Recovery of short tussock and woody species guilds in ungrazed Festuca novae-zelandiae short tussock grassland with fertiliser or irrigation

In a Festuca novae-zelandiae short tussock grassland in South Island, New Zealand, we tested the propositions (1) that present regional trends in vascular plant species-richness in tussock grasslands are independent of current pastoral management, and (2) that grazing retards the invasion and dominance of nonnative species, particularly where soil resources are not limiting. Sheep and rabbit-grazed, ungrazed, ungrazed+fertilised and ungrazed+irrigated treatments were applied in a replicated experiment that was sampled annually from 1988 to 2000. Native species richness and native forb cover decreased, and exotic grasses increased in all treatments, with no significant differences between grazed and ungrazed treatments in either trends or final cover. Exotic species richness decreased in the ungrazed, ungrazed+fertilised and ungrazed+irrigated treatments but showed no trend in grazed vegetation. Cover of native tussock grasses and the tall shrub Carmichaelia petriei decreased in the grazed treatment, remained steady in the ungrazed treatment and increased in the ungrazed+fertilised and ungrazed+irrigated treatments. Native subshrubs decreased in the grazed, ungrazed+fertilised and ungrazed+irrigated treatments but not in the ungrazed treatment. The invasive forb Hieracium pilosella increased with time in grazed, ungrazed, and ungrazed+irrigated treatments, but after 10 years it decreased in the ungrazed+fertilised treatment and its cover was negligible there after 12 years. Grazing appeared to reduce the cover of tussocks and certain woody species, and we conclude that current management affected vegetation trends. Grazing did not decrease the dominance of exotic species, or maintain native species richness at a higher level than in ungrazed vegetation. There was limited recovery of taller native species with grazing removal alone. However, grazing removal plus 12 years of resource enrichment promoted the growth of native tall shrubs and tussocks and did not result in physiognomic dominance by exotic species. Succession towards taller native tussock-shrubland communities may be an appropriate goal for conservation management of short tussock grasslands, and nutrient enrichment in the absence of grazing may be an appropriate management device in some circumstances.