Modified dispersal-related traits in disjunct populations of bird-dispersed Frangula alnus (Rhamnaceae): a result of its Quaternary distribution shifts?

Many European tree species survived Pleistocene glaciations in Mediterranea n refugia and rapidly recolonized temperate Europe afterwards. Inter- and p ostglacial migration processes are assumed to have catalized evolutionary o ptimizations of dispersal-related traits, but up to now empirical evidence is lacking in vertebrate-dispersed plants. We investigated if south Iberian glacial relict and central European "colonizer" populations of the bird-di spersed tree Frangula alnus have experienced differentiations of dispersal- related traits which increase the mobility of northern populations. A compa rison of lifetime reproductive strategy, disperser guilds, ripening phenolo gy, and fruit design revealed considerable differences. Compared to south I berian conspecifics, central European plants were considerably smaller and experienced a highly accelerated generation turnover, In south Iberian popu lations seed dispersal was carried out almost completely by resident birds which occurred in constant abundances throughout the ripening season. In co ntrast, central European seeds were dispersed by migrants whose abundances changed considerably during the ripening season. Several bird species were involved in both study areas but rendered different importance for seed dis persal. The fruit ripening pattern was highly asynchronous throughout the r ipening season in south Iberia, while central European trees showed a compl ex ripening sequence which resulted in a significant correlation between fr uit abundance and changing disperser availability. Central European fruits were smaller and showed a considerably smaller seed load than south Iberian fruits, thus presumably being more attractive for their small-sized main d ispersers (Sylvia warblers). Chemical analyses revealed significant differe nces in contents of water, glucose, fructose, proteins, ash, and phenolic c ompounds. The extensive differentiation of dispersal-related traits in F. a lnus suggests that even weak selective pressures by frugivores may induce e volutionary adjustments of dispersal traits over large time scales. We sugg est that the differences we observe today evolved during the species' distr ibution shifts in the Quaternary.