Seasonal changes in fire behaviour in a tropical Savanna in Northern Australia

In a landscape-scale experiment, fires were lit in replicate catchments 15- 20 km(2) in area, either early in the dry season (June) or late in the dry season (September) between 1990 and 1994. For each fire, Byram-intensity wa s determined in representative one ha areas of Eucalyptus miniata - E. tetr odonta open-forest, with a ground stratum dominated by annual grasses. Fuel weights were measured by harvest, fuel heat content was assumed to be cons tant, and the rate of spread was determined using electronic timers. Fuels consisted primarily of grass and leaf litter, and ranged from 1.5 to 13 t h a(-1); in most years, average fuel loads were 2-4 t ha(-1). Rates of spread were generally in the range of 0.2-0.8 ms(-1). The mean intensity of early dry season fires (2100 kW m(-1)) was significantly less than that of the l ate dry season fires (7700 kW m(-1)), primarily because, in the late dry se ason, there was more leaf litter, fuels were drier, and fire weather was mo re extreme. Crown fires, a feature of forest fires of high intensity in sou theastern Australia, were not observed in the Kapalga fires. Fire intensity was a very good predictor of both leaf-char height and leaf-scorch height for fires between 100 kW m(-1) and 10,000 kW m(-1), the range in which the majority of experimental fires fell.