Ohia rain forest study: ecological investigations of the ohia dieback problem in Hawaii

Abstract

This final report summarizes the more important results of a two year study of the ohia (Metrosideros collina subsp. polymorpha) rain forest, extending from within Hawaii Volcanoes National Park north across the east flank of Mauna Kea, Island Hawaii. The study focus was on the ohia dieback which occurs in many areas of this terrain. A 1:48,000 vegetation map was produced, which is included in selected copies of this report. In addition, an independent habitat classification was developed from physical soil and moisture regime differences occurring in the area. Over 35 ohia forest stands were sampled in detail for their ohia population structures and 39 releves were analyzed for their floristic content. Five different forms of dieback were recognized. Two of these, called the Dryland and Wetland Diebacks appear to be the more rapid and dramatic forms. Their causes are not from disease or insect attack, but are presumed to be from climatic triggers, acting through the soil. These diebacks are clearly associated with ohia-stand rejuvenation. A third form of dieback, here called Bog-formation Dieback, appears to be a slower form of stand dieback related to permanent site changes. An Ohia-displacement Dieback occurs in the Olaa Tract area, where tree ferns seem to gradually take over the habitats. Here the dieback cause appears to be overmaturity. Individual tree dieback, the fifth form of dieback, is found as an isolated, but common phenomenon in many non-dieback stands examined. All diebacks appear to have natural causes, which are suggested in detail. A new theory is presented, which proposes that there are a number of dynamic phases, including the dieback, which provide for the perpetuation of the shade-intolerant, dominant tree species (ohia) in this rainforest ecosystem.