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During the 1997 to 1998 El Niño, drought conditions triggered widespread increases in fire activity, releasing CH4 and CO2 to the atmosphere. We evaluated the contribution of fires from different continents to variability in these greenhouse gases from 1997 to 2001, using satellite-based estimates of fire activity, biogeochemical modeling, and an inverse analysis of atmospheric CO anomalies. During the 1997 to 1998 El Niño, the fire emissions anomaly was 2.1 +/- 0.8 petagrams of carbon, or 66 +/- 24% of the CO2 growth rate anomaly. The main contributors were Southeast Asia (60%), Central and South America (30%), and boreal regions of Eurasia and North America (10%).

We measured organic-layer (OL) recovery and carbon stocks in dead woody debris a decade after wildfire in black spruce (Piceamariana (Mill.) B.S.P.) forests of interior Alaska. Previous study at these research plots has shown the strong role that landscape position plays in governing the proportion of OL consumed during fire and revegetation after fire. Here, we show that landscape position likely influences fire dynamics in these stands through changes in mineral soil texture. The content of fine-textured materials in underlying mineral soils was positively related to OL depths measured 1 and 10 years after fire, and there was an interaction between soil texture and elevation in governing OL consumption and OL recovery a decade following fire. OL depths 10 years after fire were 2 cm greater than 1 year after fire, with a range of 19 cm of accumulation to 9 cm of subsidence. Subsidence was inversely related to the percentage of fine textures within the parent material. The most influential factor determining the accumulation of OL carbon stocks a decade following wildfire was the interaction between landscape position and the presence of fine-textured soil. As such, parent material texture interacted with biological processes to govern the recovery of soil organic layers. Key words: Picea mariana, parent material, clay, loess, wild fire. Nous avons mesure le retablissement de la couche organique (CO) et les reserves de carbone dans les debris ligneux 10 ans apres un feu d'origine naturelle survenu dans des forets d'epinette noire (Picea mariana (Mill.) B.S.P.) de l'interieur de l'Alaska. Une etude anterieure realisee dans ces placettes de recherche a montre que la position dans le paysage a un effet determinant sur la proportion de la CO consumee pendant le feu et la revegetalisation apres le feu. Dans la presente etude, nous montrons que la position dans le paysage influence probablement la dynamique du feu dans ces peuplements via des changements de texture du sol mineral. Le contenu en materiaux de texture fine dans le sol mineral sous-jacent a ete positivement relie a la profondeur de la CO mesuree 1 et 10 ans apres le feu, et il y avait une interaction entre la texture du sol et l'altitude concernant la consumation et le retablissement de la CO 10 ans apres le feu. Dix ans apres le feu, la CO etait 2 cm plus profonde qu'un an apres le feu, avec une etendue des observations de 19 cm d'accumulation a 9 cm de subsidence. La subsidence etait inversement reliee au pourcentage de materiaux de texture fine dans le materiau parental. Le facteur qui influencait le plus l'accumulation des reserves de carbone dans la couche organique 10 ans apres le feu etait l'interaction entre la position dans le paysage et la presence de sol a texture fine. Ainsi, la texture du materiau parental interagit avec les processus biologiques pour determiner le retablissement des couches organiques du sol. [Traduit par la Redaction] Mots-cles: Picea mariana, materiau parental, argile, lress, feu d'origine naturelle., Introduction Boreal forests currently harbor about a third of the Earth's rapidly cycling soil organic carbon (C) (McGuire et al. 1995; Pan et al. 2013), which is a pool size [...]

Satellite fire products have the potential to construct inter-annual time series of fire activity, but estimating area burned requires considering biases introduced by orbiting geometry, fire behavior, and the presence of clouds and smoke. Here we evaluated the performance of fire counts from the Advanced Thermal Scanning Radiometer (ATSR) for the boreal forest region using area burned information from other sources. We found ATSR detection rate varied between regions and different years, being higher during large fire years than during small fire years. The results show ATSR fire counts do not represent an unbiased sample of fire activity, and independent validation may be required prior to using this data set in studies of global emissions from biomass burning.

