Wildlife and Woodchips by David Lindenmayer
UNSW Press - 1996.
Leadbeater's Possum. A Test Case for Sustainable Forestry.
"Chapter 7
Changing the Landscape: The Impacts of Logging
The forests in which Leadbeater's Possum occurs are extremely valuable to the timber and paper industries because they contain tall, straight, fast-growing trees. Forestry is an important part of the local economy in many small towns in the Central Highlands including Marysville, Alexandra, Healesville, Powelltown, Yarra Junction and Noojie.
More than 75% of the forests in the known range of Leadbeater's Possum are broadly designated for timber harvesting and most have already been cut extensively for logs and pulpwood for the paper industry. Given this, there has been great concern for the past 30 years over the impacts of logging on the conservation of the species. These concerns are particularly pronounced because the possum has such a restricted distribution and efforts to conserve it must be focussed on areas where the timber and paper industry also have strong interests.
The Impacts of Clearfelling
Montane ash forests are cut using the clearfelling method. First, virtually all trees on a site or coupe of between 15 and 40 hectares (ha) are removed in a single timber harvesting operation that may take several months. After the crowns, bark and branches have been removed, the trunks are taken to be sawn for timber or pulped to produce paper. Once all the trees have been removed, the debris on the forest floor is burned to provide a bed of ashes in which the next crop of trees can grow. The regeneration fire may be very intense, with flames soaring up to 50 m high. Seeds are then scattered from the air on to the burnt site. The cycle of clearfelling, regneration and clearfelling again is presently planned to take 50-80 years, that is, a given patch of forest will regrow for about 50-80 years before it is harvested again. The interval between two successive timber harvests at a given site is called the 'rotation time'. The forest available for timber harvesting is divided into management units called 'forest blocks', of which there are 35-40 in the Central Highlands, ranging from 3000-10 000 ha each.
Clearfelling results in major changes in the vegetation structure of the forest, affecting (1) the availability of nest-trees for Leadbeater's Possum, (2) the suitability of forest sites for the species and (3) the arrangement of suitable habitat patches in the forest landscapes. Logging affects Leadbeater's Possum at several spatial scales in the forest, ranging from the sdmall scale (2 m x 2m, eg nest-trees) to the large-scale (10km x 10km, eg forest blocks).
...trees used by Leadbeater's Possum are typically between 200 and 400 years old, sometimes even older, but the rotation time between logging operations is only 50-80 years. Trees on logged sites never become old enough to develop the hollows that are suitable for Leadbeater's Possum and this paucity of hollow trees in logged forests further exacerbates the problems associated with the rapid, natural decay and collapse of hollow trees in uncut areas.
Not only is the type of large, hollow tree important for Leadbeater's Possum but the number and spacing of potential nest-sites is also critical. Patches of clearfelled and regenerated forest have significantly fewer hollow trees than unlogged areas. In general, there are insufficient potential nest-trees in clearfelled forest to meet the possum's needs. This is a major problem because each time the forest is cut, at least another 200 years will pass before it becomes potentially suitable possum habitat. While the possum favours very old and highly decayed trees at the end of their ageing cycle, these trees have passed through various earlier stages of development. Therefore, to ensure a perpetual supply of nest-sites, the forest must support trees of different ages with many in less decayed stages, which will eventually progress to more advanced old-age and thus provide suitable nesting places. This sequence does not occur in clearfelled forests because all trees are cut at the same time and all regenerated trees are the same age.
The spacing of hollow trees is important for Leadbeater's Possum and other hollow-dependent forest animals. One negative impact of forestry operations is that they transform the spacing of hollow trees from a random to a clustered or clumped arrangement. Possums favour forests in which hollow trees are randomly or regularly distributed. Conversely, if hollow trees are retained on logged areas, they are often clumped together to better protect them during logging and regeneration burns. However, there are significantly reduced rates of occupancy of clumped hollw trees, possibly as a result of the possum's territorial behaviour. As they defend their nest-trees and the surrounding forest against members of their own and other species, when hollow trees are clustered together, competition for hollows and aggressive behaviour between animals may substantially reduce the number of occupied trees.
There have been attempts to conserve nest-sites in clearfelled forests, but in many cases, these potential habitats are badly damaged in the hot regeneration fires. This is particularly true for large dead trees, which are highly flammable. These trees, and many living ones, often collapse soon after they have been burnt. In addition, retained trees are exposed to the wind and large temperature fluctuations after the surrounding forest stand has been cut, both of which may hasten their collapse. Whatever the cause, the loss of such retained trees leads to a considerable reduction in the abundance of potential nest-sites for Leadbeater's Possum.
