SYNOPSISThroughout much of the world's subtropics, eucalypts have become favoured plantation trees for commercial forestry. The reasons for their popularity are similar to those that made Norway spruce the favoured plantation tree in Central Europe. As is well known, however, many Norway spruce stands all over Europe are now in a state of decline, suffering from mineral deficiencies. Such a fate can be predicted also for repeated eucalypt plantations. Moreover, eucalypts are in a state of decline in East Australia from Canberra to Brisbane. Here the decline is related to insects that devour eucalypts because of intensified agricultural development. High-yielding eucalypts are, of course, attainable in the short run. For long-term sustainability, however, a number of ecological parameters need to be observed. Among these is an important biological component, which will be the central theme of this paper.