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To recruit to reproductive size in fire-prone savannas, juvenile trees must avoid stem mortality (topkill) by fire. Theory suggests they either grow tall, raising apical buds above the flames, or wide, buffering the stem from fire. However, growing tall or wide is of no advantage without stem protection from fire. In Litchfield National Park, northern Australia, we explored the importance of bark thickness to stem survival following fire in a eucalypt-dominated tropical savanna. We measured bark thickness, prefire height, stem diameter and resprouting responses of small stems under conditions of low to moderate fire intensity. Fire induced mortality was low (<10%), topkill was uncommon (<11% of 5 m to 37% of 1 m tall stems) and epicormic resprouting was common. Topkill was correlated only with absolute bark thickness and not with stem height or width. Thus, observed height and diameter growth responses of small stems are likely different pathways to achieving bark thick enough to protect buds and the vascular cambium. Juvenile height was traded off against the cost of thick bark, so that wide stems were short with thicker bark for a given height. The fire resilience threshold for bark thickness differed between tall (4–5 mm) and wide individuals (8–9 mm), yet tall stems had lower P_Topkill for a given bark thickness. Trends in P_Topkill reflected eucalypt versus non-eucalypt differences. Eucalypts had thinner bark than non-eucalypts but lower P_Topkill. With deeply embedded epicormic buds eucalypts do not need thick bark to protect buds and can allocate resources to height growth. Our data suggest the only ‘strategy' for avoiding topkill in fire-prone systems is to optimise bark thickness to maximise stem bud and cambium protection. Thus, escape height is the height at which bark protects the stem and a wide stem per se is insufficient protection from fire without thick bark. Consequently, absolute bark thickness is crucial to explanations of species differences in topkill, resprouting response and tree community composition in fire-prone savannas. Bark thickness and the associated mechanism of bud protection offer a proximate explanation for the dominance of eucalypts in Australian tropical savannas.

Item Type:	Refereed Article
Keywords:	diameter-response, epicormic sprouting, eucalypts, height-response, stem death, topkill, tropical savanna
Research Division:	Environmental Sciences
Research Group:	Ecological applications
Research Field:	Ecosystem function
Objective Division:	Environmental Management
Objective Group:	Fresh, ground and surface water systems and management
Objective Field:	Assessment and management of freshwater ecosystems
UTAS Author:	Murphy, B (Dr Brett Murphy)
ID Code:	117190
Year Published:	2011
Web of Science® Times Cited:	150
Deposited By:	Plant Science
Deposited On:	2017-06-01
Last Modified:	2017-07-20
Downloads:	133 View Download Statistics