...the bark strips which hang from the branches of montane ash trees are a key habitat component for Leadbeater's Possum because they provide a microhabitat for insects, particularly large tree crickets, which are a valuable source of protein for the animal. However, it may be a long time before these flightless insects can re-colonise logged and regenerated sites. This problem may be exacerbated by the fact that young trees have only a limited amount of peeling bark. It may therefore take many years for trees to develop suitable microhabitat conditions for the crickets. Tree crickets are themselves carnivorous and are dependent on suitable habitat conditions for the insects on which they must feed. Given this, clearfelling may have a serious effect on the availability of food for Leadbeater's Possum.
Post-Fire Salvage Logging
Stands of trees burnt in wildfires are often harvested in what are called 'salvage logging' operations. They are cut because the economic value of fire-damaged trees is considerably lower than that of unburnt trees. Trees killed by fire are also removed by salvage logging. During the 15 years of extensive salvage logging that followed the 1939 wildfires, stands of burnt forest were cut for sawlogs and later, pulpwood. Salvage logging also followed the 1983 Ash Wednesday wildfires. Modern salvage logging is a clearfelling operation, ...
The effects of wildfires are clearly very different from those of clearfelling because some individual trees and patches of forest may survive. Wildfires may even stimulate the development of cavaties in older trees and large, dead trees may stand for more than 50 years after they have been burnt, during which time they provide nest sites for a range of hollow-dwelling fauna.
However as old trees are destroyed during salvage logging, both this type of operation and standard clearfelling operations will have similarly severe impacts on hollow-dependent fauna.
Clearfelling and Regenerating 'Unstocked' Forest
Some areas of montane ash forest are characterised by extensive patches of wattle trees with relatively few eucalypts. These areas are sometimes called 'unstocked forest' because they support a smaller number of potential sawlogs and pulplogs than elsewhere in the forest. They are increasingly targeted for clearfelling by forest managers, who aim to regenerate the forest with a new stand with more eucalypts. The problem with this practice is that 'unstocked forests' which support some large eucalypts and extensive areas of wattle trees can provide important habitat for Leadbeater's Possum. Therefore, clearfelling and regenerating them can eliminate valuable habitat for the species as well as many other forest animals.
Forest Fragmentation
The detrimental impacts of logging on Leadbeater's Possum may extend well beyond the actual sites that are cut. Fragmentation of forests, which has been recognised as a problem for wildlife in many parts of the world, is also a major threat to this species, particularly where there are many logged areas in a forest landscape, as is commonly the case in the Central Highlands. Under these conditions, patches of suitable habitat, and the populations of Leadbeater's Possum therein, become increasingly isolated by stands of clearfelled forest that may never support the species. Thus, if possum populations in these remnant patches of suitable habitat become extinct (eg as a result of predation by owls), the patches may be too isolated for new populations to re-establish themselves. This may, in turn, mean that there may be fewer source populations of animals available to disperse to other remnant patches. Several 'localised' extinctions could result in the remaining patches becomimg even more isolated, eventually leading to the loss of the species from entire forest blocks.
Robert Lacy and I simulated the problems faced by small, isolated populations of Leadbeater's Possum using computer modelling techniques. When factors that cause fluctuations in population size, such as drought, affect small popluations (25-50 animals), there is high risk of extinction. There is an increased likelihood of loss of genetic variability among small populations, which may, in turn, impair the animals' resistance to factors like disease. Small populations in restricted areas are more vulnerable to catastrophes like fires and even a localised disaster such as small, intense fire can eliminate an entire population. All of these factors mean that small, isolated populations are mush more vulnerable to extinction than larger ones. The fact that Leadbeater's Possum was absent from 40% of sites with apparently suitable habitat suggests that the effects of fragmentation may already be apparent.
The problems of forest fragmentation are highlighted by the predicted fate of Leadbeater's Possum in the Murrundindi Forest Block, a 3500-ha wood production area near Healesville and Toolangi in the western part of the Central Highlands. This block has been logged extensively and intensively for more than a century. Because hollow trees are collapsing rapidly in regrowth forest, stands of old-growth are the most important remaining areas of habitat for the possum. There is only 25 ha of old-growth forests left in the entire block and this is scattered in almost 40 separate patches. Computer simulation models devised by Hugh Possingham and Robert Lacy predict that the remaining possum populations in this area are not viable and face almost certain extinction in the next 20 years. This pessimistic forecast is the result of limited areas of old growth and the small and highly fragmented nature of the remnant habitat patches.
