Multi-aged mountain ash forest, wildlife conservation and timber harvesting

Development of multi-aged stands of mountain ash trees (Eucalyptus regnans) in south-eastern Australia was examined with a spatial model of disturbance by fire. Stands were represented as spatially-explicit aggregations of cells that were exposed to spatially-correlated disturbance. The model predicted that approximately 10% of 3-ha sites would have trees both older than 200 yr and younger than 100 yr. At the scale of areas cut during timber harvesting operations (40 ha), the model predicted that 22% of sites would be multi-aged. As the size of cells became smaller, the scale of the analysis became finer grained and the frequency of development of multi-aged forest tended to increase to an asymptote. Predictions of the model were compared to survey data collected from 373 field sites each measuring 3 ha in area. Surveyed sites were considered to be multi-aged if at least 15% of the stems on a site belonged to a second (or sometimes third) age-class. Under this definition 9% of 3-ha sites were multi-aged. 28% of sites had large living trees with fire scars, indicating that these trees had survived at least one fire. While even-aged stands are common in mountain ash forests, multi-aged stands are an important ecological component. Multi-aged mountain ash forest provides important habitat for a suite of arboreal marsupials, some of which are recognised as endangered. The current silvicultural system in mountain ash forests is based on the paradigm that mountain ash trees rarely survive fires. Timber harvesting operations reflect this by using clearfelling techniques that remove most trees. Clearfelling operations are largely deterministic, of high intensity, and have prescribed rotations of 50-80 yr. Removing trees at such short intervals will preclude the development of cavities, an important habitat component in mountain ash forests. Without appropriate prescriptions to retain a suitable number of trees, multi-aged stands in timber production areas will not develop and key components of wildlife habitat will be lacking. In contrast to managed disturbance, natural disturbance in these forests is highly variable. It has random elements, operating over a range of intensities and spatial scales. If managed disturbance is to mimic natural disturbance more closely, timber harvesting operations will need to be less intense and more variable in at least some areas.