Variation in eucalypt bark allometry across Australia

Abstract

Positive bark allometry (hyperallometry), characterised by rapid early bark growth, is expected where fire selects for thicker bark to resist cambial damage and topkill. We examine this prediction for 52 Australian eucalypt species. An effective bark allometric coefficient (α) was estimated from the first segment of breakpoint regression, which included fire-affected young trees. Eucalypts presented a negative–positive bark allometry continuum. Contrary to expectation, 73% of species (n = 38) displayed negative effective bark allometry. Early rapid bark growth was observed (α = 0.92 ± 0.04, x ± s.e., n = 168 sites), but was slower than isometry and bark was thinner overall than expected. Fire type (crown-fire propensity) and net primary productivity (resource availability) most influenced bark allometry. Productive ecosystems experienced crown-fire and bark was thicker at standardised diameter (x ± s.e. = 2.04 ± 0.20 cm) than in less productive ecosystems under surface fire (x ± s.e. = 1.68 ± 0.18 cm). Bark morphology types did not differ in their stem diameter (x ± s.e. = 21.47 ± 1.06 cm) or bark thickness (x ± s.e. = 1.88 ± 0.08 cm) thresholds, representing putative stable alternative evolutionary solutions. Fundamentally, many eucalypts possess negative bark allometry with a relatively thin bark compared to non-eucalypts, because unique bark bud traces sustain post-fire resilience through epicormic resprouting. The resource allocation trade-off to thin bark v. height growth, selected by fire and resource limitation, accounts for eucalypt persistence and domination of Australia’s flammable ecosystems.