Early vs. Late Succession
The four Frankia soil clades discerned in our phylogenetic
analysis were non-randomly distributed across early and late-succession
soils derived from A. tenuifolia rhizospheres (Table 1,
pseudo-R2=0.09, likelihood ratio (LR) P <
0.0001). Between early and late succession, proportional representation
of AT clade sequences decreased significantly (Fisher’s exact P
< 0.0001), representation of soil clade B increased
significantly (Fisher’s exact P < 0.0001), and ‘typicalAlnus ’ representation trended toward a significant increase
(Fisher’s exact P =0.07).
Distribution of sub-clades was also non-random between early and late
succession (Table 2) (pseudo-R2=0.17, LR P <
0.0001) soils. The largest difference occurred for the early-succession
nodule specialist RF7 (Fisher’s exact P < 0.0001), which was
not found in any late succession soils but made up >25% of
the clones derived from early-succession soils (Table 2). Significant
differences also occurred for soil sub-clades B1 (P =0.01), B3 (P
< 0.0001), and B4 (P = 0.007), which yielded 6.1%, 16.7%,
and 7.0% of clone sequences from late succession alder soils, but were
not found in early succession soils. Sequences related to the ‘typicalAlnus ’ group that is dominant in nodules of A. viridis in
our study sites were found in seven (6.1%) clones from late succession
alder soils, but only one (0.9%) early succession clone. This
difference trended toward significance (P = 0.07). Late succession
soils, both within and outside of A. tenuifolia rhizospheres,
appear to support higher OTU richness than early succession soil (Figure
2A).