Spatial patterns of 'ohi'a mortality associated with rapid 'ohi'a death and ungulate presence

Authors: PERROY, RYAN - University Of Hawaii; SULLIVAN, TIMO - University Of Hawaii; BENITEZ, DAVID - Hawaii Volcanoes National Park; HUGHES, FLINT - Forest Service (FS); Keith, Lisa; BRILL, EVA - University Of Hawaii; Kissinger, Karma; DUDA, DANNY - Hawaii Volcanoes National Park

Submitted to: Forests
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2021
Publication Date: 8/4/2021
Citation: Perroy, R., Sullivan, T., Benitez, D., Hughes, F., Keith, L.M., Brill, E., Kissinger, K.R., Duda, D. 2021. Spatial patterns of 'ohi'a mortality associated with rapid 'ohi'a death and ungulate presence. Forests. 12(8). Article 1035. https://doi.org/10.3390/f12081035.
DOI: https://doi.org/10.3390/f12081035

Interpretive Summary: An integrated method for forest health monitoring and testing at the individual-tree level from Hawai'i Island, which has been undergoing widespread native forest declines since 2010 associated with Ceratocystis wilt of 'ohi'a (Metrosideros polymorpha Gaudich.), was developed. This approach, which includes multi-platform remote sensing in conjunction with field sampling and confirmatory laboratory testing, has been used to quickly respond to new outbreaks and has helped illustrate the importance of fencing and ungulate activity in determining 'ohi'a mortality levels in affected regions.

Technical Abstract: Effective forest management, particularly during forest disturbance events, requires timely and accurate monitoring information at appropriate spatial scales. In Hawai'i, widespread 'ohi'a (Metrosideros polymorpha Gaud.) mortality associated with introduced fungal pathogens has affected 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 'ohi'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 'ohi'a trees suspected of infection by the fungal pathogens responsible for rapid 'ohi'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 'ohi'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 'ohi'a mortality from aerial imagery alone. Spatial patterns of ROD-associated 'ohi'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. Suspect 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 had previously been 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 'ohi'a stands from Ceratocystis-induced mortality and repeat monitoring to detect forest changes and resource threats.