WEED INFESTATION AND FACTORS AFFECTING WEED INCIDENCE IN SPRING CEREALS IN FINLAND - A MULTIVARIATE APPROACH

Weed vegetation of spring cereal fields in southern and central Finlan d was analyzed by ordination methods to provide a community level desc ription of weed populations. Attention was paid particularly to the re lative importance of environmental factors affecting weed incidence su ch as crop management, soil properties and weather conditions. A data set of 33 weed taxa from 252 fields was subjected to both indirect and direct gradient analysis. Indirect ordination was obtained with corre spondence analysis (CA), and direct gradient analyses were performed w ith redundancy analysis (RDA) and with canonical correspondence analys is (CCA) relating environmental factors to the occurrence of weeds. Am ong several management factors, continuous herbicide use explained bes t the variation in the species composition of weed flora. Weed vegetat ion was also associated with soil type, moisture conditions and soil p H(H2O). Ordination diagrams visualized the species-environment interac tions and detected characteristic weed species for different geographi cal regions. In addition to ordination analyses of weed flora, the lev el and structure of weed infestation are described. The density of wee ds averaged 170 plants m-2(median=124) and the air-dry weight of weeds 320 kg ha-1(median=183). The average weed density was the same in dif ferent soil types, but the weed biomass was lower in clay soils than i n coarse mineral and organic soils.