Differing regeneration patterns after catastrophic fire and clearfelling: Implications for future stand dynamics and forest management

Abstract

Silvicultural systems in native forests typically attempt to mimic natural disturbance regimes to ensure that the dominant tree species and associated biodiversity persist. Where the disturbance regime is driven by catastrophic, stand-replacing disturbances, such as fire and windstorms, even-aged (or single-cohort) silvicultural systems such as clearfelling (clearcutting) are commonly applied. However, there are important differences between stand-replacing natural disturbances and clearfelling. Considerable attention has focused on the biological legacies that persist after these disturbances (e standing dead trees, coarse woody debris). However, the inherent variability in regeneration patterns, which will drive long-term stand development patterns over the coming decades and centuries, has been largely overlooked. In this study we take advantage of a catastrophic fire event to compare patterns of regeneration for a dominant tree species after a high-intensity natural disturbance and after clearfelling. We focused on the mountain ash (Eucalyptus regnans F. Muell.) forests of southeastern Australia that were burned in the 2009 Black Saturday fire complex and are typically managed through clearfelling. We surveyed mountain ash seedling density in 24,200 quadrats across 121 plots (35 clearfelled, 86 burnt). We developed a statistical model to characterise the abundance and variability of seedling density. Our results demonstrated that natural regeneration density after the 2009 fires was nearly two times greater than regeneration at clearfelled sites that was aerially sown by helicopter. More importantly, however, the over-dispersion parameter in our model was nearly four times greater in the clearfelled sites than in the fire sites, meaning that regeneration in the clearfelled sites was much more spatially uniform than in the fire-affected sites. More than 50% of the surveyed quadrats in the fires sites had no mountain ash seedlings. The highly patchy distribution of natural regeneration has important implications for future stand development. Over time, patchy regeneration leads to structural variability that can benefit wildlife habitat and forest resilience to disturbances. A comparison of diameter distributions of stands that established at very different initial densities after the catastrophic 1939 fires shows the importance of early density on future stand structure. Our results provide an ecological basis for modifying current regeneration practices to better align contemporary forest management with natural disturbance regimes.