Effects of discontinuous marginal habitats on the genetic structure of common ash (Fraxinus excelsior L.)

Common ash (Fraxinus excelsior L.) has, in comparison to other tree species such as spruce (Picea), beech (Fagus) and oak (Quercus), poor competitive ability and is rarely dominant under natural conditions. This study, performed at the northern limit of the species distribution on islands in Southern Finland, investigates the genetic structure of four populations of ash which differ in their degree of isolation. Our hypothesis was that the patchy distribution of suitable habitats in terms of presence of islands and suitable soils leads to low genetic diversity within and strong genetic differentiation between populations. This study using nuclear microsatellites provides first information that increasing isolation goes along with decreasing genetic variability, an almost total interruption of gene flow and increasing population genetic differentiation. Furthermore the rarity of suitable ecological niches for F. excelsior seem to have fundamental effects on the mode of colonisation.