Metrosideros polymorpha ('ōhi'a, 'ōhi'a lehua) is an important foundation species in Hawaiian forest habitats. The genus originated in New Zealand and was dispersed to the Hawaiian archipelago approximately 3.9 million years ago. It evolved into five distinct endemic species and one of these, Metrosideros polymorpha, further differentiated into eight varieties across what are now the main Hawaiian Islands. 'Ōhi'a is a tree that has great significance in indigenous Hawaiian culture. It is considered a physical manifestation of several principal Hawaiian deities, and serves a broad range of uses in Hawaiian material culture. It occupies a wide diversity of habitats, extending from sea level to over 2,200 m elevation, occupying habitats that range from extremely wet to dry rainfall zones. It is the dominant or co-dominant tree species in wet and mesic forests and is also one of the first woody species to become established on young lava flows. Although 'ōhi'a is a dominant forest tree it also exhibits many characteristics of a pioneer species. 'Ōhi'a provides the matrix for a wide diversity of endemic plants and animals found in these habitats and functions as the primary vegetation cover on native Hawaiian watersheds, facilitating groundwater recharge and regulating surface runoff. 'Ōhi'a has shown remarkable resilience by recolonizing forests that were opened up by disturbance, such as the widespread 'ōhi'a canopy dieback that occurred on East Maui in the 1900s and on the east side of the Island of Hawai'i in the 1970s. Several human-related conditions threaten the continued stability of Hawaii's native ecosystems, including invasive plants, plant diseases, introduced animals, and changing climate. The research and conservation legacy of Dr. Dieter Mueller-Dombois helped to expand our knowledge of the ecology and importance of 'ōhi'a forests, and to increase awareness and appreciation of the remarkable Hawaiian ecosystems that are unique to the world.

Questions (a) Have Metrosideros polymorpha trees become re-established in Hawaiian forests previously impacted by canopy dieback in the 1970s? (b) Has canopy dieback expanded since the 1970s? (c) Can spatial patterns from this dieback be correlated with habitat factors to model future dieback in this area? Study Site An 83,603 ha study area on the eastern slopes of Mauna Loa and Mauna Kea volcanoes on the island of Hawaii, USA. Methods We analyzed very-high-resolution imagery to assess status of Metrosideros polymorpha forests across the eastern side of the island of Hawaii. We generated 1,170 virtual vegetation plots with a 100-m radius; 541 plots in areas mapped in 1977 with trees dead or mostly defoliated (dieback), and 629 plots in adjacent wet forest habitat, previously mapped as non-dieback condition. In each plot we estimated the frequency of M. polymorpha trees that were dead or mostly defoliated, and the frequency of trees with healthy crowns. These results were combined with habitat data to produce a spatial model depicting probability of canopy dieback within the study area. Results Seventy-nine percent of plots mapped in 1977 in dieback condition recovered their canopy and were now considered in non-dieback condition. Ninety-one percent of plots in previous non-dieback areas were found to still have a healthy M. polymorpha canopy in 2015. A spatial model allowed us to identify areas within the study area with high, medium, and low probability of experiencing this same type of canopy dieback in the future. Conclusions Most former dieback areas mapped within the study area in 1977 now show recovery of the tree canopy through growth of new cohorts of young M. polymorpha trees. This suggests these forest communities are resilient to this type of canopy loss and tree death so long as other factors do not disrupt the natural regeneration process.

'Ohi`a Lehua (Metrosideros polymorpha) is the most abundant tree species in the native wet and mesic forests throughout the main Hawaiian Islands. In the late 1960s and early 1970s large areas on the wet, eastern side of Hawai`i island appeared to have extensive defoliation and death of the 'ohi`a trees. The dieback on Hawai`i island extended to approximately 49,000 ha of which 24,000 ha was considered to be in heavy to severe dieback (>50% of the canopy trees dead or defoliated), and 25,000 ha characterized as having slight to moderate dieback (25-50% of the canopy trees dead or defoliated). Research was initiated in 1976 by a team led by Professor Dieter Mueller-Dombois to assess both extent and ecological characteristics of the forest impacted by canopy dieback relative to areas that did not experience dieback in this same forest zone. To assess the spread or recovery of the 'ohi` a dieback forest over time, twenty-six permanent plots were established across the study area. The results from the monitoring of the 26 permanent plots indicate that many of the original dieback sites are now showing strong recovery of the 'ohi` a tree canopy through recruitment of new seedlings that have now grown into saplings and even taller trees (Boehmer et J Veg Sci 24(4):639-650, 2013). However, it was not clear if these results truly represented the conditions across the entire original dieback area. Therefore, we conducted a much larger survey of response of the 'ohi`a forest to that dieback event, across the entire wet forest region on the eastern side of the island of Hawai`i. We did this by analyzing very high-resolution aerial imagery (<10 cm pixels) taken by Pictometry International (POL), to assess both canopy and understory change throughout this region. The POL imagery proved to be an effective and efficient tool to use for assessing the status of 'ohi`a forest across the eastern Hawai`i Island study area. The results of this large area survey, using the POL imagery, agree closely with the conclusions presented by Boehmer et al. (J Veg Sci 24(4): 639-650, 2013), that most of the 'ohi` a forests on the eastern side of the island of Hawai`i that were affected by canopy dieback in the 1960s and 1970s have started to recover their tree canopy, as a new cohort of young trees are growing back in these sites.