Other forest blocks in the Central Highlands support more old growth forest. For example, the Steavenson Block near Marysville contains about 250 ha, about 5% of its total area of 5000 ha. This is small but is more than almost any other forest block where timber harvesting is currently permitted.
Cumulative Effects of Logging Over Several Cutting Rotations
Multiple clearfelling rotations may have cumulative effects on populations for forest wildlife, the full impacts of which will not be apparent for a long time. For example, some populations may persist after the first cutting because they have been able to recolonise logged areas from refuges in neighbouring uncut patches. This will not occur after several successive opertions in nearby areas. Indeed, if key habitat refuges are destroyed or become highly fragmented or isolated, survival would be impossible as there would be no further sources of animals to recolonise logged and regenerated areas. Therefore, the response of populations to a second logging rotation may be very different from that to the first. This is particularly true where forests that were cut in the past using low-intensity selective harvesting are subsequently logged using methods like clearfelling. This is the case in montane ash forests where clearfelling operations have been employed for only the past 30 years.
Unsuitability of Some Unlogged Parts of the Forest
Although some areas of montane ash forest are not logged, they may be of limited benefit to Leadbeater's Possum. Forest on steep terrain is retained but we know already that the possum is significantly less abundant in such locations. The reasons for this are unclear but may be related to the smaller numbers of hollow trees on steep slopes. Further, the vegetation strips retained along watercourses to maintain water quality are often dominated by cool temperate rainforests comprised of trees such as Myrtle Beech and are usually not suitable for Leadbeater's Possum.
Even is half a forest block is reserved, there may still be significant negative effects of timber harvesting on Leadbeater's Possum, because those areas most suitable for animals are also the ones most liekly to be logged. Our climatic analysis of the possum's distribution shows that sites where the species is found are characterised by warm, wet conditions. These are the most productive areas and are often targeted for logging. Forest animals seem to have found such highly productive pockets of forest millions of years before forest managers identified them as best places to harvest and regrow trees. Given this, a simple estimate of the amount of unlogged forest in an area may be a very misleading reflection of the quality and quantity of remaining habitat for a given species and a poor indicator of the animal's chances of survival in response to logging.
Differences Between Wildfire and Clearfelling
Most eucalypts are highly resistant to fire and badly burnt trees often sprout new growth, but montane ash trees differ from most other species as they are more susceptible to destruction by wildfire. Millions of trees were killed in the 1939 wildfires but seedlings germinated underneath the burnt forest and created new stands of regrowth in which most trees were of uniform age, now 55 years old. For this reason, it has been argued that clearfelling mimics wildfires, because all the trees are cut down and replaced by a new stand of young trees of the one age. Moreover, this arguement has been used to justify clearfelling as an 'ecologically-sensitive' way to log forests.
There are actually major differences between the effects of clearfelling and wildfires, many of which are related to the fact that clearfelling removes all trees from a site but the intensity of wildfires is highly variable and some areas of forests, such as those in wet gullies, may escape destruction. These differences were highlighted by Dr David Ashton in his maps of ash forest on the slopes of Mt Donna Buang.
In a wildfire, some trees are only partially burnt and subsequently survive. This is common in old growth forests where trees have thick bark and exhibit some fire resistance. Our survey of large living trees in montane ash forests showed that many were fire-scarred, indicating that although thay had been burnt, they survived past fires. In another study, an examination of the growth rings of two very old Mountain Ash trees showed that they had survived seven fires in the past 500 years.
The structure of some fire-damaged stands of montane ash forests can be very complex. Such stands are usually comprised of trees of several different ages, and the older trees typically contain hollows. Such structural complexity is important for Leadbeater's Possum and many other forest animals because they area dependent on hollow-bearing trees as a key part of their habitat requirements. It is important to understand that clearfelling does not create potential nest-sites in old trees or bring about the structural complexity needed by Leadbeater's Possum.
Conclusion
Our studies of the habitat and nest-tree requirements of Leadbeater's Possum indicate that clearfelling operations as they are currently practised will have a serious detrimental impact on populations of the species. In fact, the possum may be totally eliminated from timber production forests under current management regimes. This means there is a major conflict between the twin goals of continued timber and woodpulp production in montane ash forests and the long-term conservation of Leadbeater's Possum."