The genus Alnus hosts strikingly specific communities of ectomycorrhizal (ECM) fungi. This specificity, however, has been noted largely in relation to co-occurring, distantly related plants. We sought to understand whether the evolutionary history of the host might explain Alnus ECM specificity by comparing Alnus and Betula ECM communities. If Alnus ECM fungi are specific to the Betulaceae family, but not to Alnus itself, they would be expected to associate promiscuously with these sister host genera. To test this hypothesis, we examined the ECM communities on Alnus rhombifolia and Betula occidentalis in western Idaho. We characterized the communities at a field site where the hosts co-occur and in a growth chamber bioassay, comparing the ECM communities present when Alnus and Betula roots overlapped to those present on each host alone. Molecular techniques were used to identify ECM fungal taxa and host roots.
In the field, our results resembled those of prior studies: Alnus supported a small, homogenous ECM community, while Betula hosted a diverse group of fungi. Only four of the 27 ECM taxa in the field study (Laccaria, Russula, Tomentella, and Tuber species) occurred on both Alnus and Betula, and only Laccaria was found on both host genera in a single soil sample. In the growth chamber assay, however, the ECM communities on Alnus and Betula included shared taxa in both the single-host and mixed-host arrangements. In four of five cases, overlapping root systems shared at least one fungal taxon, but these shared taxa comprised only two of six taxa present (an Alnicola and a Tomentella species).
This result challenges the paradigm of absolute genus specificity among Alnus ECM fungi. At least six taxa (Alnicola, Laccaria, Russula, Tuber, and two Tomentella species) can associate with both Alnus and Betula, but no taxon except Laccaria associated with both hosts simultaneously in the field. The reciprocal specificity of Alnus and its ECM fungal associates appears to be strong, but not absolute, and may be driven by selectivity of the host, specialization of the fungi, and the influence of other root symbionts.