PERSPECTIVES FOR AN ETIOLOGY OF STAND-LEVEL DIEBACK

Abstract

When trees are dying in large numbers in a forest stand, it is usually assumed they are dying from a disease. Therefore, the study of stand-level dieback has traditionally been the concern of the forest pathologist and pest entomologist. However, stand-level dieback is not always caused by biotic agents, and the term as defined in this paper refers to stands of dead or dying trees whose dieback cause is not obvious and that typically occur in several locations of a larger forest ecosystem. In such stands, adjacent trees as well as spatially separated trees die more or less simultaneously, and loss of foliage usually begins from the top downwards. Dieback trees may be identified either as standing snags or as still living but stag-headed trees. Such dieback stands are common in many indigenous Pacific forests (41), and they have received attention also in several North American (32) and European forests (9). The recent emphasis on the dieback of forests in industrial countries has enlarged the group of researchers to include soil scientists, foresters, and pollution specialists. In both the pathological and air pollution research of dieback, the structure of the forests with dieback, their spatial and temporal patterns, their habitat relationships, and their associated vegetation and successional re­ sponses have received less attention than the more immediate symptoms and assumed causes. However, to arrive at a more complete etiology, such vegetation structural aspects should be analyzed with equal effort. Without such effort, it is difficult to distinguish commonalities and discrepancies in stand-level dieback of different forest biomes. For example, Jimenez et al (21) described stand-level dieback in mangrove forests as and suggested that mass diebacks always have an abiotic rather than biotic cause. Their distinction between mass mortality and normal mortality is helpful in understanding natural dieback, but when larger and smaller areas of