The losers are turning green: Expanding Arctic riparian tall shrub cover indicates a shift in stream hydrologic regime

Shrub expansion has become a conspicuous pan-Arctic phenomenon in recent decades. Increasing summer air temperatures are often cited as the primary driver of shrub increase, which in turn is documented in the literature to warm soils in winter due to increased snow accumulation. Complex interactions between shrubs and permafrost raise the question of whether it is the shrubs that are initiating shifts in ground temperature and hydrologic regime, or whether the shrubs are taking advantage of already improved soil conditions brought on by climate change. Here we present late summer and winter field measurements of vegetation, hydrology, permafrost, and soil microbial communities along stream corridors in the foothills region of the North Slope, Arctic Alaska. Our results show that tall riparian shrub cover is an effective indicator of an influent stream regime, i.e. stream sections that lose water to the ground. A high leaf area index (LAI) and shrub height, deep taliks, and distinctive soil microbial communities were found along influent stream sections. Streams that lacked a riparian tall shrub cover were gaining (effluent) and did not have a deep talik. LAI was linearly correlated to change in streamflow, with denser canopies (LAI > 2) coinciding with losing stream sections. The circumpolar scale of modern shrub expansion is particularly pronounced in riparian corridors. At the same time, winter discharge is observed to have increased in large Arctic rivers. Our findings therefore suggest that talik formation and the resultant stream hydrology shifts, from gaining to losing discharge regime, are occurring across the Arctic with potential impacts to larger river hydrology.