The evolving threat of rapid 'ohi'a death (rod) to Hawai'i's native ecosystems and rare plant species

Authors: FORTINI, LUCAS - Us Geological Survey (USGS); KAISER, LAUREN - University Of Hawaii; Keith, Lisa; PRICE, JONATHAN - University Of Hawaii; HUGHES, FLINT - US Department Of Agriculture (USDA); JACOBI, JAMES - Us Geological Survey (USGS)

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/14/2019
Publication Date: 9/15/2019
Citation: Fortini, L., Kaiser, L., Keith, L.M., Price, J., Hughes, F., Jacobi, J. 2019. The evolving threat of rapid 'ohi'a death (rod) to Hawai'i's native ecosystems and rare plant species. Forest Ecology and Management. 448:376-385. https://doi.org/10.1016/j.foreco.2019.06.025.
DOI: https://doi.org/10.1016/j.foreco.2019.06.025

Interpretive Summary: Ceratocystis lukuohia is the main fungal pathogen responsible for widespread mortality of rapidly dying 'ohi'a on Hawai'i Island. We projected the potential distribution of C. lukuohia across the main Hawaiian Islands. 122 (52%) of threatened or endangered plant species have more than half of their current range within areas suitable for C. lukuohia. Protecting habitat by fencing reduces incidences of C. lukuohia. Dynamic web portal hosts updated results online at http://rapidohiadeath.com/.

Technical Abstract: Hawai'i’s most widespread native tree, 'ohi'a (Metrosideros polymorpha), has been dying across large areas of Hawai'i Island mainly due to two fungal pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) collectively known as Rapid 'Ohi'a Death (ROD). The speed at which ROD appears to be spreading presents an urgent problem for managers and a pressing need to understand the changing risks of ROD in a timely manner. Here we examine patterns of positive detections of C. lukuohia as it has been linked to the larger mortality events across Hawai'i Island. Our analysis compares the environmental range of C. lukuohia and its l spread over time through the known climatic range and distribution of 'ohi'a. We further modeled the potential distribution of C. lukuohia across the Hawaiian Archipelago to estimate the risk of ROD to other islands. Given the potential for C. lukuohia to alter the structure of 'ohi'a dominated forests, we used our projected potential distribution of C. lukuohia to assess the risk of ROD to threatened and endangered plant species across Hawai'i. Analyses show this fungal pathogen generally encompassed the core, but not the extremes of the climatic range of 'ohi'a. We also found evidence that protecting habitat by fencing out introduced feral ungulates reduces the incidence of the disease likely by reducing physical damage caused by these animals to 'ohi'a trees - a precondition for Ceratocystis infection. Our distribution models indicate that there are highly suitable areas for the disease across the archipelago. Many native plants are likely vulnerable to these types of large 'ohi'a mortality events: of 234 endangered native plant species considered, 122 (52%) have more than half of their range within current and expanding C. lukuohia suitable areas. The large number of vulnerable native plants across the archipelago, along with results suggesting habitat management with fencing reduces Ceratocystis incidence through reduced ungulate damage, lends further support for improved biosecurity and native habitat fencing efforts across the state. Given the ongoing spread of C. lukuohia, we expect more 'ohi'a forests to be impacted by the disease, as exemplified by the recent detection of C. lukuohia in Kauai. In response, we developed a dynamic web portal to host our results online, where models and analyses are updated with new lab-confirmed detections to provide managers with a useful tool to help monitor and assess the risk of C. lukuohia as it continues to spread.