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How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness

Lawes, Michael J., Adie, Hylton, Russell-Smith, Jeremy, Murphy, Brett P. and Midgley, Jeremy J. (2011). How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness. Ecosphere,2(4):Article 42.

Document type: Journal Article
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Title How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness
Author Lawes, Michael J.
Adie, Hylton
Russell-Smith, Jeremy
Murphy, Brett P.
Midgley, Jeremy J.
Journal Name Ecosphere
Publication Date 2011
Volume Number 2
Issue Number 4
ISSN 2150-8925   (check CDU catalogue open catalogue search in new window)
Scopus ID 2-s2.0-83055161036
Start Page Article 42
Total Pages 13
Place of Publication United States
Publisher Ecological Society of America (ESA)
Abstract 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 PTopkill for a given bark thickness. Trends in PTopkill reflected eucalypt versus non-eucalypt differences. Eucalypts had thinner bark than non-eucalypts but lower PTopkill.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 fireprone 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.
Keywords diameter-response
epicormic sprouting
stem death
tropical savanna
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Additional Notes Copyright: © 2011 Lawes et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits restricted use, distribution, and reproduction in any medium, provided the original author and sources are credited

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