Ecosystem dynamics altered by pathogen-mediated changes following invasion of Banksia woodland and Eucalyptus marginata forest biomes of south-western Australia by Phytophthora cinnamomi
Shearer. B. L Crane. C. E Fairman. R. G Dunne. C. P
Changes in plant species richness and composition, vegetation structure, ecosystem functioning and soil nutrients following invasion of Banksia woodland and Eucalyptus marginata forest biomes by the introduced soilborne multihost plant pathogen Phytophthora cinnamom were determined using space-for-time substitution of diseased and adjoining healthy areas. In most study areas, canopy closure, basal area and number of plant species were significantly lower in old diseased compared with adjoining healthy areas, with diseased front intermediate between the two. In half of the study areas, percentage ground cover and total plant species cover were significantly lower in old diseased compared with adjoining healthy areas, with diseased front intermediate between the two. Evenness, Shannon-Weiner H and Simpson’s D did not significantly change between disease status for the majority of study areas. For ordination of percentage canopy closure and ground cover there was separation of study areas along a disease status gradient and a weak soil fertility gradient. There was significantly less percentage organic carbon, extractable phosphorus and extractable and total potassium in old diseased areas compared with adjoining healthy areas for one-quarter to a third of the study areas. Total phosphorus changed significantly between disease status, but this was due to higher levels in diseased front compared with the old diseased or adjoining healthy areas. For all study areas there was no significant effect of disease status on percentage total nitrogen and pH. The cover of a majority of plant species did not change significantly between disease status with 16% of the total number of perennial species in healthy areas significantly decreasing and 10% significantly increasing in cover in old diseased compared with adjoining healthy areas. As with significant differences in cover between disease status, change in cover of a majority of plant species was not significantly correlated with canopy closure. Cover of 20% of the total number of perennial species in healthy areas was significantly linearly positively correlated with canopy closure and the cover of 10% of plant species was significantly negatively correlated with canopy closure. These species were herbs and shrubs from a range of families with a mixture of functional properties such as Phytophthoria cinnamomi susceptibility, response to fire, rooting type and mycorrhizal association. The paper concludes with a conceptual analysis of core issues and those that cascade out of core issues involved in pathogen and patch plant community interactions.
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