Fire is a major disturbance in the boreal forest, and has been shown to release significant amounts of carbon (C) to the atmosphere through combustion. However, less is known about the effects on ecosystems following fire, which include reduced productivity and changes in decomposition in the decade immediately following the disturbance. In this study, we assessed the impact of fire on net primary productivity (NPP) in the North American boreal forest using a 17-year record of satellite NDVI observations at 8-km spatial resolution together with a light-use efficiency model. We identified 61 fire scars in the satellite observations using digitized fire burn perimeters from a database of large fires. We studied the postfire response of NPP by analyzing the most impacted pixel within each burned area. NPP decreased in the year following the fire by 60-260g CM-2 yr-1 (30-80%). By comparing pre- and postfire observations, we estimated a mean NPP recovery period for boreal forests of about 9 years, with substantial variability among fires. We incorporated this behavior into a carbon cycle model simulation to demonstrate these effects on net ecosystem production. The disturbance resulted in a release of C to the atmosphere during the first 8 years, followed by a small, but long-lived, sink lasting 150 years. Postfire net emissions were three times as large as from a model run without changing NPP. However, only small differences in the C cycle occurred between runs after 8 years due to the rapid recovery of NPP. We conclude by discussing the effects of fire on the long-term continental trends in satellite NDVI observed across boreal North America during the 1980s and 1990s.

A synthesis was carried out to examine Alaska's boreal forest fire regime. During the 2000s, an average of 767 000 ha*[year.sup.-1] burned, 50% higher than in any previous decade since the 1940s. Over the past 60 years, there was a decrease in the number of lightning-ignited fires, an increase in extreme lightning-ignited fire events, an increase in human-ignited fires, and a decrease in the number of extreme human-ignited fire events. The fraction of area burned from human-ignited fires fell from 26% for the 1950s and 1960s to 5% for the 1990s and 2000s, a result from the change in fire policy that gave the highest suppression priorities to fire events that occurred near human settlements. The amount of area burned during late-season fires increased over the past two decades. Deeper burning of surface organic layers in black spruce (Picea mariana (Mill.) BSP) forests occurred during late-growing-season fires and on more well-drained sites. These trends all point to black spruce forests becoming increasingly vulnerable to the combined changes of key characteristics of Alaska's fire regime, except on poorly drained sites, which are resistant to deep burning. The implications of these fire regime changes to the vulnerability and resilience of Alaska's boreal forests and land and fire management are discussed. Resume: Une synthese a ete effectuee pour etudier le regime des feux de la foret boreale en Alaska. Durant les annees 2000, le feu a detruit en moyenne 767 000 ha*[an.sup.-1], soit une superficie 50% plus grande qu'au cours de n'importe quelle decennie precedente depuis les annees 1940. Au cours des 60 dernieres annees, le nombre de feux de foudre a diminue, les feux de foudre majeurs ont augmente, les feux d'origine humaine ont augmente et le nombre de feux majeurs d'origine humaine a diminue. La proportion de la superficie brulee par des feux d'origine humaine a chute de 26% au cours des annees 1950 et 1960 a 5% au cours des annees 1990 et 2000 a cause d'un changement de politique de gestion du feu qui accordait la plus haute priorite a la suppression des feux qui surviennent pres des etablissements humains. La superficie brulee par les feux de fin de saison a augmente au cours des deux dernieres decennies. Les horizons organiques de surface ont ete brules plus en profondeur dans les forets d'epinette noire (Picea mariana (Mill.) BSP) par les feux de fin de saison et sur les stations les mieux drainees. Toutes ces tendances contribuent a rendre les forets d'epinette noire plus vulnerables aux changements combines des caracteristiques cles du regime des feux en Alaska, a l'exception des stations mal drainees qui sont resistantes aux feux de profondeur. La discussion porte sur les consequences de ces changements des caracteristiques du regime des feux sur la vulnerabilite et la resilience des forets boreales de l'Alaska et sur la gestion des feux de foret. [Traduit par la Redaction], Introduction Wildland fire is one of the most widespread and important disturbances affecting Alaska's boreal forests. Its role in regulating ecosystem processes across the North American boreal forest has long [...]

