Articles in The West Australian newspaper on 15th and 17th September 2007 suggested that global warming will lead to the virtual disappearance of Western Australia’s iconic karri forest. The articles quote Dr Ray Wills, a research scientist at the University of Western Australia’s Geography Department, who asserts that karri forests could be reduced to small pockets and marginal remnants in the years to come. He bases this view on projections that the southwest of Western Australia (WA) will become warmer by 2 to 3 degrees in the years ahead, and on the assumption that this warming will in turn lead to a decline in rainfall to the extent that karri will basically die out.
Karri forests are part of the so-called “southern forests” of Australia’s southwest corner. They comprise about 1.3 million hectares of pure karri and karri mixed with jarrah, marri and red and yellow tingle. Apart from several outliers, such as at Boranup (near Margaret River) and Porongorup (east of Mt Barker), all of the present karri forest is found in areas with a long-term annual rainfall of >1100 mm.
However, the present karri forest is also a remnant. Analysis of pollen in geological strata has demonstrated that karri once occupied a very much wider area; indeed it is still possible to find typical karri forest understorey in moist gullies in the northern jarrah forest. The shrinkage of the karri forest appears to have resulted mainly from a decline in rainfall many thousands of years ago.
Karri is well able to survive much higher temperatures than those predicted. The species is adapted to a present-day climate which every summer experiences well above the average temperature, including days over 40 degrees. I have successfully grown karri in Perth and the Darling Ranges, regions with much warmer average temperatures than the lower southwest, and I even succeeded in establishing karri in my arboretum in the Avon Valley where the temperature exceeds 40 degrees day after day from January through to March. Karri was unaffected by these high temperatures. What killed them was winter frosts not summer heat. A feature of the current natural distribution of karri is that frost is very rare and when it does occur it is relatively mild and short-lived.
I believe that a predicted rise in average annual temperature of 2-3 degrees per se will not worry karri, especially if this occurs as a result of milder winters rather than hotter summers.
The problem of lower rainfall is another matter, and already forests all over the southwest of WA (especially wandoo and tuart) are observed declining in the face of below-average rainfall in recent years. The karri forest has also experienced a similar reduction in rainfall, but is not yet showing the same drought symptoms as wandoo and tuart. If there is another substantial decline in the current rainfall pattern, it probably will, unless some action is taken by forest managers.
Luckily something can be done to ameliorate the impact on the karri forest of lower rainfall. This is a well-planned and professionally conducted program of thinning of overstocked regrowth forests plus regular (7-9 year rotation) mild prescribed burning across the whole forest area. Such a program will lead to a higher proportion of rainfall getting through to recharge soil moisture, and will ensure less competition for water at the root zone. Prescribed burning will also reduce bushfire fuels and render old growth forests less susceptible to conversion to dense rainfall-gulping regrowth by high intensity summer fires.
Opponents of thinning and prescribed burning will immediately rise up and condemn this strategy, claiming that it will cause “a loss of biodiversity”. There is no scientific basis for this fear. But if no action is taken and Dr Wills’ doomsday predictions are correct, the biodiversity is going down the tube anyway. Even a 60% reduction in greenhouse gas emissions will not stop the 2-3 degree killer temperature rise according to Dr Wills.
It is my understanding that the jury is still out on the link between a projected higher temperature due to global warming and a projected lower rainfall. Never mind. Even if “normal” rainfall patterns return to south-western WA, the forests will be healthier and more biologically diverse if overstocked regrowth stands have been thinned and mild burning undertaken to reduce fuels and thus minimise high intensity wildfires. And if the predictions of Dr Wills and his colleagues are right, well-managed forests will be better able to cope if a still-drier climate eventuates. The other good thing is that both thinning and burning are standard forestry operations which have been conducted for generations and subject to a great deal of research and monitoring. We know how to do it and that it will work, with no environmental downside.
Incidentally, Dr Wills is by not the first distinguished scientist to predict the extinction of Australia’s southwest forests. In the 1970s geography Professor Arthur Connacher predicted that logging for woodchip-quality logs would result in the “desertification” of the karri forest. Thankfully this has not occurred. And in the 1980s ecologist Dr Wardell-Johnson warned of the imminent loss of the tingle forests on the south coast due to “continental drift”. Australia was at that time thought to be drifting towards the equator at a rate of a few millimetres per century. It has also been too early to detect any evidence of this calamity.
Roger Underwood worked as a forester in the karri forest in the 1960s and 1970s.