Ecological impediments to the establishment of trees by direct seeding on pastoral sites in the Midlands of Tasmania

Soil degradation resulting from land clearing, tree decline and landuse practices is a serious environmental problem in Australia. While changes in land management practices are required to overcome many degradation problems, and mechanical stabilization techniques will be necessary in many instances, the establishment of trees and other woody vegetation is considered to be an important tool for combating land degradation. Direct seeding is increasingly considered to be a cheap but effective method of re-establishing woody vegetation in rural areas in Australia. It has been used with good results on many rural sites, but in some areas, such as the Tasmanian Midlands, there has been less success. Broadacre field trials and intensive field and glasshouse experiments were established to investigate direct seeding techniques in the Midlands, and to identify possible factors contributing to results. Results in the field were consistently poor, with low rates of emergence and survival, and very slow growth. In many instances, treatment effects appeared to be masked by harsh environmental conditions. It was demonstrated that germination and survival of three eucalypt species could be significantly increased by irrigation and the resultant increase in available soil moisture. Fertilizer addition, however, was found in glasshouse experiments to depress Eucalyptus amygdalina germination. Pronounced growth check in E. amygdalina seedlings grown in both pasture and residual woodland soil in the glasshouse was not overcome by adding moisture. The combination of nutrient addition and weed control did, however, overcome this growth inhibition in pasture soils, as did heat sterilization at 70°C. From this result it was hypothesized that growth check in E. amygdalina seedlings grown in pasture soil was a result of a soil nutrient imbalance and/or an unfavourable soil microflora. Competition studies revealed that both a grass, Lolium perenne, and a broadleaf, Leontodon taraxacoides, significantly decreased the seedling growth of one eucalypt species. The importance of long term weed control in promoting growth was clearly demonstrated both in the field and glasshouse. It was found that long term weed control could be improved by soil scalping or the use of residual herbicides. E. amygdalina growth was stimulated by competition from acacia seedlings, which may have been a result of increased nitrogen nutrition as a consequence of the nitrogen fixing capacity of acacias, or of a more favourable environment for the development of appropriate soil microflora resulting from an increase in root density. Experimental results suggest that eucalypt seedling emergence, survival and growth may be enhanced by site preparation techniques which modify the microenvironment to increase soil moisture levels and provide protection from extreme temperatures; by good long term weed control; by the the post-emergence addition of fertilizer; and by sowing high rates of acacias in combination with eucalypts. Results may also be improved by sowing at a time of year when soil moisture conditions are more favourable, or when the activity of seed harvesting organisms is low. Until there is a greater understanding of the processes causing growth inhibition in eucalypt seedlings in the Midlands, however, direct seeding may not be an appropriate technique on many long term exotic pasture sites, or in some remnant stands of native vegetation.