Invading rain forest pioneers initiate positive fire suppression feedbacks that reinforce shifts from open to closed forest in eastern Australia

Abstract

Question: Suppression of Aboriginal burning and wildfire from forests in which fires were historically frequent may trigger environmental changes that further suppress fire frequency and intensity. In high-rainfall regions of eastern Australia, long-unburnt open forests are frequently invaded by rain forest pioneer trees, which in turn modify open-forest understorey structure and composition. We examine altered understorey fuel properties as potential fire suppression feedback mechanisms reinforcing switches from fire-dependent eucalypt open forest to fire-resistant closed forest. Location: Northeastern New South Wales, Australia. Methods: We characterized forest understorey leaf traits, plant crown architecture and vegetation structure to test for differences in flammability across a mosaic of contrasting times since fire (recently burnt — 4 years; long unburnt — 16 years) and midstorey composition (open, sclerophyll, rain forest) in subtropical shrubby dry sclerophyll eucalypt forest. Results: Drivers of flammability were lower in long-unburnt open forest with a rain forest pioneer midstorey at multiple scales (leaf, foliage clump, crown and stratum) than either long-unburnt open forests with a sclerophyll midstorey or recently burnt open forest with an open midstorey. Grassy (graminoid) and sclerophyllous shrub fuels declined sharply beneath a rain forest pioneer midstorey, consistent with the fire suppression feedbacks regulating tropical savanna to closed forest transitions. Rain forest pioneers also reduced understorey flammability drivers through floristic compositional changes that increased leaf and foliage clump separation in shrub and midstorey fuel strata, and increased leaf moisture in the shrub stratum. Conclusion: Our results suggest that rain forest pioneers are ecosystem engineers that modify open-forest fuel properties, initiating a positive fire suppression feedback that facilitates their persistence and continued transition to a fire-resistant closed forest. The fire suppression feedbacks we identify may help explain paleoecological and modern observations of rain forest expansion under high CO2 atmospheres, despite increased temperatures and seasonal drought severity.