Decline of Eucalyptus tereticornis Near Bairnsdale, Victoria: Insect Herbivory and Nitrogen Fractions in Sap and Foliage

Abstract

Eucalyptus tereticornis growing along roadsides and in pastures in eastern Victoria were often in poor health and were repeatedly defoliated by herbivorous insects. Epicormic buds sprouted following bouts of defoliation and the first epicormic leaves produced from such buds were rich in nitrogen and particularly in nitrogenous solutes such as proline compared with adult leaves. Xylem sap collected from declining trees was richer in nitrogenous solutes than that from healthy trees. Concentrations of total nitrogen and specific solutes in foliage were not closely related to pressure potential in shoots or to nitrogen availability in soil. In glasshouse-grown seedlings, foliar concentrations of total nitrogen and of a number of nitrogenous solutes were increased by reduced water availability. Chronic insect infestations and periodic insect outbreaks may be supported by high concentrations of nitrogenous solutes in sap and foliage, especially epicormic foliage, which in turn may be a response to drought or increasing salinity ('physiological drought').