Uncommon ectomycorrhizal networks: richness and distribution of Alnus ‐associating ectomycorrhizal fungal communities

The level of specificity is a key evolutionary and ecological parameter of any symbiotic interaction. Where diverse classes of mutualists interact, extracting the drivers of spatial–temporal variation in community composition is also fundamental to understanding the interaction. Ectomycorrhizal (EM) plants and fungi typically associate with many symbionts both as individuals and across a species’ range. Trappe (1977) estimated that Pseudotsuga menziesii is compatible with some 2000 species of EM fungi, an impressively high number that has stood the test of time but could be revisited. At the low end among autotrophic plants is the genus Alnus. Although it is known that the EM fungi associated with Alnus are not a rich community (Molina et al., 1992), there is still debate about the factors that might drive this phenomenon, as well as its implications. In this issue of New Phytologist, two contributions on Alnus EM fungi help to fill this gap. Roy et al. (pp. 1228–1238) provide a detailed investigation of the fungal diversity of Alnus associates at a regional scale, while P~olme et al. (pp. 1239–1249) assess fungal biogeography at a global scale. Both studies carefully evaluate the influences of host phylogeny, geographic distance, climate and other environmental variables on patterns of plant–fungus association. We have entered a new era of molecular ecology research in which advanced molecular tools for documentation of microbial diversity and genetic variation are increasingly combined with sophisticated statistical tools of population genetics, phylogenetics and community ecology to provide deeper insights with rigorous scientific support. The simultaneous publication of two studies using the full range of these tools focusing on the same model system at two spatial scales exemplifies this trend, together providing important new insight into host and habitat factors driving host specificity and community dynamics. Roy et al. sampled 165 trees belonging to five species in France and Corsica. P~olme et al. include root samples of 22 Alnus species collected from 96 stands across the broad geographic range of the genus. Surprisingly, this substantial difference in sampling scale yielded only a modest difference in EM fungal richness. Roy et al. report 86 molecular operational taxonomic units (MOTUs) from 1178 sequences in France and Corsica, while P~olme et al. report 146 MOTUs from 1172 sequences across the globe. One of the important questions in EMecology is what drives the composition and species richness of EM fungi found in plant communities (Bruns, 1995). The genus Alnus, with its low richness of EM fungi, provides an excellent model system to address this question. P~olme et al. found host species’ phylogenetic relatedness had the greatest impact on EM fungal community composition, suggesting fungus-to-plant specificity at the subgeneric level. The second most important factor driving EM community composition was biogeography. The fungal communities recovered from northwest America shared the mostMOTUs with those fromAsia, while those from northeast America shared the mostMOTUs with those from South America. The biogeographic patterns are not particularly well supported, but are not unique to Alnus fungi: the authors suggest they reflect host migration after the last glacial maximum. Surprisingly, higher richness at the global scale was correlated with increasing soil calcium (Ca) concentration, highlighting the need for additional research focused on mycorrhizal interactions and Ca biogeochemistry.