Cirsium arvense (creeping thistle) (L.) Scop. is an important perennial weed of pasture, arable and conservation areas in the temperate regions of both hemispheres (Holm et al. 1977; Peel & Hopkins 1980; Donald 1990). It is a major problem as it restricts the area available for livestock grazing and reduces crop yields (Hartley & James 1979; O’Sullivan et al. 1982; references in Donald 1990). It is a particularly troublesome weed as new plants can recruit from both seed and small root fragments, and because one established plant can infest large areas by recruitment of shoots from adventitious buds on a creeping root system (Moore 1975; Grime, Hodgson & Hunt 1988).
In the management of C. arvense populations, chemical herbicide-based control measures are becoming increasingly unpopular, either because they are ineffective or uneconomic, or due to environmental concerns (e.g. effect on pasture legumes; Hartley & Thomson 1981), and non-herbicidal control methods are sought (Bourdôt et al. 1995). One approach that has been advocated is to enhance interspecific plant competition, so as to prevent the creation of conditions conducive to the recruitment of C. arvense shoots (Bourdôt 1996). The sowing of competitive smother crops is a method that utilizes this approach, and this has been shown to be effective in controlling C. arvense in some cases (Donald 1990). A further method is to manage interspecific competition through changes in the timing and intensity of grazing. The benefits of altering grazing management (e.g. lax vs. heavy grazing) will depend on the degree to which recruitment, growth and survival of C. arvense seedlings and shoots are inhibited by competing plants and how C. arvense is directly affected by grazing (e.g. seedling or shoot herbivory, removal of flowers and seeds; Amor & Harris 1975; Mitchell & Abernethy 1993). These demographic processes in C. arvense should, in principle, depend on the species and life-history stage (e.g. seedling vs. adult) of competing plants, as these will differ in their competitive ability (Thrasher, Cooper & Hodgson 1963; Hallgren 1976) and tolerance or palatability to herbivores relative to C. arvense (Crawley 1983).
A further approach advocated for the control of C. arvense is to enhance interspecific competition through fertilizer application (Donald 1990). Reductions in C. arvense abundance with fertilizer application might result from competing species being able to use and assimilate the added nutrients better, or from changes in the rate at which gaps appear and close in the vegetation (Bourdôt 1996). In studies to date, however, the response of C. arvense to fertilizer application has been inconsistent; for instance, nitrogen fertilizer has been shown to increase (Reece & Wilson 1983; Nadeau & Vanden Born 1990), decrease (Thrasher, Cooper & Hodgson 1963; Hume 1982) or have no effect (Hay & Ouellette 1959) on the abundance of C. arvense for reasons that are not clear (Donald 1990). Few studies have considered the effect of nutrients other than nitrogen (e.g. phosphorus and potassium; Donald 1990) or how fertilizer application might interact with grazing. There is increasing evidence from a wide range of plant communities that the grazing and diet selection behaviour of herbivores, and the impact that herbivores have on plant communities, alters as soil fertility changes along natural and artificial fertility gradients (Bazely 1990; Nams, Folkard & Smith 1996; John & Turkington 1997).
This paper reports on two factorial experiments conducted in rabbit-grazed acid grassland that examined the impact of herbivory and plant competition on shoot and seedling recruitment of C. arvense. Both experiments examined the effects of vertebrate and invertebrate herbivory on shoot and seedling recruitment by erecting rabbit fences and applying insecticides and molluscicides. The experiments differed, however, in how the effect of plant competition was examined. In the first experiment, the effect of competition from mature plants was examined using a cultivation treatment to remove the perennial cover, while the effect of competition from seedlings was examined by sowing a wildflower seed mix soon after cultivation. In the second experiment, the effect of different kinds and intensities of plant competition on C. arvense was examined by removing grass or herb species from the grassland with selective herbicides. Also in the second experiment, the effects of soil fertility and soil pH on C. arvense abundance were examined by applying nitrogen, phosphorus, potassium and magnesium fertilizers, and lime. Using these various treatments, we sought to test the idea that C. arvense abundance would be reduced in areas with increased growth or accumulation of competing vegetation (e.g. fenced vs. grazed, grassland vs. cultivated, sown vs. unsown, fertilized vs. non-fertilized) because of decreased recruitment of shoots and seedlings. We use our results to point out possible non-herbicidal control measures for C. arvense on recently cultivated soil and in intact grassland.