The ecology of bark thickness (2): another twist
Sometime ago we proposed that “at the global scale, a significant proportion of the variability in bark thickness is explained by the variability in fire regimes”, and specifically predicted that frequent low intensity fires select for thick bark . In addition, we suggested that differentiating between inner and outer bark thickness would help to better understand the functional role of bark, especially in non-fire prone ecosystems. The paucity of available data at a global scale limited an empirical demonstration of the proposed framework.
A recent paper has now provided evidence for the fire hypothesis of bark thickness at a global scale [2, 3]. Specifically, Rosell  regressed bark thickness against fire frequency and climate parameters and showed that the most sensitive part of the bark in relation to fire was the outer bark, while the inner bark was quite variable and slightly related to both fire and climate . In the early paper  we also mentioned that little was known about the role of bark thickness in arid ecosystems. Recent research support the role of bark as a fire protection mechanisms in some arid ecosystems [4, 5].
To advance in the relationship between bark thickness and fire, it is necessarily to consider not only fire frequency, but also fire intensity, and to scale these fire characteristics with plant life-histories (, see figure below). This is because the relationship between fire regime and bark thickness is not expected to be simple and linear, but a bit more complex, including some threshold-type relationships (figure below).
Little by little we are improving our understanding on the role of bark as a fire-protection mechanism, and how fire regimes has shaped bark thickness in many ecosystems.
Figure: Bark thickness as a function of fire regime: flame height (an indicator of fire intensity) and mean fire return interval (fire frequency). Fire regime is scaled by the characteristics of the plant (height to the base of the crown and longevity, respectively). The shaded area represents the areas where thick bark is adaptive for fire protection, i.e., when return intervals are shorter than the lifespan of the plant and fires are of low intensity (flame height is shorter than the distance to the base of the crown, e.g., surface fires); the shade area is limited thresholds (values of 1 in the axes). The unshaded area represents the conditions where thick barks are not adaptive (thin bark is more likely), i.e., when fires are crown-fires or when the return interval is long (in relation to the longevity of the plant). From .
 Rosell J.A. 2016. Bark thickness across the angiosperms: more than just fire. New Phytologist 211: 90–102
 Schubert, A. T., Nano, C. E. M., Clarke, P. J. & Lawes, M. J. 2016. Evidence for bark thickness as a fire-resistance trait from desert to savanna in fire-prone inland Australia. Plant Ecol. 217: 683-696.
 Cousins, S. R., Witkowski, E. T. F. & Pfab, M. F. 2016. Beating the blaze: Fire survival in the fan aloe (Kumara plicatilis), a succulent monocotyledonous tree endemic to the Cape fynbos, South Africa. Austral Ecol. 41:466-479.