Survival of native plants of Hawkesbury Sandstone communities with additional nutrients: effect of plant age and habitat

Australian soils are naturally low in nutrient concentrations, particularly nitrogen (N) and phosphorus (P). Native plants are well adapted to low-nutrient soils, and can be adversely affected when exposed to higher concentrations of nutrients. The Hawkesbury Sandstone soils in northern Sydney are naturally low in nutrients, but often receive additional nutrient input from urban stormwater run-off. Increases in soil nutrients in urban bushland are associated with the presence of exotic species, and the decline in the diversity of native species. This study tested the hypothesis that high concentrations of nutrients, in particular P, in the disturbed soils of urban bushland, reduce survival of native plants. We examined the survival of native species under five different nutrient concentrations that are typical of nutrient-enriched urban bushland soil, in two glasshouse experiments. The experiments examined both survival of seedlings and survival of 6-month-old plants. We used native species that are adapted to both nutrient-poor and nutrient-rich soils. In general, the survival of native plants decreased with increasing nutrient concentrations. At soil total-P concentrations >200 mg kg^-1, most plants died. Seedlings were more sensitive to added nutrients than the 6-month-old plants. Species that were from higher-nutrient soil had consistently higher survival than species from low-nutrient soils, under the nutrient addition treatments. These results suggest that at high soil nutrient concentrations typical of stormwater-affected urban bushland, native plant species of low-nutrient soils will be unable to survive. If ecological restoration works are to be done in such areas, replanting with more mature plants from naturally high-nutrient habitats is likely to be the most successful. However, restoration of these areas may have limited success and they are likely to remain dominated by exotic plant species.