CSIRO PUBLISHING www.publish.csiro.au/journals/ajb Australian Journal of Botany, 2004, 52, 653–658 Potential ballistic dispersal of Cytisus scoparius (Fabaceae) seeds Juan E. Malo Departamento Interuniversitario de Ecolog´ ıa, Facultad de Ciencias, Universidad Aut´ onoma de Madrid, E-28049 Madrid, Spain. Email: je.malo@uam.es Abstract. Cytisus scoparius (L.) Link (Fabaceae) has a ballistic type of primary seed-dispersal mechanism in which its legumes dehisce suddenly when they dry. This paper presents an experimental approach to the measurement and modeling of distances reached by seeds under optimum dispersal conditions. Branchlets of the species carrying mature legumes were collected and attached to 1.20-m-high platforms on a flat roof terrace. For 2 weeks, daily measurements were made of distances attained by seeds ejected from legumes in the previous 24 h. Seeds were found at a distance of 2343 ± 113 mm (mean ± s.e., n = 245), with 10.2% of recorded distances greater than 5 m and some about 7 m. The mixture model fitted to the distances traveled by seeds allows the mathematical isolation of two underlying processes, the ballistic projection of seeds by dehiscent legumes (nearly 49% of seeds, reaching 3686 ± 1797 mm) and the barochorous dispersal of the remaining seeds in the platform environs (1254 ± 1254 mm). Modeling shows that seeds dispersed ballistically reach locations at some distance from the shrub crown, with low densities of sibling seeds—a potential advantage for the establishment of new individuals. Introduction Seed dispersal is a fundamental process in plant reproduction as it affects the plants’ ability to colonise areas at different distances from those occupied by established individuals (Howe and Westley 1997; Nathan and Muller-Landau 2000). Plants have thus evolved numerous adaptations that facilitate the mobilisation and transport of seeds to ‘safe sites’ (areas with a relatively high probability of germination and establishment of new individuals). These adaptations and the spatial distribution of the dispersed seeds (seed shadows) have been the focus of much research (Ridley 1930; Van der Pijl 1982; Howe and Westley 1997; Willson and Traveset 2000) that has nevertheless irregularly covered the range of dispersal modes and has made little progress on the synthesis and production of predictive models. Frugivorous dispersal is thus disproportionately more analysed than the other modes, although research on it is to some extent bogged down in descriptive studies (Levey and Benkman 1999). Spatial analysis and modeling of dispersal, on the other hand, have made little progress, and thus present major challenges for the future (Nathan and Muller-Landau 2000). Attempts to model dispersal are focusing increasingly on the tails of distance distributions, as the small proportion of seeds that travel large distances plays a key role for the colonisation of new areas and in the responses by plant populations to habitat changes (Portnoy and Willson 1993; Cain et al. 1998; Lavorel 1999; Nathan and Muller- Landau 2000). Traditional models often underestimate the importance of these tails, a problem that may be addressed by the application of mixture models (Higgins and Richardson 1999). Mixture models combine two or more probability distributions and therefore fit observations of distances traveled by seeds more precisely because each distribution is able to represent an underlying dispersal process. Moreover, their use could be of outstanding interest for research into the reproductive biology of plants with heteromorphic seeds (McEvoy and Cox 1987; Gibson and Tomlinson 2002) or multiple dispersal systems (Liddle and Elgar 1984). Little is known about seed dispersal of early successional dry-fruited species, despite the important role played by colonisation of new sites in their life-history strategies. Dry- fruited shrubs (e.g. species of Leguminosae, Cistaceae and Labiatae) form a rich but relatively homogeneous species guild in the Mediterranean, where they are predominant in early successional scrubs (Herrera 1984; Aronne and Wilcock 1994). This species guild is also predominant in other Mediterranean areas (Keeley 1991; Cowling et al. 1996) and dry-fruited shrubs are abundant outside this climatic context too. In general, these species have small seeds with abiotic dispersal and most lack morphological adaptations for this purpose. Many dry-fruited shrubs form highly resilient thickets because of their pyrophillous nature (Pausas 2001; Valbuena and Trabaud 2001), but they also have a remarkable ability to colonise new areas (Parker 2000; De Simone and Zedler 2001). Scotch broom, Cytisus scoparius (L.) Link. (Fabaceae), grows naturally in most of Europe and is now naturalised © CSIRO 2004 10.1071/BT03162 0067-1924/04/050653