THE DISTRIBUTION AND ABUNDANCE OF HIERACIUM SPECIES (HAWKWEEDS) IN THE DRY GRASSLANDS OF CANTERBURY AND OTAGO

Abstract

Summary: We examined the distribution and abundance of the invasive Hieracium species (hawkweeds) in the dry grasslands of the Upper Waitaki Basin (Canterbury) and Otago, using measures of Hieracium species frequency and hawkweed cover from 301 vegetation plots. Average hawkweed cover was significantly less in Otago than Canterbury. Hawkweed cover was also lower on drier sites, with hawkweeds having less cover at lower elevation, on more xeric sites and, in Canterbury, on soils with a lower moisture holding capacity. Environmental variables (elevation, moisture index, soil class, and measures of rabbit activity and the amount of recent pasture development) explained 43 % of the variation in hawkweed cover in Canterbury, but only 25 % in Otago. Furthermore, having controlled for environmental differences among plots, hawkweed cover in Otago was significantly spatially autocorrelated such that plots on the same property had similar hawkweed cover, but tended to differ from plots on other properties. This pattern is consistent with the hypothesis that variation in property management influenced the degree of hawkweed infestation. However, the same pattern was not found in Canterbury. We propose a general model to explain the observed patterns. The probability that a site will be invaded by hawkweeds is a function of: (1) the suitability of the site for hawkweed establishment (a function of the environment and past management), and (2) the size of the hawkweed propagule rain. The probability of