Increasing understanding and capacity to manage and mitigate Ceratocystis-caused Rapid ʻŌhiʻa Death (ROD) across Hawai’i

ʻŌhiʻa forests support a wide diversity of plant and animal species found only in Hawaiʻi. ʻŌhiʻa-dominated forests also protect the many watersheds that provide Hawaii’s households with abundant, clean drinking water and that sustain the complex coral reefs of Hawaii’s coastal ecosystems. ʻŌhiʻa is also a cultural keystone species for Hawaiʻi and in Native Hawaiian worldview is considered a family member. Today, ʻōhiʻa forests face three major threats:

1. Rapid ʻŌhiʻa Death (ROD), a lethal disease caused by the recently introduced fungal pathogen, Ceratocystis lukuohia. Since its discovery in 2014, C. lukuohia has caused stand level mortality across more than 20% of Hawai’i Island’s ʻōhiʻa-dominated forests. 
2. Strawberry guava (Psidium cattleianum), a widespread, invasive, non-native tree brought to Hawaiʻi from Brazil nearly 200 years ago. This species is now Hawaii’s most abundant tree and forms dense thickets that outcompete native tree species including ʻōhiʻa. 
3. Feral hooved animals (wild pigs, cattle, sheep, and goats), introduced to Hawaiʻi in the past two centuries and now affecting nearly all of Hawaii’s forests. Their movement and eating patterns harm native species and their habitats in numerous ways. Hooved animals are also associated with the spread of Ceratocystis inoculum and wounding of ʻōhiʻa trees that allow the fungus to enter and kill individual trees and entire stands.  

Each of these threats on their own exert devastating impacts to the integrity of ʻōhiʻa forests, and our work is showing that their interaction amplifies and exacerbates those individual impacts. For example, feral pigs disperse strawberry guava seed, they disturb soil and so facilitate invasion by non-native plants, and importantly they wound ʻōhiʻa trees and thus expose them to infection by Ceratocystis. Feral hooved animals like pigs carry and so can disperse living Ceratocyctis spores, which can be deposited on wounds pigs create while foraging, rooting in soil, and rubbing against trees. Managing the interactions among ROD, hooved animals as both agents of wounding and disease vectors, and strawberry guava requires a holistic approach to: (i) understanding the nature of the interactions; (ii) developing effective treatments that simultaneously consider multiple threats; and (iii) effectively implementing these treatments across highly variable landscapes.

Objectives

This research will address the following management needs:

1. Understand the interacting effects of ROD, invading plants, and feral animals;
2. Identify methods to mitigate the impacts of these interacting threats on remaining ʻōhiʻa forests;
3. Quantify the presence and strength of ʻōhiʻa resistance to Ceratocystis lukuohia across Hawaiʻi, the spatial
4. Increase capacity to restore ROD-impacted forests ʻōhiʻa forests that are infested by strawberry guava and impacted by feral ungulates; 
5. Develop a framework that integrates forest pathology, plant genetics, and biocontrol of invasive plants to advance restoration practices needing to target multiple threats. 

Research Activities

Hooved mammals (ungulates) and ROD infection

Growing evidence indicates that non-native feral hooved animals (ungulates) play a pivotal role in the spread of C. lukuohia inoculum and wounding of ʻōhiʻa trees, speeding up tree infection and mortality. Managers have proposed that fencing and animal removal as the most effective way to protect ʻōhiʻa forests from ROD. However, fence construction and maintenance are expensive and Hawaii’s hunting communities can view fencing and ungulate eradication as threats to their lifeway. This research is building knowledge about the efficacy of fencing and ungulate eradication as a primary tool in the protection of ʻōhiʻa forests from ROD. This project is rigorously testing the extent to which feral ungulates act as disease vectors of ROD into and across ʻōhiʻa forests, and whether ungulate control is effective for protecting ʻōhiʻa forests from ROD. This work includes:

• Game camera deployment to document feral hooved animal activity and behavior;
• Soil sampling for C. lukuohia inoculum in fenced, ungulate-free forests and in unfenced forests where hooved animals are present;
• Installation and monitoring of Forest Monitoring Plots to track ROD mortality.

ʻŌhiʻa resistance to Ceratocystis lukuohia 

ʻŌhiʻa trees have existed in Hawai’i for at least 4 million years, and during that time they have evolved populations that occupy the varied environmental conditions of the archipelago. Relying on the incredible diversity of ʻōhiʻa genotypes, the ʻŌhiʻa Disease Resistance Program (ʻŌDRP) has been exploring how this genetic variation includes individuals that tolerate or even resist C. lukuohia infection and/or subsequent disease. ʻŌRDP seeks to build seed orchards of local ʻōhiʻa that are ROD-resistant for use in ʻōhiʻa forest restoration. To achieve this outcome, BIL-supported research includes:

• Propagate and prepare plants for greenhouse incoulation trials following ʻōhiʻa-specific disease screening protocols developed by ʻŌDRP. 
• Expand sampling of seed from new ʻōhiʻa populations to increase our understanding of resistance across Hawaiʻi. This work is being carefully coordinated with federal, state, and Native Hawaiian-serving organizations.
• Propagate putatively resistant material identified from the disease screening process so it can be outplanted.
• Seeking to understand the mechanisms of disease resistance, and what anatomical or chemical characteristics contribute to resistance.

Reducing strawberry guava infestation 

Ongoing IPIF-led research has found that many ʻōhiʻa forests heavily impacted by ROD – particularly those below 3500’ elevation on Hawaiʻi Island – are also dominated by dense understories of the highly invasive, non-native tree, strawberry guava. Such thickets severely constrain ʻōhiʻa recruitment and create daunting barriers to effective and efficient restoration, including reestablishment of self-sustaining ʻōhiʻa populations. Decades-long IPIF research spearheaded by Dr. Tracy Johnson has discovered that the galling insect, Tectococcus ovatus, is a highly effective biocontrol agent of strawberry guava. Johnson has carefully tested this agent for host specificity, and we are propagating it for targeted distribution. Given the very wide distribution and highly variable densities of P. cattleianum, deployment must not only be effective across a range of density conditions, but must be highly efficient as well. IPIF is developing and testing approaches designed to introduce T. ovatus rapidly, efficiently, and effectively to strawberry guava stands at large spatial scales. Research activities include:

• In collaboration with partners at the University of Hawaiʻi at Hilo, design and demonstrate aerial dispersal methods of T. ovatus using drones and helicopters;
• Test and monitor impacts of T. ovatus on strawberry guava growing in ROD-impacted study sites being used for ungulate and ʻŌDRP work; 
• Continue to quantify impacts of strawberry guava biocontrol at long-term study sites.

Outputs

• Consistent updates to collaborators, partners, and stakeholders via monthly Rapid ʻŌhiʻa Death Working Group meetings, bi-monthly Rapid ʻŌhiʻa Death Science Team meetings, Quarterly ʻŌhiʻa Disease Resistance Program meetings, at the annual Hawaiʻi Conservation Conference, and at national professional society and FS workshops addressing forest pathology and forest restoration.
• Outreach strategy, materials, and gatherings to inform Hawaiʻi hunters and land managers of the best available science, summarizing major findings and management applications. These materials will be intended to promote informed decision-making and actions.
• Scientific journal publications and conference presentations.