There’s an interesting paper out recently in PeerJ by Roy et al., Alder and the Golden Fleece: high diversity of Frankia and ectomycorrhizal fungi revealed from Alnus glutinosa subsp. barbata roots close to a Tertiary and glacial refugium. It follows a simple but interesting puzzle. There’s been a lot of work on glacial refugia for plants. When the glaciers expanded during the Ice Age, there were a lot fewer locations plants could grow. It’s possible to identify these locations as the shelters for genetic diversity for the plants. However, much less has been done on symbionts, like ectomycorrhizal fungi.
A. glutinosa is a good tree to work with for this. Readers of Annals of Botany will be familiar with recent work by Mandák et al. examining the palaeohistory of A. glutinosa. In fact, Roy et al. are very specific in working with A. glutinosa subsp. barbata, and not just alders in general. This makes a lot of sense as Mandák et al. the past history of the species varies with the species of alder you’re looking at.
Alder also seems to be a good tree to examine as Roy et al. have found that Alnus ectomycorrhizal fungi communities are low in diversity, highly conserved at a regional scale, and only partly shared between congeneric host species. in their PeerJ paper, they note: “Alnicola and Alpova are two genera that appear to be strictly associated with alders because they have never been found on any other tree species. Several species of Lactarius, Russula, Amanita and Cortinarius are also exclusive to alders.” They also look at nitrogen-fixing actinobacteria in the genus Frankia, which tend to differ with ectomycorrhizal fungi communities.
Did the fungi and Frankia shelter with the A. glutinosa subsp. barbata? If they did then you’d expect to find more diversity and more endemic organisms in the Colchis region of Georgia where the alders lived. To test their idea Roy et al. sampled the colonies in the roots of alder in three areas of Georgia. They looked for endemic species and the results were interesting.
There isn’t really any endemism for ectomycorrhizal fungi in Colchis. If you were looking for a glacial refuge for Alder – based purely on examining their fungal partners – you wouldn’t expect it here. On the other hand, there were five endemic species of Frankia. So the soil symbionts both support and refute the hypothesis that symbionts sheltered with the Alder trees. There were also endemic ectomycorrhizal fungi and Frankia found outside the Alnus refugia. It’s a bit of a headache. In the discussion Roy et al. note that while both ectomycorrhizal fungi and Frankia need Alnus to survive in the soil, they disperse in very different ways. It would appear that ectomycorrhizal fungi are a lot more mobile.
Another factor they raise is Alder populations can live as isolated islands. In this situation specific symbiont species may be trapped on their island where there may become extinct from their relations on other islands. This could leave some locations as microbe refugia. Roy et al. note that there are other hotspots of microbe biodiversity, so their findings are consistent with a patchwork of microbial diversity, rather than all microbes being present in the soil. This, they add, has importance for biodiversity conservation.