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Measurements of treeline white spruce fine root (<2 millimeters (mm)) carbon, nitrogen concentrations, along with stable isotopes of carbon and nitrogen. Fine roots (<2 mm) were collected from soil cores taken at the dripline below focal trees. The purpose of this dataset was to examine spatial variation in white spruce root nutrition across the Brooks Range and in relation to local microclimates. The dataset contains data for treeline trees that were treated with NPK (nitrogen, phosphorus, potassium) fertilizer at a sub-set of the research sites. 

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. 

Tree-ring widths from 1576 white spruce trees growing at 78 treelines distributed across 19 research sites along a longitudinal gradient in the Brooks Range of northern Alaska. The general purpose of the sampling was to examine west-east variation in white spruce growth responses to changes in climate. Trees were sampled at both high ("alpine") and low elevation treelines ("arctic"). Increment cores were collected in August and September of 2022 as low as possible on each tree (~25 centimeter height). Trees indicated with an "f" were treated with NPK (nitrogen, phosphorus, potassium) fertilizer in June of 2019 and June of 2021. 

Photograph-derived counts of seed cones on white spruce (Picea glauca) trees at elevational (alpine) & lower (arctic) treelines at 19 research sites along a west-east gradient in Alaska's Brooks Range. Photographs are from a single angle and thus represent a "cone production index" rather than a complete count of all the cones on a tree. The general purpose of the sampling was to examine west-east variation in white spruce growth and reproduction responses to changes in climatic gradients and ongoing climate change. 

Measurements of air temperature at 2 meter height made along elevation gradients from the valley bottom to the alpine at 16 sites along a west to east gradient in the Brooks Range. The purpose of this dataset was to examine spatial variation in white spruce needle nutrition and gas exchange physiology across the Brooks Range and in relation to local microclimates. 

Measurements of treeline white spruce annual branch primary growth during 2019, 2020, 2021 and 2022. Measurements were made using digital calipers on branches at breast height with persistent apical dominance. The purpose of of this dataset was to examine spatial variation in treeline white spruce branch growth across the Brooks Range and in relation to local microclimates. 

Photograph-derived counts of seed cones on white spruce (Picea glauca) trees at elevational (alpine) & lower (arctic) treelines at 19 research sites along a west-east gradient in Alaska's Brooks Range. Photographs are from a single angle and thus represent a "cone production index" rather than a complete count of all the cones on a tree. The general purpose of the sampling was to examine west-east variation in white spruce growth responses to changes in climate. 

Soil temperature measured at 10 centimeter depth beneath the drip-line of mature white spruce trees at treelines across the Brooks Range. The purpose of the data was to examine spatial variation in soil temperature during different seasons across the Brooks Range. 

Measurements of treeline white spruce needle carbon, nitrogen and phosphorus concentrations, along with stable isotopes of carbon and nitrogen. Samples were collected of the most recent needle cohort at breast height in August and September. The purpose of this dataset was to examine spatial variation in white spruce needle nutrition and gas exchange physiology across the Brooks Range and in relation to local microclimates. The 2021 and 2022 datasets contain data for treeline trees that were treated with NPK fertilizer at a sub-set of the research sites. 

Measurements of treeline white spruce needle length and needle mass per unit projected needle area on needles produced and sampled 2019, 2021 and 2022. Measurements of needle length and projected area were made using WinSeedle software on scans of the needle samples. The purpose of of this dataset was to examine spatial variation in treeline white spruce needle attributes across the Brooks Range and in relation to local microclimates.