Soil phosphorus and tree cover modify the effects of livestock grazing on plant species richness in Australian grassy woodland

The effects of grazing on the richness of understorey plant communities are predicted to vary along gradients of resources and tree cover. In temperate Australia livestock management has involved phosphorus addition and tree removal but little research has examined how the effects of grazing on plant species richness may vary with these management regimes. Patterns of understorey plant species richness were examined in 519, 0.09 ha quadrats in grazed pastures and remnant grassy forests and woodlands in southern Australia. Sheep grazing was the primary land use and sites varied widely in grazing frequency and density, tree cover and phosphorus fertiliser history. Using an information theoretic approach the available data provides strong evidence that the effect of grazing on total species richness varies according to available phosphorus and tree cover. Intermittent grazing and no grazing were associated with high total and native plant richness, but only at low phosphorus concentrations. Phosphorus was strongly negatively correlated with richness, particularly at low grazing frequency. Total species richness was positively correlated with tree cover except under frequent grazing at high stocking rates, suggesting that heavy grazing eliminates spatial and temporal heterogeneity imposed by trees. Native plant species richness was negatively correlated with a history of cultivation, positively correlated with tree cover and varied according to landscape position and geological substrate. Frequent high density grazing, particularly when associated with clearing, cultivation and fertiliser addition, was associated with the persistence of very few native plant species. In contrast, the richness of exotic plant species was relatively invariant and performance of the best model was low. While several studies have highlighted the importance of the grazed and cleared matrix for the conservation of native plant species, this benefit may be limited in landscapes where intensive grazing management systems dominate. Strong evidence for interactions between grazing, phosphorus and tree cover suggest that failure to consider other land use practices associated with grazing management systems could lead to erroneous conclusions regarding vegetation responses to livestock grazing.