Primary productivity of tundra plants is strongly limited by nitrogen (N) availability, so plants capable of symbiotic nitrogen fixation have the potential to alter plant, soil and microbial interactions in these rapidly arctic warming ecosystems. We examined the above- and belowground traits of alder, an N-fixing deciduous shrub, at the Kougarok hillslope site located on Alaska’s Seward Peninsula. Our goal was to assess the current and past impact alder has on N cycling within a tundra landscape. Alder grows within two of the six of plant communities at Kougarok: 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 by these two alder ecotypes by measuring nodule biomass and N fixation rates within nodules using 15N-N2 . A suite of aboveground trait data (height, basal area, foliar chemistry) was also collected. Maps of the current distribution of alder shrublands were generated by applying a multi-sensor fusion approach to remote sensing datasets from multiple sensor platform. Historic cover of alder shrubland at the site was derived from aerial images of the site collected in 1956, 1978, 2006 and 2014.
Results/Conclusions:
Alder growing in shrublands at the Kougarok hillslope site elevated local soil N availability and 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). Differences between these annual N fixation rates was primarily driven by differences in nodule biomass between the two ecotypes. Nodule biomass was low in mixed shrub-tundra, potentially due to P limitation. Nodule biomass, n fixation rates within nodules, and alder aboveground traits exhibited a strong bimodal distribution. This dichotomy across above- and belowground observations is unusual and suggests that N fixation in this system could be inferred from aboveground alder traits. At Kougarok, alder shrubland cover has increased 40% since 1956 and we estimate this increase was associated with a 28% increase in annual N fixation. Overall, our results suggest that 1) the inclusion of a high latitude N-fixing shrub in dynamic vegetation models will be crucial for capturing the changing N cycle of high latitude ecosystems and 2) the structure and function of this plant functional type will need to take into account bimodal distributions of above and belowground traits.