Passive clearing of native vegetation: Livestock damage to remnant jarrah (Eucalyptus marginata) woodlands in Western Australia

In south-western Australia, areas of native forest and woodland on farmland left after agricultural clearing (remnants) have in recent times been subject to chronic disturbance by grazing livestock. We analysed adjacent grazed and ungrazed sites to assess the effects of grazing disturbance on the scleropyhll woodland community. Species richness and diversity were reduced in heavily-grazed sites and floristic dissimilarity between grazed and ungrazed sites was high. In the heavily-grazed sites, exotic species were 46% to 48% of the species recorded. Frequency and cover of native perennial species was significantly reduced in the heavily-grazed sites and in a lightly-grazed site. There was a significant increase in the frequency and cover of exotic annual grasses and herbs in the heavily grazed sites. Other life form groups such as geophytes, native perennial grasses and native annuals were not significantly affected by livestock grazing. Grazing also resulted in a significant increase in surface soil compaction and water repellency as well as concentration of soil N and P. Size-class distribution of the overstorey indicated that no recruitment had taken place for many years and, although germination of overstorey species occurred each year, no seedlings survived at the heavily grazed sites. In these sclerophyll woodlands, grazing has altered plant community structure, from a understorey dominated by perennial shrubs to one dominated by exotic annual grasses and forbs. Resilience of native perennial species is dependent on reproductive strategy, life form and morphology, life history and palatiblity. Annual exotic species are favoured by increases in soil nutrients and disturbance, reduction in competition and an ability to withstand a high level of disturbance.