ASU researchers use airborne remote sensing in the hunt for pathogen killing Hawaiian tree

According to legend, Pele, Hawaiian goddess of volcanoes and fire, once fell in love with the handsome warrior ʻŌhiʻa and asked him to marry her. But ʻŌhiʻa had already pledged his love to another, Lehua. Pele, heartbroken and furious in rage, turned ʻŌhiʻa into a twisted tree. 

The gods took pity on Lehua, turning her into a flower on the twisted tree that was once her love. The two, forever joined together, would become the ʻŌhiʻa lehua tree. 

The ʻŌhiʻa lehua (metrosideros polymorpha) is a native tree to six Hawaiian islands, tied to both the environment and culture. But today, a new culprit has begun wreaking havoc on the trees and surrounding ecosystem. Rapid ʻŌhiʻa Death (ROD) is a fungal pathogen that is both unpredictable and swift in causing tree mortality. Each year, an average of 10% of trees die due to ROD.

“ʻŌhiʻa lehua is an amazing plant. It is the first to recolonize barren lava and can be found growing as different life forms, shrubs to trees, in all environments across the Hawaiian Islands, creating the foundation for the forests. Native birds have evolved to utilize its flowers. In Hawaiian culture, the ʻŌhiʻa is deeply embedded in traditional moʻolelo (stories), mele (songs) and oli (chants),” said Robin Martin, mentoring author and associate professor at Arizona State University’s Center for Global Discovery and Conservation Science. 

With this native tree being threatened, researchers are seeking answers to understand the spread of ROD across Hawaii. The disease is both unpredictable and swift, driving an average annual tree mortality rate of 10% in ʻŌhiʻa, compared with the natural background rate of less than 1%. Current efforts include the ASU team’s mapping of canopies that show browning trees, a symptom of ROD. However, once a canopy has turned brown, the tree typically dies within a few weeks. 

Therefore, developing methods to identify trees at risk to ROD is critical to ensure there is enough time to act and limit spread. 

A new study published in Ecological Applications by researchers from the Center for Global Discovery and Conservation Science discovered aircraft-measured spectral differences in the foliar traits of trees that would later develop visible signs of ROD. This finding suggests that their unique aircraft mapping system may provide early detection of trees affected by the pathogen.

“We used repeat laser-guided imaging spectroscopy of forests on Hawaiʻi Island collected by the ASU Global Airborne Observatory to derive maps of foliar characteristics previously found to be important in distinguishing between ROD-infected and healthy ʻŌhiʻa canopies,” said Erin Weingarten, lead author, and now a PhD student at Colorado State University. “Data from these maps were used to develop a prognostic indicator of tree stress prior to the visible onset of browning.”

The ASU Global Airborne Observatory, or “lab in the sky,” mapped the two forest canopies from 2018 to 2019. Over the course of the year, one group of trees remained green. The other group changed from green to brown due to the onset of ROD symptoms. The team then assessed and compared spectral data of each group in 2018 for the foliar traits, or properties, of the leaves that have been linked to ROD. After analysis, the researchers observed key spectral differences in the leaf traits before the canopy turned brown. 

"Our results indicate that airborne laser-guided imaging spectroscopy, a technology totally unique to the Global Airborne Observatory, can be used to identify infected ʻŌhiʻa prior to any visible symptoms. This will result in more time to combat a pathogen that has changed Hawaiian ecosystems forever," said Greg Asner, senior author of the study and director of the Center for Global Discovery and Conservation Science.

The study was supported by the United States Forest Service and the State of Hawai‘i Department of Land and Natural Resources.