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.

The availability of viable seed can act as an important constraint on plant regeneration following disturbance. This study presents data on seed quantity and quality for black spruce (Picea mariana (Mill.) B.S.P.), a semiserotinous conifer that dominates large areas of North American boreal forest. We sampled seed rain and viability for 2 years after fire (2005)(2006)(2007) in 39 sites across interior Alaska that burned in 2004. All sites were dominated by black spruce before they burned. Structural equation modeling was used to assess the relative importance of prefire spruce abundance, topography effects, canopy fire severity, and distance to unburned stands in explaining variations in black spruce seed rain. Prefire basal area of spruce that remained standing after fire was a significant predictor of total seed rain, but seed viability was more strongly related to site elevation, canopy fire severity, and distances to unburned stands. Although positive relations between tree basal area and the size of the aerial seed bank may place a first constraint on seed availability, accurate prediction of postfire viable seed rain for serotinous conifers also requires consideration of the effects of abiotic stress and canopy fire severity on seed viability.Résumé : La disponibilité de graines viables peut constituer une contrainte importante pour la régénération des végétaux après une perturbation. Cette étude présente des données sur la quantité et la qualité des graines d'épinette noire (Picea mariana (Mill.) B.S.P.), un conifère à cônes semi-sérotineux qui domine de vastes zoes de la forêt boréale de l'Amérique du Nord. Nous avons échantillonné la pluie de graines et estimé la viabilité des graines pendant 2 ans (2005)(2006)(2007) dans 39 stations brûlées en 2004 à l'intérieur de l'Alaska. Toutes les stations étaient dominées par l'épinette noire avant d'être brûlées. Une modélisation par équation structurelle a été utilisée pour estimer l'importance relative de l'abondance de l'épinette avant le feu, des effets de la topographie, de la sévérité du feu dans la canopée et de la proximité de stations non brûlées sur la variation de la pluie de graines d'épinette noire. La surface terrière avant le feu des épinettes noires qui sont demeurées sur pied après le feu était une variable prédictive significative de la pluie de graines totale, mais la viabilité des graines était plus fortement reliée à l'altitude de la station, à la sévérité du feu dans la canopée et à la proximité des peuplements non brûlés. Quoique des relations positives entre la surface terrière des arbres et la taille de la banque aérienne de graines puissent constituer une première contrainte sur la disponibilité des graines, on doit aussi tenir compte des effets des stress abiotiques et de la sévérité du feu dans la canopée sur la viabilité des graines pour prédire avec exactitude la pluie de graines viables des conifères sérotineux après un feu.[Traduit par la Rédaction]

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