'Sax-sess'-- genetics of primary succession in a pioneer species on two parallel glacier forelands.

The primary succession on glacier forelands is characterized by a sequence of early and late successional species, but whether there is also a chronosequence at the intraspecific, genetic level is a matter of debate. Two opposing hypotheses differ in their prediction of genetic diversity in colonizing populations due to founder effects and postcolonization gene immigration. The development of genetic diversity in the pioneer Saxifraga aizoides was investigated along a successional gradient on two parallel glacier forelands, in order to test whether populations from older successional stages were less genetically diverse than populations from younger successional stages, and to locate the sources of the propagules that originally colonized new glacier foreland. Genetic diversity was determined with amplified fragment length polymorphisms, and potential sources of colonizing propagules were assessed via assignment tests. Our results indicate considerable postcolonization gene flow among populations on glacier forelands, since population differentiation was low and genetic diversity within populations was significantly higher. Molecular diversity and differentiation of populations did not develop linearly. Dispersal events within the glacier foreland, from the adjacent valley slopes, and from parallel glacier valleys were identified. In summary, it seems that the colonization of glacier forelands in the European Alps is highly dynamic and stochastic.