VEGETATION-ECOLOGICAL ASPECTS OF “DESERTIFICATION” IN THE SEMI-ARID REGIONS OF AUSTRALIA

In the Australian Continent, in those areas occupied by Eurpoeans during the past 200 years, the natural environment has been modified; the change are most severe in the semi-arid regions. Prior to Eurpoean settlement, for some tens of thouthands years of Aboriginal occupation, there were slow changes mainly due to occasional burning of vegetation so that the Australian flora was gradually moved towards fire-resisted species. With the introduction of agriculture and grazing drastic changes in the density, floristics, and structure of native vegetation occurred. Disturbance or removal of vegetation has also affected surface and sub-surface hydrology. These changes have resulted in water or wind erosion and salinization. Where vegetation has been regenerated the species established are often those which are unpalatable to grazing stock or can withstand frequent burning. This leads to “desertification”.<br> The concept of “desertification” is ecologically defined as follows: The natural vegetation in equilibrium with the potentiality of regional climate (climatic climax) is replaced by the vegetation which grows naturally and originally in more arid region due to environmental stress directly induced by man and the accompanying land degradation.<br> The Murray-Mallee Region in the southern part of Australia, where the late Pleistocene and Holocene sand dunes are widely distributed, is a semi-arid region with annual rainfall of 250-400 mm; it is wholly covered with mallee (Eucalyptus spp.) scrub. The sand dunes which were stable covered totally with such vegetation during the pre-European days have become unstable at many sites due to vegetation clearing, farming and grazing associated with European settlement since the late 19th Century. The remobilization of sand dunes is one, of the typical events of “desertification” and, in extreme cases, the surface remains unvegetated. Various stages in vegetation change can be observed showing “desertification” or regeneration process. The authors describe the processes involved in land and vegetation degradation, and discuss the measures to prevent “desertification” based on the examination of the relationships between activity of sand drifting, thickness of loose sand overlying the stable substrata, floristic composition, community height, and activity of rabbits (Olyctolagus cuniculus).<br> The natural mallee scrub in the conservation park is dominated by Eucalyptus spp. accompanied by rich lower shrub layer species such as Casuarina stricta, Callitris verrucosa and Hakea leucoptera. The mallee scrub remaining on farmland, however, is very poor in understory resulting from grazing. Triodia spp. and Acacia spp. invade in the understory of the mallee scrub after bushfires. Acacia spp. regenerate easily and are dominant on the land where the surface is disturbed by vegetation clearance and/or wind erosion. Natural mallee scrub will regenerate through sprouting from so-called mallee roots (lignotuber) persisting after surface disturbance. When the lignotuber is exposed by wind erosion or covered by sand accumulation, the recovery to natural mallee scrub becomes very difficult. On such sites, Triodia colonies and/or Acacia scrub stand as disclimax communities with a life cycle of 30 to 50 years (Figs. 6 and 7).<br> The thicker the loose sand is, the more active and continuous is the sand drifting (Fig. 8). In particular, on many dunes with loose sand thicker than 80 cm, serious drifting has continued for recent tens of years exacerbated by the large amount of mobile sands and the difficulty of vegetation invasion. At the sites with thick loose sands, the population of rabbit introduced from Europe is larger, suggesting that the reactivation and acceleration of sand drifting is induced by rabbit infestation (Fig. 9).