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Summary Snow is an important insulator of Arctic soils during winter and may be a 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.We explored fine root dynamics at three contrasting treelines in northwest Alaska. We used snowfences to increase snow depth relative to control and minirhizotrons to estimate fine root growth, standing crop, and overwinter loss.Experimental deepening of snowpacks led to warmer winter soils but did not affect growing season soil moisture. Deeper snow reduced fine root standing crop with no significant effects on overwinter fine root loss. 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 standing crops.We hypothesize that deeper snow improves plant access to soil nutrients, resulting in reduced investment in fine roots, potentially leaving additional resources to support aboveground growth and reproduction. Our results suggest one mechanism by which deeper snow could promote northern treeline advance. 

Climate-induced northward advance of boreal forest is expected to lessen albedo, alter carbon stocks, and replace tundra, but where and when this advance will occur remains largely unknown. Using data from 19 sites across 22 degrees of longitude along the tree line of northern Alaska, we show a stronger temporal correlation of tree ring growth with open water uncovered by retreating Arctic sea ice than with air temperature. Spatially, our results suggest that tree growth, recruitment, and range expansion are causally linked to open water through associated warmer temperatures, deeper snowpacks, and improved nutrient availability. We apply a meta-analysis to 82 circumarctic sites, finding that proportionally more tree lines have advanced where proximal to ongoing sea ice loss. Taken together, these findings underpin how and where changing sea ice conditions facilitate high-latitude forest advance.