Relationships among moisture stress, insect herbivory, foliar cineole content and the growth of river red gum Eucalyptus camaldulensis.
Influence of moisture stress on tree growth, foliage development and incidence of insect herbivory in a native forest of river red gum (Eucalyptus camaldulensis) in southern New South Wales was examined. Trees standing within intermittently flooded waterways were compared with trees at an average of 7.5 and 37.5 m from the edge of waterways. As distance from water increased to 37.5 m, there was a threefold increase in plant moisture stress, and diameter at breast height, crown condition and leaf area was significantly reduced. On average, 13.9% of leaf area was lost via insect herbivory. There were significant correlations between moisture stress and trunk diameter increment, leaf area, potential leaf area, leaf length/width ratio and % leaf area lost to insect herbivory. Mean leaf area was strongly correlated with tree growth. Moisture stress appeared to influence tree growth rate and leaf size and shape but not physiological foliage parameters of % total nitrogen, total terpenoid yield or % 1,8-cineole of total terpenoid yield. In moisture-stressed trees, increased herbivory appeared to be related to smaller leaf size and not to significant changes in levels of foliar nitrogen or cineole. There were, however, two distinct populations of trees, independent of distance from waterways, in relation to total foliar terpenoid and cineole content. One population produced a mean terpenoid yield (2.12% ± 0.13 w/w, leaf dry wt) and cineole content (71.7% of total terpenoid yield), significantly higher than the other (1.12% ± 0.22 and 12.7% respectively). High oil-yielding trees were subjected to significantly less mean herbivory than trees with the lower yields (11.4% and 18.0% leaf area missing, respectively). It is proposed that genetic selection of trees with high 1,8-cineole content will reduce insect herbivory.