Forest decline and soil nutritional problems in Pacific areas

Abstract

Soil nutrient stresses have been determined to contribute to stand-level dieback in two Pacific forest biomes, the Hawaiian Metrosideros rain forest and remnants of the eastern Australian Eucalyptus forest. In the Hawaiian dieback, low levels of N limit indigenous forest development early in primary succession on volcanic soils, while later in primary succession, stresses appear to be associated with soil aging, acidification, loss of cations, decreasing levels of P, increases in soluble Al, and, under poor drainage, sharp increases in soluble Fe. These nutrient limitations put a ceiling on stand development and growth and are considered as one of the three causes predisposing stands to dieback. In the rural or New England dieback of eastern Australia, indigenous eucalypts are adapted to ancient soils with very low levels of P, but pasture improvement with clover and fertilization with superphosphate has imposed different stresses on remnant eucalypts in pastures and nearby forest islands. After fertilization, the trees grow faster initially, but their foliage becomes highly nutritious for insects. Other factors also contribute to a build-up of insects as pests, which now threaten the remaining eucalypts. The rural dieback represents an example of how forests with low canopy species diversity, simplified structure, lack of successional species, and which are prone to dieback under natural conditions, can be destroyed by intensification of agricultural development. The paper closes with a summary of generic factors that were found to cause forest dieback under natural conditions and compares these to the anthropogenically superimposed stresses that led to aggravated tree and forest decline in the Australian rural or New England dieback. The suggestion is made that comparative dieback research at the global level will lead to an improved understanding of natural forest dynamics as an aid in interpreting the new stresses. imposed on forests by human activity.