Changes in evapotranspiration following wildfire in resprouting eucalypt forests
Forests that recover from disturbance predominately via vegetative resprouting may be expected to have different catchment water balance dynamics following wildfire than forests recovering from seed. However, the impacts of wildfire on forest water use are largely unknown in resprouting forest types. This is despite their dominance across the majority of southern Australia's forested catchments and the large areas burnt in recent years. We hypothesized that postfire changes in evapotranspiration (Et) would be a function of fire severity and topography and that partitioning of Et would change after fire because of altered stand structure. We tested these hypotheses by monitoring Et and component fluxes across different topographic positions and fire severities in a mixed eucalypt species forest located in water supply catchments for the city of Melbourne. For this forest type, wildfire triggers vegetative resprouting from lignotubers and epicormic shoots on the bole and branches of the overstorey trees, in addition to prolific seedling germination. Monitoring was undertaken over 1-3 years following the 2009 Black Saturday wildfires. We found that Et was on average 41% lower in forest burnt at high severity compared with unburnt forest, whereas Et from forest burnt at moderate severity was only 3% lower than unburnt forest over 1-2 years postfire but on average 9% higher over 2-3 years postfire. Et losses were driven by tree and shrub mortality in conjunction with lower transpiration in surviving trees. Lower Et was partially offset by regenerating seedlings that drove increases in forest floor Et and interception loss. Finally, we found that topography, through its effects on evaporative demand and forest structure, was a strong determinant of total Et but did not affect the nature of postfire recovery.