Nutrient Studies in Relation to Habitat Types and Canopy Dieback in the Montane Rain Forest Ecosystem, Island of Hawai'i

Abstract

A soil and foliar nutrient analysis was carried out in the Hawaiian Metrosideros rain forest for the purposes of elucidating a previously published physical habitat classification and for finding an explanation for the widespread canopy dieback, which is not caused by a biotic agent in this ecosystem. Soil elements analyzed were C, N, P, Ca, AI, Mn, Fe, and other parameters such as pH. Foliar analysis was restricted to N, P, Ca, Mn, and Fe and assessed only for the canopy M. polymorpha and its major associate, the tree ferns, in the undergrowth. It was found that the earlier recognized two-way breakdown into oligotrophic and eutrophic nutrient regimes had to be enlarged for the habitat classification to a mesotrophic category. Young, deep-ash soils were recognized as mesotrophic on account of their moderate pH and high Ca levels, but they showed severe limitations in available nitrogen and phosphorus that were reflected also in the foliage. Intermediate-aged (1000 yr) ash soils were found to be nutritionally well supplied and balanced and thus considered eutrophic, while older ash soils were designated as oligotrophic on account oftheir extreme acidity and associated higher and potentially toxic levels of soluble AI, Mn, and Fe. Except for the mesotrophic 'a'a lava site, other shallow rock outcrop habitats on pahoehoe were recognized as oligotrophic, because of very low available N and high acidity and, in the poorly drained sites, also high levels of potentially toxic metals. The bog habitats had the highest levels of potentially toxic metals, but surprisingly moderate levels of available N and P. The canopy dieback phenomenon cannot be entirely explained from the nutrient imbalances found in most of the substrate types, but these imbalances are considered contributory in the sense that they become increasingly stressful with stand development, thereby decreasing tree vigor and predisposing stands to dieback.