Fire-induced changes to vegetation of tall-tussock Chionochloa rigida grassland at Deep Stream and Mount Benger, Otago, New Zealand

  • Published source details Payton I.J. & Pearce H.G. (2009) Fire-induced changes to the vegetation of tall-tussock (Chionochloa rigida) grassland ecosystems. Science for Conservation 290


In Zew Zealand, spreading pastoralism from the 1850s onward led to increased fire frequency with consequent changes in composition and stature of native fire-induced grasslands. Especially in drier areas, previously dominant tall tussock species have been replaced by shorter tussock or prostrate species, often including a range of invasives. Management by fire is a contentious issue in New Zealand. Some view it as environmentally damaging and others as essential for maintaining livestock grazing land. The effects of fire burn season were examined in tall-tussock Chionochloa rigida grasslands at two sites in Otago, South Island.

The two sites (ungrazed by livestock and unburned for over 10 years) were Deep Stream (640-700 m a.s.l) and Mt Benger (1,100-1,180 m). At each, nine 100 × 100 m plots (surrounded by earth firebreaks) were established. Groups of three adjacent plots were blocked, and treatments (unburned, spring or summer-autumn burn) were randomly allocated. Each plot was divided into 25 (20 × 20 m) subplots, one randomly allocated for: destructive harvest (to assess plant biomass, carbon and nutrient pools); or C.rigida flowering and seedling establishment (counted annually in 10, 1 m² quadrats).
At Deep Stream, spring burns (simulating pastoral practice) took place on 2 October 2001, and summer burns on 7 March. At Mt Benger, spring burns took place on 3 November 2000, and summer burns on 31 March 2006 (delayed due to fire restrictions). Burn temperatures were measured using thermocouple sensors and/or heatplates marked with temperature-indicating paint.
The percentage of tussocks killed by spring fires was determined 6-8 weeks after burning. For summer burns (as winter frosts also affect survival) assessments were made in the subsequent spring.

Fire temperatures reached 1010°C at the ground surface but were of short duration (4-8 min) thus heated the soil little. Biomass, carbon and nutrient losses were lowest when burning took place under damp conditions.
Spring burns under damp conditions (Mt Benger) killed on average 35% of tussock tillers but caused no tussock deaths. Spring burns in drier conditions (Deep Stream) killed nearly 80% of tussock tillers and 21-70% of tussocks. Summer burns killed 83% of tillers and 51% of tussocks at Deep Stream and 87% of tillers at Mt Benger; by the following spring, tiller mortality had increased (over-winter death of many resprouted tillers) to 93% at Deep Stream and 92% at Mt Benger.
Seedling density and flowering were least affected when burning took place under damp spring conditions; under drier conditions, both were greatly reduced and showed little sign of returning to pre-burn levels even 4-5 years after fire. If retaining tall-tussock cover is a management objective, summer/autumn fires, or fires when conditions are dry, should be minimised.

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