Effects of fire and drought in a tropical eucalypt savanna colonized by rain forest
Aim This study documents the effects of multiple fires and drought on\nthe woody structure of a north Australian savanna never grazed by\ndomestic stock.\nLocation The study was conducted in a 500 ha pocket of\nEucalyptus-dominated savanna surrounded by a late Quaternary lava flow.\nThe flow is known as the Great Basalt Wall, located c. 50 km northeast\nof Charters Towers in semi-arid north-eastern Australia. This region was\nexposed to the largest 5-year rainfall deficit on record between 1992\nand 1996.\nMethods All individual woody plants were tagged within a 1.56 ha plot.\nSpecies were segregated into their habitat affinities (rain forest,\necotone, savanna) and regeneration strategy (resprouter, seeder). The\nsurvivorship of plants within these categories was analysed in relation\nto fire intensity from the first fire, and to each of four fires lit\nbetween 1996 and 2001.\nResults Before the first fire, the plot contained thirty-one tree\nspecies including twenty-one typical of the surrounding dry rain forest.\nThese rain forest species were represented by small individuals and\nconstituted <1% of the total basal area of woody plants. The basal area\nof savanna trees was 7.5 m(2) ha(-1) at the commencement of monitoring,\nalthough 31% had recently died and others had major crown damage.\nFurther death of the drought debilitated savanna trees was substantial\nduring the first year of monitoring and the basal area of live savanna\ntrees declined to 1.1 m(-2) ha(-1) after 5 years. Most species from both\nrain forest and savanna were classified as resprouters and are capable\nof regenerating from underground organs after fire. Species without this\nability (rain forest seeders and ecotone seeders) were mostly eliminated\nafter the first two consecutive fires. Among resprouters, survivorship\ndeclined as fire intensity increased and this was more pronounced for\nrain forest than for savanna species. Repeated burning produced a\ncumulative effect of decreasing survivorship for rain forest resprouters\nrelative to savanna resprouters.\nMain conclusions The study provides evidence that savanna and rain\nforest trees differ in fire susceptibility and that recurrent fire can\nexplain the restricted distribution of rain forest in the seasonally\narid Australian tropics. The time of death of the savanna trees is\nconsistent with the regional pattern after severe drought, and\nhighlights the importance of medium term climate cycles for the\npopulation dynamics of savanna tree species and structure of Australian\nsavannas.