Throwing fuel on the fire? Contrasting fine and coarse fuel responses to windthrow in temperate eucalypt forests in south-eastern Australia

Abstract

Windthrow is a disturbance in south-eastern Australian forests which significantly re-arranges forest and fuel structure by removing canopy trees. The implications for fuel dynamics and fire behaviour remains unquantified, making it a concern for fire managers. This study quantifies changes to forest and fuel structure caused by windthrow of varying severity, and uses this data to consider potential implications for fire behaviour. Thirty sites were established across dry eucalypt forests in south-eastern Australia ∼2 years after a severe windstorm resulted in extensive areas of windthrow. Ten sites were selected within each of unimpacted, low and high severity windthrow areas. Fine fuels were assessed using visual estimates of height, cover and percent dead across various fuel strata (surface; near-surface; elevated; bark) to develop fuel hazard scores using established protocols. Coarse fuels (fallen branches, logs) were assessed using a line-intercept survey. Fuel data were used to calculate expected fire behaviour (rate of spread, flame height, total heat output and residence time) across windthrow severity. The trends in both fine and coarse fuels were largely reflective of the forest structural changes induced by windthrow. Surface fuel (i.e. dead litter on the forest floor) decreased in depth and cover, while near-surface fuels increased in cover, height and proportion of dead with increasing windthrow severity. This reflects a reduction in litter inputs from reduced canopy cover, and an increase in the abundance of grasses and fallen canopy debris. Elevated fuels (i.e., shrubs) were unchanged across windthrown and unimpacted sites;bark fuel decreased with increasing windthrow severity, reflecting a reduction in standing basal area at severely impacted sites. Coarse fuel loads substantially increased with windthrow severity: high severity sites had ∼300 Mg/ha of coarse fuel loads compared to ∼20 Mg/ha at unimpacted sites. The changes in fine and coarse fuels resulted in a 1.2-fold increase in predicted flame height and a 17-fold increase in total heat output and residence time in high severity windthrow sites compared with unimpacted sites. Our research shows that windthrow produces complex and contrasting patterns in fine and coarse fuel loads, and the increase in near-surface and coarse fuels confers greater potential fire intensity via increased flame height, total heat output and residence time. Windthrow represents a conundrum for fire managers who are required to balance the reduction in fire risk while also retaining the important habitat resource fallen logs represent.