Effects of inter-planted wattles on the establishment of eucalypts planted on forest sites affected byPhytophthora cinnamomi

Abstract

Summary The feasibility of using wattles as a nurse crop to reduce the impacts of Phytophthora cinnamomi on planted eucalypts was examined in two field experiments established on forest sites in South and East Gippsland severely affected by ‘dieback’. In the Mullungdung State Forest, South Gippsland, E. sieberi, a disease-sensitive species, was planted alone and in a mixture with each of four Acacia species. On a similar site in the Waygara State Forest, East Gippsland, E. sieberi and two tolerant species of eucalypt (E. botryoides and E. sideroxylon) were inter-planted with and without A. longifolia. Within three years of planting, about 50 and 60 per cent of the E sieberi at Mullungdung and Waygara respectively were dead, with most losses being recorded within the first six months. At Waygara, subsequent mortality was associated with periods of heavy rain during spring and summer. Losses among the disease-tolerant species were low. For each eucalypt species at each site, mean height growth was significantly greater when inter-planted with wattles. This response was also reflected in diameter growth at Mullungdung. Spot measurements of soil moisture after heavy rain at the Mullungdung site showed that the soils were considerably drier under the wattle canopy. At Waygara, a delay in the wetting cycle of the soil was also recorded in plots containing wattle. Defoliation of A botryoides due to psyllid infestation on the Waygara site was found to be significantly less when grown in association with A. longifolia. The nurse crops of wattles were unable to protect Phytophthora-susceplible eucalypts on disease-prone sites due to high early mortality. However, on the basis of the observed growth responses, inter-planting of wattles when establishing eucalypts on infertile sites is worthy of further examination.