Distance-based redundancy analysis (db-RDA), a recently developed ordination technique useful for both multivariate hypothesis testing and data interpretation, was used to evaluate treatment effects on weed communities in a long-term study of alternative potato cropping systems. The experiment consisted of a factorial arrangement of three pest management systems, conventional (CON), reduced input (RI), and biointensive (BIO), two soil management systems (amended vs. unamended), and two crop-rotation entry points. Soil samples collected in the spring of 1998 were subjected to exhaustive germination as a means of characterizing the weed community. Using partial ordinations, each factor in the factorial treatment structure was tested separately, revealing a significant interaction between pest and soil management systems. An ordination diagram of the pest by soil management interaction was used to interpret the results. Weed species that were highly correlated with the first two ordination axes included: common lambsquarters, broadleaf plantain, oakleaf goosefoot, common hempnettle and a complex of the Brassicaceae that included wild mustard, birdsrape mustard, and wild radish. Univariate analyses confirmed the response of these species to the factors examined. The BIO pest management system showed a different response to soil amendments than the other systems. Soil amendments caused an increase in the total weed density in the CON and RI systems, but caused a decrease in the BIO system. Given the need for better multivariate hypothesis testing and data interpretation in many types of weed science research, the use of db-RDA is expected to grow.
Nomenclature: Birdsrape mustard, Brassica rapa L. BRARA; broadleaf plantain, Plantago major L. PLAMA; common hempnettle, Galeopsis tetrahit L. GAETE; common lambsquarters, Chenopodium album L. CHEAL; oakleaf goosefoot, Chenopodium glaucum L. CHEGL; wild mustard, Brassica kaber (DC.) L. C. Wheeler BRAKA; wild radish, Raphanus raphanistrum L. RAPRA; potato, Solanum tuberosum L.