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The increase in deciduous shrub growth in response to climate change throughout the Arctic tundra has uncertain implications, in part due to a lack of field observations. Here we investigate how increasing alder shrub growth in alpine tundra in Interior Alaska corresponds to active layer thickness and soil physical properties. We documented increased alder growth by combining biomass harvests and dendrochronology with the analysis of remotely sensed Normalized Difference Vegetation Index and fire history. Active layer thickness was measured with a tile probe and carbon and nitrogen pools were assessed via elemental analysis. Shallower organic layers under increasing alder growth indicate that nitrogen-rich, deciduous litter inputs may play a role in accelerating decomposition. Despite the observed reduction in organic carbon stocks, active layer thickness was the same under alder and adjacent graminoid tundra, implying deeper thaw of the underlying mineral soil. This study provides further evidence that the widely observed expansion of deciduous shrubs into graminoid tundra will reduce ecosystem carbon stocks and intensify soil–atmosphere thermal coupling. Two consequences of rapid climate warming in the Arctic, where grass-like plants dominate under very cold conditions, are an increased growth and occurrence of shrubs and associated thaw of frozen ground. This exposes organic matter in soils to microbes that can decompose it into carbonaceous greenhouse gases, but some of this carbon loss may be offset by the increased plant growth. Here, we investigate the impacts of greater shrub presence on soil properties at five sites in Alaska. We documented shrub growth by analyzing satellite images, which can help us understand the productivity and/or leaf coverage at each site back in time, and annual growth rings in shrub stems, which show how old the shrubs are and how much they grow each year. We also measured the depth of soil thaw in the field and its organic matter content in a laboratory. Where shrubs were more common, we found a thinner layer of organic matter at the soil surface. Thaw depth remained the same, which may indicate that the presence of shrubs results in deeper thaw of the mineral soil. Our findings support the hypothesis that shrub expansion will further enhance warming-driven increases of greenhouse gas emissions from Arctic landscapes. Trends in dendrochronology and Normalized Difference Vegetation Index reveal increasing growth of alder shrubs in Interior Alaska. More alder cover results in the loss of the soil organic layer and thus soil C and N that is not offset by more shrub biomass. Increasing alder growth may promote permafrost thaw not captured by tile probe active layer thickness monitoring. [ABSTRACT FROM AUTHOR]

Thawing permafrost in the Canadian Arctic tundra leads to peat erosion and slumping in narrow and shallow runnel ponds that surround more commonly studied polygonal ponds. Here we compared the methane production between runnel and polygonal ponds using stable isotope ratios, ¹⁴C signatures, and investigated potential methanogenic communities through high-throughput sequencing archaeal 16S rRNA genes. We found that runnel ponds had significantly higher methane and carbon dioxide emissions, produced from a slightly larger fraction of old carbon, compared to polygonal ponds. The methane stable isotopic signature indicated production through acetoclastic methanogenesis, but gene signatures from acetoclastic and hydrogenotrophic methanogenic Archaea were detected in both polygonal and runnel ponds. We conclude that runnel ponds represent a source of methane from potentially older C, and that they contain methanogenic communities able to use diverse sources of carbon, increasing the risk of augmented methane release under a warmer climate.