Phytophthora cinnamomi infestation — a 24-year study of vegetation change in forests and woodlands of the Grampians, Western Victoria

Abstract

Changes in vegetation, pathogen population and distribution were monitored periodically in both defined infested quadrats and similar pathogen-free quadrats at six sites representing major types of forest and woodland. Assessments were recorded in May 1977, 1979, 1981, 1983–1984, 1995 and 2000. The susceptible eucalypts in the overstorey of infested sites, such as Eucalyptus obliqua, E. baxteri, E. willisii and E. macroryncha, showed severe dieback, loss of crown or deaths. All the trees died on some sites, others presented dead leaders with epicormic growth on lower branches. Dieback followed by death occurred in 54% of the understorey species, including the dominant Xanthorrhoea australis, thereby changing community structure and species composition. At the time of its greatest prevalence, the pathogen’s activity resulted in a decline in species richness in infested quadrats to a mean of 25.6 species compared with a mean of 39.2 for pathogen-free quadrats. Percentage cover and percentage contribution to the community by susceptible species were negligible. On steep sites, 65% of the ground remained bare, but on other sites the susceptible flora was replaced by field-resistant species of sedges and rushes, such as Lepidosperma semiteres and Hypolaena fastigiata, and by partly resistant tea-trees Leptospermum myrsinoides, L. continentis and L. scoparium. The dense, field-resistant understorey consisted of the ground cover of H. fastigiata, scattered clumps of various sedges and above this a mass of tea-tree scrub, approximately 1 m in height, with moderately severe dieback of the branches.

From 1976 to 1984, the pathogen was isolated from 100% of the 345 root and soil samples and from all of the infested quadrats, but then gradually declined. In 2000, Phytophthora cinnamomi Rands was rare at four sites and was not isolated from two sites. Regeneration of 30 susceptible species, previously eliminated, was recorded from infested sites and 21 of these species were growing in more than one quadrat. Vigorous regeneration of the previously dominant but highly susceptible X. australis occurred at two sites and was similar to that recorded from some recovering infested sites in the Brisbane Ranges, Victoria. The decline of the pathogen and the regeneration of susceptible species may be associated with low spring rainfall from 1995 to 2000 and the consequent reduction in zoospore production, enabling a partial recovery from dieback. The disease cycle from invasion and destruction of a susceptible indigenous flora by this virulent pathogen to the decline of the pathogen and the regeneration of that same susceptible indigenous flora was almost complete on sections of two of the six sites studied. In other areas, the post-infection colonising flora of field-resistant species remained dominant, except at one steep site where the ground remained uncolonised and subsequently eroded following the death of susceptible flora. Extinction following infection by P. cinnamomi, however, remains a grave threat to endangered, endemic species if susceptible.