Effects of phosphorus on growth and competitive interactions of native and introduced species found in White Box woodlands


Department of Biological Sciences, CW 405 Biological Sciences Centre, University of Alberta, Edmonton AB, Canada T6G 2E9 (Email: kallcock@ualberta.ca)


Abstract  White Box (Eucalyptus albens Benth.) woodlands are among Australia's most endangered ecosystems and are threatened by exotic species invasion. There is evidence from other Australian communities that phosphorus enrichment can facilitate invasion, and differential growth of native and exotic species under increased phosphorus is a possible mechanism. Two glasshouse experiments were designed to test the following three questions relating to species responses to phosphorus: (i) do exotic and native species have different patterns of growth along a gradient of increasing phosphorus?; (ii) do exotic species have a greater competitive effect on native species than do conspecifics?; (iii) does phosphorus enrichment compound the competitive effect of exotic species on native species? Four native perennial species (Themeda australis (R. Br.) Staph., Bothriochloa macra (Steud.) S. T. Blake, Austrodanthonia racemosa (R. Br.) H. P. Linder and Eucalyptus albens) and two exotic annual species (Vulpia bromoides (L) Gray and Echium plantagineum L) were used. In the first experiment, plants were grown individually under six levels of soil phosphorus ranging from 0 to 60 p.p.m. In the second experiment, individuals of Eucalyptus albens and B. macra were grown alone, with a conspecific competitor, or with an exotic (V. bromoides or Echium plantagineum) competitor under low (10 p.p.m.) and high (100 p.p.m.) phosphorus. Both exotic species showed a greater positive response to increased phosphorus than the native species in experiment 1, and Eucalyptus albens seedlings grown with Echium plantagineum were significantly smaller than individuals grown alone or with Eucalyptus albens in experiment 2. There was no evidence that high phosphorus increased the competitive effect of the exotic species, but the combination of a strong positive response to phosphorus and a strong effect on growth of a native species indicates that phosphorus enrichment could favour exotic species in woodland remnants and that field studies testing the effect of phosphorus in a broader context would be appropriate.