Exploring the impacts of Phytophthora agathidicida on soil bacterial communities associated with kauri (Agathis australis)

Abstract

Kauri trees (Agathis australis) are iconic giants in Aotearoa, New Zealand, renowned for their immense size and longevity. They are endemic to Aotearoa and hold immense ecological, cultural, and social significance as a keystone species. Kauri were once prolific across the North Island, but extensive logging by early European settlers drastically reduced their mature forested areas to a mere 7500 hectares. This alarming decline spurred efforts to safeguard these precious species. In contemporary times, kauri face a new threat, Phytophthora agathidicida, the causal agent of kauri dieback. This soil-borne oomycete pathogen causes yellowing of leaves, canopy dieback, and gum lesions on the base of the tree trunk, eventually leading to tree death. Infection of kauri by P. agathidicida occurs through colonisation of the host’s roots and vascular tissue, leading to changes in the flow of nutrients and water throughout the tree and within the surrounding soil. Research efforts have unveiled the consequences of kauri dieback disease on kauri health, many of which have broader ecological implications for forest health. However, few studies have considered how P. agathidicida influences the soil microbial communities that maintain forest ecosystem and plant health, e.g., through carbon and nutrient cycling and pathogen protection. It is essential to consider the implications that P. agathidicida has on the soil bacterial communities beneath infected kauri, as this could directly impact kauri health, as well as kauri forest ecosystem health and functioning. My study used 16S rRNA gene sequencing to examine the impact of P. agathidicida on the soil bacteria of ~700 kauri trees in the Waitākere Ranges, an area severely affected by dieback disease. I found that healthy and infected kauri trees possess significantly different core soil bacterial communities that are deterministically selected by the environment of infected trees. The roles of environmental, climatic and soil physicochemical variables on soil bacterial composition were explored to determine which caused or are related to the differences in soil bacterial communities I observed between healthy and infected trees. Soil pH was the most important predictor of soil bacterial structure and composition, with infection being more common in soils with higher pH. I employed random forest models to determine whether soil bacterial communities could be harnessed as bioindicators for P. agathidicida presence. Key environmental attributes such as soil pH could be predicted from the analysis of the bacterial community data, and P. agathidicida could be predicted also, though with less accuracy. Future investigations should explore fungal or metagenomic data to enhance predictive models, assisting in detecting P. agathidicida in the environment and related biological interactions. My thesis greatly advances our understanding of the broader ecological impacts of P. agathidicida infection on soil bacterial communities and contributes to efforts to conserve these vital kauri forests.