Dynamics of pasture invasion by three weeds, Avena fatua L., Carduus tenuiflorus Curt. and Onopordum acanthium L

Abstract

Three weeds, Avena fatua L., Carduus tenuiflorus Curt. and Onopordum acanthium L., were deliberately introduced to two 1 ha fields in which they had not been recorded previously. One field was mown annually after weed seed fall, the other was grazed by sheep in late summer-autumn. The population growth and spread of flowering individuals of each species from sites of 10 original mother plants in each field was recorded for two years of spread. The location of daughter plants was described using a single parameter from fitted curves.Grazing compared with mowing had no significant effect on population growth, dispersal or area occupied for any species. C. tenuiforus populations had greater dispersing fractions than the other species, and this appeared to be influenced by wind. This species also had a greater initial population growth and spread than the other species. In the second year, the population of C. tenuiflorus declined in grazed and ungrazed treatments, which was attributed to its population's tendency to establish each year from a single cohort. Although the population size and dispersal gradients of A. fatua and O. acanthium were similar after two years of spread for each species, O. acanthium occupied a greater area owing to a higher frequency of low density patches. Spread rates for O. acanthium, A. fatua and C. tenuifloms respectively, increased, remained constant and decreased with time.