Influence of herbivory, competition and soil fertility on the abundance ofCirsium arvense in acid grassland

1. The extent to which the weed Cirsium arvense (creeping thistle) may be c ontrolled by manipulating interspecific competition and herbivory was exami ned in two factorial experiments in order to identify non-chemical herbicid e-based control methods for the weed. 2. In the first experiment, a single spring cultivation of grassland intens ively grazed by rabbits led to a 25-fold increase in C. arvense cover withi n 3 months, the effects of which were still present the following summer. A s well as destroying the competing perennial vegetation, cultivation create d and dispersed small root fragments (3-5 cm in length) from which almost a ll shoot recruitment occurred. 3. Fencing the cultivated plots against rabbits decreased the cover of C. a rvense because ungrazed regrowth from palatable/grazing intolerant species reduced recruitment of C. arvense seedlings and shoots. Seedling competitio n, in the form of a wildflower seed mix sown soon after cultivation, reduce d C. arvense cover on fenced plots to pre-cultivation levels. 4. In the second experiment, conducted in a permanent grassland, C. arvense shoot densities on plots fenced against rabbits and treated as a hay meado w were about one-eighth of those found on rabbit-grazed plots where competi ng vegetation was kept short. Adventitious shoot recruitment was greater on soil disturbances such as molehills and rabbit scrapes than in intact vege tation. Seedling recruitment occurred only on soil disturbances such as mol ehills. 5. Lime and nitrogen fertilizer application to the fenced grassland increas ed the standing biomass of competing species, which reduced C. arvense shoo t density. Outside the fences, rabbit grazing was so concentrated on the co mpeting species of the nitrogen-fertilized and limed areas that C. arvense benefited from competitive release, exhibiting increased shoot density. Cir sium arvense showed pronounced competitive release from grasses, with great er shoot densities where grasses were removed with selective herbicides tha n where no plant species were removed. 6. Exclusion of insects and molluscs with chemical pesticides had no effect on shoot or seedling recruitment or overall shoot density on cultivated so il or in permanent grassland. 7. It is concluded that combinations of management procedures that encourag e interspecific competition, such as sowing crops soon after cultivation an d delaying grazing of them, and nitrogen fertilizer application and non- or reduced grazing of intact grasslands, will help reduce C. arvense abundanc e.