Hydrology (i.e., inundation and soil moisture) is the most important abiotic factor controlling wetland function and extent, and scientists predict that wetland hydrology can be significantly altered over relatively short timescales due to climate change and anthropogenic impact. Whereas broadscale hydrology is difficult to monitor in forested wetlands with ground-based and optical remote sensing methods, C-band synthetic aperture radar (SAR) systems have the potential to improve the capability to monitor forested wetland hydrology. In this study, we examined the use of Environmental Satellite Advanced SAR (C-HH and C-VV) data for monitoring levels of inundation and soil moisture throughout the year in a typical Mid-Atlantic floodplain and some of the main limitations inherent to C-band data (i.e., polarization and plant phenology) in this environment. The relationships between the backscatter coefficient [[sigma].sup.o] and inundation, soil moisture, tree basal area, tree height, and forest canopy closure were examined. Significant differences in C-HH [[sigma].sup.o] were found between forested areas of varying hydrology (0%-60% area inundated) throughout the year and in C-VV [[sigma].sup.o] during the leaf-off season. As expected, C-HH SAR backscatter was better correlated with inundation and soil moisture than was C-VV SAR backscatter, and the correlations between both polarizations of backscatter and hydrology were stronger during the leaf-off season (C-HH leaf-off [r.sup.2] = 0.50, leaf-on [r.sup.2] = 0.39; C-VV leaf-off [r.sup.2] = 0.21, leaf-on [r.sup.2] = 0.19; all significant at p < 0.0001 level). Based on our findings, we concluded that the C-HH data are useful for monitoring hydrology beneath forest canopies throughout the year, whereas the C-VV data can be used during the leaf-off season. Our findings support previous studies that concluded that C-band imagery can be used to monitor forested wetland hydrology in large floodplains that are fully inundated. However, this study used detailed in situ measurements and demonstrated that C-band SAR data can also be used to monitor forested wetland hydrology in smaller partially inundated floodplains, which are more common in the Mid-Atlantic. Index Terms--Hydrology, radar applications, synthetic aperture radar (SAR), water resources.

A multifrequency, multipolarization airborne SAR data set was utilized to examine the relationship between radar backscatter and the aboveground biomass in loblolly pine forests. This data set was also used to examine the potential of SAR to estimate aboveground biomass in these forests. The total aboveground biomass in the test stands used in this study ranged from

An approach is presented to physically describe the tree and canopy components within loblolly pine forests in studies of microwave backscattering from forested canopies. The approach is based on a set of algorithms which describe the biomass characteristics of individual trees using measurements of tree diameter, height, and canopy depth. These algorithms predict bole, branch, and needle biomass; number, sizes, and orientations of primary and smaller branches; and number and density of needle clumps. Methods are discussed to aggregate the individual tree measurements into stand measurements (i.e., measurements per unit area). The results of the model clearly show how biomass distribution between tree components and canopy layers varies as a function of stand age for loblolly pines. The results also show the approach can be used to predict changes in the branch size and orientation within the canopy as a function of stand age, both on an individual tree basis as well as an entire stand basis.

Investigations have shown that fires in boreal forests result in characteristic responses which are recorded on ERS-1 SAR imagery. Using one of the many fire signatures observed on ERS-1 SAR imagery, the analysis of the data revealed there is > 10 dB in variation in image intensity within the burned areas on the SAR images. Possible explanations for this variation include increases in soil moisture, increased surface roughness exposed to the incoming microwave radiation, and damage to the vegetation canopy by the fire.

The calibration of a polarimetric SAR system is achieved through use of radar reflectors with a significant cross-polarized reflection of known properties. Three such reflectors are the dihedral, the active radar calibrator (ARC), and the gridded trihedral. Each of these devices has been used as calibration radar reflectors for the three-frequency (1.25, 5.30, and 9.35 GHz) ERIM/NADC P-3 SAR System. The focus of this paper is on the characteristics of the gridded trihedral. The gridded trihedral is a normal trihedral with one of the conducting faces replaced with a grid of closely spaced (relative to a wavelength) parallel conductors over a layer of microwave absorber. This grid changes the incident wave's polarization and results in a calibration target with a significant cross-polarized reflection. The advantages of the gridded trihedral are that it has a broad backscatter beamwidth (unlike the dihedral), and that it is passive and simple to construct (unlike the ARC). The performance characteristics of the gridded trihedral reflector will be reviewed using basic theoretical models and calibrated P-3 SAR imagery.