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Annals of Botany 89: 525-536, 2002
© 2002 Annals of Botany Company

Sexual and Vegetative Reproduction of Hieracium pilosella L. under Competition and Disturbance: a Grid-based Simulation Model

E. WINKLER*,1 and J. STÖCKLIN2

1UFZ-Centre for Environmental Research Leipzig-Halle, Department of Ecological Modelling, POB 500 135, D-04301 Leipzig, Germany and 2Universität Basel, Botanisches Institut, Schönbeinstr. 6, CH-4056 Basel, Switzerland

* For correspondence. E-mail ewi{at}oesa.ufz.de

Received: 21 August 2001; Returned for revision: 29 October 2001; Accepted: 18 January 2001.

We used a spatially explicit simulation model to examine the relative importance of vegetative and sexual reproduction in Hieracium pilosella L. Based on an understanding of the complex life cycle of this species and on data from in situ population dynamics in a calcareous grassland in NW Switzerland, we simulated growth and the relative contribution of clonal reproduction by stolons and reproduction by seeds across a gradient of increasing soil fertility. Competition by a clonal grass resulted in nearly complete exclusion of H. pilosella from the more fertile part of the simulation plot. Under low soil fertility, when grass could not survive, H. pilosella largely persisted by vegetatively produced rosettes. This pattern of a sharp separation of both species was shifted slightly in favour of H. pilosella by introducing random disturbances. Only by adding: (1) long-distance seed dispersal, and (2) facilitation of seedling establishment in the vicinity of grass tussocks in vegetation gaps was a more realistic representation of field observations realised, with rosettes of H. pilosella grown from seeds occasionally distributed within dense grass vegetation. Phenotypic plasticity of stolon length was a decisive factor for the maintenance of H. pilosella populations. We conclude that a mixed strategy of clonal growth and reproduction by seeds in H. pilosella is necessary to maintain populations of this species in the presence of high interspecific competition and a shortage of open space.

Key words: Clonal growth, competition, Hieracium pilosella, mouse-ear hawkweed, reproductive strategies, seed dispersal, sexual reproduction, spatially explicit model.


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