ʻŌhiʻa (Metrosideros polymorpha), a dominant native tree of high ecological and cultural importance, is experiencing widespread mortality across Hawaiʻi Island due to two distinct fungal species in the plant pathogenic genus Ceratocystis. As its name suggests, “Rapid ʻŌhiʻa Death” (ROD) causes infected trees to ‘brown’ and completely defoliate within weeks to months. To date, over 50,000 acres have been impacted on Hawai’i Island and there is high concern these virulent fungal pathogens will spread to neighboring islands. The objective of our study was to track the spatial and temporal patterns of ROD mortality on an individual tree basis across a 100 acre study area within the Waiakea Forest Reserve. We collected cm-scale resolution imagery on a bi-weekly to monthly basis via small unmanned aerial systems (sUAS) beginning in February, 2016, and extended our imagery time-series back to 2010 (prior to the beginning of the disease outbreak) through a combination of aerial pictometry and satellite imagery. We sampled 45 trees for Ceratocystis species using quantitative polymerase chain reaction (qPCR) to confirm results obtained from imagery.
Prior to 2013 there is no evidence of active ʻŌhiʻa mortality within the study area, at least dating back to 2010. The ROD outbreak began in this location between 2013 and 2015, with over 335 new cases of identified tree mortality. By 2016, an additional 500 trees were infected or dead, comprising 26% of the ʻŌhiʻa forest in this area. Since the beginning of sUAS imagery collection efforts in early 2016, there has been a decline in the rate of new cases of trees exhibiting active ROD symptoms (characteristic browning of leaves) and the spatial distribution of new cases is progressing in a westerly direction. qPCR results revealed that both fungal species are present within the study area, and sUAS imagery analysis suggests that it may be possible to differentiate between the two based on the temporal expression of ROD symptoms. None of the 11 asymptomatic trees sampled were positive for Ceratocystis, even those completely surrounded by infected trees. Additional work on the early identification of infected trees via spectral signature changes is ongoing. The ability to collect repeat, on-demand sUAS imagery allows us to monitor dramatic and rapid changes in forest health in near real-time.