Congruence between natural and human forest disturbance: a case study from Australian montane ash forests

Abstract

We examined the hypothesis that the impacts of human disturbances like logging on forest biodiversity will be minimised if there is close congruence with the effects of natural disturbances (e.g. wildfire). The montane ash forests of the Central Highlands of Victoria (south-eastern Australia) are used as a case study to examine several themes associated with congruence between natural (wildfire) and human disturbance (clearfell logging) regimes. These themes include: (1) the types of biological legacies left or created following wildfires and their use as habitat components by vertebrates, (2) the use of biological legacies to reconstruct pre-European stand structural conditions, (3) contrasts between stand and landscape conditions created by wildfire and clearfelling, and (4) approaches to create greater consistency between the effects of wildfire and logging at the stand and landscape levels. Extensive empirical studies reveal major differences between clearfelled forests and those burned by wildfires, particularly with respect to vegetation structure, plant species composition, and landscape patchiness. Clearfelling simplifies stand structure and creates a single cohort of regrowth trees. In contrast, natural disturbance by wildfire varies in its intensity and has variable effects—all trees are killed in some stands and large dead trees remain. In others a multi-aged stand develops which is a mixture of fire-damaged living and dead trees. Many of the biological legacies typical of stands damaged by wildfires are also those which are severely depleted in young clearfelled forest. Differences between burned and clearfelled forests can have significant negative impacts for many forest-dependent vertebrates. The widespread application of clearfelling operations is not consistent with the effects of natural disturbance regimes. This suggests there is a need to change cutting and regeneration methods to more closely resemble natural disturbance regimes and promote structural complexity in stands of harvested forest to enhance their value for wildlife. Such changed silvicultural systems should lead to: (1) the retention of more living and dead trees that remain standing through several cutting events and eventually develop cavities, (2) the protection of intact thickets of logging-sensitive understorey vegetation, (3) the recruitment of more large logs to the forest floor, (4) longer rotation times more akin with the natural life cycle of montane ash stands, and (5) the creation of more stands of truly multi-aged forest.