There is a report in today’s Age about the decline of Leadbeater’s possum in the face of fires and timber harvesting. Professor David Lindenmayer of the Australian National University notes that the situation is dire, and that timber harvesting should be ceased in mountain ash forests. It is worth noting here that these forest are the main source of wood for native forestry in Victoria. The forest industry representative, Lisa Marty, says that fire is the problem, not timber harvesting. Who is correct?
Leadbeater’s possum relies on old forest for its survival. While it can persist in recently burnt forest, it only does so where large hollow bearing trees that existed before the fire remain present. However, if these trees are killed by the fire, most will collapse within decades. As a consequence, in a forest that is exposed to unplanned fires, only a fraction of that forest will be high quality habitat – it will be those areas that are currently old, and those that are young but were previously old.
It is possible to do a back-of-the-envelope calculation to determine that proportion. Let’s assume for simplicity that high quality habitat only occurs in forest where the previous fire occurred more than 200 years ago. The average fire interval in these forest is approximately 100 years. If fires occur at any site randomly in time, then the probability that a site will escape a fire and reach 200 years of age will be exp(-200/100), which is approximately 14%.
This means if an area of forest is conserved from timber harvesting, in the long term we would expect that only ~14% of it would be old enough to support high quality habitat. The actual proportion would fluctuate around that number over time, but it is a realistic (albeit rough) assessment of the amount of forest we would expect to occur in an old state at some time in the future.
So, if we set aside 10,000 ha for conservation, we would expect less than 1,500 ha to be suitable. The more that is set aside, the greater the amount we expect to be suitable in the future. Further, a larger conservation reserve also provides a larger buffer against fluctuations caused by irregular large fires.
Therefore, in a world exposed to unplanned fires, any harvesting reduces the expected area of forest that will become old because harvesting reduces the age of trees. In this light, timber harvesting certainly reduces the production of older forest, thereby contributing to declines of old-growth dependent species.
This is basically an issue about how we should plan to conserve biodiversity in the presence of unplanned fires or other random (but expected) events. It is a topic I studied during my PhD, leading to the following paper:
McCarthy, M.A., and Burgman, M.A. (1995) Coping with uncertainty in forest wildlife planning. Forest Ecology and Management 74: 23–36. [Email for PDF]
In fire-prone environments, forest managers should assume that fires will occur, but at a time they cannot predict. Thinking of a fire regime as a stochastic process provides a framework for predicting impacts.