Abstract:
Results are summarized for a number of separate studies which investigated the effects of killing the trees, and of grazing by domestic livestock on herbage biomass, densities of grasses, forbs and woody plants and some surface soil properties in poplar box woodlands. Some temporal changes in vegetation and spatial patterns in soil properties occurred in all treatments. Forb and grass densities increased from 1966 to 1970 and 1972 respectively and then declined. Surface soil pH, micro-relief, salt content and litter biomass were generally higher at the tree bole than between the tree canopies. Herbage biomass was highest where trees had been killed but the killing of trees did not have a direct effect on the densities of either grasses or shrubs. The increased growth of herbage is attributed to an increase in the availability of water and nitrogen. The vegetation changes following tree killing with livestock excluded are summarized in the form of changes in the proportions of the grasses, shrubs and trees; grass proportion increased for the first six years following tree killing after which grasses tended to be replaced by shrubs. The density of shrubs increased following the exclusion of livestock. Shrub densities were less in experimental grazed plots than in ungrazed plots but still increased with time. With grazing by livestock, herbage biomass was highest adjacent to trees and the percentage of bare soil increased away from trees. Changes resulting from interactions between the grazing and killing treat- ments mainly related to soil compaction and water infiltration. It is suggested that shrub encroachment in the poplar box lands may arise because of fluctuations in grazing pressure resulting from large paddock and property sizes and variability in rainfall. Little of the available herbage is grazed during abnormally wet years, thus there is an effective release from grazing at a time when germination of shrubs is likely to occur.