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Relative Growth of Six Thistle Species Along a Nutrient Gradient with Multispecies Competition

M. P. Austin, R. H. Groves, L. M. F. Fresco and P. E. Kaye
Journal of Ecology
Vol. 73, No. 2 (Jul., 1985), pp. 667-684
DOI: 10.2307/2260503
Stable URL: http://www.jstor.org/stable/2260503
Page Count: 18
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Relative Growth of Six Thistle Species Along a Nutrient Gradient with Multispecies Competition
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Abstract

(1) Seedlings of six species of thistle (Carduus nutans, C. `pycnocephalus', Carthamus lanatus, Cirsium vulgare, Onopordum aff. illyricum and Silybum marianum) common in mixed pastures of south-eastern Australia, were grown in pots of sand to each of which one of twelve concentrations of nutrient solution was applied regularly for 14 weeks. The species were grown either as monocultures or in mixtures. (2) The total yield of all species increased in monocultures and in mixtures with increasing total nutrients. Analyses of variance confirmed that nutrients, species and nutrient × species interactions were significant for both monocultures and mixtures. (3) Polynomial regressions were fitted to the responses of individual species to the nutrient-concentration gradient. High degree (quartic) functions were necessary to describe some responses. For individual species, the response curves in monoculture and mixture were different. (4) Non-linear smoothing with running medians was used to provide a more appropriate functional relationship. Complex response curves to nutrient concentration were obtained. Curves of shoot/root quotient (S/R) were also complex, although S/R quotients were generally higher in mixtures than in monocultures, especially for some taxa at the higher end of the nutrient gradient. (5) When species performance in monoculture was expressed on a relative basis (relative physiological performance, Rij), most showed a characteristic response; only Onopordum aff. illyricum and Silybum marianum were similar. Carthamus lanatus achieved maximum yield at low, Carduus `pycnocephalus' at intermediate and Cirsium vulgare at high nutrient concentrations. (6) There were differences between species in terms of their normalized ecological performances (EiJ) in mixtures--Carthamus lanatus was dominant at low, Carduus `pycnocephalus' at intermediate and Silybum marianum at high nutrient concentrations. (7) Prediction of thistle performance in multispecies mixture was shown to be possible from monoculture performance. However, the predictive model differed, for each of three zones along the nutrient concentration gradient. The significant regression models required the shoot/root quotient and nutrient concentration as well as the RiJ value of a species in order to predict the normalized ecological performance. (8) The results are discussed in terms of the known ecology of thistles in south-eastern Australia. Some suggestions as to the limitations to the predictive value of the results obtained from performance in monoculture are made.

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