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Issues ConditionThreatening Processes |
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At a glance 
The issue Tree decline is defined as the premature death of forest and woodland trees. In Tasmania, tree decline particularly affects eucalypts on drier areas of agricultural land. There appears to be no single cause of tree decline. Many stresses have caused the slow attrition of eucalypts from the dry rural landscapes over the last few decades. Tree decline mainly affects white gum (Eucalyptus viminalis), however, it also affects black peppermint (E. amygdalina), black gum (E. ovata), swamp peppermint (E. rodwayi); mountain gum (E. dalrympleana), silver peppermint (E. tenuiramis), cabbage gum (E. pauciflora), blue gum (E. globulus), candlebark (E. rubida) and white-top stringybark (E. delegatensis) (Neyland 1996). While there is new recognition of the importance of moisture stress in rural tree decline, many other factors are also implicated: frost, vegetation clearance, browsing animals (e.g. possums), soil compaction, insect attack, soil-borne diseases, reduced soil fungi (e.g. ectomycorrhizal fungi), loss of understorey, lack of seedling regeneration, old age (i.e. mature trees dying and no young trees establishing underneath), and agricultural practises such as grazing, clearing, tilling and the addition of fertilisers. This 'At a glance' section provides an overview of rural tree decline. More detailed information and references are available in the Rural Tree Decline Issue Report. The report contains three indicators: Distribution and Severity of Tree Decline, Daily and Extreme Rainfall, and Possum Numbers. A recommendation outlining possible responses to the problem of Rural Tree Decline is also provided. Favourable news - More is now known about rural tree decline since the previous SoE Report (1997) because of research at the University of Tasmania and the Cooperative Research Centre for Sustainable Production Forestry. This research indicates that tree decline occurs in response to a complex interaction of factors associated with climate and rural land use. These factors include nutrient balance, competition with introduced pasture species, pest/predator balance, fire regimes, moisture stress, and tree age (Close & Davidson 2001). Views differ on the most important single cause, although both moisture stress and tree age are recognised as key factors (Rees 2000; Close & Davidson 2001).
- Recognition of the problems caused by dieback has led to a significant amount of work being done by landowners and managers to re-establish trees in affected areas. National programs such as Landcare and One Billion Trees have supported this by providing resources for planting trees, and fencing to prevent grazing pressure on seedlings.
- Maintaining the remaining vegetation cover is a key response to managing rural tree decline. The Private Forests Reserve Program, the Protected Areas on Private Land Program, and the North Facing Slopes Project under the Natural Heritage Trust have all provided an effective response through the protection of remnant vegetation in areas susceptible to rural tree decline.
- It has been reported that farmers have found that the property-based game management program run by the DPIWE is an effective way of managing possum numbers (Bushcare 2001).
Unfavourable news - Tree decline is the most extensive form of land degradation in Tasmania, estimated to be affecting 42% (861,000 ha) of private land. It is prevalent throughout the low rainfall districts (below 900 mm per year), with the worst areas being in the Midlands and the upper Derwent Valley (Grice 1995).
- The record of success in revegetation in the dry rural areas of Tasmania has been low.
- There is concern that tree decline may threaten sustainable agriculture and lead to a reduction in the profitability of agricultural production. Tree decline, and the resultant land degradation, is also impacting on biodiversity values, due to habitat loss. Tree decline is also reducing the overall quality of the State's landscape along an important tourist corridor.
Uncertain news - The information provided in this SoE Report summarises only one part of the 'tree-decline' problem. A more complete assessment would also need to consider the current state of knowledge of other forms of tree decline attributed to factors such as pests and diseases.
- There has been no additional Statewide information gathered on the areas affected by tree decline since the previous SoE Report (SDAC 1997).
- The only systematic Statewide survey of tree decline was conducted in 1992. The survey was part of the Statewide soil and land degradation assessment coordinated by Grice (1995). These assessments were conducted on private land only.
- There remains uncertainty about the relative significance of the factors implicated in rural tree decline. On healthy trees, for example, possums (Trichosurus vulpecular) may have little impact, but on trees that are severely drought-stressed, browsing by possums can be fatal. Possum guards in combination with coppicing (a type of pruning) are effective in improving tree health. However, possum guards alone produced only a marginal improvement in tree health on trees showing evidence of decline. This improvement was not statistically significant (Rees 2000).
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