Structural features of old-growth Australian montane ash forests

Abstract

We examined structural features of montane ash stands that varied from 15 to 300+ years of age in the Central Highlands of Victoria, south-eastern Australia. Extensive field data were gathered for our investigation and were drawn from >3700 survey plots on >625 sites located throughout the study region. Much of our study focussed on understorey features because extensive past studies in these forests have highlighted their importance as key habitat components for wildlife. A wide range of stand features, including the abundance of shrubs, the prevalence of tree ferns, the presence of Acacia spp., the presence of Myrtle Beech ( Nothofagus cunninghamii), and the number of vegetation strata (a measure of vertical heterogeneity) varied considerably between age classes; most attributes were significantly ( p < 0.001) less likely to occur or were significantly less abundant in young logged forests. Highly significant differences in these structural features also were found in stands dominated by different tree species (Mountain Ash [ Eucalyptus regnans] and Alpine Ash [ E. delegatensis]). The traditional view of disturbance in montane ash forests is that of high-intensity stand-replacing wildfires which produce even-aged regrowth forests. However, our data suggest that low-intensity, non-stand-replacing fires or only partial stand-replacing fires, have occurred in many old-growth montane ash stands, both to create multi-aged stands and to re-juvenate components of the understorey — a process leading to asynchrony in the ages of understorey and some elements of the overstorey. Thus, the structural variability, and the patterns of natural disturbance leading to such conditions, appears to be more complex than often previously recognised. This has important implications for both nature conservation strategies and timber harvesting practices in montane ash forests. Existing high-intensity clearfell logging operations produce an even-aged dominant overstorey as well as understorey and overstorey plants of the same age. If logging effects are to more closely resemble natural disturbance regimes and promote structural complexity in stands of harvested forest to enhance wildlife habitat values, then a wider range of types of harvesting methods (in addition to clearfelling) need to be employed.