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

Abstract

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.

Expanding beaver pond distribution in Arctic Alaska, 1949 to 2019

“…5 and 6 ). In the tundra, we expect that these processes surrounding beaver ponds increase heat absorption and thaw permafrost, some of which is evident in images as widening channels and thermokarst formation surrounding beaver ponds 42 (Fig. 6 , Supplemental Fig.…”

Do beaver ponds increase methane emissions along Arctic tundra streams?

“…Beaver impacts to permafrost are most evident at the surface where active layer thickness is relatively shallow (e.g [13]), reflecting cold soil thermal conditions. In these locations, beaver engineering and expansion of surface water over near-surface permafrost leads to enhanced heat transfer into the underlying permafrost, leading to thermokarst in ice-rich terrain, and high potential for thaw and C release [13]. The presence of groundwater flow paths through gravel bars and floodplain soils likely reduces permafrost presence in braided river corridors prior to beaver arrival.…”

“…North American beavers (Castor canadensis) have colonized the Arctic tundra during the last half-century [12]. In the Arctic, beaver dams often trigger local inundation and subsequent permafrost thaw [12][13][14], a mechanism known to mobilize permafrost carbon (C) to the atmosphere in the form of the greenhouse gasses carbon dioxide (CO 2 ) and methane (CH 4 ). Shortterm and long-term measurements linking beaver pond construction in the Arctic with CH 4 release are lacking.…”

“…Beaver engineering, where prevalent, is the dominant control on increases in surface water, which is closely linked to permafrost dynamics and ecosystem processes [14]. Time-series images of beaver colonization in the Arctic tundra show that inundation and hydrologic route diversion associated with pond construction may trigger permafrost thaw and thermokarst within and surrounding the nascent wetland [12][13][14]. Beaver ponds alter the local thermal balance and increase heat transfer to the underlying permafrost [38,39].…”

“…This hydrologydriven heat transfer is more effective at warming permafrost than the pre-existing vegetation cover, driving increased permafrost thaw via both lateral pond expansion and subsidence, vertical talik formation, and formation of thermokarst ponds. A case study from an ice-rich permafrost stream in western Alaska demonstrates how rapidly beaver hydrologic engineering can trigger thermo-erosional processes to thaw permafrost and restructure lowland streams [13]. Once initiated, actively thawing permafrost margins on ponds and lakes can quickly become sites of high CH 4 emissions, which can remain active for decades [40].…”