Abstract
Ice-wedges are large subsurface ground-ice formations commonly found in northern Arctic tundra regions with continuous permafrost (ground that is frozen for two consecutive years or more). They are created by repeated cycles of frozen ground cracking, spring meltwater seepage, and meltwater freezing, resulting in vertical ice-vein growth occurring over hundreds to thousands of years. The spatially heterogeneous microtopography associated with interconnected networks of ice-wedges generates visible surficial polygonal patterns in northern Arctic tundra landscapes. This, in turn, produces high spatial variability in topography, hydrology, energy balance, vegetation distribution, nutrient fluxes, and ecosystem biogeochemical processes on scales of centimeters to meters. Recent warming has resulted in ice-wedge thaw and dynamic environmental changes on landscape scales, contributing to uncertainty in Arctic ecosystem trajectories with climate warming. Frozen ground thaw, coupled with altered hydrology, influences the availability and transport of nutrients. Nutrient availability can vary substantially across stages of ice-wedge degradation and stabilization, and the resultant changes in vegetation can lead to further ice-wedge degradation or facilitate stabilization. As tundra ecosystems are nitrogen-limited, N regulates carbon cycles at high latitudes as well as dictates the distribution, composition, and productivity of Arctic vegetation communities, which has important implications for Arctic C and N stocks, C-sequestration, and greenhouse gas emissions. A refined understanding of fine-scale spatial and temporal nitrogen dynamics among ice-wedge polygon trajectories will improve the ability to predict Arctic response to warming, but such assessments are further complicated by the influence of local environmental conditions on ice-wedge dynamics.
To assess whether ice-wedge degradation results in increased nutrient availability – and whether this increase is reflected in vegetation uptake – this study aimed to identify and quantify C and N concentrations in the soil and surface water of various stages of ice-wedge degradation and stabilization, as well as examine concurrent changes in aboveground vegetation biomass and plant functional group foliar N (Chapters 2 & 3). To explore the impact of local conditions on these patterns, trends among ice-wedge stages were compared between two north Alaskan tundra sites at different latitudes (Jago and Prudhoe Bay, Alaska) (Chapter 3). Additionally, field vegetation data from both sites were then used to train and validate a nutrient-based Arctic vegetation model to incorporate the presence of ice-wedges and ice-wedge degradation into an Arctic landscape. The model was used to extrapolate changes in vegetation biomass and vegetative C and N stocks over larger spatial and temporal scales to glean insight into how ice-wedge dynamics may alter future Arctic vegetation communities and nutrient stocks. Finally, a conceptual model of N inputs and fluxes within ice-wedge polygon systems outlines existing research and highlights current knowledge gaps requiring further study. In Chapters 2 and 3, I found that there were ice-wedge stage-specific differences in soil and surface water nutrients, with concurrent changes in vegetation community composition, vegetation biomass, and foliar N content. According to modeled temporal extrapolations in Ch 4, warming and ice-wedge degradation may result in substantial biomass increases due to the proliferation of aquatic moss. In more southern latitudes, moss proliferation is so great that it could potentially exceed belowground biomass loss from the loss of shrubs, thereby increasing Arctic C and N stocks in degraded landscapes. While the pronounced effect that ice-wedge polygon microtopography and ice-wedge dynamics can have on local to landscape-scale N cycles is apparent, it remains relatively understudied. The synthesis of scant literature addressing ice-wedge polygon N dynamics in Chapter 5 suggests that thawing permafrost is the main contributor of N to ice-wedge systems and that lateral flow and vegetation uptake and turnover are the main N fluxes, but these processes are still poorly understood. Previous studies stress the importance of characterizing, quantifying, and incorporating these fine-scale processes into future Arctic ecosystem modeling. The spatial heterogeneity of ice-wedge landscapes will likely play a considerable role in determining future Arctic carbon and nitrogen stocks, and ultimately contribute significantly to the evolution of Arctic ecosystem functioning with climate warming.
