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

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Abstract

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.

Introduction

Natural and human disturbances are very important events in many ecosystems, influencing plant species composition, vegetation structure, species assemblages of animals and fluxes and cycles of nutrients (Bormann and Likens, 1979; Pickett and Thompson, 1978; Franklin et al., 1985; Noble and Slatyer, 1980; Hobbs and Huenneke, 1992; Lugo, 1995). Various authors have reviewed disturbances by fire in forest ecosystems (Kozlowski and Ahlgren, 1974; Attiwill, 1994a, Attiwill, 1994b; Williams and Gill, 1995), and such events may vary markedly in intensity, location, size and frequency (Baker, 1992; Glitzenstein et al., 1995). Much attention has focused on effects of the average time between disturbance events (e.g., Chou et al., 1993; Wadleigh and Jenkins, 1996), although variability in disturbance regimes may be equally important (McCarthy and Burgman, 1995; Morrison et al., 1995; Clark, 1996).

Intense stand-replacing fires occur throughout the world, including parts of the boreal forests and the forests of the Pacific north-west (Heinselman, 1981; Barrett et al., 1991; Agee, 1993). Natural stochastic fire regimes (Gill, 1975) are a major form of natural disturbance in the mountain ash forests in the Central Highlands of Victoria, south-eastern Australia (Ashton, 1981; Attiwill, 1994b). Mountain ash trees (Eucalyptus regnans F. Muell.) are generally believed to be fire-sensitive (McArthur, 1968) making them different from many other species of Eucalyptus trees. As a result, it is often assumed that mountain ash trees occur as even-aged stands that regenerate after high-intensity fires (e.g., Loyn, 1985; Griffiths, 1992; Attiwill, 1994b). Stand-replacing fires are, in turn, used as a template for the design of clearfelling operations in mountain ash forests (Cremer et al., 1984; Squire et al., 1991; Attiwill, 1994b). As part of such activities, most merchantable trees are removed in a single harvesting event and a new stand is initiated with the aid of a high-intensity regeneration burn (Squire et al., 1991). Such fires often destroy trees that are retained on the site (Lindenmayer, 1996). However, parts of mountain ash forests may be multi-aged (Cunningham, 1960; Ashton, 1981; Ambrose, 1982; Smith, 1984; Macfarlane, 1988; Chesterfield et al., 1991; Lindenmayer et al., 1991a, Lindenmayer et al., 1991b; Ough and Ross, 1992; Attiwill, 1994b), and these areas can be extremely important habitat for a wide variety of forest-dependent vertebrates (Lindenmayer et al., 1993a; Lindenmayer, 1996). The abundance of arboreal marsupials increases with the number of age-classes present on 3-ha sites (Lindenmayer et al., 1993a). Additionally, sites composed of multi-aged forest are likely to contain both a large number of hollow-bearing trees and a dense Acacia understorey, which are necessary habitat components for species such as the endangered Leadbeater's possum (Gymnobelideus leadbeateri; Lindenmayer et al., 1991bLindenmayer et al., 1993a).

Although it is recognised that multi-aged mountain ash stands occur, the area of a `stand' is rarely defined, and the frequency of occurrence is rarely quantified. Qualitative expressions are commonly used to describe the prevalence of multi-aged mountain ash forest, often leading to ambiguity. For example, Attiwill (1994b)(p. 306) stated that “two or three age classes are sometimes found in small patches”, but on the same page wrote “trees at a given site are even-aged”. Cunningham (1960)(p. 2) noted that “stands of two or sometimes more distinct age classes are relatively common in Victoria”, but Cremer et al. (1984)(p. 106) regarded mountain ash trees as being “of uniform age”. Ashton and Attiwill (1994)(p. 170) stated that “a 2-, 3-, or 4-aged forest may result, depending on the exact fire regime”. Where the most recent fire was of high intensity, mountain ash forests will be even-aged. The intensity of a fire will vary spatially (Smith and Woodgate, 1985), and the incidence of low intensity fire at a site will be to some extent a chance event, depending in part on the weather at the time. Although particular areas may be more likely to develop into multi-aged forest, its incidence depends on the nature of previous disturbance events. Thus, multi-aged stands of mountain ash appear to be dynamic, their development depending on the chance occurrence of fires of the appropriate intensity.

