SPATIAL AND TEMPORAL ANALYSIS OF WEED SEEDLING POPULATIONS USING GEOSTATISTICS

An intensive field survey of an eastern Nebraska corn and soybean fiel d was conducted to characterize the spatial structure and temporal sta bility of broadleaf weed seedling populations over two growing seasons , Anisotropy, the effect of direction on the relationship between obse rvations, is present in the semivariogram for the velvetleaf and commo n sunflower populations in 1992 and 1993, The directional trends in ag gregation are visible in kriged maps as elliptical shapes oriented eas t to west across the study area, In addition, there are two distinct z ones of aggregation from north to south, These two distinct areas of a ggregation are reflected as a 'plateau' in the north-south semivariogr am. The distance over which this plateau extends indicates that the sh ape or size of the patch is contracting in the north-south direction ( perpendicular to the crop row), The slope of the semivariogram in the east-west direction (aligned with the crop row) remains consistent fro m 1992 to 1993 suggesting that the shape of the patch is not changing, For sunflower populations, the slope of the north-south empirical sem ivariogram changes at 20 m, similar to the velvetleaf population semiv ariograms. This change, however, is reflected as a downward trend in t he empirical semivariogram. The distance over which this trend occurs increases from 1992 to 1993 suggesting that seedling patch size was sm aller in 1993 compared to 1992, Weed seedling establishment resulting from seed dispersal, differential seed and seedling mortality, or emer gence may have resulted in the observed patch dynamics.