Browsing College of Natural Science and Mathematics (CNSM) by Subject "Permafrost"
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Assessment of LiDAR and spectral techniques for high-resolution mapping of permafrost on the Yukon-Kuskokwim Delta, AlaskaThe Yukon-Kuskokwim Delta (YKD) is one of the largest and most ecologically productive coastal wetland regions in the pan-Arctic. Formed by the Yukon and Kuskokwim Rivers flowing into the Bering Sea, nearly 130,000 square kilometers of delta support 23,000 Alaskan Natives living subsistence lifestyles. Permafrost on the outer delta commonly occurs on the abandoned floodplain deposits. Ground ice in the soil raises surface elevations on the order of 1-2 meters, creating plateaus on the landscape. Better drainage on the plateaus supports distinct Sphagnum-rich vegetation, which in turn protects the permafrost from rising air temperatures with low thermal conductivity during the summer. This ecosystem-protected permafrost is thus vulnerable to disturbances from rising air temperatures, vegetation mortality, and inland storm surges, which have been known to flood up to 37 km inland. This thesis assesses several novel techniques to map permafrost distribution at high-resolution on the YKD. Accurate baseline maps of permafrost extent are critical for a variety of applications, including long-term monitoring. As air and ground temperatures rise across the Arctic, monitoring landscape change is important for understanding permafrost degradation processes (e.g. thermokarst) and greenhouse gas dynamics from the local to global scales. This thesis separately explored the value of Light Detection And Ranging (LiDAR) and spectral datasets as tools to map permafrost at a high spatial resolution. Furthermore, this thesis sought to automate these processes, with the vision of high-resolution mapping over large spatial extents. Fieldwork was conducted in July 2016 to both parameterize and then validate the mapping efforts. The LiDAR mapping extent assessed a 135 km² area (~15% permafrost cover), and the spectral mapping extent assessed an 8 km² area (~20% permafrost cover). For the LiDAR dataset, the use of a simple elevation threshold informed by field ground truth values provided a permafrost map with 94.9% accuracy. This simple approach was possible because of the extremely flat terrain. For the spectral datasets, an ad-hoc masking technique was developed using a combination of texture analysis, principal component analysis, and morphological filtering. Two contrasting workflows were evaluated with fully-automated and semi-automated methods with mixed results. The highest mapping accuracy was 89.4% and the lowest was 79.1%, though the error of omission in mapping the permafrost remained high (7.02 - 59.7%) for most analyses. The spectral mapping algorithms did not replicate well across different high-resolution images, raising questions about the viability of using spectral methods alone to track thermokarst and landscape change over time. However, incorporating the spectral methods explored in this analysis with other datasets (e.g. LiDAR) has the potential to increase mapping accuracies. Both the methods and the results of this thesis enhance permafrost mapping efforts on the YKD, and provide a good first step to monitoring landscape change in the region.
Controls on ecosystem respiration of carbon dioxide across a boreal wetland gradient in Interior AlaskaPermafrost and organic soil layers are common to most wetlands in interior Alaska, where wetlands have functioned as important long-term soil carbon sinks. Boreal wetlands are diverse in both vegetation and nutrient cycling, ranging from nutrient-poor bogs to nutrient- and vascular-rich fens. The goals of my study were to quantify growing season ecosystem respiration (ER) along a gradient of vegetation and permafrost in a boreal wetland complex, and to evaluate the main abiotic and biotic variables that regulate CO₂ release from boreal soils. Highest ER and root respiration were observed at a sedge/forb community and lowest ER and root respiration were observed at a neighboring rich fen community, even though the two fens had similar estimates of root biomass and vascular green area. Root respiration also contributed approximately 40% to ER at both fens. These results support the conclusion that high soil moisture and low redox potential may be limiting both heterotrophic and autotrophic respiration at the rich fen. This study suggests that interactions among soil environmental variables are important drivers of ER. Also, vegetation and its response to soil environment determines contributions from aboveground (leaves and shoots) and belowground (roots and moss) components, which vary among wetland gradient communities.
The effects of permafrost degradation on soil carbon dynamics in Alaska's boreal regionHigh-latitude regions store large quantities of organic carbon (C) in permafrost soils and peatlands, accounting for nearly half of the global belowground C pool. Projected climate warming over the next century will likely drive widespread thawing of near-surface permafrost and mobilization of soil C from deep soil horizons. However, the processes controlling soil C accumulation and loss following permafrost thaw are not well understood. To improve our understanding of these processes, I examined the effects of permafrost thaw on soil C dynamics in forested upland and peatland ecosystems of Alaska's boreal region. In upland forests, soil C accumulation and loss was governed by the complex interaction of wildfire and permafrost. Fluctuations in active layer depth across stand age and fire cycles determined the proportion of soil C in frozen or unfrozen soil, and in turn, the vulnerability of soil C to decomposition. Under present-day climate conditions, the presence of near-surface permafrost aids C stabilization through the upward movement of the permafrost table with post-fire ecosystem recovery. However, sensitivity analyses suggest that projected increases in air temperature and fire severity will accelerate permafrost thaw and soil C loss from deep mineral horizons. In the lowlands, permafrost thaw and collapse-scar bog formation resulted in the dramatic redistribution of soil water, modifying soil thermal and C dynamics. Water impoundment in collapse-scar bogs enhanced soil C accumulation in shallow peat horizons, while allowing for high rates of soil C loss from deep inundated peat horizons. Accumulation rates at the surface were not sufficient to balance deep C losses, resulting in a net loss of 26 g C m⁻² y⁻¹ from the entire peat column during the 3000 years following thaw. Findings from these studies highlight the vulnerability of soil C in Alaska's boreal region to future climate warming and permafrost thaw. As a result, permafrost thaw and soil C release from boreal soils to the atmosphere should function as a positive feedback to the climate system.
Processes controlling thermokarst lake expansion rates on the Arctic coastal plain of Northern AlaskaThermokarst lakes are a dominant factor of landscape scale processes and permafrost dynamics in the otherwise continuous permafrost region of the Arctic Coastal Plain (ACP) of northern Alaska. Lakes cover greater than 20% of the landscape on the ACP and drained lake basins cover an additional 50 to 60% of the landscape. The formation, expansion, drainage, and reformation of thermokarst lakes has been described by some researchers as part of a natural cycle, the thaw lake cycle, that has reworked the ACP landscape during the course of the Holocene. Yet the factors and processes controlling contemporary thermokarst lake expansion remain poorly described. This thesis focuses on the factors controlling variation in extant thermokarst lake expansion rates in three ACP regions that vary with respect to landscape history, ground-ice content, and lake characteristics (i.e. size and depth). Through the use of historical aerial imagery, satellite imagery, and field-based data collection, this study identifies the controlling factors at multiple spatial and temporal scales to better understand the processes relating to thermokarst lake expansion. Comparison of 35 lakes across the ACP shows regional differences in expansion rate related to permafrost ice content ranging from an average expansion rate of 0.62 m/yr on the Younger Outer Coastal Plain where ice content is highest to 0.16 m/yr on the Inner Coastal Plain where ice content is lowest. Within each region, lakes vary in their expansion rates due to factors such as lake size, lake depth, and winter ice regime. On an individual level, lakes vary due to shoreline characteristics such as local bathymetry and bluff height. Predicting how thermokarst lakes will behave locally and on a landscape scale is increasingly important for managing habitat and water resources and informing models of land-climate interactions in the Arctic.