Eucalypt dieback in eastern Australia: a simple model

Abstract

Summary Eucalypt dieback is widespread throughout Australia and affects an increasing range of species. In addition to salt, a 1993 study listed 13 types of insects, five types of fungi, five kinds of vertebrate animals, four climatic perturbations and a parasitic plant that had been implicated as major agents of dieback in southeastern Australia. Repeated defoliation by insects has usually been identified as a major factor in rural and forest diebacks, while mesic understorey development is often an important feature of forest diebacks. Different mechanisms of initiation and reinforcement have been proposed to account for many different forms of dieback. High rates of folivory leading to both rural and forest diebacks, have been related either to high resource availability and tree vigour or to low resource availability and tree stress. A simple model of eucalypt dieback is proposed to account for both rural and forest dieback, including an increasing range of 'susceptible' species and sites. It associates eucalypt dieback with increased soil moisture and nitrogen status that stresses the roots of established eucalypt trees. These changes affect the physiology of the trees and encourage high rates of folivory and/or fungal pathogenicity. This model can encompass dieback from dryland salinity, 'high-altitude' dieback in Tasmania, 'bellbird' dieback, 'koala' dieback in Victoria and South Australia, phasmatid outbreaks in New South Wales and Victoria, and potentially extends to 'regrowth' dieback in Tasmania. Reduced application of low- intensity fire is a common agent of changed soil conditions. Additional factors that may apply are fertilisation and modifications to runoff and soil drainage. Keywords: diebackfiremodels Eucalyptus Australia