Dieback of rural trees in Australia

Much of Australia's woody vegetation has been cleared since European settlement, allowing extensive agricultural and pastoral industries to develop. Although scattered stands of native trees were often left on farmland, many of these trees are now in decline, particularly in regions where farm management is intensive. Rural dieback is a dramatic manifestation of this decline, and is characterized by a widespread, relatively recent upsurge of premature dieback and death of trees on farms. Comparatively little is known of its aetiology. Possible causal agents include insects, pathogens, drought and changes in the properties of soils or groundwater. We review and assess the evidence implicating each of these factors, and outline a conceptual model that demonstrates how several of them may be undesirable side effects of an intensification of pastoral management. The model shows how several different factors may interact to contribute to tree stress, and how dieback may eventuate if any one of them intensifies. Negative feedbacks operate between insect damage and the epicormic foliage that eucalypts produce in response to it, and also between dieback-related tree deaths and a worsening of the environmental degradation that contributed to the onset of dieback. These negative feedbacks will have most impact in those rural areas in which trees have become predisposed to chronic defoliation because of management practices or other environmental factors.