Experimental evidence for the effects of additional water, nutrients and physical disturbance on invasive plants in low fertility Hawkesbury Sandstone soils, Sydney, Australia

Abstract

Summary The environmental conditions of a site are thought to influence its invasibility by exotic plants. We tested the effects of physical disturbance, water and nutrient addition on growth and survival of a total of 28 plant species from urban bushland remnants on low fertility Hawkesbury Sandstone soils in Sydney, Australia. Species were classified as native (to Australia, including the local community) or exotic (from outside Australia) and as non‐invasive or invasive. In a glasshouse experiment the treatments were control (natural soil), added water, added nutrients and added water and nutrients. We found significant differences in response of the plant types to additional nutrients but not to water. Under additional nutrients, exotic invasive species had better survival than all other plant types. There were strong biomass responses to added nutrients for all plant types except native non‐invasives. In contrast, the survival and growth of all plant types were similar under control and additional water treatments. In a field experiment the factors were site (high and low nutrient soils) and physical disturbance but survival differed between plant types only in relation to site. Exotic species had much better survival than native species at the high nutrient site only. Exotic invasive species had the strongest biomass responses to the high nutrient site while native and exotic non‐invasive species showed relatively smaller growth responses. Differences between invasive and non‐invasive plants were not consistent between exotic and native species. Exotic invasives had greater survival than exotic non‐invasives under high nutrient conditions in both experiments, and only exotic invasives showed consistently strong biomass responses to nutrient addition. For native species, there were no differences between invasives and non‐invasives in survival in high nutrient conditions in the glasshouse or field; however, only invasive species showed a positive growth response to high nutrient conditions. This study provides clear evidence that the success of exotic invasive species in this low fertility vegetation community is facilitated by the addition of nutrients. It would appear that on relatively infertile (particularly phosphorus‐limited) sites, invasive species cannot take advantage of additional resources provided by physical disturbance or water addition due to overwhelming nutrient‐limitation. We suggest that a combination of the environmental conditions (nutrient‐enrichment) and invading species having traits that allow high survival and faster growth in response to nutrients, may allow successful invasion in Hawkesbury Sandstone communities.