Tree Decline in the Absence of Fire

This project represented the first national study of tree decline caused by reduced frequency or absence of fire. In New South Wales, Victoria, Western Australia and Tasmania there are large tracts of native moist-to-dry forest in advanced stages of decline. In some instances there is strong evidence linking this decline to change in fire management, from historically frequent low intensity burns to a current management of fire exclusion.

Suitable study sites were identified in NSW, WA and Tasmania where the same forest type occurs as long unburnt stands adjacent to sites where there are good records of frequent burning.

The research was led by Dr Neil Davidson from the University of Tasmania in partnership with State Forests NSW, University of NSW, Conservation and Land Management WA, Murdoch University WA, Tasmanian Fire Service, Forestry Tasmania, Department of Tourism, Parks, Heritage and the Arts Tasmania, and University of Melbourne.

This project hypothesised that an absence of fire leads to the increased development of woody mid-storey that out-competes eucalypts for soil water and/or alters tree nutrient availability. 

The project took a ‘space-for-time’ approach of paired sites with a history of either being long unburnt or relatively frequently burnt. The sites were established in E. gomphocephala woodland in Western Australia and in E. delegatensis forest in Tasmania.

It found that in the long unburnt sites in both WA and Tasmania there was a greater percentage cover of midstorey (tall shrub layer) and that eucalypts were exposed to greater water stress (measured as higher water-use efficiency). Also, in WA eucalypts of long unburnt sites were deficient in foliar copper (Cu), and in Tasmania eucalypts of long unburnt sites had lower levels of foliar phosphorus (P).

Using an index of tree health with crown cover as the measure, the project showed that tree decline was correlated with increased midstorey shrub cover, and water stress and nutrient deficiency in eucalypt trees.

It concluded that the midstorey that develops in the long absence of fire alters ecological processes, leading to premature eucalypt decline. A decision support tool for fire managers was developed.

The Manager of Fire Management at Forestry Tasmania, Tony Blanks, stressed the importance of this work. “This is a project is of national significance.  The research report will be of value to all managers of natural and long rotation eucalypt forests.

 “The possibility of the premature decline and loss of the overstorey eucalypts from even a proportion of the southern Australian forests is of major concern aesthetically and ecologically, but also for the effect this process will have on carbon sequestration.

“It is important that forest managers understand the processes which affect the longevity of eucalypts, as a step in finding ways to protect the massive quantities of carbon stored in the stems of our eucalypt forests.

Bryony Horton’s PhD study at of the University of Tasmania explored the links between ectomycorrhizal fungi, fire and eucalypt dieback in forests with different understorey vegetation.

Fire managers now have a decision support tool to aid in their management of the forest’s health.