Potential Effects of an Emerging Disease on Tree Communities Around a Susceptible Foundation Tree Species

Disease caused by plant pathogens can shape plant community structure and composition, especially if the host species plays important ecological roles. The New Zealand kauri, Agathis australis, is a foundation species that modifies its soil environment and, by doing so, it acts as an ecological filter selecting for a distinctive suite of plant species that co-occur preferentially with it. Kauri is threatened by kauri dieback disease caused by Phytophthora agathidicida. Despite kauri's ecological importance in structuring plant communities, little is known about the consequences of kauri dieback on the plant species ecologically associated with kauri—kauri associates. This study examined (1) the environmental drivers of diseased kauri occurrences, (2) the correlations between the occurrences of kauri associates and environmental factors and (3) which kauri associates frequently co-occurred with diseased kauri as a result of shared environmental responses. A joint species distribution model with nine environmental factors was used to analyse the occurrences of diseased kauri and 16 kauri-associated plant species located within a 10-m radius of 761 kauri trees in lowland kauri-podocarp-broadleaved forests of Waitākere Ranges, Auckland, New Zealand. Our results showed that diseased kauri occurred more frequently in sites at lower elevation, with shallower organic soil layers and a history of logging or timber processing disturbance. Additionally, successional strategies influenced the relationship between kauri associate occurrences and environmental factors, with kauri associates characteristic of early succession frequently co-occurring with diseased kauri due to environmental factors. This suggests that kauri dieback could either cause the retrogression of succession in kauri forests or that kauri in younger stands were more likely to be diseased. Both of these possibilities indicate that kauri dieback would likely change the successional trajectory of kauri forests. Future studies should integrate longitudinal demographic data with disease progression to investigate the mechanistic effects of disease on kauri community dynamics.