Fire and water: water quality impacts of landscape-scale disturbance by wildfire.

Background : Wildfire plays an important role in determining ecosystem processes, composition, structure and condition, and is forecast to play a greater role under climate change. Wildfire affects the physicochemical and habitat characteristics of waterways, and the response in freshwater systems depends on characteristics of the fire, landscape and climate. Knowledge of fire effects on freshwater physiochemistry and habitat is predominantly based on suboptimal designed and short-medium term studies. Using a rigorous before-after-control-impact (BACI) study design and up to 28-years timeseries data, we examined if physicochemical and habitat variables changed following wildfire, and the duration of changes relative to unburnt sites in sub-alpine (hereafter alpine) and montane and sub-montane (montane hereafter) environments in south-eastern Australia. Results: Of the variables hypothesised to change in response to fire, 8 out of 33 variables at alpine sites, and 7 out of 12 variables at montane sites, changed in line with our predictions. Four variables changed in the opposite direction to predictions. Of 11 variables measured at both sites in alpine and montane environments, 3 variables responded to the fire in only one environment (montane zone) and 1 variable (electrical conductivity) responded in both environments but in different directions. For 90% of response variables examined at both alpine sites (33 variables) and montane sites (12 variables) effects were not detectable beyond 2 years post-fire. The remaining 10% of variables examined were detected up to 8 years post fire at alpine sites, and for 2.5 years at montane sites. The duration of detectable effects was greater at alpine sites than montane sites. Conclusions : We found no single consistent effect of fire on stream physicochemistry. Although some variables were found to respond to wildfire in a consistent way, the magnitude and duration of effects varied by site group (alpine versus montane) and site type (site burnt versus catchment burnt), illustrating the complexity of responses to wildfire. The complexity and inconsistency of responses of water physicochemical and habitat variables to wildfires reinforces the need for a better mechanistic understanding of the effects of fire on streams.