Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

Ecological conditions shape natural distribution of plants. Populations are denser in optimal habitats but become more fragmented in the areas of suboptimal environmental conditions. Usually, fragmentation increases towards the limits of species distribution. Fragmented populations are often characterised by decreased genetic variation, and this effect is frequent in peripheral populations, mostly due to the reduced effective population size. Interestingly, the genetic consequences of fragmentation seem to be relatively weak in forest trees. Using microsatellite markers, we assessed the impact of population fragmentation on the genetic structure of a European tree species Acer campestre. Within the study area, this medium-size wind-dispersed and insect-pollinated tree reveals a gradual decrease in population density towards the northern range limit. Over the distance of 150 km, we detected the significant decrease in allelic richness, heterozygosity as well as an increase in the rate of population divergence along with latitude. On the other hand, we failed to show that the observed patterns of genetic structure result from the variation in population densities. Moreover, inbreeding levels revealed no association with both density and geographic location, suggesting that pollen limitation does not occur, even at the range margin. As we showed that there is no difference in a dispersal scale between low- and high-density populations in the study species, we argue that the genetic structure is a result of postglacial recolonization. However, unlike many other forest trees, A. campestre showed the sharp latitudinal genetic pattern at a very restricted spatial scale. Limited dispersal and high fragmentation are likely the reasons.