The bushfire which took place just under a month ago close to Prevelly, in the Wallcliffe sector of Margaret River, is a reminder that Christmas usually means the start of the 'bushfire season' in Australia. Only 5 ha of vines were actually singed in this case, but smoke taint is, as usual, as significant a threat. The chemical markers for this are guaiacol and 4-mythylguaiacol.

Interestingly, these chemicals are naturally produced in small quantities by some grape varieties (including Cabernet Sauvignon and Shiraz), and exposure to toasted oak can also up their levels in wine. You don’t need thick smoke to taint a crop, and a major frustration is that it’s hard to gauge levels of contamination from grapes alone; borderline taint can sometimes only become apparent after fermentation. Nor, alas, can smoke taint be ‘washed off’ by rain or water sprays in the vineyard. It’s more of a problem for red wines (where skin maceration is necessary) than for whites, though the fact that taint can also be transferred to fruit from the leaves themselves means that no fruit is inviolate.

Ironically, this particular fire was deliberately started – by the authorities themselves. Back burning or controlled burning is regularly used to lighten the fuel load prior to the bushfire season, and this particular fire ‘got away’ from the Leeuwin-Naturaliste National Park further south due to unexpectedly strong winds. Anyone growing vines in Australia, though, knows that fire will, sooner or later, be a threat.

This is nothing new. The study of sediment cores from Lake George and elsewhere has shown that fire due to natural ignition became a major element in Australia’s ecosystem over the last 100,000 years, and the arrival of humans in Australia between 60,000 and 40,000 intensified the fire record. James Cook, spying the plumes of smoke by day and the glimmering fires by night from the deck of the Endeavour, called Australia ‘the continent of smoke’. All of the early explorers of the continent describe the extensive use by indigenous Australians of small-scale fires.

This ‘fire-stick farming’ (a phrase coined in 1969 by the Welsh-Australian prehistorian Rhys Jones) had two principal purposes: to optimise feeding conditions for the small marsupial mammals on which humans in turn fed, and to avoid the build-up of dense, fuel-rich vegetative masses which could lead to catastrophic, large-scale fires. (It was also useful for signalling, travel and killing vermin, and sometimes a hunting aid, too.) Many of those smaller marsupials are now extinct, though, and ‘the bush’ is often thicker, more impenetrable and more fuel-rich than it was at the time of European colonisation.

Australia weather systems cross the country from west to east. During summer, slow-moving high-pressure systems are often followed by cold fronts. These conditions allow hot winds from the interior to roar southwards, through the gap between the two weather cells. Desiccating heat spikes and colossal bush-fire risks in wine-growing regions of South Australia, Victoria and New South Wales are the result.

These conditions indirectly or directly led to both the Ash Wednesday fires of 16th February 1983 (when there were 180 separate fires burning across South Australia and Victoria) and the Black Saturday fires of February 7th 2009 (which killed 173 people and burned over a million acres in Victoria). There have, though, been many others: the fires in North East Victoria in the 2006-2007 season caused AUD300 million’s worth of damage in lost grapes and wine.

Nor is Australia the only vulnerable location. The California wildfires of 2008 (2,780 individual fires taking hold in June and July, mostly started by lightning on tinder-dry forest on June 20th) illustrated California’s periodic vulnerability: Anderson Valley Pinot was badly smoke-affected. Fires, too, burn around the Mediterranean in most summers – as they did near our house in Languedoc at the end of August 2009, destroying vineyards I regularly cycle past.

The relationship between fire and vines, in sum, is an unhappy one. We seem destined to become more familiar with it as rising carbon levels lift temperatures, and create more extreme weather events, in the future.

Written by Andrew Jefford