COS 193-4 - CANCELLED - Fungal pathogen inoculum in Ohia forest soils and its relationship to ungulate activity: effective management of Rapid Ohia Death in Hawaii

Abstract: Among forest disturbances, tree pests and pathogens may be among the most impactful, both ecologically and economically. That is particularly so when affected trees are keystone species and resultant diseases lack effective management solutions. ʻŌhiʻa lehua (Metrosideros polymorpha) is Hawai’i’s dominant native tree, accounting for 33% of Hawaii Island’s trees and 50% of the carbon mass stored in her forests. Further, ʻŌhiʻa comprises 80% to >90% of the trees across Hawaiʻi's native forests (i.e., 865,000 ha statewide and 620,000 ha on Hawai'i Island). These forests also overwhelmingly constitute the watersheds which provision Hawaiʻi's communities with abundant, clean drinking water. Today, the health of ʻŌhiʻa forests face a major threat in the form of Rapid ʻŌhiʻa Death (ROD), a lethal disease caused by the recently introduced fungal pathogen, Ceratocystis lukuohia, which threatens ʻŌhiʻa trees, stands, and forests statewide. Since its discovery as ROD’s causal agent in 2014, C. lukuohia has hastened stand level mortality across more than 20% of Hawai’i Island’s ʻŌhiʻa-dominated forests. Prior research indicates that in C. lukuohia infected stands ROD-related mortality averages 10% per year, with some stands exhibiting levels of mortality >95%. However, feasible management actions may exist to protect extant ʻŌhiʻa-dominated forests from C. lukuohia infection and eventual widespread mortality. C. lukuohia is a characterized as a wound pathogen, and fresh wounds are required to allow the pathogen’s ingress into, and infection of, a given tree. Further, feral ungulates have been implicated in the spread of C. lukuohia inoculum and wounding of ʻŌhiʻa trees that allow the fungal pathogen to enter and kill individual trees and entire stands. In forests where feral ungulates are excluded – typically by fencing - C. lukuohia-induced mortality is suppressed when compared to adjacent areas of ungulate activity. Here we present research results revealing mechanisms underlying documented patterns of differential ‘Ōhiʻa stand mortality regarding feral ungulate presence. Evaluating forest soils for the presence and abundance of C. lukuohia inoculum (i.e., viable spores) via culturing and molecular analysis techniques, we found significantly lower amounts of inoculum in soils (e.g., 0 to 4% of samples) where feral ungulates were excluded compared to adjacent forests where ungulates were present and active (e.g., 40 to 86% of samples). Our results provide insight and strong evidence supporting the need to eliminate exposure of such forests to feral ungulate damage in order to suppress the spread and impact of Rapid ‘Ōhiʻa Death across Hawaii's native forests.