SPATIAL AND TEMPORAL VARIABILITY IN DEFOLIATION OF AUSTRALIAN EUCALYPTS

Insect defoliation is commonly associated with and assumed to be a cause of mortality in Australian eucalypts, particularly in rural regions where trees suffer from the eucalypt dieback syndrome. To test this, leaf growth and defoliation were measured in the canopies of Eucalyptus trees from June 1982 to June 1986, and related to tree health and eucalypt dieback. Over 5000 leaves were monitored, including replicates of branches, canopy heights, individual trees, species, and sites. Three types of sites were selected, representing the most common conditions in the eastern Australian tablelands: woodlands (comprised of healthy trees but with slight signs of dieback), healthy trees in pastures, and dying trees in pastures. Within each site the commonest native tree species were selected for study; species were not always the same between adjacent sites because of interspecific differences in distribution patterns. Some eucalypt species suffered much greater defoliation than others, and this was related to their observed susceptibility to dieback. Eucalyptus nova-anglica (peppermint) and E. stellulata lost up to 300% and 274% of leaf surface area, respectively, in one year, and these trees suffered severe dieback. In contrast, E. caliginosa and E dalrympleana annually lost as little as 20% and 8%, respectively, and showed little evidence of dieback. Other aspects of leaf growth were quantified for each canopy, including numbers of leaves flushing, amounts of leaf area affected by galls and miners, and leaf longevities. More than half of the leaves monitored senesced from causes other than insect damage. Defoliating insects appeared to have a greater impact on eucalypts in Australia than they do on forest and pasture trees of other continents.