Evaluating the effects of two newly emerging plant pathogens on northern Aotearoa-New Zealand forests using an individual-based model

Globally, forest ecosystems face many threats to their resilience. During the last 20 years, two plant pathogens have become of widespread concern in Aotearoa-New Zealand (NZ): kauri dieback (a soil-borne disease caused by Phytophthora agathidicida and affecting Agathis australis, a large and long-lived gymnosperm) and myrtle rust (a wind-borne disease caused by Austropuccinia psidii and affecting members of the Myrtaceae). The long-term consequences of these pathogens are unclear, but they could drive significant changes in forest composition and ecosystem function. Here, we use an individual-based forest model for northern NZ, enabling us to explore some of these stressors. The model has previously been used to explore the dynamics of northern forests in NZ. To examine how myrtle rust and kauri dieback might affect forest dynamics under various scenarios we refined the model by (i) representing additional species, (ii) including an underlying edaphic gradient in addition to competition for light, (iii) simulating the effects of the plant pathogens kauri dieback and myrtle rust and (iv) porting the model from NetLogo 6 to Julia. Our simulation experiments suggest that myrtle rust may hasten the decline of two early successional species that we evaluated but has less effect on carbon storage. On the other hand, kauri dieback may lead to the stand-level loss of all Agathis australis and a median decline in aboveground live carbon storage of up to 55 % compared to undiseased stands after 500 years. The model experiments do not identify any interactive effects between the two pathogens. As with other efforts to model NZ's forest ecosystems, the model struggles to capture the regeneration dynamics of very long-lived species. Regeneration dynamics and evaluating a broader pool of the tree species common in the forests of northern NZ are where we will focus on future model development.