14 July 2021

How have land-use changes affected the growth and survival of Phytophthora agathidicida, the invasive oomycete that causes kauri dieback?

Kauri (Agathis australis), one of New Zealand’s most iconic native trees, is culturally significant to Māori and functions as an ecological foundation species. However, after logging and clearance for agriculture only around 1% of the original kauri forest remains. These remnant, fragmented kauri forests are now threatened by the spread of kauri dieback.

Based in Waipoua and in partnership with Te Roroa Iwi, we set out to understand what part these land-use changes have played.

We found that surrounding land uses of exotic pine (Pinus radiata) and pasture (common pasture species) led to higher growth and sporulation rates of P. agathidicida than in than kauri forests soils, creating potential disease reservoirs. Pine also significantly altered soil microbial community composition, including the loss of microbial taxa linked to plant health. This highlights that historical disturbance of the soil environment surrounding kauri forests may increase their susceptibility to invading plant pathogens.

We also investigated how kauri dieback may cause secondary, cascading impacts that affect long-term forest ecosystem functioning. We looked at the differences between symptomatic and asymptomatic mature kauri in terms of diversity, taxonomic composition and functional genes related to carbon and nitrogen cycling of soil microbial communities.

We found significant differences in the fungal diversity and the fungal and bacterial community composition between asymptomatic and symptomatic kauri. Several microbial taxa known for supressing plant disease, such as Penicillium, Trichoderma, Enterobacteriacae and Pseudomonas, were significantly more abundant in asymptomatic kauri soils.

These results provided a promising direction for the discovery of micro-organisms that suppress kauri dieback. As well, significant differences in the composition and abundance of microbial genes related to carbon and nitrogen cycling highlight the potential long-term impacts of dieback disease on the health and functioning of kauri forests.