Evidence for bark thickness as a fire-resistance trait from desert to savanna in fire-prone inland Australia

Abstract

Although bark thickness and fire-activity are correlated in many ecosystems worldwide, substantial data-gaps remain, especially for dryland biomes, preventing generalisation of this relationship at the global scale. We examined bark thickness trends in trees and shrubs across a large-scale fire-rainfall gradient from desert to dry savanna in northern Australia. Bark thickness increased with local fire activity but was unrelated to mean annual rainfall or the mean normalised difference vegetation index (surrogates of resource-productivity). In high-fire regions, thin-barked trees were restricted to localised low-fire patches. Thicker bark was associated with sites characterised by flammable 'Triodia' hummock grassland (spinifex). Within this flora, bark thickness reflected a trade-off in trait allocation to fire resistance versus fire resilience. For trees, thicker bark (fire resistance) was strongly associated with epicormic resprouters. In contrast, fire-resilient species that were either basal resprouters or reseeders had thinner bark. With increasing aridity there was a shift in dominance from epicormic resprouters to thinner-barked shrub and mallee species that either basally resprout or are killed by fire. Pairwise congeneric species comparisons showed a consistent relationship of thicker bark under high fire activity. This same pattern also emerged from a multi-species comparison within the dominant tree genus 'Eucalyptus'. Overall, for this system, species with thick bark at the sapling stage dominate where fire is frequent. Thus, we confirm that fire can be a major driver of plant traits in fire-prone drylands.