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 Mediterranean refugia and rapidly recolonized temperate Europe afterwards. Inter- and postglacial migration processes are assumed to have catalized evolutionary optimizations 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-dispersed tree Frangula alnus have experienced differentiations of dispersal-related traits which increase the mobility of northern populations. A comparison of lifetime reproductive strategy, disperser guilds, ripening phenology, and fruit design revealed considerable differences. Compared to south Iberian conspecifics, central European plants were considerably smaller and experienced a highly accelerated generation turnover, In south Iberian populations seed dispersal was carried out almost completely by resident birds which occurred in constant abundances throughout the ripening season. In contrast, 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 dispersal. The fruit ripening pattern was highly asynchronous throughout the ripening season in south Iberia, while central European trees showed a complex ripening sequence which resulted in a significant correlation between fruit 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 dispersers (Sylvia warblers). Chemical analyses revealed significant differences in contents of water, glucose, fructose, proteins, ash, and phenolic compounds. The extensive differentiation of dispersal-related traits in F. alnus suggests that even weak selective pressures by frugivores may induce evolutionary adjustments of dispersal traits over large time scales. We suggest that the differences we observe today evolved during the species' distribution shifts in the Quaternary.