Vegetation Dynamics in a Grazed Mulga Shrubland Community. I. The Mid-Storey Shrubs

Abstract

The dynamics of the major mid-storey shrub species in a grazed mulga shrubland community were studied on a senes of sites in the arid winter rainfall zone of Western Australia Of the major specles present, Eremophila leucophylla decreased with increasing grazing pressure while Maireana planifolia and Eremophila sp. declined after a temporary increase. All three species are strongly aggregated around old plant remains and under the canopies of existing trees and tall shrubs Their aggregation in this microenvironment results from enhanced seedling survival. Common preference for this restricted part of the total space apparently leads to an exclusion type of Interspecific competition and to species populations that are segregated relatwe to one another The importance of this microenvironment, and the apparent nature of interspecific competition within it, suggest that a stable state may develop under moderate levels of grazing in this community. The dynamics of the community are unlikely to be explained satisfactonly by traditional Clementsian concepts Rather, they would appear to be more consistent with the concept of alternative stable states. Any beneficial influence of management on community composition, from a pastoral point of view, is thus likely to be limited to dlscrete opportunities under specific seasonal conditions.