Studies on vegetation and insect predation of eucalypts in forest and woodland at Ridgeway, Tasmania

Studies on vegetation and insect predation were conducted during the period 1982-84 at the Ridgeway Park Reserve, Tasmania. The vegetation of the area is eucalypt forest and woodland which varies in composition, dominance and structure. Soil type, topography, the level of grazing and incidence of fire also vary within the study area. The influence of soil type on moisture availability was investigated by comparing the vegetation on sites receiving the same incident solar radiation. The majority of species experience a shift in distribution over the three soil types such that with increasing radiation the species reach their peak frequency first on the brown earth, then on the podzolic soil and finally on the podsol. This pattern conformed to a model of moisture availability devised for the three soils. Species that did not conform to the model were either particularly vulnerable to the effects of grazing or fire, opportunists or restricted to one or other of the three soil types and hence a particular set of soil nutrient conditions. The effects of insect grazing on sapling eucalypts on north and south-facing slopes, which differed in geology, were assessed by monthly monitoring of insect damage and by deterring insects from grazing certain trees through the use of insecticide. On every tree percentage damage, and the amount of different types of damage, leaf loss, mean shoot length, the number of leaves per shoot and the percentage of damaged leaves, were measured on the current season's shoots. A limited sample of the same data was collected on leaves approximately one year old. Overall, there were few significant differences in the level of these variables between the different species and sites. Percentage damage figures were comparatively low (7-12%) and there was no significant difference between the species. However, damage was significantly higher in the most mesic, least fertile site. Percentage damage, and leaf and shoot loss were combined to derive a figure for the total damage to each species (29-41%). This was significantly higher in E. obliqua compared with the other species. The patterns of insect defoliation may have been considerably altered as a result of a drought experienced during the course of the study. Insect activities may effect forest community composition, but only as part of a web of inter-relationships whose effects vary, between the species, in space and time.