Fire regime, fire intensity and tree survival in a tropical savanna in northern Australia

Dry season fires are a feature of the tropical savannas of northern Australia. As part of a landscape-scale fire experiment, we examined the effects of fire regimes on tree survival in a tropical savanna in Kakadu National Park, northern Australia. The fire regimes were annual early dry season (June) fires, annual late dry season (September) fires, and, no fire (control). Prescriptive, experimental fires were lit annually, between 1990 and 1994, in replicate compartments, each 15-20 km(2). In addition to the prescribed fires, however, one of the control compartments, which had been unburnt for seven years, was burnt by an unplanned, high intensity fire (similar to 20 000 kW m(-1)) in September 1994. This provided an opportunity to compare the impacts on the tree stratum of frequent, prescribed burning at various intensities, and a single unplanned fire. In all fire regimes, stem survival was substantially lower than whole-plant survival, and decreased linearly with increasing fire intensity. Significantly, stem death following the single, high intensity 20 000 kWm(-1) fire (75%) was comparable to that of a regime of annual late dry season burning for five years, at an average intensity of c. 8000 kWm(-1). In the high intensity unplanned fire, stem survival showed a non-linear response to stem size, being least in the small (< 10 cm DBH) and large (> 40 cm DBH) size classes, and highest in the intermediate size classes. Stem survival was also species-dependent, being higher in the dominant Eucalyptus miniata than in the subdominant, broad-leaf deciduous trees. In the absence of fire for 5-10 years, the structure and composition of the tree stratum of these savannas tends to become more complex than in sites burnt more frequently, especially by high intensity fire. Such a long-term absence of fire may be a conservation objective for some areas of savanna. However, build-up of fuel to near maximal levels can occur in 2-4 years without fire. This may predispose the savannas to high-intensity, late dry season fires. Whatever the fire-management goal within a given patch of savanna, whether it be the prescribed use of fire on a biennial basis, or the exclusion of fire at a semidecadal scale, careful attention still needs to be given to the consequences of fuel build-up in fire-excluded sites.