Relationships Among Moisture Stress, Insect Herbivory, Foliar Cineole Content and the Growth of River Red Gum Eucalyptus camaldulensis

Abstract

1. The influence of moisture stress on tree growth, foliage development and the 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 the waterways. 2. There was a threefold increase in plant moisture stress, expressed as xylem water pressure potential, as the distance from water increased to 37-5 m. Growth, expressed as change in diameter at breast height, fell significantly (1 2, 0 8, 0 1 cm, respectively) with increased distance (0, 7 5, 37 5 m) from the waterways. Crown condition also fell significantly. Mean leaf area was significantly reduced from 13 7 to 8-7 cm2 with increasing distance from floodwater. On average, 13 9% of leaf area was lost via insect herbivory. 3. 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. 4. Moisture stress appeared to influence tree growth rate and leaf size and shape but not the 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 the levels of foliar nitrogen or cineole. 5. There were, however, two distinct populations of trees, independent of distance from the 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), while the other had a significantly lower terpenoid yield (1 12% + 0 22) and cineole content (12 7% of total terpenoid yield). The higher oil-yielding trees were subjected to significantly less mean her- bivory than trees with the lower total oil and cineole 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