Eucalypt decline in Australia, and a general concept of tree decline and dieback

Abstract

Decline and dieback of eucalypts have been attributed to an exotic pathogen, various native organisms, climatic factors and agricultural or urban pollution. Where particular biotic or abiotic factors could not be singled out, they have been regarded as predisposing, inciting or contributing factors in ‘diseases of complex etiology’. Ongoing monitoring of eucalypt decline during recent droughts in eastern Australia, together with extensive one-time observations across temperate Australia, provided opportunities to further examine some hypotheses of decline and dieback that were largely based on retrospective investigations. Episodes of dieback can be distinguished from the process of chronic decline. Dieback episodes were associated with natural climatic extremes whereas chronic decline was associated with human management. Decline of forests in nature reserves was associated with exclusion of fire and grazing, while decline of rural trees was mostly associated with pasture improvement. Trees growing low in the landscape on soils with poor drainage and aeration were especially predisposed to decline. It appears that chronic abiotic stress causes tree decline when the function of roots is impaired by changes in soils. Climatic extremes can accelerate chronic declines associated with human management. A variety of pests, ‘pathogens’ and parasites can take advantage of trees that are stressed by environmental changes, especially eutrophication. Similarities between diebacks and declines in the Atlantic and Pacific regions suggest a simple unifying concept of tree decline and dieback. The implications for management of forest health are discussed.