Dieback of rural eucalypts: Response of foliar dietary quality and herbivory to defoliation

Abstract

Rural dieback of Eucalyptus blakeiyi trees growing on pastoral properties near Canberra is associated with chronic defoliation by in.sects. In order to test the effect of defoliation on subsequent herbivory. I artificially defoliated three healthy trees by clipping their terminal branchlets. The foliage that regrew on the clipped trees was nutritionally superior to the foliage it replaced, and was much more heavily damaged by grazing insects. There was a transient increase In the tannin content ofthe regrowth foliage, but this was apparently ineffective in defending it from subsequent herbivory. Compared with the foliage on nearby E. blakeiyi trees that also produced major flushes of leaf growth during the same period, the regrowth on the clipped trees had enhaneed dietaty qualities and suffered more insect damage. Leaf age contributed to many of the differences in dietary quality, but when adjustments were made for the effects of leaf age the same trends remained. Five ofthe nearby trees were suffering from the chronic insect grazing associated with rural dieback, and the other five appeared healthy. The dietary quality of regrowth foliage on the elipped trees was qualitatively more similar to that of foliage on the dieback trees. Thus the chronic herbivory associated with rural dieback may be partly selfperpetuating, given this positive feedback between defoliation and dietary quality, and an apparent absenee of other effective controls on insect populations.

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

“…Folivores can reinforce eucalypt decline because defoliation stimulates new epicormic leaves that are more nutritious and palatable to the folivores than mature leaves (Landsberg, 1990). Parasitic plants including mistletoes, cherries and dodder laurel vines (Cassytha spp.)…”

Koala conservation and habitat requirements in a timber production forest in north-east New South Wales

“…A relative shortage of protein could limit koala distribution to habitats with abundant and year round supplies of new leaf growth, habitats with exceptional mature foliage protein levels such as those on fertile soils, or habitats with high nitrogen dietary supplements such as flowers and understorey casuarinas. Such habitats include partially cleared and diebackaffected rural lands where tree foliage is dominated by young and epicormic growth (Landsberg 1990); rural and urban lands treated with fertilizers; alluvial soils along rivers and drainage lines; escarpment forests on soils of moderate fertility with casuarina understoreys that have not been subject to frequent control burning; and forests and woodlands on fertile soils and geologies.…”

Genetic resistance of Eucalyptus globulus to autumn gum moth defoliation and the role of cuticular waxes

“…A large array of foliar characteristics have been reported to directly affect eucalypt susceptibility to insect attack including foliar oils (e.g., Stone and Bacon 1994), waxes (Edwards 1982), phenolics (e.g., Landsberg 1990), nitrogen content (Ohmart and Edwards 1991), and leaf toughness . However, their role in determining the resistance of E. globulus to autumn gum moth defoliation is currently unknown.…”