Burning in a New Zealand Snow-tussock Grassland: Effects on Vegetation and Soil Fauna

Although New Zealand has a fire history extending back at least 20 million years, natural fires were rare in the Pleistocene and Holocene and the contemporary vegetation shows few adaptations to burning. A dramatic rise in charcoal occurrence about 1000 years ago, coincident with Polynesian colonization, presaged a widespread phase of deforestation that reduced the area of forest by about 40%. With European settlement in the mid-19th century a further 20-30% of the primary forest, and much of the regenerating lands, were cleared, burned and converted into farmland (Basher et al., 1990; Wardle, 1991). Many areas of New Zealand, especially in the eastern South Island high country, previously supported Nothofagus forest which was displaced during Polynesian settlement and later burning of the tall tussock grassland (Tate, 1992; Wardle, 1991). Fire continues to be used as a cheap and effective but controversial management tool in tussock grassland and scrublands. Fire is widely used to suppress shrub development. Tall tussock grassland is also burnt to improve stock access and induce nutritious regrowth. Studies of post-fire recovery in New Zealand tussock grasslands demonstrate that after an initial change in species dominance, floristics and structure return to approach the pre-burn condition within two decades (Basher et al., 1990; Allen and Partridge, 1988; Gitay et al., 1992; Payton et al., 1986). Physical sustainablity of agro-ecosystems in the eastern South Island hill and high country of New Zealand, as elsewhere in the world, is dependent on the maintenance of the soil resource as a medium for supplying water and nutrients for plant growth. Objectives in managing the soil resource are to maintain or enhance the physical and chemical characteristics of the soil. Depth of soil, texture, organic matter and structural stability control the ability of the soil to supply water and nutrients for plant growth, and to resist the erosive forces of wind and water. Maintenance of soil fertility is dependent on adequate organic matter levels, a diverse population of soil organisms, and efficient nutrient cycling. Burning without replacement of nutrients by fertiliser, or inputs from the atmosphere and weathering, will ultimately result in soil degradation that will limit plant growth (Basher et al., 1990); the combination of loss of cover and erosion from intense rainfall events could also be important in the context of possible global climate change (Meyer et al., 1992). G.W. YEATES and WILLIAM G. LEE Manaaki Whenua Landcare Research, Private Bag 11052, Palmerston North, New Zealand; and Manaaki Whenua Landcare Research, Private Bag 1930, Dunedin, New Zealand.