Which trees dominate in savannas? The escape hypothesis and eucalypts in northern Australia

Abstract

In humid savannas, the transition from juvenile to mature tree sizes is thought to be a major demographic bottleneck because smaller plants are topkilled by frequent fires. Species with the highest net rates of sapling growth should dominate the tree component of savannas by reaching fire-proof sizes, 'escape size', more rapidly than competitors. However, tests of this prediction have failed to explain eucalypt dominance in Australian savannas as eucalypt mean growth rates are low. We tested the escape hypothesis directly by recording the number and identity of tagged individuals reaching escape size in a long-term study. Results were consistent with the escape hypothesis with greater than six times more eucalypts reaching escape height than non-eucalypts in frequently burnt savannas. The pattern was reversed where fires were excluded for 5 years, with more non-eucalypts emerging than eucalypts.We conclude that mean growth rates alone are a poor predictor of the rates at which juvenile savanna trees transition to mature tree size. Only the fastest growing individuals make the transition to mature trees whereas mean values include many suppressed individuals. Measures of maximum growth rates will provide more robust estimators of demographically important effects on mature tree cover. The mechanistic basis for the remarkable ability of juvenile eucalypts to escape frequent fires is not yet understood.

How to tell a shrub from a tree: A life‐history perspective from a South African savanna

“…However, all dominant plants in the studied savanna system resprout after fire and thus a dichotomous classification does not seem appropriate in this context (see Appendix S1 in Supporting Information) even though results from semi-arid Australian ecosystems suggest resprouting ability and woody plant growth form are linked (Vesk et al 2004). (iii) Escaping fire may involve investing resources in rapid growth to ensure rapid attainment of fire proof size and delaying reproduction until fire resistant size-classes are attained (Higgins et al 2000;Hoffmann & Solbrig 2003;Bond et al 2012). In our system shrubs do not get large enough to escape fire (Higgins & Scheiter 2012;).…”

Defining the fire trap: Extension of the persistence equilibrium model in mesic savannas

“…Frequent fire can trap these small trees in a cycle of above-ground biomass loss followed by resprouting, which results in a demographic bottleneck limiting transition to the canopy (Hoffmann 1998;Higgins et al 2000;Bond 2008;Prior et al 2010;Bond et al 2012;Werner & Prior 2013). Frequent fire can trap these small trees in a cycle of above-ground biomass loss followed by resprouting, which results in a demographic bottleneck limiting transition to the canopy (Hoffmann 1998;Higgins et al 2000;Bond 2008;Prior et al 2010;Bond et al 2012;Werner & Prior 2013).…”

The lanky and the corky: fire‐escape strategies in savanna woody species

“…This effect generates a demographic bottleneck that increases the dominance of short and juvenile woody plants (the 'Gulliver' syndrome; Bond & van Wilgen 1996;Higgins et al 2007). The transition from below to above this 'fire trap' is a major constriction on tropical savannas (Bond, Cook & Williams 2012;Hoffmann et al 2012) and is largely dependent on fire regimes (Gill & Ashton 1968;Gignoux, Clobert & Menaut 1997;Bond 2008;Hoffmann et al 2012;Murphy & Bowman 2012).…”

“…9b in Archibald & Bond 2003 for an example) and failure to grow enough during the inter-fire period results in top-killing followed by basal resprouting. Because building a tall canopy is also adaptive against large browsers, it should also be particularly important in heavily browsed savannas (Bond, Cook & Williams 2012). The second strategy includes plants allocating resources to thick heat-insulating barks that enable plants to stay and resist fire (Gignoux, Clobert & Menaut 1997; Fig.…”