Variation in the effects of vertebrate grazing on fire potential between grassland structural types

P>1. Vegetation fires can have major social, economic and ecological consequences. Research into fire behaviour has aimed to give managers greater ability to predict and control fires. Fire and grazing are important and interacting disturbances in grasslands. Fire is known to widely affect grazing patterns, but the effects of grazing on the incidence of fire are less well known. There have been few tests of the idea that 'grazing reduces blazing', which has popular and political currency in some countries. This study addresses the hypothesis that grazing affects fire potential in native grasslands. 2. Paired grazed and ungrazed quadrats were established at five lawn and five tussock grassland sites. Fuel load, percentage dead fuel and the number of days sustaining fires where possible were compared between treatments. 3. In lawn grasslands, grazing markedly reduced fire potential through the removal of plant biomass and by preventing the vegetation escaping into the unpalatable and flammable tussock state. Grazing led to increased fire potential in tussock grasslands where animals selectively removed live shoots, leaving a high proportion of dead fuel. 4. The difference in flammability responses to grazing between lawn and tussock grassland appeared to be due to differences in palatability that in turn may relate to soil fertility and the constancy of intense grazing. These differences mean that grazing lawns and tussock grasslands are likely to be subject to differing disturbance regimes. This association between disturbance regime and vegetation structural type suggests that lawns and tussock grasslands represent alternative stable states within the grassland ecosystem. 5. Synthesis and applications. Grazing is only likely to reduce the probability of fire where the bulk of the vegetation consists of potential food for grazing animals. It is likely that the negative relationship between vegetation palatability and fire potential applies to grasslands generally and possibly to many other vegetation types. Grassland managers may need to manipulate disturbance regimes in order to maintain vegetation structural heterogeneity and thereby promote landscape-scale biodiversity.