Effects of forest management on structural features important for biodiversity in mixed-age hardwood forests in Australia's subtropics

Abstract

The contiguous forest landscapes of the Brigalow Belt Bioregion in Australia's subtropics, known as ‘the western hardwood forests’ are thought to be important for biodiversity conservation through the provision of continuous habitat features in an otherwise extensively cleared region. However, these forests have a long management history, as they are also important for timber production and stock grazing. Knowledge of the effect of these and other disturbances on the distribution and abundance of key structural or habitat features are limited and forest management activities thus continue uninformed. To address this knowledge gap, we investigated the effects of prescribed and non-prescribed disturbances on the distribution and abundance of six key habitat features (live and dead trees with hollows, large mature live trees, shrub abundance, downed coarse woody debris and litter cover) at 120 sites located in spotted gum Corymbia citriodora dominant forests within the western hardwoods.Logged stands had a lower mean abundance of live trees with hollows (2.5±0.6ha−1) than unlogged stands (6.2±0.8ha−1). This is below current timber harvesting prescriptions which outline the retention of 6 live trees with hollows ha−1. Since only 13.5% of western hardwood forest under state forest tenure remains unlogged, live trees with hollows are currently limited throughout the majority of the western hardwood forests. Recent wildfire in the region also meant reduced numbers of live trees with hollows. Logging affected the abundance of large, live trees, with highest densities occurring in the less intensively logged stands. Given the close relationship between increased tree diameter and hollow occurrence across a number of tree species, the small area of unlogged forest remaining in the region suggests that the pool of trees that may eventually form hollows may also be limited. The density of dead trees with hollows was strongly reduced by fire; by both low-frequency but high-intensity fires (wildfire) and by high-frequency but low-intensity burns associated with stock grazing management. Increased grazing was also associated with decreased abundance of shrubs and less downed coarse woody debris. Other than decreasing with increased logging intensity, litter cover did not appear to be influenced by other disturbance regimes, at least not by the long term correlational approach of our investigation. Overall, our results suggest that although forest management in the western hardwood forests could currently be described as low-intensity, it is extensive and impacts are discernible and cumulative. We highlight the need for a landscape approach to be taken in managing the various land uses for both production and biodiversity within the contiguous western hardwood forests.