ABSTRACT: Fire severity can be defined as the amount of biomass combusted by wildfire. Stored carbon (C) and nitrogen (N) are emitted into the atmosphere as wildfires consume vegetation and soil organic layers, thus C and N emissions should be related to fire severity. Since boreal forests store 30% of the world's terrestrial C and are subject to high-intensity, stand-replacing wildfires, it is critical to be able to estimate C fluxes from wildfires. Furthermore, quantifying fire severity is important for predicting post-fire vegetation recovery and future C sequestration. We reconstructed pre-fire organic soil layers and quantified fire severity levels from the 2004 wildfires in Interior Alaska with the adventitious root height (ARH) method. We tested the ARH method in unburned stands and by comparing our reconstructed values in burned stands with actual prefire measurements. We found that ARH correlated to organic soil height in unburned stands (with a small offset of 3 cm). We measured organic soil (using the ARH method) and stand characteristics in boreal black spruce forest and estimated the amount of soil and canopy biomass consumed by fire. We compared these results to the composite burn index (CBI), a standardized visual method, which has not been widely used in the boreal forest. CBI assessments were significantly related to our ground and canopy fire severity estimates. We calculated C and N pools using C and N concentration and bulk density estimates from soils sampled in burned and unburned stands. We conclude that the ARH method can be used to reconstruct pre-fire organic soil depth, C and N pools and to assess fire severity. Furthermore, CBI shows promise as a way of estimating fire severity quickly and is a reasonably good predictor of biomass and soil C loss.

"The boreal forest is the largest terrestrial ecosystem in North America, one of the least disturbed by humans, and most disturbed by fire. This combination makes it an ideal system to explore the environmental controls over species composition, the relative importance of abiotic factors and floristic composition in governing ecosystem processes, and the importance of legacy effects at a large regional spatial scale. In the boreal region of interior Alaska, Picea mariana (black spruce) is the predominant tree species and spans a wide range of habitats, including north-facing slopes with permafrost, lowland bogs, and high dry ridge-tops. This research uses a combination of site description and analysis from both locally near Fairbanks (54) and across a large region and number of sites (146) to answer questions about the regional variability and biodiversity of the black spruce forest type. Based on the relationships between species composition and environmental factors, topography and elevation were the most important gradients explaining species composition locally in the Fairbanks region, and mineral soil pH was the overriding environmental gradient across interior Alaska. To describe the floristic variability, I separated the black spruce forest type into three floristically-based community types and five community subtypes. Variability in ecosystem properties among black spruce stands was as large as that documented previously among all forest types in the central interior of Alaska. The variability in plant community composition was at least as effective as environmental or abiotic factors and stand characteristics as a predictor of soil C pools in the black spruce forest type of interior Alaska. The variability in species composition at the community subtype-level was related to a combination of environmental factors and fire history. Together, these results provide a foundation for future work in black spruce ecosystems of interior Alaska, and contribute to our understanding of the regional variability and biodiversity of the black spruce forest type"--Leaves iii-iv.

Black spruce (Picea mariana (Mill) B.S.P) is the dominant forest cover type in interior Alaska and is prone to frequent, stand-replacing wildfires. Through impacts on tree recruitment, the degree of fire consumption of soil organic layers can act as an important determinant of whether black spruce forests regenerate to a forest composition similar to the prefire forest, or to a new forest composition dominated by deciduous hardwoods. Here we present a simple, rule-based framework for predicting fire-initiated changes in forest cover within Alaska's black spruce forests. Four components are presented: (1) a key to classifying potential site moisture, (2) a summary of conditions that favor black spruce self-replacement, (3) a key to predicting postfire forest recovery in recently burned stands, and (4) an appendix of photos to be used as a visual reference tool. This report should be useful to managers in designing fire management actions and predicting the effects of recent and future fires on postfire forest cover in black spruce forests of interior Alaska.

"As the climate changes, Alaska's boreal forest faces the simultaneous threats of rising invasive plant abundances and increasing area burned by wildfire. Highly flammable and widespread black spruce forest represents a boreal habitat that may be increasingly susceptible to non-native plant invasion. In other biomes, non-native plant invasions are generally greatest in high severity burns that are only a few years old. The relationship between fire and non-native plant invasion has not been investigated in the northern boreal forest. To assess the invasibility of burned black spruce forests, I used burned field sites that spanned a gradient of burn severities, moisture levels, and burn ages. I conducted both field surveys and a greenhouse experiment using soil taken from burn sites. Contrary to generalizations from other biomes, I found soils from low severity burns and burns between 10 and 20 years old support greater invasive plant growth in black spruce forests than do high severity and more recent burns. In addition, regional differences between burn complexes outweighed burn severity and site moisture in determining the invasibility of burned black spruce sites. Finally, rebounding native vegetation appears to offer burned areas a level of resistance to invasive plant establishment"--Leaf iii.