University of Hawaii Hilo environmental scientist Ryan Perroy recently published a paper highlighting the connection between hoofed animals and the rapid death of the state's ‘ōhi‘a trees.
The “Spatial Patterns of ‘Ōhi‘a Mortality Associated with Rapid ‘Ōhi‘a Death and Ungulate Presence” report was made in collaboration with the National Park Service, U.S. Forest Service and U.S. Department of Agriculture. It suggests that excluding hoofed animals, known as ungulates, from an area is effective at lessening the impact of rapid ‘ōhi‘a death (ROD) in forested areas of Hawaii.
“The results from this work show us that the impacts of ROD can vary across the landscape,” Perroy said in a statement. “We hope this information can be useful in managing and caring for our native forests.”
ROD, caused by two invasive fungi, Ceratocystis huliohia and Ceratocystis lukuohia, was discovered killing the native ‘ōhi‘a tree in 2014 in the Big Island of Hawaii. It was also found on Kauai, Maui, and Oahu.
To complete his report, Perroy used aerial imagery to detect ‘ōhi‘a mortality at an individual tree level.
“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,” the report's authors wrote in the paper's summary.
The aerial images were able to show significant differences in areas with and without ungulates.
The researchers write that “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.”
The authors of the study advise fencing, removal of feral ungulates and the felling of infected trees to mitigate the mortality of ‘ōhi‘a trees in affected regions.
Perroy directs the UH Hilo Data Visualization Laboratory. In 2019 he won an award from the National Park Service for his aerial image work on ROD.