Ash to ashes – what could the 2013 fires mean for the future of our forests?
In the high country of Victoria, firefighters are presently battling a large bushfire that is moving through the forests south of Harrietville and past the second highest mountain in Victoria, Mt. Feathertop.
It has threatened small communities, hikers, come within 50m of the boundary of the Hotham Heights alpine resort, and – most devastating of all – sadly resulted in the loss of two firefighters’ lives.
But aside from social impacts and blackened trunks, what have been some of the ecological costs?
Alpine Ash (Eucalyptus delegatensis) is a tall forest tree species found near Mt. Feathertop and other areas of high rainfall and elevation in Victoria, New South Wales and Tasmania (where a distinct subspecies is present).
Like its lower elevation relative, Mountain Ash (Eucalyptus regnans), Alpine Ash is classified as an “obligate-seeder“. This means in the event of high-intensity fire, the mature individuals of the species are invariably killed. The next generation of trees arise from seeds that fall and germinate – after a period of dormancy – in the nutrient-rich ash that follows the fire.
This is in contrast to a number of many other Eucalypt species which rely less on seed regeneration and more on post-fire resprouting from the trunk and branches.
When Alpine Ash regenerates from seed, it takes roughly 15 years before the trees are old enough to have their own seeds. This means the population of saplings is somewhat vulnerable to disturbance during that period.
As of mid-February 2013, the alpine fire burning near Harrietville has burned well over 31,000 hectares of the Alpine National Park. According to the forest type information provided by the Department of Sustainability & Environment, at least 11,000 ha of Alpine Ash have been burned so far.
Fire in Australian ecosystems is by no means a new concept, of course – the flora of Australia have been living and evolving with it for at least 60 million years. But it is important to consider not just the extent, but the frequency, intensity and timing of fire – what is known as the “fire regime“.
Over the past 10 years, at least 2.5 million hectares of the mountainous forested areas of Victoria, extending north into NSW, have been burned in bushfires of varying intensity. There are few forests one can visit without seeing a scorched and blackened landscape.
This is particularly significant when considering the 2013 season in Victoria. The area burning in the Harrietville region was burned in the 2003 and (in some places) 2007 bushfires. This means forest burned barely ten years ago has been burned again. Before 2003, it had not been burned since the 1939 Black Friday conflagrations.
The DSE has found over half (55%) of the Alpine Ash burned near Harrietville this year had severe crown scorch or crown burn in 2003, the two highest severity classes of bushfire.
The University of Melbourne looked at the severity of the 2009 wildfires and found that overstorey survival was close to nil for Alpine Ash in those two highest severity classes. For lower severity classes, overstorey survival tends to rely on terrain and forest age class. We can conclude that around 6,000ha of alpine ash burned this season were saplings regenerating from the 2003 fires, and not yet at seed-bearing maturity. Pending post-fire surveys to confirm the above assumptions, this could be the death knell for this forest type in those areas.
There are ecological consequences of fires such as these burning so frequently in tall mountain forests. And the consequences not just restricted to the Alpine Ash forest that we are using as an example here.
Researchers from the Australian National University recently postulated that with an increase in disturbances (they looked at logging) in certain ecosystems, a “landscape trap” could arise. That is, vegetation is converted to a type which perpetuates frequent fire. The forests’ environmental services – such as water, carbon and habitat – are severely compromised or at least heavily modified.
Applied to the example here, we can argue that at least 6,000 ha of Alpine Ash forest is likely to be converted to shrublands or potentially even grasslands, unless the area is actively replanted.
It would be a potentially costly exercise for the managing authorities. But its arguably worth doing as the consequences are far greater than a mere shift in vegetation community.
The role of forests in sequestering carbon has received much attention over the past few years. Highly productive ecosystems such as Alpine Ash are gradually being recognised for the role that they play in storing carbon dioxide.
There is a reasonable degree of variation in how much they’re estimated to store in above-ground carbon – anywhere from 197 to 339 tonnes of carbon per hectare, though much higher has been estimated. Shrublands, in contrast, have been estimated to store significantly less – around 60 tonnes of carbon per hectare.
Therefore, if that 6,000 hectares are converted from highly productive forest to shrublands, we will see a transition from a forest that could cumulatively store anywhere between 4.3 and 7.4 million tonnes of CO2 at maturity to shrublands that would store only a fraction of that (1.3 million tonnes of CO2).
Fire is often portrayed as a devastating force in the landscape. But this situation highlights it is not the mere presence of fire, but the regime in which it operates, that is one of the greatest threatening processes to the integrity of our forests.
Furthermore, this ecological shift is occurring within national parks – a landscape tenure many people would consider safeguarded from such threats. It also raises questions whether, presently and in the future, humans may need to intervene and actively manage certain ecosystems to ensure they retain the values we want them to have.
These are by no means easy questions or propositions, but that does not mean we should shy away from them. This example may seem localised and specific, but in a warmer and drier future, as predicted under climate change scenarios, could very well mean that it could become the parable of our future forests.
Originally Published at http://theconversation.com/ash-to-ashes-what-could-the-2013-fires-mean-for-the-future-of-our-forests-12346