Nutrient dynamics of the southern and northern BOREAS boreal forests

Ecophysiological response of aspen (Populus tremuloides) and jack pine (Pinus banksiana) to atmospheric nitrogen deposition on reconstructed boreal forest soils in the Athabasca oil sands region

Oil and gas extraction in the Athabasca Oil Sands Region of northeastern Alberta, Canada has increased anthropogenic nitrous oxide (NO_x) and ammonia (NH₃) emissions over the past three decades, leading to a potential increase in N deposition. Deposition on reclaimed sites was hypothesized to be higher than in surrounding boreal forests, but had not been quantified. The objective of this study was to assess the implications of this potentially increased deposition on reclaimed aspen (Populus tremuloides Michx.) and pine (Pinus banksiana Lamb.) ecosystems through the use of several N status indicators, including N deposition, total and available concentrations in plants and soils, and δ¹⁵N values in deposition and plants and soils. Atmospheric N deposition, which was dominated by ammonium (NH₄⁺), averaged 24 kg N ha^-1 year^-1 as bulk precipitation and 6 kg N ha^-1 year^-1 as throughfall. Increased N deposition influenced the N cycle in both aspen and pine stands. Aspen appeared to be actively biocycling N as indicated by a closed N cycle, resulting in minimal N losses. Whereas the N cycle in pine may be more open as indicated by the dominance of soil nitrate (NO₃^-), and enrichment of ¹⁵N in available soil NH₄⁺, root and foliar N. Therefore, we suggest that pine stands on reclamation sites may be at kinetic N saturation where the rate of N inputs exceeds vegetation and soil N net sinks, and do not require additional N fertilizer inputs.

Effects of canopy-deposition interaction on H+ supply to soils in Pinus banksiana and Populus tremuloides ecosystems in the Athabasca oil sands region in Alberta, Canada

Soil acidification has been of concern in the oil sands region in Alberta due to increased acid deposition. Using the canopy budget model, and accounting for H(+) canopy leaching by organic acids, we determined sources and sinks of H+ in throughfall in jack pine (Pinus banksiana) and trembling aspen (Populus tremuloides) stands in two watersheds from 2006 to 2009. In pine stands, H+ deposition was greater in throughfall than in bulk precipitation while the opposite was true in aspen stands. The annual H+ interception deposition was 148.8-193.8 and 49.7-70.0 molcha(-1) in pine and aspen stands, respectively; while the annual H+ canopy leaching was 127.1-128.7 and 0.0-6.0 molcha(-1), respectively. The greater H+ supply in pine stands was caused by greater interception deposition of SO4(2-) and organic acids released from the pine canopy. Such findings have significant implications for establishing critical loads for various ecosystems in the oil sands region.

Quantifying fire severity, carbon, and nitrogen emissions in Alaska's boreal forest

The boreal region stores a large proportion of the world's terrestrial carbon (C) and is subject to high-intensity, stand-replacing wildfires that release C and nitrogen (N) stored in biomass and soils through combustion. While severity and extent of fires drives overall emissions, methods for accurately estimating fire severity are poorly tested in this unique region where organic soil combustion is responsible for a large proportion of total emissions. We tested a method using adventitious roots on black spruce trees (Picea mariana) in combination with canopy allometry to reconstruct prefire organic soil layers and canopy biomass in boreal black spruce forests of Alaska (USA), thus providing a basis for more accurately quantifying fire severity levels. We calibrated this adventitious-root-height method in unburned spruce stands and then tested it by comparing our biomass and soils estimates reconstructed in burned stands with actual prefire stand measurements. We applied this approach to 38 black spruce stands burned in 2004 in Alaska, where we measured organic soil and stand characteristics and estimated the amount of soil and canopy biomass, as well as C and N pools, consumed by fire. These high-intensity quantitative estimates of severity were significantly correlated to a semiquantitative visual rapid assessment tool, the composite burn index (CBI). This index has proved useful for assessing fire severity in forests in the western United States but has not yet been widely tested in the boreal forest. From our study, we conclude that using postfire measurements of adventitious roots on black spruce trees in combination with soils and tree data can be used to reconstruct prefire organic soil depths and biomass pools, providing accurate estimates of fire severity and emissions. Furthermore, using our quantitative reconstruction we show that CBI is a reasonably good predictor of biomass and soil C loss at these sites, and it shows promise for rapidly estimating fire severity across a wide range of boreal black spruce forest types, especially where the use of high-intensity measurements may be limited by cost and time.