Fuel dynamics and vegetation recovery after fire in a semiarid Australian shrubland

Abstract

Understanding fuel dynamics in fire-prone ecosystems is important because fuels play a central role in shaping fire hazard and behaviour. There is ongoing debate over whether fire hazard continually increases with time since fire in shrublands of Mediterranean-type climates, and studies of the temporal changes in fuel loads can contribute to this discussion. We used a chronosequence of fire ages to investigate fuel dynamics and recovery of vegetation structure in the Acacia-dominated shrublands of interior south-west Western Australia. We collected and measured fuels from vegetation with fire ages ranging from 6 to 80+ years and then fitted linear, negative exponential, quadratic and logarithmic models to explore temporal patterns of fuel accumulation. Components of fine (<1 cm) fuel (ground, aerial live, aerial dead) and total fine fuel levels were found to accumulate rapidly in the first few years following a fire and then gradually increase for many decades thereafter. On average, total fine fuel was ~10 t ha–1 at 10 years post fire, and ~20 t ha–1 after 40–60 years. Akaike’s Information Criterion did not confidently discriminate between linear models and those that plateau at a certain fire age. However, all models showed gradual accumulation of fuel between 10 and 60 years post fire. Dead fine fuel (both litter and aerial) was virtually absent from young shrubland (<10 years) but accumulated slowly with age and comprised around 40% of total fine fuel in long-unburnt stands (>50 years). Although there is some evidence of shrub senescence in very long-unburnt vegetation (>60 years), no corresponding decline in fuel levels was detected, suggesting lag effects or inter-fire recruitment to maintain vegetation structure and fuel levels. Fuel structure and quantity varied considerably across the landscape, even within areas of the same landform and time since fire. We found that some of this variation was attributable to soil depth but suggest that other environmental factors may also cause variation in vegetation and fuel characteristics.