Competitive exclusion of Cirsium arvense in pasture: a simulated neighbour grazing-height experiment.

The role of interspecific competition in regulating Cirsium arvense in newly sown pasture was investigated in two pot experiments in Canterbury, New Zealand, where it was grown in the presence and absence of Lolium perenne and Trifolium repens. In Experiment 1 (2006–07), where C. arvense was established as transplanted seedlings, it competitively excluded its neighbours when they were maintained between 20 and 60 mm in height. Coexistence occurred when the neighbour height was maintained between 100 and 150 mm, while competitive exclusion of C. arvense resulted when the neighbours were not trimmed. In Experiment 2 (2007–08), where C. arvense was established from small root cuttings, it was out-competed regardless of neighbour height. The results support the existence of a ‘zone threshold’ along a gradient of increasing pasture height (competitiveness) within which the weed and sown species can coexist, and below and above which the weed or the sown species exclude the other.