Uncharted terrain: the phylogeography of arctic and boreal lichens

Lichens represent symbioses between a fungus and a cyanobacterium and/or a green alga and form an important part of the biodiversity in arctic, subarctic and boreal terrestrial ecosystems. Most lichens occupy large, frequently underexplored distribution areas with frequent intercontinental and sometimes bipolar disjunctions. Because of this and the fact that lichens are often disregarded or overlooked in vegetation surveys, distribution maps are usually sketchy and the composition of lichen floras of remote regions is often poorly known. Three main problems have impeded studies on the biogeography of lichens in the past: fuzzy species concepts, incomplete knowledge of the distribution of species and a lack of fossil records. The application of molecular methods in phylogeographic studies on lichens provides some first insights into the population structure and history, but so far most studies have concentrated on boreal and bipolar taxa. The few available examples show that possible glacial refugia for some boreal species existed in western North America. The wide distribution of arctic and boreal species seems to be matched by a comparable wide distribution of single DNA haplotypes. Whether this is due to incomplete lineage sorting or ongoing genetic exchange among populations is so far unresolved. More phylogeographic studies, including studies on lichen photobionts, are necessary to render a more coherent picture of the postglacial history of lichen populations.