A conceptual model for the population dynamics of Cirsium arvense in a New Zealand pasture

Based on a critical review of published research we have developed a conceptual model for a pasture population of Cirsium arvense (Californian thistle) focusing on plausible mechanisms for the regulation of population size: 1) plants overwinter as creeping roots with adventitious buds and subterranean shoots; 2) a main spring cohort of shoots supported by mobilized carbohydrates is regulated numerically by overwintered root mass and apical dominance; 3) shoot death is promoted by interspecific competition, disease and weed control practices; 4) new creeping roots formed throughout the growing season support a low level of shoot recruitment; 5) overwintered root mass is a linear function of photosynthetic opportunity; 6) shoot growth predominates over root growth during the first half of the growing season; 7) root buds, elongating as apical dominance subsides with autumnal shoot senescence, produce the main spring cohort of shoots; 8) creeping root formation is reduced by intraspecific competition; and 9) shoot growth declines with interspecific competition. The validation of these mechanism models should be the focus of ongoing research.