Multi-Dimensional Remote Sensing Analysis Documents Beaver-Induced Permafrost Degradation, Seward Peninsula, Alaska

Beavers have established themselves as a key component of low arctic ecosystems over the past several decades. Beavers are widely recognized as ecosystem engineers, but their effects on permafrost-dominated landscapes in the Arctic remain unclear. In this study, we document the occurrence, reconstruct the timing, and highlight the effects of beaver activity on a small creek valley confined by ice-rich permafrost on the Seward Peninsula, Alaska using multi-dimensional remote sensing analysis of satellite (Landsat-8, Sentinel-2, Planet CubeSat, and DigitalGlobe Inc./MAXAR) and unmanned aircraft systems (UAS) imagery. Beaver activity along the study reach of Swan Lake Creek appeared between 2006 and 2011 with the construction of three dams. Between 2011 and 2017, beaver dam numbers increased, with the peak occurring in 2017 (n = 9). Between 2017 and 2019, the number of dams decreased (n = 6), while the average length of the dams increased from 20 to 33 m. Between 4 and 20 August 2019, following a nine-day period of record rainfall (>125 mm), the well-established dam system failed, triggering the formation of a beaver-induced permafrost degradation feature. During the decade of beaver occupation between 2011 and 2021, the creek valley widened from 33 to 180 m (~450% increase) and the length of the stream channel network increased from ~0.6 km to more than 1.9 km (220% increase) as a result of beaver engineering and beaver-induced permafrost degradation. Comparing vegetation (NDVI) and snow (NDSI) derived indices from Sentinel-2 time-series data acquired between 2017 and 2021 for the beaver-induced permafrost degradation feature and a nearby unaffected control site, showed that peak growing season NDVI was lowered by 23% and that it extended the length of the snow-cover period by 19 days following the permafrost disturbance. Our analysis of multi-dimensional remote sensing data highlights several unique aspects of beaver engineering impacts on ice-rich permafrost landscapes. Our detailed reconstruction of the beaver-induced permafrost degradation event may also prove useful for identifying degradation of ice-rich permafrost in optical time-series datasets across regional scales. Future field- and remote sensing-based observations of this site, and others like it, will provide valuable information for the NSF-funded Arctic Beaver Observation Network (A-BON) and the third phase of the NASA Arctic-Boreal Vulnerability Experiment (ABoVE) Field Campaign.

High Temporal and Spatial Nitrate Variability on an Alaskan Hillslope Dominated by Alder Shrubs

In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems is of critical importance, yet many details about the form, location, and availability of N from these shrubs remain unknown. To address this knowledge gap, the spatiotemporal variability of nitrate (NO3−) and its environmental and edaphic controls were investigated at an alder (Alnus viridis spp. fruticosa) dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was collected from locations within alder shrubland growing along a well-drained hillslope and compared to soil pore water collected from locations outside (upslope, downslope, and between) the alder shrubland. δ15N and δ18O of soil pore water were consistent with the predicted range of NO3− produced through microbial degradation of N-rich alder shrub organic matter. Soil pore water collected within alder shrubland had an average NO3− concentration of (4.27 ± 8.02 mg L−1) and differed significantly from locations outside alder shrubland (0.23 ± 0.83 mg L−1; p < 0.05). Temporal variation in NO3− within and downslope of alder shrubland corresponded to precipitation events, where NO3− accumulated in the soil was flushed downslope during rainfall. Enrichment of both δ15N and δ18O isotopes at wetter downslope locations indicate that denitrification buffered the mobility and spatial extent of NO3−. These findings have important implications for nutrient production and mobility in N-limited permafrost systems that are experiencing shrub expansion in response to a warming Arctic.

Distribution and Population Size of Emperor Geese during the Breeding Season on the Seward Peninsula, Alaska

The Emperor Goose (Anser canagicus) is a year-round occupant of northern latitudes, spending its entire annual cycle in coastal habitats of western Alaska and the Russian Far East. Over the last several decades, the Emperor Goose population underwent a pronounced decline, prompting 30 consecutive years of harvest closures, followed by a protracted recovery and the recent reopening of harvest. This recovery was primarily documented on the Yukon-Kuskokwim Delta in western Alaska, where an estimated 80% – 90% of the world’s Emperor Goose population breeds. However, the size and status of their population on the Seward Peninsula, Alaska, which is their only other significant breeding area in North America, remains almost completely unknown. Therefore, to better inform population and harvest management of Emperor Geese in western Alaska, we conducted extensive aerial surveys of Emperor Geese along the northern coast of the Seward Peninsula during the breeding season. During the summer of 2018, we surveyed 150 transects totaling 351 km2, for a total sampled fraction of 7.2% of the 4853 km2 survey area. Using a double-observer technique that accounted for detection probability, we estimated a population of 1226 (95% CI: 792 – 1660) Emperor Geese on the Seward Peninsula, of which 614 (95% CI: 416 – 811) were considered breeding birds based on their observed status as singles or pairs. Most Emperor Geese (61%) were found on barrier islands, even though these islands accounted for just 3.5% of the total survey area; the remaining geese were found in lowland coastal habitats (23%) or upland tundra (16%). Overall, our surveys indicate a small breeding population of Emperor Geese on the Seward Peninsula, which raises some conservation concern. Further reductions or extinction of this small population would leave Emperor Geese with only one significant breeding area in North America. Because Emperor Geese typically display high breeding site fidelity and female natal philopatry, any future growth of this small population will likely to need to come from within.