Environmental characteristics affecting Helianthus annuus distribution in a maize production system

Population density and seedling growth stage were measured over three years along transects passing through patches of feral Helianthus annuus in fields of maize. The effect of topographic position on seed re-dispersal via erosion was investigated, and density of H. annuus was significantly correlated (P < 0.01) to percent soil organic carbon (SOC) (positive) and relative elevation (negative), but not to soil pH. Mean seedling growth stage did not change with distance from the patch center. Seedling density was always highest at the patch center with no reduction in mean seedling size. Marked seed dispersed at various topographic positions were recovered two weeks after dispersal. A small percentage of H. annuus seeds was recovered, most of which were found at lower topographic positions - their movement being facilitated by water flow down slope. Seeds at the lowest topographic positions were not moved to other topographic positions. Prevailing wind moved seeds to a lesser degree. The association of weedy H. annuus with SOC and the dispersal to lower topographic positions appear to be useful factors in explaining its distribution and might be generalized to other weed species and systems.