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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.
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This data set represents 23 study sites across 22 degrees of longitude along treeline in Alaska's Brooks Range. Each study site was approximately 50 square kilometers in area and sampled with approximately n = 3,000, randomly-placed 6 meter (m) diameter disks. Each of the 63,224 disks were classified by humans into coarse vegetation classes twice, once using 1970s aerial orthophotos (e.g., Alaska High-Altitude Photography) and again using 2010s very-high resolution satellite imagery (e.g., MAXAR World-View). In addition, annual maximum Landsat normalized difference vegetation index (NDVI) values were extracted from a subset of these disks (n = 27,835 from 12 study sites) for Theil-Sen regression on greening trends using the R package LandsatTS.more » « less
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Measurements of treeline white (Picea glauca) and black (P. mariana) spruce abundance during 2019, 2020, 2021 and 2022 as sampled within n = 695 5-meter (m) radius plots (area = 78.5 square meters), each centered on a selected white spruce adult called "Focal Tree". Measurements included height of stems between 0.5 and 1.4 m tall and diameter at 1.4 m for individuals taller than 1.4 m. Those individuals taller than 1.4 m tall were also stem mapped in polar coordinates with r = distance and theta as magnetic direction from a focal center white spruce tree. The purpose of this dataset was to examine spatial variation in treeline white spruce densities and basal area across the Brooks Range and in relation to local microclimates. It also provides a baseline for future stem mapping to determine changes in abundance.more » « less
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Cover rankings of ground cover during 2019, 2020, 2021 and 2022 as sampled within n = 553 5-meter (m) radius plots (area = 78.5 square meters), each centered on a selected white spruce adult called "Focal Tree." The purpose of this dataset was to examine spatial variation in ground cover (less than 0.2-0.3 m tall) of the most important genera across the Brooks Range and in relation to local microclimates. It also provides a baseline for future ground cover rankings to determine changes in abundance. Taxa were ranked in order of cover from Rank1 with the most cover down to Rank5 with the lowest cover in most cases.more » « less
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This dataset includes measurements and estimated variable values from treelines in Alaka's Brooks Range mountains. It includes locations of colonists above treelines found in 2018, 2019, 2020, 2021, and 2022; forest advance rates from the 1970s to 2010s from repeat imagery; growth rates of leaders of juveniles during 2015-2020 and lateral branch growth of adults in 2019; counts of saplings; temperatures of air at 2 meters (m) and soil at 10 centimeters (cm) from 2019-2022; soil moisture in 2019; estimated snow depth in January 2020, 2021, and 2022; foliar nitrogen and phosphorous; and foliar stable isotope ratios for nitrogen (15N:14N) and carbon (13C:12C). The purpose of the dataset is to show the effect of sea ice retreat on treeline advance.more » « less
