Effects of soil and landscape characteristics on the population dynamics of wild Helianthus annuus L.

Since mechanization and chemical herbicides came into wide use, conventional weed management has been uniformly applied to whole fields, both to areas where weeds were present and areas where they were not. The distribution of many weed species, however, is far from uniform. The patchy nature of H. annuus was previously well described, but explanations for the aggregated distribution were hypothetical and only beginning to be tested. A better understanding of the population dynamics of weed species in relationship to environmental variability may allow better prediction and management of weeds with patchy distributions. Variability in the characteristics of sub-field environments results in niches with combinations of characteristics that are more or less favorable to H. annuus. Soil organic carbon (SOC) and landscape position were believed to be principal factors affecting H. annuus niche favorability and population dynamics. Soil-applied herbicide, a major tactic in the integrated weed management strategies employed in row crop agriculture, is attenuated in its bioavailability due to sorption to SOC. Seed dispersal and movement of seeds after dispersal were believed to be affected by landscape position. Seed germination in the field was not affected by stratification in High versus Low SOC locations. However, seedling survival was on average 40% higher in areas characterized by High SOC levels. No differences in growth were observed between juvenile and flowering stages as a consequence of High and Low SOC locations. Seeds from H. annuus did not move out of low landscape positions (depressions) due to wind or water movement, but a fraction of seeds originally dispersed to higher landscape positions moved downslope (as far as 105 m). Observations on existing H. annuus populations provided an index of SOC and relative elevation that was used to accurately identify areas (niches) favorable to patch development. Increased knowledge about the relationships of other species to the same or other niche characteristics may enable identification of niches favorable to the weed and at higher risk for weed interference with the crop