Bark is a vital and very visible part of woody plants, yet the functional and evolutionary ecology of bark is still poorly understood. This summary is for only one bark property: thickness. Bark thickness is very variable among woody plants and the authors hypothesize that fire is the most important factor selecting for a thick bark. This is because bark is a very good heat insulator and under low intensity fires, small increases in bark thickness provide a great increase in stem protection and survival. Consequently, at the global scale, most of the variability in bark thickness should be explained by variability in fire regimes. Here I provide evidence supporting the role of fire regime in shaping bark thickness (in conjunction with other plant traits) on a global scale.
Forest environments with very frequent (and low intensity) understory fires select for trees with thick bark at the base of the bole. In some savannas, trees do not have specially thick bark as they tend to growth quickly to escape the height affected by grass fires. Savanna trees living in poor soils may not be able to grow quickly and thus trees can only survive if the whole plant (including the thin branches) has a very thick bark . In Mediterranean ecosystems, fires are less frequent than in savannas, and there is time for the accumulation of fine woody biomass. Consequently, fires burn intensely and thus small differences in bark thickness do not increase stem survival; in such conditions, most species have relatively thin bark. In wet tropical forests, tree bark is very thin because fires are very rare and thus a thick bark is not advantageous. In very arid ecosystems, fuels are too sparse for fire spread, and thus thick barks might not be expected, although some thick barks may occur as a response to water stress.
In conclusion, fire regimes can explain a large proportion of the variability of bark thickness at the global scale, and thus this trait varies across ecosystems in a predictable manner.