Anthropogenic forcing on the spatial dynamics of an agricultural weed: the case of the common sunflower.
Establishment and spread are central in weed invasion. In this study we quantified the impact of harvest and weed management practices on these processes in row-crop agriculture. Quantifying the dynamics of patch expansion can direct management aimed at containment of weed populations. We assessed annual patterns of common sunflower Helianthus annuus seedling recruitment to determine the influence of management on seed dispersal and patch expansion. Weed seed banks were sowed at three initial densities and exposed to either high or low intensity weed management. Fields were maintained in a maize-soybean crop rotation, with cultivation and harvest orientated in a single direction. We analysed spatial pattern in annual seedling recruitment using geostatistics and an integro-difference model to determine treatment effects on spatial dynamics. The two spatial analyses allowed us to separate and quantify the contributions of natural and anthropogenic dispersal to seedling emergence and patch expansion. Expansion was predominantly isotropic, and estimated rates of isotropic spread (i.e. diffusion) were consistent between analysis methods. We also confirmed that directional management practices can effect significant anisotropy on dispersal and expansion. Crop rotation had the greatest impact on expansion; anisotropy in patch expansion was more pronounced in maize compared with soybean. However, the scale of weed seed dispersal by machine (combine harvester) was greater following harvest of soybean. Simulation of patch dynamics indicated that harvest can increase expansion rate nearly fourfold. Synthesis and applications. Patches of H. annuus originating from intermediate and high seed bank densities had the potential to spread rapidly from the source population. Patches with a low initial seed bank experienced extinction when subjected to high levels of chemical weed management. The combined approach of spatial modelling and geostatistics was particularly effective for quantifying admixed modes of dispersal from sequential data of population distribution. Independence of the two methods provided a system for cross-validating model assumptions and estimated parameters. The scales of spatial dynamics we assessed were well suited to these methods. The results of the analysis underscore the importance of managing populations in order to keep local populations at low densities. Helianthus annuus is difficult to manage and once intermediate or high local densities are reached, the rate of spread is accelerated, distributing this weed widely across fields. A targeted site-specific approach to maintaining populations at low levels would reduce the necessity for more costly field-wide management.