Fuel dynamics and rarity of fire weather reinforce coexistence of rainforest and wet sclerophyll forest

Abstract

Rainforest, wet sclerophyll forest and eucalypt forest exist as Alternative Stable States moderated by fire. Eucalypt forest is flammable and subject to regular fire. Insights into the flammability of rainforest and wet sclerophyll forest are difficult to obtain because of the rarity of fire, particularly in sub-tropical and tropical environments. This study aimed to determine the relative flammability of rainforest and wet sclerophyll forest with a canopy of Lophostemon confertus in subtropical Australia. Fires in 2019, coinciding with extreme fire weather and following a long period of drought, burnt rainforest and wet sclerophyll forest in south-east Queensland. The weather record indicates these conditions recur about three to four times a century. Remapping of the vegetation overlaid with the fire-scar mapping, indicated that the 2019 fires burnt 23 % of rainforest, much of which was disturbed, but 60 % of wet sclerophyll forest suggesting greater flammability of the latter. This study revealed that above-ground biomass and litter fuel loads are more than twice as high in wet sclerophyll forest than rainforest. The litter of Lophostemon confertus, a Myrtaceae tree dominating the upper canopy, on average decomposes at less than half the rate of rainforest litter, contributing to the higher fuel loads and thus greater flammability of wet sclerophyll forest compared to rainforest. The Alternative Stable States of wet sclerophyll and rainforest are partly stabilised by the fire retardance of rainforest, the rarity of weather conditions suitable for fire in the subtropical environment and the higher fuel loads in wet sclerophyll forest that allow occasional fire. Wet sclerophyll forest requires rare fire events and the legacy of these fires over hundreds of years is evident in the multiple cohorts of ‘giant’ canopy trees.