Factors affecting the re-establishment of native plant species on abandoned pastures

This thesis examines the re-establishment of native plant species on abandoned pastures in Tasmania adjacent to undisturbed native forest. The rate of native species re-establishment on these sites is typically very slow and limited to a small subset of species present in the adjacent forest. The aim of this study was to investigate the factors that limit or facilitate this re-establishment. With the continuing decline of remnant native vegetation in rural Australia, research in this area is required to promote the process of re-establishment. Re-establishment on improved pastures was restricted to the edge of the pasture, whereas on unimproved (native) pastures reestablishment extended further into the pasture. In both cases, native trees and shrubs were relatively infrequent compared with herbaceous species. The effects of fire, soil disturbance and grazing were examined. The exotic pasture species resprouted quickly following disturbance, but there was typically a reduction in the dominance of exotic grasses. However, very little germination or growth of native species occurred following disturbance of the pasture. Significant inputs of seed were detected for native tree and shrub species (Eucalyptus spp., Leptospermum scoparium and Epacris spp.). However, relatively few seeds of native species were found in the soil seed bank in the pastures. Rates of seed predation were high in all seasons except winter. For some native species, the rate of predation may be sufficiently high to counter the large quantity of seed-fall. Seedlings of native trees and shrubs (Eucalyptus pulchella, Leptospermttm scoparium and Bedfordia salicina) were planted in the pasture with different levels of competition from the pasture grasses. Seedlings were more likely to survive or grow faster if grass root competition was reduced. Frost and herbivory contributed to the mortality of seedlings. The soil nutrient status of the pasture was not significantly different from the adjacent forest soil, except for available and total phosphorus which were higher in the pasture. Under glasshouse conditions, the growth rates of the three selected native species (see above) were better on the soil collected from the pasture than forest soil. However, the formation of mycorrhizal associations was poor on seedlings grown in pasture soil, except for soils collected from the pasture edge and from beneath isolated native trees in the pasture. In conclusion, the opportunities for the establishment of native seedlings are likely to be rare. This seems to be mainly due to the presence of exotic grasses that were highly resilient to disturbance and highly competitive.