Vegetation Changes Associated With Invasion by Phytophthora cinnamomi of Defined Plots in the Brisbane Ranges, Victoria, 1975-1985

Abstract

Changes in plant species composition over a 10 year period were measured by biennial counts of numbers and areas on seven quadrats at each of three sites; one site pathogen free, one in the process of invasion by Phytophthora cinnamomi and one diseased since 1970. Susceptible species died and field-resistant species increased. Partly susceptible species fluctuated in growth. The plant community changed from open forest with sclerophyllous understorey dominated by Xanthorrhoea australis to open forest with large gaps and sedge-dominated ground flora. Tree numbers increased by 25% on the pathogen-free site but decreased by 42.9 and 45.3% on the two infested sites. Susceptible shrub species increased 10% on pathogen-free quadrats but decreased in both numbers and diversity with infestation. The high percentage of bare ground on the old diseased site was gradually colonised by graminoids and legumes. At the end of the 10 year period P. cinnamomi could no longer be isolated from this site, tree crowns showed vigorous growth and seedlings of some susceptible species were observed.

The epidemic caused by P. cinnamomi in the Brisbane Ranges may be finite, with peak death periods in 1979 for the invaded site and in 1972 for the old diseased site. The bare ground was later colonised by field-resistant species and the disease potential of the pathogen declined. Regeneration has commenced on the old diseased site and may eventually become complete for the tree stratum, but incomplete for the understorey because Xanthorrhoea australis, formerly dominant, has a very slow growth rate.