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

By M. Lawes, H. Adie, Jeremy Russell-Smith, B. Murphy, J. Midgley. 2011

198 citations recorded by the source corpus.

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 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.

Subfields: Global and Planetary Change, Nature and Landscape Conservation

View Paper: https://www.semanticscholar.org/paper/3018faa989b8467483392ea64d796b2e1851667f

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Citations in the corpus

Costs and benefits of relative bark thickness in relation to fire damage: a savanna/forest contrast (2013, methodology): “Peterson & Ryan (1986) found this relationship for conifers to be: sc ¼ 2:9B2T: eqn 4 While for trees in the tropical savannas of northern Australia, Lawes et al. (2011b) report: sc ¼ 0:0327B1:982T : eqn 5 Here, we use the general relationship sc ¼ B2T to model the benefit of thick bark.”
Ecology and long-term history of fire in New Zealand. (2014, background): “The relationship between bark thickness and trunk diameter varies between species, but thickness tends to increase with diameter whether linearly (Hengst & Dawson 1994) or as a power function (Lawes et al. 2011).”
The lanky and the corky: fire‐escape strategies in savanna woody species (2013, background): “1; Medeiros & Miranda 2008; Hoffmann et al. 2009; Lawes et al. 2011a; Dantas et al. 2013; Dantas, Batalha & Pausas in press).”
Fitness and evolution of resprouters in relation to fire (2011, background): “Measures that prevent heat death of the meristems or sunken buds, such as thick non-combustible bark (Gill and Ashton 1968; Lawes et al. 2011) and leaf bases (Lamont et al. 2004), might be considered as fire adaptations if it can be shown that they are an evolutionary response to fire, rather than,…”
Bark thickness determines fire resistance of selected tree species from fire-prone tropical savanna in north Australia (2011, background): “2006), and on stem survival in relation to bark properties and height and diameter growth rates (Lawes et al. 2011).”
Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie (2011, background): “Likewise, the thinner bark and lower stature of young woody plants makes them more susceptible to fire than adults (Scholes and Archer 1997, Higgins et al. 2000, Bond 2008, Lawes et al. 2011).”
Fire and tree death: understanding and improving modeling of fire-induced tree mortality (2018, background): “Damage to the conductive tissues in the stemmay be themost important primary cause ofmortality in small trees and thin-barked species (Michaletz and Johnson 2008, Lawes et al 2011a, 2011b).”