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African savannas are fire-prone, and fire is important in determining the composition and structure of these ecosystems (Bond & Van Wilgen 1996; Anderson, Cook & Williams 2003). Without fire, considerable areas of African savannas could potentially develop into closed woodlands under the current climate, and the occurrence of fires over the past c. 8 million years has also seen the evolution of a fire-tolerant and fire-dependent flora (Bond, Woodward & Midgley 2005). The appropriate use of fire in savannas is therefore an important consideration for managing these ecosystems. Tree mortality in savannas, and the recruitment of trees into larger size classes, is strongly affected by fire intensity. An understanding of the relationship between fire intensity and tree mortality has been used for some time by managers of African savannas to decrease tree dominance and encroachment by selecting conditions that lead to more intense fires (Trollope 1974).
The ability of trees and grasses to coexist is central to the understanding of savanna ecology. This coexistence is traditionally explained by either equilibrium or disequilibrium models (Scholes & Archer 1997). Equilibrium models propose that grass–tree coexistence is possible, for example because of separation of the rooting niche, with trees having sole access to water in deeper soil horizons and grasses having preferential access to, and being superior competitors for, water in the surface soil horizons (Walter 1971). In this equilibrium model, climatic variability precludes dominance by either life form, and coexistence is possible in a variety of states (Walker & Noy-Meir 1982). Disequilibrium models, on the other hand, propose that there is no stable equilibrium and that frequent disturbances prevent the extinction through competition of either grasses or trees by periodically biasing conditions in favour of alternative competitors. Higgins, Bond & Trollope (2000) have proposed a disequilibrium model in which interactions between life-history characteristics of trees (sprouting ability, fire survival at different life stages and mortality) and the occurrence of fires (which prevent recruitment of trees into adult life classes) could explain coexistence. This model identified the critical need for variability in fire intensity as a prerequisite for grass–tree coexistence and suggested that the imposition of fire regimes of homogeneous intensity (such as those associated with regular prescribed burning) could lead to dominance by grasses.
The fire regimes that characterize fire-prone ecosystems are normally described in terms of their frequency, season, intensity and type of fire (Gill 1975). While season and frequency are relatively easily measured features of a fire regime, the accurate determination of the range of intensities of fire that occur is more problematic. There are several broad measures of fire intensity: heat per unit area, reaction intensity and fire-line intensity (Biswell 1989). Heat per unit area measures the total energy released by a fire per unit area, while reaction intensity measures the rate of release per unit area. Byram's (1959) fire-line intensity measures the rate of energy released along the fire front, and is strongly correlated with the above-ground impacts of fire. Fire-line intensity is not correlated with soil temperatures experienced during fires (and thus is not related to, for example, variations in seed germination patterns; Bradstock & Auld 1995). It is, however, significantly correlated with damage to above-ground plant parts, especially ‘topkill’ in woody plants (Higgins, Bond & Trollope 2000).
Fire-line intensity is calculated as the product of the heat yield of fuels, the amount of fuel consumed and the rate of spread of the fire. Heat yields are measured in J g−1, fuel loads in g m−2 and rates of spread in m s−1, providing units of kW m−1. Of these factors, rate of spread has the greatest range in vegetation fires, varying from 0·1 to 100 m min−1. The value for fuel consumed in savanna fires can vary from about 20 to 100 g m−2. Heat yields vary so little (by about 10%) that they can be considered as almost constant at about 18 000 J g−1 (Stocks, Van Wilgen & Trollope 1997). Fire intensity in savannas thus has a potential 100-fold range of < 500 to > 50 000 kW m−1, primarily because of the large variation of possible spread rates (Stocks, Van Wilgen & Trollope 1997). This variation (largely because of variation in the spread rates of fires burning in the grass layers of the vegetation) has significant consequences for the post-fire survival of trees and shrubs in African savannas. The direct measurement of fire intensity is not always possible, and post-fire indicators such as leaf and bark scorch height and percentage topkill of trees are often used as surrogate measures.
The determination of fire regimes is dependent on good fire records. Such records are seldom kept, and the reconstruction of fire histories in savannas is normally dependent on satellite remote-sensing. Where fire records are kept, they normally provide only the date and extent of fires. Both physical records (Van Wilgen et al. 2000) and remote sensing (Russell-Smith, Ryan & Durieu 1997) allow for the determination of frequency and season, and more recently of intensity (Smith et al. 2005). A recent analysis of different approaches to fire management in an African savanna (Van Wilgen et al. 2004) concluded that management had little real impact on fire return periods (which were dependent on grass biomass, in turn determined by the amount of, and variability in, rainfall). On the other hand, season of fire, and possibly fire intensities, constituted elements of the fire regime that could be influenced by management.
Fire intensities have been recorded for more than two decades on experimental burning plots in the savanna ecosystems of the Kruger National Park, South Africa. These fires included a range of seasonal and post-fire age treatments, and they allowed for the derivation of general principles relating to the factors influencing fire intensity. We used fire intensity measurements from 956 experimental fires between 1982 and 2003 to derive such principles. We then used the principles to examine the probable historic effects of changing management approaches on the fire intensity regimes in the park, using the comprehensive fire records available for the park from 1957 to 2001.