COMMUNITY ECOLOGY - ORIGINAL PAPER Are the eucalypt and non-eucalypt components of Australian tropical savannas independent? M. J. Lawes • B. P. Murphy • J. J. Midgley • J. Russell-Smith Received: 11 December 2009 / Accepted: 15 October 2010 / Published online: 10 November 2010 Ó Springer-Verlag 2010 Abstract Eucalypts (Eucalyptus and Corymbia spp.) dominate ( [ 60%) the tree biomass of Australia’s tropical savannas but account for only a fraction (28%) of the tree diversity. Because of their considerable biomass and adaptation to environmental stressors, such as fire, the eucalypts may drive tree dynamics in these savannas, possibly to the exclusion of non-eucalypts. We evaluated whether the eucalypt and non-eucalypt components in tropical savannas are dependent so that changes in one component are matched by opposite trends in the other. Using tree inventory data from 127 savanna sites across the rainfall and fire frequency gradients, we found that euca- lypt and non-eucalypt basal area and species richness had a negative relationship. This relationship was maintained across the rainfall gradient, with rainfall having a positive effect on the basal area and species richness of both components, but with a greater effect in non-eucalypts. Fire frequency negatively affected basal area, but not species richness, although basal area and species richness of eucalypts and non-eucalypts did not differ in their response to fire. Rainfall appears to set the upper bounds to woody biomass in these mesic savannas, while fire maintains woody biomass below carrying capacity and facilitates coexistence of the components. The magnitude of the component responses, particularly for non-eucalypts, is determined by rainfall, but their dependence is likely due to their differential response to both rainfall and fire, but not to competition for resources. Thus, while eucalypts domi- nate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low. Keywords Savanna dynamics Á Fire regimes Á Rainfall Á Mesic savanna Introduction The tropical savannas of northern Australia have high plant diversity comprising many endemic species (Woinarski et al. 2006). The Myrtaceae, and the eucalypts (Eucalyptus and Corymbia spp.) in particular, dominate the tree bio- mass of Australia’s tropical savannas, yet they account for only a small fraction of the tree diversity (Taylor and Dunlop 1985; Bowman and Prior 2004; Prior et al. 2006, 2009; Russell-Smith et al. 2010). Among the key processes affecting tree abundance of tropical savannas are fire and herbivory (Werner 2005; Prior et al. 2006; Werner et al. 2006; Russell-Smith et al. 2010), although complex syn- ergistic interactions with other environmental determinants are likely, such as the seasonality and amount of rainfall Communicated by Peter Clarke. M. J. Lawes (&) School for Environmental Research, Charles Darwin University, Darwin, NT 0909, Australia e-mail: Michael.Lawes@cdu.edu.au B. P. Murphy School of Plant Science, University of Tasmania, Hobart, Australia e-mail: brettpatrickmurphy@hotmail.com J. J. Midgley Botany Department, University of Cape Town, P bag Rondebosch 7701, South Africa e-mail: Jeremy.Midgley@uct.ac.za J. Russell-Smith Bushfires NT, Northern Territory Government, Darwin, Australia e-mail: Jeremy.Russell-Smith@nt.gov.au 123 Oecologia (2011) 166:229–239 DOI 10.1007/s00442-010-1829-4