Influence of herbivory, competition and soil fertility on the abundance of Cirsium arvense in acid grassland.
The extent to which the weed Cirsium arvense (creeping thistle) may be controlled by manipulating interspecific competition and herbivory was examined in two factorial experiments. In the first experiment performed in 1996-97, a single spring cultivation of UK grassland intensively grazed by rabbits led to a 25-fold increase in C. arvense cover within 3 months, the effects of which were still present the following summer. As well as destroying the competing perennial vegetation, cultivation created and dispersed small root fragments (3-5 cm in length) from which almost all shoot recruitment occurred. Fencing the cultivated plots against rabbits decreased the cover of C. arvense because ungrazed regrowth from palatable/grazing intolerant species reduced recruitment of C. arvense seedlings and shoots. Seedling competition, in the form of a wild flower seed mix sown soon after cultivation, reduced C. arvense cover on fenced plots to pre-cultivation levels. In the second experiment (initiated in 1991), conducted in a permanent UK grassland, C. arvense shoot densities on plots fenced against rabbits and treated as a hay meadow were about one-eighth of those found on rabbit-grazed plots where competing vegetation was kept short. Adventitious shoot recruitment was greater on soil disturbances such as mole hills and rabbit scrapes than in intact vegetation. Seedling recruitment occurred only on soil disturbances such as mole hills. Lime and nitrogen fertilizer application to the fenced grassland increased the standing biomass of competing species, which reduced C. arvense shoot density. Outside the fences, rabbit grazing was so concentrated on the competing species of the nitrogen-fertilized and limed areas that C. arvense benefited from competitive release, exhibiting increased shoot density. C. arvense showed pronounced competitive release from grasses, with greater shoot densities where grasses were removed with selective herbicides than where no plant species were removed. Exclusion of insects and molluscs with chemical pesticides had no effect on shoot or seedling recruitment or overall shoot density on cultivated soil or in permanent grassland. It is concluded that combinations of management procedures that encourage interspecific competition, such as sowing crops soon after cultivation and delaying their grazing, nitrogen fertilizer application, and non- or reduced grazing of intact grasslands, will help reduce C. arvense abundance.