Long-term retirement of New Zealand snow tussock rangeland: effects on canopy structure, hawkweed (Hieracium spp.) invasion and plant diversity.
Effects of up to 46 yr retirement from grazing Chionochloa spp. rangeland were assessed in exclosure plots on the Old Man Range, Central Otago. Exclosures at 910 and 1220 m have been ungrazed since 1960, one at 1230 m since 1964, and another at 1590 m remained stock-proof from 1960 until the mid 1980s. Burning history varied between sites. Vegetation structure and composition, including invasive Hieracium spp. and topsoil chemistry, were measured in the exclosures and adjacent grazed sites. Retirement had no significant effect on topsoil chemistry but was associated with increased indigenous plant diversity. There was negligible increase in the woody component under long-term protection. Density and cover of Hieracium lepidulum were significantly higher in the ungrazed 910 and 1230 m exclosures, but not in the 1220 m exclosure. Its invasiveness decreased with altitude, probably through frosting of maturing inflorescences. H. pilosella and H. praealtum were common only on a recently burnt, heavily grazed site at 1230 m. Strategic light grazing of these grasslands may facilitate removal of flower heads of H. lepidulum and reduce its invasiveness, but maximizing recovery of snow tussock following burning is recommended to retard establishment and/or spread of H. pilosella and H. praealtum.