Search for: All records

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. 

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. 

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. 

We destructively sampled up to 15 white spruce (Picea glauca) seedlings (<140 centimeters tall) from 2 study treelines (1 upper elevation, Alpine, and 1 lower elevation, Arctic) at 16 sites across Alaska's Brooks Range in August-September 2021. We harvested 3 seedlings each in 5 height classes (0-10, 10-30, 30-60, 60-100, 100-140cm). Height (cm) was measured in the field before harvest as vertical distance from ground to tallest living tissue. The target of harvest was the root-shoot boundary, where above ground stem interfaces with roots. The earliest years of a seedling should be located in the wood of this section. For very small seedlings we harvested whole seedlings. In the laboratory we cut ~1cm thick "cookies" of the harvest root-shoot section, progressively sanded until cells were visible under the microscope and then counted rings back to the pith. For small seedlings, we counted bud scars along the stem. We used the earliest date obtained from these counts as the germination date (the innermost year, or In.year in the data csv). 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. 

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. 

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. 

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. 

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.