Abstract:
The dieback of Agathis australis (D. Don) Lindl. in Loudon (kauri) across large parts of its range has been attributed to a soil borne pathogen first observed in the 1970s. While the evidence is often clear that the phytopathogen Phytophthora taxon Agathis (PTA) is resulting in kauri mortality, there is considerable uncertainty as to how PTA interacts with and affects its host and environment. The purpose of this study was to explore the dynamics of PTA in affected stands of regenerating kauri forest. Studies were based at two field sites; Huia and Piha, in the Waitakere Ranges, west of Auckland. The health status of individual kauri was evaluated and assessed at both field sites, to inform further sampling methods and to evaluate the diagnostic tools utilised in identifying PTA disease in kauri. In addition, all individual kauri were mapped within an area of approximately one km2 at Huia; these data were used to examine disease epidemiology of PTA within a stand of kauri. The use of a novel disease diagnostic tool, dendropathology, was explored and assessed to determine whether PTA symptoms are expressed in the kauri growth- ring pattern during illness and prior to death. Eight tree cores were collected from dead, symptomatic and asymptomatic kauri and were analysed using dendrochronological techniques. It was determined that there were inconsistencies between the diagnostic methods, and to determine if a kauri was affected by PTA, the diagnostic methods would need to be used complimentarily. Diseased and symptomatic kauri were found to be locally clustered at Huia, and the clustering was a result of interaction between trees, suggesting local modes of spread, such as root to root transmission of the disease. No clear signal from of PTA disease was seen in the growth rings of kauri. Ring anomalies, possibly attributed to site factors or the youth of kauri, prevented successful cross-matching and cross-dating. Understanding how the phytopathogen Phytophthora taxon Agathis is affecting Agathis australis is essential to making appropriate decisions so that its effects on kauri and associated ecosystems can be mitigated.