Frankia diversity in sympatrically occurring red alder (Alnus rubra) and Sitka alder (Alnus viridis) trees in an early successional environment

Key Message Frankia diversity on Mount St. Helens has recovered to regionally-observed levels, but small-scale geographic heterogeneity affects host-specificity of Frankia communities in red and Sitka alder. Alders ( Alnus sp.) are key pioneer tree species in disturbed and nutrient-poor ecosystems. Due to their association with nitrogen-fixing Frankia bacteria they have a disproportionate impact on soil quality and successional processes. However, surprisingly little information exists on the host-specificity and colonization patterns of Frankia communities among sympatrically occurring alder hosts in nature. We analyzed variation in Frankia community composition in sympatric red alder ( Alnus rubra ) and Sitka alder ( Alnus viridis ) root nodules from the Pumice Plain of Mount St. Helens, WA (Lawetlat'la in the Cowlitz language; USA). Five 2500 m 2 plots containing both red ( n = 11) and Sitka alder ( n = 12) trees were sampled along a 1.5-km transect. Five root nodules were collected from each tree, and Frankia genotypes were assessed by sequencing both nifH and 16S rRNA genes. In addition to root nodules, soil samples were collected from the rhizosphere of each tree for chemical analyses. We did not observe within-tree variation as only one Frankia genotype was detected per host tree, and the overall observed Frankia diversity was low and comparable to other studies of Alnus – Frankia symbioses. The most abundant nifH genotype was observed in both alder host species, in all plots, and occurred in 70.8% of all samples (69.6% of all trees). However, community composition was significantly different among plots (PERMANOVA, p = 0.002). Comparisons of communities among plots revealed modest correlations between geographic distance and community similarity (Mantel test, p = 0.001). Our findings suggest that even small-scale spatial variation and microenvironment conditions can affect an important plant–microbe symbiosis, which may have consequences for host local adaptation.