Given the occurrence and importance of multi-aged stands, we developed a model to simulate spatially-correlated fire events within mountain ash forests. The model was used to predict the frequency of occurrence of multi-aged stands of specified sizes and at a range of spatial scales under a natural disturbance regime. We compared results of the spatial simulation modelling with extensive field surveys which examined the prevalence of multi-aged mountain ash forest in the Central Highlands of Victoria. Our objective in building the model was to improve our understanding of how fire regimes contributed to the development of multi-aged forest. The implications of our findings, both for wildlife conservation and the design of silvicultural practices in mountain ash forests, are discussed.

Section snippets

Study area

The area targeted for detailed analysis was the Ada Forest Block (AFB), a 6760-ha area of forest located approximately 100 km north-east of Melbourne in the Central Highlands of Victoria, south-eastern Australia (Fig. 1). The AFB is dominated by stands of mountain ash which comprise more than 5500 ha or 82% of the study area (Lindenmayer et al., 1995). Large areas of the AFB were burnt in the 1939 `Black Friday' fires (Noble, 1977; Griffiths, 1992). In addition, the `Ash Wednesday' fires burnt

Simulations with varying stand sizes

To investigate the effect of stand area on the development of multi-aged forest, fires were simulated in stands ranging from 1 to 1000 ha in size, using cells of 1 ha. Simulations were conducted over a period of 300 yr and the age structure of the stand at the end of the simulation was recorded. The probability of obtaining more than one age-class in a given area was calculated as the proportion of 1000 random simulations in which more than one age-class was observed. Comparisons of such an

Survey data

Of the 373 3-ha sites of mountain ash forest that were surveyed, 34 (9%) supported two or more age-classes of trees. The presence of fire scars on large (>50 cm diameter) hollow-bearing trees was assessed at a total of 183 sites, with 51 (28%) of these sites having large living trees with fire scars.

Simulations with varying stand sizes

The predicted frequency of occurrence of multi-aged stands increased with stand area (Fig. 3). Young forest frequently occurred in simulated stands of all sizes (triangles in Fig. 3), while old

Fire regimes and the creation of mixed aged stands

Values for the prevalence of multi-aged stands (approximately 9% of 3-ha sites surveyed) were highly congruent with those derived from modelling 3-ha stands with cell sizes of 0.003–0.1 ha (9–11%, circles in Fig. 4). As part of extensive floristic surveys of the vegetation in mountain ash forests, Ough and Ross (1992)found that up to 40% of their quadrats supported trees of markedly different age (see figure 5 in Ough and Ross, 1992). The occurrence of two or more age cohorts within relatively

Conclusion

Field surveys and simulation modelling both indicate that multi-aged stands often occur in mountain ash forests. The model helps to formalise our understanding of the development of multi-aged mountain ash forest. It makes explicit the notion that the occurrence of multi-aged stands is dynamic and the frequency of occurrence depends on the spatial resolution of analysis. As stand size increases, the frequency of occurrence of multi-aged stands increases. Natural disturbance does not just

Acknowledgements

The support of the Australian Nature Conservation Agency, particularly Mr. A. Taplin, is most gratefully acknowledged. Ms. J. Smith from the Victorian Department of Natural Resources and Environment kindly provided the computer-generated maps of the Ada Forest Block which were used as part of the fire modelling in this study. This study benefited greatly from our recent collaboration with Dr. A.M. Gill, Dr. B. Mackey and Mr. I. Mullen on forest ecology in ash forests, which we gratefully

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