| Summary: | The climatic drivers and landscape-scale patterns of recent Arctic shrub expansion are well understood, while the biogeochemical and physical mechanisms controlling shrub recruitment and growth remain unclear. The biogeophysical characteristics associated with tall shrub expansion near streams could potentially explain the documented increases in carbon and nutrient export from large Arctic rivers through the underlying processes of widespread permafrost degradation and increased subsurface flow. Additionally, occurrences of tall shrubs could be used as a readily measured proxy for subsurface properties both across space and time using remote sensing. If our observations hold true in other stream environments, the information has the potential to provide novel hydrological, permafrost, and biological process insights to complement the extensive literature on increasing Arctic shrub cover. Preliminary measurements of differential runoff in Arctic tundra link tall shrubs along riparian corridors to losing stream sections (streams with decreasing discharge downstream), whereas gaining or maintaining stream sections (increasing discharge downstream) lack tall shrubs. We tested whether the formation of a talik (permafrost-free ground) and a downward hydrologic gradient creates a biogeochemical subsurface environment that favors shrub recruitment and growth, which, in turn, results in patterns of tall shrub distribution coinciding along losing stream reaches. Therefore, we measured two streams with differing gradients, bed morphologies, and substrates to test the statistical significance of our preliminary observation that tall shrubs indicate losing streams. Furthermore, we expanded our measurements to include shrub height, cover, biomass, Leaf Area Index (LAI), permafrost, soil properties and soil microbial communities. To describe the soil properties and the microbial communities, we sampled soils along two Creeks (Oksrukuyik and Aesthetic creek) on the North Slope of Alaska from 21st-27th August 2016. We sampled composite soils samples (2 creeks x 2 hydrologic regimes x 3 replications= 12 samples). Soils were sampled from the upper 10 cm of the soil profile. Soil nutrients, moisture and texture were analyzed and was DNA extracted. Bacteria (V4 region) and fungi (ITS2) were sequenced on a MiSeq. The presence and relative abundance of functional genes was determined with the GeoChip 5.0 functional gene microarray.
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