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 Especially in drier areas, previously dominant tall tussock species have been replaced by shorter tussock or prostrate species, often including a range of invasives.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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