SummaryThe vegetation of weeds was sampled in matched pairs of long established organic and contemporary conventional cereal fields in summer 1987, and in spring and summer 1988, with an extended Raunkiaer circle at distances up to 50 m from the field boundary. In spring 1988, the number of plants were also counted. Species density, plant density, a proxy measure of plant density (accumulated Frequency Sum), crop and weed biomass, and occurrence of plants assigned to functional groups, are compared between farming systems. The plants were assigned to functional groups according to: (i) herbivore associations to broad‐leaved taxa and (ii) flower location in the canopy, i.e. visibility and availability to flying insects. Furthermore, crop margin and mid‐field were compared concerning species density and accumulated Frequency Sum.All variables that differed among the two farming systems had highest values ‐ often several times higher ‐ in the organic system with four exceptions: total biomass, biomass of crop, proportion of a single broad‐leaved taxon and of grasses. The differences between the two systems were largest mid‐field due to a pronounced pre‐herbicide spraying gradient in species and plant density from crop margin to mid‐field in conventional fields. A similar gradient was not found in the organic fields.

Abstract:Bank erosion was measured at 91 stream banks located in 15 Danish rural 1st and 2nd order streams over a 2-year period. Our aims were firstly to examine factors controlling spatial variation in bank erosion, secondly to estimate sediment and phosphorus (P) loss via bank erosion. The overall mean bank erosion rate was 11 mm year 1 . Bank erosion rate over the 2-year period was significantly related to a number of site-specific characteristics, including bank angle, bank vegetation cover, overhanging bank and estimated stream power. An empirical model for bank erosion based on these descriptive variables yielded a 55% explanation of the observed spatial variation in bank erosion rate. Bank erosion was higher at the lower 50-cm bank section (20 mm year 1 ) than at the upper bank (6 mm year 1 ). Cattle fencing in grazed areas and buffer zones with riparian woodland lowered bank erosion rates. We found that total P content of bank material was high (0Ð64 g P kg 1 ) and at the same level as found in agricultural topsoil along the streams. The overall annual catchment loss of bank-derived clay-silt sediment and total P to streams amounted to 58-72 kg sediment ha 1 and 0Ð23-0Ð28 kg P ha 1 , respectively. In comparison, the mean annual suspended sediment (SS) and total P losses from 14 similar sized Danish agricultural catchments were 122 kg SS ha 1 and 0Ð58 kg P ha 1 over the 2-year study period. Thus, bank erosion seems to be a major contributor of suspended sediment and P in this type of small channelized lowland stream.

Summary 0[ Surveys have shown that there has been a dramatic decrease in the weed~ora of _elds under rotational cultivation during the last 29 years[ This trend has been particularly noticeable in winter cereals\ a crop of increasing importance in the landscape[ 1[ The weed~ora of spring and winter cereals was compared in 08 unsprayed _elds during a 4!year study to test the hypothesis that cereal type exerts no e}ect on thẽ ora or on the absolute and relative abundance of single species[ 2[ Plant and species densities\ and accumulated species richness\ were lower in winter than in spring cereals[ 3[ Floristic similarity was greater among spring cereal _elds and between spring and winter cereals within the same _elds than among winter cereal _elds[ 4[ Species that occurred with unequal density in spring and winter cereals occurred at higher densities in the spring cereals^these species germinated mainly in the spring[ However\ for a few species the relative plant abundance was highest in winter cerealst hese species were able to germinate both in the spring and autumn[ 5[ Some species Ð on the relative scale Ð occurred indi}erently of season of sowingâ ll but one of these species were able to germinate both in the spring and autumn[ 6[ Plant species and taxa that are important food resources for arthropod herbivores occurred at greater densities in spring than in winter cereals and\ in addition\ occurred with the highest relative abundance in spring cereals[ 7[ Change in land use from spring to winter cereals involves not only an immediate reduction of more than 14) in the density of plants and species\ but also a change and increased uncertainty in the composition of the weed~ora[ These _ndings may have serious implications for the ecology of wildlife in the agricultural landscape[ Key!words] farmland\~oristic diversity\ plant density\ species density\ wild plants[ Journal of Applied Ecology "0888# 25\ 13Ð21

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