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 Austral ia. As part of a landscape-scale fire experiment, we examined the effects o f fire regimes on tree survival in a tropical savanna in Kakadu National Pa rk, 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 t he impacts on the tree stratum of frequent, prescribed burning at various i ntensities, 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 sin gle, high intensity 20 000 kWm(-1) fire (75%) was comparable to that of a r egime of annual late dry season burning for five years, at an average inten sity of c. 8000 kWm(-1). In the high intensity unplanned fire, stem surviva l 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 intermedi ate size classes. Stem survival was also species-dependent, being higher in the dominant Eucalyptus miniata than in the subdominant, broad-leaf decidu ous trees. In the absence of fire for 5-10 years, the structure and composi tion of the tree stratum of these savannas tends to become more complex tha n 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 o f 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, l ate dry season fires. Whatever the fire-management goal within a given patc h 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 n eeds to be given to the consequences of fuel build-up in fire-excluded site s.