Fire-induced changes to the vegetation of tall-tussock (Chionochloa rigida) grassland ecosystems

The deliberate use of fire has long been a contentious issue in the South Island high country of New Zealand, being seen by some as damaging to the environment and by others as an essential pastoral management tool. These issues were examined in tall-tussock (Chionochloa rigida) grasslands at two sites in Otago, which were burned either in spring, to simulate pastoral management practice, or late summer, to simulate accidental fires. Fire temperatures reached over 1000°C, but were of short duration (4-8 minutes) and had little heating effect on the soil. Biomass, carbon, and nutrient losses were lowest when the grasslands were burned under damp conditions, and increased as soil and plant moisture levels declined. The best predictors of biomass loss were the moisture content of the top 5 cm of soil and the base of the tussocks. Spring burns under damp conditions killed c. 35% of the tussock tillers but did not cause the death of tussocks, whereas burns under drier conditions or later in the growing season killed over 75% of tussock tillers and resulted in the death of tussocks. Seedling densities and inflorescence production were also least affected when the grasslands were burned under damp spring conditions; when conditions were drier, both were dramatically reduced and showed little sign of returning to pre-burn levels 4-5 years after the fire. Early season burns under damp conditions posed little threat to the long-term survival of tall-tussock ecosystems, whereas fires later in the season, or when conditions were drier, resulted in substantially greater biomass, carbon and nutrient losses and caused a loss of tussock dominance, at least in the short to medium term. Therefore, minimising their extent should be a priority wherever tussock cover is to be retained.