Physicochemical Characterisation of Soil from Dieback Kauri Forest

Abstract

Kauri (Agathis australis) is an ecologically and culturally significant conifer, endemic to the northern North Island of Aotearoa New Zealand. Kauri trees are designated as foundation species due to their substantial effect on the configuration of distinct surrounding plant communities and significant impact on overall species diversity in kauri forests. Kauri greatly influence the soil conditions underneath their crowns by lowering soil pH, impeding nitrogen cycle, and occasionally creating podzols. In kauri forests, soil has significant influence on the health of the ecosystem by anchoring roots, storing water and nutrients, and providing a habitat for a variety of invertebrates and microorganisms. However, these ancient and iconic kauri trees are threatened with extinction as a result of dieback disease caused by the highly pathogenic and intrusive oomycetes, Phytophthora agathidicida. This novel soil and waterborne pathogen has the ability to disturb above and below ground species composition and biogeochemical processes.

This study utilised soil samples from kauri forests located in the Waitākere Ranges Regional Park in Auckland, and Tairua and Whangapoua in Coromandel, to investigate physical and chemical characteristics of soil from beneath healthy kauri trees as well as kauri trees that display dieback disease symptoms. These soil characteristics were compared using two-sample unpaired t-tests and Wilcoxon tests to investigate whether there are differences in the physicochemical characteristics of kauri soil within and between healthy and unhealthy trees.

In both the Auckland and Coromandel regions, unhealthy kauri soil demonstrated significantly higher moisture factor (p = 0.0481 in Waitākere and p = 0.00239 in Coromandel) and water holding capacity (p = 0.0224, p = 0.0347) than healthy soil, which may indicate that wetter soil provides more favourable conditions for growth and proliferation of Phytophthora agathidicida. Moreover, unhealthy kauri soil in both regions also exhibited significantly higher total carbon (p = 0.000559, p = 0.0235), total nitrogen (p = 4.621×10-5, p = 0.0318), and total hydrogen (p = 0.00953, p = 0.0265) in comparison to healthy kauri soil. The root cause of the elevated content of these elements is likely to be thinning of kauri canopies and defoliation, both symptoms of dieback which add to the acidic, tannin-rich litter. The association with litter was confirmed by using Kendall rank coefficient test to find significant negative correlations between pH and total carbon (p = 2.58×10-5) as well as total nitrogen (p = 0.00762) in unhealthy kauri soil. Hence, these elements may be useful indicators of the presence of dieback, and thus could help map the spread of the disease.

The findings of this study contribute towards presenting more comprehensive insight into the link between soil physicochemical characteristics and kauri dieback in the Auckland and Coromandel regions. The current study also contributes to the ongoing research into the detection and prevention of dieback disease, and ultimately in the preservation of the remaining kauri stands in Aotearoa New Zealand.