Assessing spatial distribution, stand impacts and rate of Ceratocystis fimbriata induced 'ōhi'a (Metrosideros polymorpha) mortality in a tropical wet forest, Hawai'i Island, USA

Pests or pathogens that affect trees have the potential to fundamentally alter forest composition, structure and function. Throughout the last six years, large areas of otherwise healthy 'ōhi'a (Metrosideros polymorpha) trees have been dying rapidly (typically within weeks) in lowland tropical wet forest on Hawai'i Island, USA. This mortality is quite distinct from previous well-documented 'ōhi'a dieback episodes driven by cohort senescence. Ceratocystis fimbiata was identified and routinely found associated with rapidly dying individuals of 'ōhi'a, Hawai'i's most widespread native tree. Pathogenicity of this fungus was proven and M. polymorpha was recorded as a new host for C. fimbiata. Mortality of 'ōhi'a at this scale is of great concern as the understory in these forests is often occupied by invasive non-native plants capable of severely limiting 'ōhi'a regeneration. Imagery of 'ōhi'a mortality obtained in 2012 revealed large expanses of greater than expected mortality (i.e., ≥10%) across 1600 ha. By 2014 'ōhi'a mortality levels ≥10% had spread to 6403 ha, or 30% of total area classified as 'ōhi'a in our study area. Further, levels of 'ōhi'a mortality in field plots established within the study region averaged 39%, and mortality levels were comparable across size classes and forest compositions. Results from a subset of field plots re-inventoried one year after plot establishment revealed average annual 'ōhi'a mortality rates of 24% and 28% based on basal area and stem density measures, respectively; mortality rates were as high as 47% in some field plots. The dearth of 'ōhi'a seedling recruitment and characteristic understory dominance of non-native species documented within our research plots, coupled with the lethality of C. fimbriata to 'ōhi'a, suggest that these forests likely will be dominated by non-native species in the future.