Fuel moisture moderates wildfire resistance in rainforests of south-east Australia

Abstract In fire-prone forests of south-east Australia, rainforests have longer fire-return-intervals than the dominant and adjoining eucalypt forests, because rainforests occur in topographic positions which are typically too wet to burn. Thus, rainforests often act as natural barriers to fire spread. Although rare, severe drought can make rainforests available to burn, and this can promote very large and intense wildfires by increasing fuel availability across landscapes. Here, we explore how ten fuel moisture indices impact wildfire occurrence in rainforest patches of south-east Australia, when compared with wet and dry sclerophyll eucalypt forest types which are drier and have shorter fire-return-intervals. Vapour pressure deficit was the strongest and most ubiquitous moisture index predicting wildfire occurrence across all forest types, followed by soil moisture and live fuel moisture. Vapour pressure deficit thresholds facilitating a wildfire probability >0.5 also did not differ between forest types. However, the percentage of days exceeding vapour pressure deficit thresholds increased from rainforests to wet eucalypt forests and peaked in dry eucalypt forests. Collectively, our results suggest that the same fuel moisture thresholds promote wildfire in rainforests and fire-prone eucalypt forests; however, wildfire is less common in rainforests because they experience less time in a dry combustible state. Our results provide a framework to forecast wildfire probability across wet and dry forests at large spatial scales.