Effect of fire regime on plant abundance in a tropical eucalypt savanna of north-eastern Australia

Changes in plant abundance within a eucalypt savanna of north-eastern Australia were studied using a manipulative fire experiment. Three fire regimes were compared between 1997 and 2001: (i) control, savanna burnt in the mid-dry season (July) 1997 only; (ii) early burnt, savanna burnt in the mid-dry season 1997 and early dry season (May) 1999; and (iii) late burnt, savanna burnt in the mid-dry season 1997 and late dry season (October) 1999. Five annual surveys of permanent plots detected stability in the abundance of most species, irrespective of fire regime. However, a significant increase in the abundance of several subshrubs, ephemeral and twining perennial forbs, and grasses occurred in the first year after fire, particularly after late dry season fires. The abundance of these species declined toward prefire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance with fire intervals of 4 years. The density of trees (>2 m tall) significantly increased in the absence of fire for 4 years, because of the growth of saplings; and the basal area of the dominant tree Corymbia clarksoniana significantly increased over the 5-year study, irrespective of fire regime. Conservation management of these savannas will need to balance the role of regular fires in maintaining the diversity of herbaceous species with the requirement of fire intervals of at least 4-years for allowing the growth of saplings >2 m in height. Whereas late dry season fires may cause some tree mortality, the use of occasional late fires may help maintain sustainable populations of many grasses and forbs.