Multi-scale investigation of factors influencing moisture thresholds for litter bed flammability

Abstract

Fuel moisture is important to flammability. Vegetation communities vary in their moisture thresholds for ignition and fire spread. Different factors, operating at distinct spatial scales (litter vs. vegetation community) may be responsible for these variations in moisture thresholds. The relative importance of these factors at each scale remains unquantified. Our study sought to examine what factors influence moisture thresholds for flammability across two spatial scales (point vs. plot). Litter samples were collected repeatedly over one fire season (2020–21) from selected sites within temperate eucalypt forest along an aridity gradient in south-eastern Australia. Samples were reconstructed then burnt under controlled conditions. At the point-scale (0.05 m2), we quantified flammability as the probability of sustained ignition, flame spread rate and flaming duration. At the plot-scale (400 m2), we quantified flammability as the proportion of sustained ignitions. At the point-scale, moisture thresholds varied with leaf cover on the surface of the litter bed for ignition and leaf size for flame spread rate. At the plot-scale, vapour pressure deficit (VPD) was the best predictor of ignitability and moisture thresholds varied with aridity. Wetter parts of the landscape had a higher VPD threshold for ignition than more arid parts, meaning they were available to burn less often. The relationship between leaf cover and ignitability observed at the point-scale was overwhelmed by the effect of moisture at the plot-scale. Variations in ignitability between vegetation communities were driven by aridity-induced changes in canopy cover and its effect on litter moisture. Ignitability models based on VPD and aridity could be used to predict ignitability now and into the future, given anticipated increases in VPD under climate change.