Minirhizotron fine root data from a treeline snowfence experiment near the Agashashok River, northwest Alaska (2019-2021)

Snow is an important insulator of Arctic soils during winter and may be an important source of soil moisture in summer. Changes in snow depth are likely to affect fine root growth and mortality via changes in soil temperature, moisture, and/or nutrient availability, which could alter aboveground growth and reproduction of Arctic vegetation, including white spruce (Picea glauca), a dominant Arctic treeline species in North America. In this study, we explored the drivers of fine root dynamics at three contrasting treelines in the western Brooks Range of Alaska. We increased snow depth relative to control plots using snowfences, and we used minirhizotrons to estimate fine root growth, standing crop, and overwinter mortality. Nearly all fine roots were observed in the upper 25 centimeters (cm), with the greatest fine root density occurring in the top 10 cm. Experimental deepening of winter snowpacks led to warmer soils throughout the winter. However, deeper snowpacks did not affect growing season soil moisture. After five years of treatment, deeper winter snowpacks reduced fine root standing crop with no significant effects on overwinter fine root mortality. Using structural equation modelling, we tested for pathways by which experimental deepening of winter snowpacks may have affected soil microclimates which, in turn, may have affected fine root growth and standing crop. Warmer soils in late winter were associated with warmer soils in early and mid-summer. Warmer early summer soils may have promoted early root growth. However, warmer July soils were associated with reduced fine root growth and smaller fine root standing crops. We hypothesize that experimental deepening of winter snowpacks improved plant access to soil nutrients, resulting in reduced investment in fine roots, potentially leaving additional resources available to support aboveground growth and reproduction. Our results suggest one mechanism by which deeper snowpacks could promote advance of the northern treeline.