Leaf nitrogen and its effects on the rate of herbivory on selected eucalypts in the jarrah forest

Abstract

This study measured the relationship between the rate of leaf consumption by insect herbivores and the level of leaf nitrogen in selected species of eucalypts. Sampling was carried out on a monthly basis from Eucalyptus accedens, Eucalyptus resinifera and Eucalyptus marginata in areas rehabilitated following bauxite mining and from E. marginata in a forest regenerated after clearfelling. The rate of herbivory was examined on leaves aged 1–3 years. The mean monthly rate of herbivory of all species varied from 0.9 to 1.9% of the total leaf area and was positively correlated ( P < 0.05) with the level of leaf nitrogen in E. marginata. The previous month's level of nitrogen was also positively correlated to the rate of herbivory in the current month in all species. The level of leaf nitrogen was negatively correlated to the current and previous month's level of herbivore abundance and the current month's rate of herbivory in E. resinifera. These correlations suggest two possibilities. Firstly, that in some eucalyptus the level of leaf nitrogen may be lowered when the level of predation on leaves is high. Secondly, the herbivorous insects may preferentially feed on eucalypt leaves having a high amount of nitrogen.