Ground and satellite-based assessments of wet eucalypt forest survival and regeneration for predicting long-term hydrological responses to a large wildfire

Large fires can have marked impacts on forest ecology and consequently on hydrology, including catchment water yields. Decisions on levels of fire protection and post-fire forest management require accurate information on the consequences of fire, including forest hydrological responses. In February 2009, wildfires burnt large areas of wet sclerophyll eucalypt forest in catchments supplying water to the city of Melbourne, Australia. Fire intensity was highly spatially variable, ranging from low intensity surface fire to high intensity crown fire. A survey of forest survival in three catchments 10–15 months after the fires revealed a good correspondence between overstorey survival and a satellite-based fire severity rating.In forests dominated by Eucalyptus regnans and Eucalyptus delegatensis, overstorey survival was close to zero in the two highest fire severity classes (crown burn and crown scorch). However, in areas mapped as intermediate fire severity (moderate crown scorch), survival varied markedly both spatially and between forest age classes.In fire tolerant eucalypt species, in mixed stands of Eucalyptus obliqua, Eucalyptus radiata, Eucalyptus cypellocarpa, and Eucalyptus baxteri, and in pure stands of Eucalyptus nitens, survival was >90% at high and intermediate fire severity. For all overstorey eucalypt species, survival in the lowest two fire severity classes (light or no crown scorch) was >75% but in most of these areas the fire temporarily removed the understorey.The fire severity mapping and ground survey indicate large areas of senescing (approximately 250–300 years old) and smaller areas of 70, 83 and 158 year old E. regnans and E. delegatensis forest were killed. Based on past research, this would be expected to result in very dense seedling regeneration, increasing evapotranspiration and decreasing catchment yields over the coming 2–3 decades. However, 2 years after the fire there were substantial differences between the three catchments in the density of seedling regeneration in these fire-killed areas. Where seedling densities are high, water yields are likely to decline. Areas with little or no eucalypt regeneration, however, might provide a persistent water yield increase. In addition, large areas of 70–158 year old E. regnans and E. delegatensis overstorey survived the fire where fire severity was low. Removal of the understorey from these areas and suppression of understorey regrowth by the intact overstorey canopy might result in water yield increases persisting for years to a decade or more.

► Forest ecological response to fire mapped using the normalised burn ratio. ► Eucalyptus regnans and Eucalyptus delegatensis more fire sensitive than expected. ► Seedling regeneration density after fire highly spatially variable. ► Long-term water yields expected to decrease in some catchments but not others. ► Low severity fire might increase catchment yields by removing understorey.