The dieback cycle in Victorian forests: a 30-year study of changes caused by Phytophthora cinnamomi in Victorian open forests, woodlands and heathlands

Abstract

Changes in both vegetation and population and distribution of Phytophthora cinnamomi were monitored periodically between 1970 and 2000 on defined, infested quadrats and on similar pathogen-free quadrats on 13 sites representing major types of open eucalypt forest, woodland and heathland. The susceptible eucalypts in the overstorey of infested sites showed severe dieback, loss of crown or deaths. All trees died on some sites, trees at other sites presented dead leaders with epicormic growth on lower branches. Dieback of the heathy understorey, followed by death, occurred in 50–75% of the species, including the dominant Xanthorrhoea australis, thereby changing the community structure and the species composition. Species richness in infested quadrats declined, and percentage cover and percentage contribution to the community by susceptible species were almost eliminated. The ground remained bare on steep slopes, but on other sites the susceptible flora was replaced by fieId-resistant species of sedges and rushes, and by partly resistant teatrees (Leptospermum spp.) that formed a dense cover. The pathogen was isolated from 100% of the root samples from infested quadrats from 1970 to 1984, but isolation frequency then gradually declined. In 2000, P. cinnamomi was rare on six sites and not isolated from four. Regeneration of 30–40 susceptible species, previously eliminated from the quadrats, was recorded from infested quadrats and two thirds of these were growing on more than one quadrat. Substantial regeneration of the previously dominant but highly susceptible X. australis occurred at four sites. Crown recovery of the trees was not observed. It is not yet clear whether the regeneration of the understorey is stable, or whether successive cycles of disease and recovery will occur.