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Historically characterized by pristine streams that support robust populations of Arctic grayling, Dolly Varden, and chum salmon, the southern slopes of the Brooks Range provide valuable economic and subsistence resources for local communities. However, since 2019, dozens of formerly clear-running streams have turned turbid and orange with iron precipitates. Seeps have been identified in the tundra and in upland rock formations. Limited data show very low pH in seep water (less than 3.0), downslope vegetation mortality, and dramatic declines in juvenile fish abundance in affected headwaters. The causes of this rapidly spreading degradation of pristine streams remains unknown. The proliferation of turbid orange streams west of the Dalton Highway ( greater than 30 since 2019) is a threat to wilderness characteristics, drinking water, subsistence resource availability, and a growing commercial salmon fishery in northwest Alaska. With many terrestrial and aquatic species dependent upon the seasonal influx of salmon, the loss of fish habitat could induce ecosystem collapse, despite the protections afforded by a vast network of National Parks and Preserves. The degradation of formerly pristine streams is occurring at such a rapid pace that we may soon lose the opportunity to compare affected with nearby unaffected streams. This comparison is essential to develop the mechanistic, causal understanding that would allow us to predict which streams will turn next and the threats to downstream villages. The community of Kiana, for instance, sits at the confluence of the Squirrel and Kobuk Rivers. At least two streams in the Squirrel watershed have turned since 2020, while two large tributaries of the Kobuk turned in 2019. The Squirrel and large Kobuk tributaries, like the Salmon River, produce much of the fish harvested by Kiana residents. This dataset includes field measurements of pH, specific conductivity, dissolved oxygen and turbidity, along with laboratory measurements of metal concentrations in acidified water samples from tundra seeps, tributaries and rivers in the western Brooks Range. The watersheds that were sampled include Timber Creek, Tukpahlearik Creek, Salmon River, Kallarichuk River, Kobuk River and Devil's Lake, which is the drinking water source for the village of Kotzebue, Alaska. 

Abstract Climate change in the Arctic is altering watershed hydrologic processes and biogeochemistry. Here, we present an emergent threat to Arctic watersheds based on observations from 75 streams in Alaska’s Brooks Range that recently turned orange, reflecting increased loading of iron and toxic metals. Using remote sensing, we constrain the timing of stream discoloration to the last 10 years, a period of rapid warming and snowfall, suggesting impairment is likely due to permafrost thaw. Thawing permafrost can foster chemical weathering of minerals, microbial reduction of soil iron, and groundwater transport of metals to streams. Compared to clear reference streams, orange streams have lower pH, higher turbidity, and higher sulfate, iron, and trace metal concentrations, supporting sulfide mineral weathering as a primary mobilization process. Stream discoloration was associated with dramatic declines in macroinvertebrate diversity and fish abundance. These findings have considerable implications for drinking water supplies and subsistence fisheries in rural Alaska. 

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. 

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. 

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. 

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 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. 

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.