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    Author(s): Jennifer S. Mitchell; Roger W. Ruess
    Date: 2009
    Source: Biogeochemistry. 95: 215-229
    Publication Series: Scientific Journal (JRNL)
    Station: Pacific Northwest Research Station
    PDF: View PDF  (3.29 MB)

    Description

    Green alder (Alnus viridis ssp. fruticosa) is a dominant understory shrub during secondary successional development of upland forests throughout interior Alaska, where it contributes substantially to the nitrogen (N) economy through atmospheric N2 fixation. Across a replicated 200+ year old vegetation chronosequence, we tested the hypotheses that green alder has strong effects on soil chemical properties, and that ecosystem-level N inputs via N2 fixation decrease with secondary successional stand development. Across early-, mid-, and late-successional stands, alder created islands of elevated soil N and carbon (C), depleted soil phosphorus (P), and more acidic soils. These effects translated to the stand-level in response to alder stem density. Although neither N2 fixation nor nodule biomass differed among stand types, increases in alder densities with successional time translated to increasing N inputs. Estimates of annual N inputs by A. viridis averaged across the upland chronosequence (6.6 ± l.2 kg N ha-1 year-1) are substantially less than inputs during early succession by Alnus tenuifolia growing along Alaskan floodplains. However, late-succession upland forests, where densities of A. viridis are highest, may persist for centuries, depending on fire return interval. This pattern of prolonged N inputs to late successional forests contradicts established theory predicting declines in N2-fixation rates and N2 -fixer abundance as stands age.

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    Citation

    Mitchell, Jennifer S.; Ruess, Roger W. 2009. N2 fixing alder (Alnus viridis spp. fruticosa) effects on soil properties across a secondary successional chronosequence in interior Alaska. Biogeochemistry. 95: 215-229.

    Keywords

    alder, boreal, nitrogen cycling, nitrogen fixation, secondary succession

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