Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

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 eucalypt 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 dominate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low.

Understory plant communities and the functional distinction between savanna trees, forest trees, and pines

“…Pines are dominant, fire-promoting, and highly fire tolerant compared to broadleaf savanna trees, which are relatively less abundant, less flammable, and less fire tolerant (but far more fire tolerant than forest trees; Fig. With two-functional groups of fireadapted trees (one dominant and fire promoting), the longleaf pine ecosystem may have more in common with highly flammable eucalypt-dominated savannas of Australia (Lawes et al 2011, Bond et al 2012) than with savannas of Africa and South America that lack highly flammable tree species . With two-functional groups of fireadapted trees (one dominant and fire promoting), the longleaf pine ecosystem may have more in common with highly flammable eucalypt-dominated savannas of Australia (Lawes et al 2011, Bond et al 2012) than with savannas of Africa and South America that lack highly flammable tree species .…”

“…1a, b; Appendix B). Studies seeking to understand global distributions of savannas or tree coexistence with savanna understory plants should consider not only the distinction between savanna trees and forest trees (Ratnam et al 2011, Hoffmann et al 2012a), but also functional variation within fire-adapted trees as a group (Lawes et al 2011, Wenk et al 2011). Studies seeking to understand global distributions of savannas or tree coexistence with savanna understory plants should consider not only the distinction between savanna trees and forest trees (Ratnam et al 2011, Hoffmann et al 2012a), but also functional variation within fire-adapted trees as a group (Lawes et al 2011, Wenk et al 2011).…”

Advances in understanding the influence of fire on the ecology and evolution of plants: a tribute to Peter J. Clarke

“…Knox and Clarke (2012) postulated that warmtemperate rainforest species were resilient to a relatively severe fire regime and that these forests do not represent alternative community states to surrounding Eucalyptus dominated forests driven solely by the presence or absence of fire. Contrary evidence has been reported from tropical savannas in northern Australia where fire exclusion apparently promotes expansion of non-eucalypt 'monsoon rainforest' species, resulting in the eventual transition of species composition from savanna to monsoon rainforest (Woinarski et al 2004;Lawes et al 2011b). Unlike warm temperate rainforests, monsoon rainforest and tropical savanna are thus likely alternative stable states driven by fire .…”

Simplifying the savanna: the trajectory of fire‐sensitive vegetation mosaics in northern Australia

“…, 2011) savanna, occupying fire‐prone situations typically on rocky shallow to deeper soils. Eucalypts comprise the major proportion of the regional savanna basal area, but only around a quarter of component tree species richness (Lawes et al. , 2011b).…”