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The issues of fire and carbon are at the heart of forest management in Tasmania.
However they are often misunderstood due to the complex nature of eucalypt forests and because forest structure and site productivity is determined as much by local factors as by general rules.
The absence of fire in wetter eucalypt forests can at times foster the development of a rainforest understorey, reducing the fire risk. On the other hand, the absence of fire can lead to build-up of fuels and an increased fire risk.
Harvesting forest products, regeneration burns and fuel-reduction burns may alter subsequent fire frequency and/or intensity and affect CO2 levels in the atmosphere. However, there is little empirical data to support the suggestion that wildfire or harvesting increase fire risk. In North America, regional landscape analysis shows prescribed burns that reduce wildfire risk can also reduce net CO2 emissions from drier forests.
The amount of available data on carbon in forests limits our ability to develop accurate ways to measure and predict changes. More baseline information is required on:
Substantially more work is also needed to identify appropriate carbon models for fire-disturbance ecosystems such as Tasmanian eucalypt forests.
EUCALYPT FORESTS AND FIRE
Fire is the dominant controlling ecological force within eucalypt forests. Eucalypts have evolved with fire over millions of years, developing flammable oils and hanging strips of bark as well as adaptations to survive fire.
Before human presence in Australia, fires were predominantly started by lightning strikes. Contrary to what many believe, not all forests were `old-growth’ before the arrival of Aboriginals and Europeans. Old-growth forests are often seen as static, even though this is at odds with their ecology.
Inadequate levels of fire can cause as significant ecological changes as excessive burning, and in managed native forests have been linked to forest health issues such as dieback and chronic decline of eucalypts, including in Tasmania. The absence of fire in these forests has even been called the true disturbance.
The adaptation of eucalypt forests to fire as a natural disturbance makes for efficient and effective sustainable timber harvesting. Many species can regenerate without replanting, and a complex and biologically diverse and resilient forest ecosystem can develop without further intervention.
When using natural disturbance as a guide to forest management, the greatest challenge is socio-economic rather than ecological, especially for forests adapted to high-severity disturbances such as fire. Many people still see fire as an agent of destruction rather than renewal.
Forest managers need to remind the community regularly of the ecological need for regeneration burning, and that:
FIRE IN MANAGED AND UNMANAGED FORESTS
Depending on the variety, eucalypts respond to fire either by re-sprouting from buds under the bark or underground, or by regenerating as seedlings.
The dominant eucalypts on more productive sites in Tasmania, the ash eucalypts Eucalyptus regnans, E. obliqua and E. delegatensis, are classic reseeders. High-intensity fire clears competing undergrowth and litter, removes the top organic layer of soils, releases soil nutrients, promotes seed fall on to the new “ashbed”, and often reduces populations of browsing animals and ants, all of which allows massive germination of eucalypt seedlings.
Many understorey species in wet forests also regenerate from rootstocks, and so can survive all but hottest fires.
Sustainable forest management requires that both fire management and harvesting are directed at maintaining the range of forest age-classes across the landscape. Older forest can add to the complexity of adjacent regenerating younger forests. It can be more beneficial to retain forest within a harvest boundary than to add an equivalent area to a more distant reserve.
Burns vary in intensity due to different vegetation types, weather conditions, fuel loads and local topography. This results in variability in the vegetation, both naturally after a wildfire as well as after a regeneration burn. Prescribed burns also often have a patchy impact, with areas within burn boundaries often left unburnt. This can have beneficial ecological consequences as in general a diverse landscape is healthy and ecologically resilient. Variability across the landscape can also be achieved through a mix of coupes and unburnt reserves.
On extreme fire-weather days, fires can overwhelm emergency preparation and response and become major human tragedies. There is good data however that, especially under milder conditions, fuel management can reduce fire intensity, alter the run of a fire and its impacts, and on occasion even allow suppression. Studies in Victoria, NSW and WA have concluded that previous prescribed burns can reduce the size and severity of unplanned fires. Others argue that that extreme weather and ignition events are bigger drivers of fire than fuel loads.
Climate change may increase the number of days on which previous prescribed burning will have minimal protective effect against wildfire. A substantial increase in future prescribed burning may be needed to mitigate this.
Some say that there is an increased fire risk for eucalypt forests in the first couple of decades after wildfire or harvesting. This has been linked to denser young canopies, large amounts of leaf litter, residual fuel and a generally drier forest before closed-canopy conditions are re-established. However it is rare to find direct Australian data on fire events that supports these arguments.
