Weather, Eucalyptus Dieback in New England, and a General Hypothesis of the Cause of Dieback

Abstract

On the New England Tablelands in Australia between 1950 and 1980 very many eucalypts declined and died. This dieback was strongly correlated with a change in the pattern of rainfall. Starting from 1945, trees were more frequently exposed during the growing season to excess of soil moisture followed immediately by a shortage of water. Several species of Eucalyptus were affected, but those species which normally grow on poorly drained sites died first and continued, even on better sites, to be the species worst and most frequently affected. Declining trees were heavily and repeatedly attacked by defoliating insects. The same species had declined and died in the same localities approximately 100 years earlier. In this century declines and diebacks in other parts of Australia and overseas showed many similarities to that of eucalypts in New England and to each other. In particular , they have been associated with departure of rainfall from the norm and with insects and fungi attacking mostly old trees and species growing on harsh sites. It is proposed here that the primary cause of diebacks and declines is a change in the pattern of rainfall which physiologically stresses trees via changes in the availability of water to their roots. Senescing and suppressed trees and those growing on sites most prone to be flooded and dried out will be first and worst affected. Defoliating and cambium-feeding insects and root-killing fungi are secondary, successfully attacking only stressed trees. They may hasten the decline and eventual death of badly stressed trees. Predators are more successful on stressed trees because more of their young survive when they feed on tissues made more nutritious by the release of nutrients during senescence induced by water stress. The extent to which they can attack successfully depends on the frequency and amplitude of stress the trees experience. Thus declines and diebacks are but one extreme of a continuum of response of trees to physiological stress; at the other extreme are small, short-lived increases of predators on one or a few trees. Outbreaks of insects and fungi of varying duration and severity fall between these two extremes.