There is a long debate about whether logging tall wet Eucalypt forests increases or decreases the flammability of forests. On an intuitive level, it makes sense that allowing forests to become older will make them less flammable: over time the understorey thins out, the canopy closes in and creates a moister micro climate, and fire is less likely to climb up into the crown. In contrast, an area that has been logged will be filled with dense regrowth of highly flammable saplings and be exposed to the drying effect of sun and wind.

This is confirmed on a regular basis by research. New work considers the two common models which are used to describe how fire risk changes over time as the forest grows. The models are the ‘moisture model’, where fire risk initially increases, then decreases, as a stand develops after a fire, and the ‘Olson model’, where fire risk increases as a function of time since previous fire.

This new report – called Fire risk and severity decline with stand development in Tasmanian giant Eucalyptus forest – suggests that the ‘moisture model’ is correct in tall wet forests, and that over time fire risk is reduced.

Key findings are:

  • Fire risk in Tasmanian tall wet Eucalyptus forest decreases as they mature.
  • Older stands of forest have a moister understorey and less dense ladder fuels, which can carry a fire into the crown.
  • Extreme fires in older forests are unlikely unless the moist understorey ignites.
  • Tasmanian tall wet Eucalyptus forests are vulnerable to a ‘landscape trap’ effect. This refers to a situation where more regular fires ‘traps’ the forest into a more flammable form – that is it remains dominated by regrowth for a long period of time rather than moving to the less flammable old growth stage.

These findings align with earlier work and highlight various dilemmas for land managers:

  • We need to protect remaining old growth forests
  • We need to exclude fire from burnt forests as they recover

The authors are James M Furlaud, Lynda D Prior, Grant J Williamson and David MJS Bowman.

A summary of the report can be found here.

The report will be published in the journal Forest Ecology and Management.

HEADER IMAGE: Jan Corigliano.