High - resolution aerial imagery for assessing changes in canopy status in Hawai‘i’s ’Ōhi‘a (Metrosideros polymorpha) rainforest

Ōhi‘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 ’ōhi‘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 ’ōhi‘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 ’ōhi‘a tree canopy through recruitment of new seedlings that have now grown into saplings and even taller trees (Boehmer et al. 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 ’ōhi‘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 ’ōhi‘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 ’ōhi‘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