Determining the nature of root and shoot competition can elucidate the competitive ability of an invasive species and direct management strategies. In a set of competition experiments, artichoke thistle (Cynara cardunculus), an exotic invasive perennial forb, was subjected to full or shoot competition with four species: black mustard (Brassica nigra), an exotic annual forb; ripgut grass (Bromus diandrus), an exotic annual grass; purple needle-grass (Nassella pulchra), a native perennial grass; and itself. For shoot competition, a smaller pot nested in a larger experimental pot sequestered the target plant root system. A bare ground invasion experiment, in which all plants were transplanted on the same date, and a community invasion experiment, in which competitor species were planted 1 mo before targets, were conducted. In the bare ground invasion experiment, target plant size was reduced (P ≤ 0.05) when exposed to full competition with the exotic species, but not purple needle-grass. Effects on target plants included reductions in height, number of leaves, rosette diameter, and shoot and leaf dry weight. In the community invasion experiment, full competition with all species reduced target plant growth (P ≤ 0.05). Shoot competition was more important when all species were planted synchronously, whereas root competition was more important when target plant establishment was delayed. In a separate experiment, artichoke thistle was grown under four light levels simulating field conditions under canopies of the same competitors. Midday carbon assimilation decreased linearly with increased shade, indicating the likely effects of shoot competition on artichoke thistle. Results indicated that exotic species are more competitive than native purple needle-grass against artichoke thistle and that restoration directly to native grassland after artichoke thistle removal might be difficult. However, artichoke thistle seedling growth is reduced by root competition from grasses that emerge earlier, indicating that early season management of grasslands to delay artichoke thistle establishment might provide effective control.
Nomenclature: Artichoke thistle, Cynara cardunculus L. CYNCA; black mustard, Brassica nigra (L.) Kock BRANI; purple needle-grass, Nassella pulchra (A. Hitchc.) Barkworth NASPU; ripgut grass, Bromus diandrus Roth BRODI.