Estimating the dispersal capacity of the rare lichen Cliostomum corrugatum

The objective of this study was to estimate the dispersal rate in an organism assumed to be confined totree stands with unbroken continuity. We used the lichen-forming ascomycete Cliostomum corrugatum,which is largely confined to old oak stands. Five populations, with pairwise distances ranging from 6.5to 83 km, were sampled in Östergötland, south-eastern Sweden. DNA sequence data from an intron inthe small subunit nuclear ribosomal RNA gene was obtained from 85 samples. Nearly all molecular variance(99.6%) was found within populations and there were no signs of isolation-by-distance. The absolutenumber of immigrants per population per generation (estimated to 30 years), inferred by BayesianMCMC, was found to be between 1 and 5. Altogether, evidence suggests abundant gene flow in the historyof our sample. A simulation procedure demonstrated that we cannot know whether effective dispersal isongoing or if it ceased at the time when oaks started to decrease dramatically around 400 years BP. However,a scenario where effective dispersal ceased already at the time when the postglacial reinvasion ofoak had reached the region around 6000 years BP is unlikely. Vegetation history suggests that the habitatof C. corrugatum was patchily distributed in the landscape since the early Holocene. Combined with thehigh dispersal rate estimate, this suggests that the species has been successful at frequently crossing distancesof at least several kilometres and possibly that it has primarily been limited by the availability ofhabitat rather than by dispersal.