Colonization of a South Australian grassland by invading Mediterranean annual and perennial pasture species

Spatial variation in the composition of the soil seed bank was studied in a 5 ha area of pasture in South Australia dominated by naturalized annuals originating from the Mediterranean basin. The results were analysed with regression, cluster and principal component analysis. A total of 34 species was collected, including 17 grasses, seven legumes, eight broad-leaved species and two rushes. The total size of the seed bank was over 200 seeds dm-2, equivalent to about 450 kg ha-1 Although the site was nearly flat, elevation affected the distribution of all species, probably because of its effect on drainage in winter. Five legumes (Tbifilium clusii, T. ornithopioides, T. dubium, T. glomeratum and T. subterraneum) dominated the seed bank, but the dominant species changed with elevation above the zone of winter-waterlogging. Of the grasses Serrafalcus hordeaceus and Vulpia spp. were present at well drained sites, while Lolium perenne and Hordeum geniculatum were found at low sites, with Holcus lanatus intermediate. Seeds of legumes were more frequent at sites where soil nitrogen and phosphorus were low, and seeds of grasses where they were high. The results are compared with those obtained from perennial species in north Europe, and the implications for pasture deterioration and improvement in Australia are discussed. It is concluded that genetic diversity is likely to be important in the successful invasion of pasture by Mediterranean annuals. The presence of several naturalized legumes in niches incompletely occupied by subterranean clover suggests that germplasm is available to increase the legume component of many annual pastures