Dynamics of fire perturbation and exotic plant invasions in selected south-western Australian ligneous formations

The structure and composition of much of the contemporary vegetation of south-western Australia is considered to be undergoing significant changes associated with the settlement of European man. This study was undertaken to document dynamic processes perceived to be important. principally the effects of fire regime on the vegetation and the concomitant establishment of exotic plant species. In addition to providing basic documentation of species and structural changes in local vegetation. motivation for this research was provided for by the need to devise management strategies to maintain the floristic integrity of gazetted vegetation reserves. Within the landscape of the Swan Coastal Plain and Darling Plateau there are two major ligneous systems; the Jarrah (Eucalyptus marginata) forest on uplifted lateritic soils, and the Jarrah-Banksia-Tuart (Eucalyptus gomphocephala) woodland complex on coastal dune sequences. Selected formations within these systems were examined for changes in floristic composition and structure which may be associated with recent fire regimes. The E. marginata sampling unit, currently under Forest Department fuel reduction management policy, showed a regular regeneration response to a 5-7 year rotational fire cycle. This compared to a variable response to past fire regimes within typical stands of Banksia woodland communities. Changes in structure and species composition within a range of degradation phases is described. Phenological studies in both systems indicated late autumn to be the most appropriate season for regenerative burning. E. gomphocephala stands within a reserve protected from fire showed regeneration of the overstorey to be in a state of dynamic change. however this was not related to fire regime or to competitive interaction with the co-dominant tree species, Agonis flexuosa. The interface between elements of the natural ligneous vegetation and the exotic flora introduced since 1829 was examined. Many exotic species were observed to require previous disturbance of the natural vegetation before successful establishment, while other species. particularly those of South African origin, establish in relatively undisturbed vegetation. Another group of species appears facultative with respect to disturbance. The type of existing vegetation, substrate and level of disturbance each influence the distribution and establishment of exotic species. Also, examination of community characteristics of examples of natural and exotic herbaceous communities showed that high levels of site invasion are associated with low species richness; this becomes more significant when a few species attain high relative dominance. Aspects of habitat pre-adaptation, life-form characteristics, seed germinability, and soil nutrition level are discussed in relation to the establishment of exotic species. The implications for conservation and management of nature reserves from the view-point of maintaining floristic diversity and stability in a fire-prone environment is also considered