Spatial Patterns of ‘Ōhi‘a Mortality Associated with Rapid ‘Ōhi‘a Death and Ungulate Presence

Abstract

Effective forest management, particularly during forest disturbance events, requires timely and accurate monitoring information at appropriate spatial scales. In Hawai‘i, widespread ‘ōhi‘a (Metrosideros polymorpha Gaud.) mortality associated with introduced fungal pathogens affects forest stands across the archipelago, further impacting native ecosystems already under threat from invasive species. Here, we share results from an integrated monitoring program based on high resolution (<5 cm) aerial imagery, field sampling, and confirmatory laboratory testing to detect and monitor ‘ōhi‘a mortality at the individual tree level across four representative sites on Hawai‘i island. We developed a custom imaging system for helicopter operations to map thousands of hectares (ha) per flight, a more useful scale than the ten to hundreds of ha typically covered using small, unoccupied aerial systems. Based on collected imagery, we developed a rating system of canopy condition to identify ‘ōhi‘a trees suspected of infection by the fungal pathogens responsible for rapid ‘ōhi‘a death (ROD); we used this system to quickly generate and share suspect tree candidate locations with partner agencies to rapidly detect new mortality outbreaks and prioritize field sampling efforts. In three of the four sites, 98% of laboratory samples collected from suspect trees assigned a high confidence rating (n = 50) and 89% of those assigned a medium confidence rating (n = 117) returned positive detections for the fungal pathogens responsible for ROD. The fourth site, which has a history of unexplained ‘ōhi‘a mortality, exhibited much lower positive detection rates: only 6% of sampled trees assigned a high confidence rating (n = 16) and 0% of the sampled suspect trees assigned a medium confidence rating (n = 20) were found to be positive for the pathogen. The disparity in positive detection rates across study sites illustrates challenges to definitively determine the cause of ‘ōhi‘a mortality from aerial imagery alone. Spatial patterns of ROD-associated ‘ōhi‘a mortality were strongly affected by ungulate presence or absence as measured by the density of suspected ROD trees in fenced (i.e., ungulate-free) and unfenced (i.e., ungulate present) areas. Suspected ROD tree densities in neighboring areas containing ungulates were two to 69 times greater than those found in ungulate-free zones. In one study site, a fence line breach occurred during the study period, and feral ungulates entered an area that was previously ungulate-free. Following the breach, suspect ROD tree densities in this area rose from 0.02 to 2.78 suspect trees/ha, highlighting the need for ungulate control to protect ‘ōhi‘a stands from Ceratocystis-induced mortality and repeat monitoring to detect forest changes and resource threats.

Ceratocystis lukuohia‐infested ambrosia beetle frass as inoculum for Ceratocystis wilt of ʻōhiʻa (Metrosideros polymorpha)

“…In Hawaiʻi, these wounds may provide infection courts for Ceratocystis ‐laced frass dispersed by wind, rain and soil movement. Perroy et al (2021) reported that the presence of ungulates (i.e., pigs and cattle) in previously animal‐free areas was associated with rapid increases in ROD mortality. Direct transmission of pathogen‐infested frass by animal bark‐stripping, rooting and rubbing, or by insect vectors carrying the pathogens, have not been verified, but these alternative vector pathways are currently under investigation.…”

“…Additionally, high wind events can damage branches and stems, simultaneously affecting large numbers of trees across the landscape. In Hawaiʻi, feral ungulates can rub, chew or strip patches of bark off M. polymorpha, and their presence in forests is strongly associated with elevated ROD mortality (Perroy et al, 2021). Prior studies with other hosts have demonstrated that Ceratocystis platani can travel between trees through functional root grafts (Tsopelas et al, 2017).…”

A framework for establishing a rapid ‘Ōhi‘a death resistance program

“…Humans can also create infection courts when they wound ʻōhiʻa. Similarly, evidence suggests that feral ungulates are suspected of creating wounds on ʻōhiʻa trees, leading to increased mortality rates of ʻōhiʻa in stands where ungulates are present versus ungulate-free stands (Perroy et al 2021). Understanding how these potential transport and wounding mechanisms contribute to the spread of the ROD pathogens is integral for the long-term management of Hawaiʻi's native forests.…”

“…Since there is limited ability to control windblown frass, efforts are also being made to reduce injuries to trees that can subsequently become infected. Fencing more pristine forests and removing feral ungulates such as cattle, goats, sheep, and pigs can reduce injury to trees, and fenced and protected forests have shown much lower levels of disease than forests with high populations of feral animals (Perroy et al 2021). Natural ʻōhiʻa regeneration is occurring at higher elevation forests (above 1,000 m), and seedlings seem less susceptible to the diseases than larger trees (R. F. Hughes pers comm).…”

Environmental drivers of seasonal shifts in abundance of wild pigs (Sus scrofa) in a tropical island environment

“…Based on limited data, Pimental (2007) estimated that wild pigs cost the United States $1.5 billion annually as a result of damages caused by wild pigs and the costs to control these wild populations. Their ability to carry and transmit pathogens also poses a considerable and yet vastly understudied threat to humans, livestock, and resident flora and fauna (Bevins et al 2014;Perroy et al 2021).…”