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Alder Distribution and Expansion Across a Tundra Hillslope: Implications for Local N Cycling

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Frontiers in Plant Science
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We investigated the impact alder, an N-fixing deciduous shrub, has on tundra N cycling at the Kougarok hillslope site located on Alaska’s Seward Peninsula. Alder (Alnus viridis spp fruticosa) grows within two of the six plant communities at this site: alder shrublands located on well-drained, rocky outcroppings near the crest of the hill and mixed shrub-tundra located along the moist toeslope of the hill. We quantified symbiotic N fixation in these two alder ecotypes by measuring nodule biomass and N fixation rates within nodules using 15N-N2. A map of current alder shrubland distribution was generated using a multi-sensor fusion approach. Historic cover of alder shrubland was also derived from aerial images of the site collected in 1956 through 2014. Alder growing in shrublands at the Kougarok hillslope site elevated local soil N availability. Annual N fixation by alder shrublands was 1.95 ± 0.68 g N/ m2/ year. Alder growing in mixed shrub-tundra communities fixed less N annually (0.53 ± 0.19 g N/ m2/ year). The differences between these rates was primarily driven by low nodule biomass in mixed shrub-tundra which may be due to low P availability and poor drainage in these organic soil profiles. Nodule biomass, N fixation rates within nodules, and alder aboveground traits all exhibited a strong bimodal distribution. Such a dichotomy across above- and belowground observations is unusual and suggests that N fixation in this tundra ecosystem could be inferred from aboveground alder traits. Analysis of historic images showed that alder shrubland cover at Kougarok has increased 40% since 1956 and we estimate this resulted in a 28% increase in N fixation. Overall, our results suggest that alder expansion has an important impact on N availability in warming tundra ecosystems and that ecotypic variation is crucial for quantifying N fixation at the landscape scale.