bank. For most of the species with a mean higher than 0.05 per core, the seed pattern (with two exceptions) was aggregated as indicated by various indices of pattern detection. Among the common species, only Amaranthus viridis exhibited a random pattern. Aggregated pattern of seeds was generally due to seeds settling in the neighbourhood of the mother plant and the clumped distribution of plant species in the overlying vegetation. Suggestions have been made regarding the improvement of the precision of the soil seed bank sample, in view of the aggregated distribution of seeds in soil. Seed bank showed considerable qualitative similarity with the above-ground vegetation, which is presumably the consequence of recurrent disturbances in the arable field.