• The effects of permafrost degradation on soil carbon dynamics in Alaska's boreal region

      O'Donnell, Jonathan A. (2010-12)
      High-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.
    • The snowshoe hare filter to spruce establishment in boreal Alaska

      Olnes, Justin; Kielland, Knut; Ruess, Roger; Juday, Glenn; Genet, Helene; Mann, Daniel (2018-05)
      Interior Alaska is a heterogeneous landscape within the circumpolar boreal forest and is largely composed of black and white spruce (Picea mariana and P. glauca). Improving our understanding of the factors affecting patterns in spruce regeneration is particularly important because these factors ultimately contribute to shaping the boreal forest vegetation mosaic. Herbivory by snowshoe hares (Lepus americanus) is one factor that likely drives patterns in spruce establishment. The interaction between spruce and snowshoe hares provides an opportunity to study how plant-herbivore interactions can affect succession, vegetation community composition, and consequently, how herbivory influences landscape heterogeneity. I explored how herbivory by snowshoe hares alters the survival and growth of spruce seedlings across Interior Alaska's boreal forest. I hypothesized that the survival and growth rate of regenerating spruce is significantly reduced by snowshoe hare herbivory and that snowshoe hare herbivory influences the pattern of spruce establishment across time and space. To address this hypothesis, I conducted research in three distinct vegetation communities across the region: productive lowland floodplains (Chapters 1 and 2), treeline (Chapters 3 and 4), and recently burned stands of black spruce (Chapter 5). Together these five chapters reveal that snowshoe hares affect spruce establishment across much of boreal Alaska. Where and when hares are abundant, spruce can be heavily browsed, resulting in suppressed seedling growth and increased seedling mortality. The results of these studies also reveal a consistent and predictable pattern in which this plant-herbivore interaction takes place. The snowshoe hare filter acts as a 'spatially aggregating force' to spruce establishment, where the potential for optimal regeneration is highest during periods of low hare abundance and where hares are absent from the landscape.