Forest regenerating from wildfire generally passes successfully through any period of higher early flammability, and there are extensive stands of older forest across south-eastern Australia. There is no evidence that forest regenerating from harvesting would not do the same.
The public needs to be made more aware of some the features of fire already familiar to forest managers from experience and the supporting science. These include that fire:
These are complex concepts to get across to communities that simply want fires put out.
CARBON
The forest carbon debate is not simple. The issue is not simply the store of carbon in the forest. Forest management activities also affect atmospheric CO2 levels through:
The issues of carbon and fire are also intimately linked. Fire is the major driver of ecological dynamics in eucalypt forests as well as being a major CO2-emitting event in its own right.
Managing solely for old-growth forest would not necessarily give the maximum reduction of atmospheric CO2, as old-growth forests have the lowest CO2 sequestration rate of any forest stage. Old-growth forests also have the largest emissions when they burn, and do not offer the chance to accumulate wood products. It is also impossible to maintain old-growth status indefinitely in a eucalypt ecosystem without fire, as fire is needed for regeneration. Over time, without fire rainforest would dominate with smaller trees and a lower carbon stock.
Only a small proportion of state forest is capable of producing the most carbon-dense forest types, typically Eucalyptus regnans. A whole-of-estate view is therefore needed that captures the range of forest types and ages present, includes typical fire regimes, covers the off-site half-life of wood products and the emissions consequences of their varied uses, and predicts carbon implications forward through time through normal forest dynamics across productive and reserved forest.
In the absence of good site carbon data, modelling is used to estimate carbon stocks from other tree and forest data. This can be problematic. The variability of forests, with their vastly different carbon levels, has lead to incorrect extrapolations in a number of studies.
The MBAC report to Forestry Tasmania is one of the only pieces of work nationally that approaches the proper standard for a landscape and whole-of-life-cycle approach. It combined a number of established general carbon modelling techniques, a scheduling model of Tasmanian state forest, and coupe-based inventory data, and used high-level assumptions to predict that the total carbon stock would increase from 156 million tonnes in 2007 to 187 million tonnes in 2050. This increase is essentially driven by growth of reserved and other native forest that is currently relatively young because of historical fires and timber harvesting. However the MBAC report does not cover the carbon consequences of the substitutional use of wood products for construction or energy generation. Currently there are no precedents or protocols for such an analysis.
Provisional results of a similar study in Victoria indicate that wildfire is the biggest landscape-scale influence on carbon stocks. The results also show that, while harvesting has an impact in limited areas, it has a small overall impact.
MANAGEMENT IMPACTS ON FOREST CARBON
Forests management can have a significant impact on carbon levels, but the impact can vary according to the time-scale of analysis.
For example, a US study shows that management that involves a short-term carbon loss, such as thinning or fuel-reduction burning, can decrease the intensity of a subsequent large fire and lead to smaller carbon emissions over the long-term. Equally, fire-suppressed forests can contain more carbon vulnerable to catastrophic release.
US studies also show that:
Similar arguments for the use of controlled fire have been made for Europe and northern Australia but as yet there is no systematic analysis for southern Australia.
Pest outbreaks can also have a very significant effect on landscape carbon storage as demonstrated by the effects of the mountain pine beetle in Canada where forests have changed from being a carbon sink to a carbon source.
Realistic carbon accounting for long-term sequestration projects will therefore need to be able to deal with losses through fire, windstorms, insect outbreaks, with climate change, and with changing landowner behaviour.
Large amount of carbon are released to the atmosphere by wildfire. It was estimated that 150 million tonnes of carbon were added to the atmosphere in the 2003 and 2006/7 fires in Victoria, with similarly large amounts being released from fire annually in the United States. Changes in carbon stocks in wetter eucalypt forests due to management thus need to be viewed in the context of the occasional, landscape-scale fires that have shaped and will continue to shape Tasmania’s forests.
Conserving and increasing carbon storage in forests may be achieved by establishing plantations on agricultural land, reforesting unstocked land, harvesting for long-lived forest products, using management techniques that promote soil carbon accumulation such as minimising site disturbance, retaining slash, and using prescribed burns either to reduce the risk of more extreme fires or even to produce charcoal that is stored in the soil.
