From unburnt to salvage logged: quantifying bird responses to different levels of disturbance severity.
Forests world-wide are increasingly subject to natural and human disturbances, including wildfires and logging of varying intensity and frequency. Understanding how biodiversity responds to different kinds and combinations of natural and human disturbance is critical to enhanced forest management. We completed an 8-year study of bird responses across a spectrum of disturbance types in Australian mountain ash (Eucalyptus regnans) forests following wildfires in 2009. We found evidence of a gradient in bird species richness over the study duration. It was highest in unlogged and unburned (least disturbed) sites, decreasing through burnt unlogged forest (subject to high or low intensity fire), lower still in logged forest, and lowest in the most disturbed sites (subject to salvage logging without island retention). Retention of uncut islands within logged areas increased bird species richness above that found in areas that had been clearcut. The greatest rate of increase per year after disturbance in bird species richness was on sites burnt by high-severity fire but which were not subject to any form of logging. The level of disturbance affected the composition of the bird assemblage. Sites that were unlogged and unburned were more likely to support species that were larger, more mobile, and nested at greater heights above the ground. Synthesis and applications. All forms of logging on burned sites impaired recovery in bird species richness relative to sites subject to high-severity fire. Alterations in stand structure and plant species composition (and hence modification in bird habitat suitability) due to logging are the most likely reasons for reduced bird species richness and delayed patterns of recovery. This study highlights the importance for native bird species of retaining patches of unlogged forest not only within otherwise clearcut forest, but also in areas that are burned and subject to salvage logging. We therefore suggest that the adoption of retention harvesting be expanded to include stands disturbed by wildfires.