Increasing weed flora in Danish arable fields and its importance for biodiversity

A number of political initiatives have been implemented to meet growing concerns about negative side effects of the intensification of land‐use practice over the past 25 years. Declining species diversity and abundance and pollution of the environment have been reported worldwide. In order to assess the overall effect on wild flora of the changing management of arable land, national surveys are necessary. Surveys of the weed flora in Danish arable fields were conducted in 1987–89 and 2001–04. The frequency of common weed species in spring barley, spring rape, winter rye, winter wheat and grass leys was compared between time periods. Weed frequency increased overall during the interval. Species that are of particular importance as food sources for invertebrates and birds increased considerably in some of the crops. The frequency of some grass species increased remarkably, possibly because of increased areas of winter crops. Some perennial weeds also became more frequent, probably caused by less mechanical weed control and a reduction in the use of specific herbicides. The dominant weed species in 2001–04, with frequency higher than 10%, were largely the same as in 1987–89, but included three new species in 2001–04. Only a few species occurred less frequently in 2001–04 than in 1987–89 and only in some crops. As common weed species make up an important food reservoir for wildlife, new trends in the management of arable land in Denmark seem to have led to more biodiversity.