Population dynamics in mature stands of Hieracium pilosella in New Zealand

The dynamics of a mature population of Hieracium pilosella under four different treatments (± irrigation, ± fertiliser, ± defoliation, and ± fungicide & insecticide, in a full factorial design) were compared in the Mackenzie Basin, New Zealand. H. pilosella had been growing at the site for at least 30 years. Recorded variables included rosette density, inflorescence and stolon production, recruitment and mortality, and plant cover. The population appears to be regulated through density-dependent mortality interacting with density-independent reproduction. In the treated plots, irrigation and fertiliser had the greatest effects, increasing both the proportion of established rosettes producing stolons and stolon length. Irrigation also increased the likelihood of a daughter rosette becoming reproductive and decreased inflorescence abortion. Fertiliser increased the probability of an established rosette reproducing and the number of daughter rosettes produced. In unmanipulated plots H. pilosella appeared to be at or near an equilibrium density of ca. 3200 rosettes m⁻², but over longer time scales the low reproductive rates shown in this study may presage slow population decline due to plant-induced environmental degradation. This revised version was published online in August 2006 with corrections to the Cover Date.