| Summary: | Because of its high phosphorus (P) demands, it is likely that the abundance, distribution, and N-fixing capacity of <i>Alnus</i> in boreal forests are tightly coupled with P availability and the mobilization and uptake of soil P via ectomycorrhizal fungi (EMF). We examined whether <i>Alnus</i> shifts EMF communities in coordination with increasingly more complex organic P forms across a 200-year-old successional sequence along the Tanana River in interior Alaska. Root-tip activities of acid phosphatase, phosphodiesterase, and phytase of <i>A. tenuifolia</i>-associated EMF were positively intercorrelated but did not change in a predictable manner across the shrub, to hardwood to coniferous forest successional sequence. Approximately half of all <i>Alnus</i> roots were colonized by <i>Alnicola</i> and <i>Tomentella</i> taxa, and ordination analysis indicated that the EMF community on <i>Alnus</i> is a relatively distinct, host-specific group. Despite differences in the activities of the two <i>Alnus</i> dominants to mobilize acid phosphatase and phosphodiesterase, the root-tip activities of P-mobilizing enzymes of the <i>Alnus</i>-EMF community were not dramatically different from other co-occurring boreal plant hosts. This suggests that if <i>Alnus</i> has a greater influence on P cycling than other plant functional types, additional factors influencing P mobilization and uptake at the root and/or whole-plant level must be involved.
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