(Un)certainty of timber production

There is a new plan to provide longer term contracts for supplying timber from public native forests in Victoria. Part of this seems to involve moving responsibility for timber management from the Environment Minister to the Agriculture Minister.

Some environment groups seem concerned about these changes. Locking in timber supply on longer-term contracts is viewed by some as irresponsible, especially in the face of climate change; regeneration of the main timber species, mountain ash, might be compromised in hotter and drier climates. However, longer-term contracts need to be viewed from a perspective that considers current timber management in Victoria.

The current approach to timber management in Victoria has been to estimate timber growth and yield in the absence of fires, and then re-assess timber supply and contracts over time, and especially after any major fires. It is an approach that has been criticised for some time. Ignoring unplanned fires would seemingly lead to ever diminishing estimates of the sustainable timber yield.

It is possible that longer-term contracts will force a more strategic assessment of timber yields that explicitly accounts for losses from fire. It is possible to do some simple calculations to determine how wrong estimates of timber yield might be if fires are ignored. Here I show how.

The aim here is to determine what proportion of forest stands reach their rotation age prior to a fire – that is the age of the forest stand at which it is planned to be harvested for timber. For the sake of simple illustration, I will make some simplifying assumptions, but these could be relaxed to include more realism (at the cost of less generality).

The first assumption is that the annual probability of a stand-replacing fire is constant from year to year, regardless of the age of the stand. I’ll call the annual probability of fire p, and 1/p is the average time between fires. In Victorian mountain ash forest, p is approximately 0.01.

Then, the probability that the stand will escape fire for R years, the rotation age of the forest, is (1-p)R. So, the probability that the stand will be burnt prior to the rotation age is 1 – (1-p)R. For R = 80, and p = 0.01, the chance of the stand burning is 0.55; there is worse than a 50% chance that the stand will reach rotation age. Not accounting for timber from salvage harvesting, this simple calculation suggests that approximately half the timber predicted to occur in the absence of fire will not be available in the presence of unplanned fires.

The same calculation can be repeated for different rotation ages and fire probabilities. Ignoring fires will lead to consistent over-estimates of timber availability, even for short rotation ages and low annual fire risks:

Figure: The probability of a forest stand reaching rotation age as a function of the rotation age, and for three different annual probabilities of fire (0.005, 0.01, 0.02), which correspond to average fire intervals of 200, 100 and 50 years.

Hopefully, longer-term contracts will account for these uncertain losses so that greater certainty can indeed be provided to Victoria’s timber industry. But this is also a nice example of how simple models can help focus thoughts on the issues.

About Michael McCarthy

I conduct research on environmental decision making and quantitative ecology. My teaching is mainly at post-grad level at The University of Melbourne.
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