THE ROLE OF SPATIAL PATTERN IN THE POPULATION BIOLOGY OF CENTAUREA-DIFFUSA

Mortality among seedlings and those established plants that did not flower increased with proximity to establish plants. The percentage of established plants that flowered was <5% for plants with root-crown diameters <4 mm, but rose abruptly to >90% for plants with root-crown diameters of 6 mm. C. diffusa, like other semelparous perennials, has a critical size requirement for flowering. A spacing experiment demonstrated the effects of interference on the life history. About 40% of the most crowded plants died pre-reproductively in their 1st year. Plants that experienced an intermediate level of crowding survived but did not reach the critical size threshold, and therefore persisted as rosettes in their 2nd year. Intermediate interference in concert with a critical size requirement for flowering prolonged the duration of the rosette stage. Uncrowded plants flowered in the 2nd year and died post-reproductively. Pre- and post-reproductive mortality produce opposite effects on the aggregation of the spatial pattern. Spatial pattern in this knapweed monoculture is created by 1) recruitment, which can either create greater regularity or contagion; 2) the regularizing process of rosette (pre-reproduction) mortality; and 3) the aggegating process of post-reproductive mortality. -from Author