Questions: To what extent can spatial structure and its causes be determined in a highly disturbed environment? What are the main determinants of pattern and are these species-specific? How much do spatial patterns change over generations?
Location: Wimmera region of southern Australia.
Methods: Broad-leaved weeds were counted in 225 000 contiguous 20-cm square quadrats. A substantial number of these quadrats were recorded again after two and four years. An hierarchical ‘adaptive analysis’ approach was used to select spatial analytical methods to examine specific aspects of pattern and variation in pattern from year to year.
Results: Patterns varied among species and included both dense and sparse patches surrounded by areas of zero density, diffuse gradations of density and clear anisotropy. Patterns in Erodium botrys and Oxalis pes-caprae persisted over years, whereas patterns in Arctotheca calendula were less pronounced and varied over time. Edaphic factors appeared to have only a minor influence over the spatial distribution of the weed community as a whole. In Oxalis pes-caprae, whose patches were hypothesized to have been shaped by cultivation, there was no spread in four years, despite further tillage. Outlying plants of O. pes-caprae failed to establish new patches, even in the year of greatest population increase. Little evidence of localised recruitment events was found.
Conclusions: Despite repeated annual disturbances by natural and anthropogenic mechanisms, clear and interpretable spatial structure develops in annual weeds over a range of spatial resolutions. Adaptive analysis is a useful approach to the characterization of such patterns.
Abbreviations: ECe = Electrical conductivity; ECa = Apparent electrical conductivity; DTM = Digital Terrain Model.
Nomenclature: Walsh & Entwisle (1994–1999).