Large Fire Years Research Articles

System-level feedbacks of active fire regimes in large landscapes

BackgroundClimate is a main driver of fire regimes, but recurrent fires provide stabilizing feedbacks at several spatial scales that can limit fire spread and severity—potentially contributing to a form of self-regulation. Evaluating the strength of these feedbacks in wildland systems is difficult given the spatial and temporal scales of observation required. Here, we used the REBURN model to directly examine the relative strengths of top-down and bottom-up drivers of fire over a 3000-year simulation period, within a 275,000-ha conifer-dominated landscape in north central Washington State, USA.ResultsWe found strong support for top-down and bottom-up spatial and temporal controls on fire patterns. Fire weather was a main driver of large fire occurrence, but area burned was moderated by ignition frequencies and by areas of limited fuels and fuel contagion (i.e., fire fences). Landscapes comprised of >40% area in fire fences rarely experienced large fire years. When large fires did occur during the simulation period, a recovery time of 100–300 years or more was generally required to recover pre-fire vegetation patterns.ConclusionsSimulations showed that interactions between fire weather, fuel contagion, topography, and ignitions manifest variability in fire size and severity patch size distributions. Burned and recovering vegetation mosaics provided functional stabilizing feedbacks, a kind of metastability, which limited future fire size and severity, even under extreme weather conditions. REBURN can be applied to new geographic and physiographic landscapes to simulate these interactions and to represent natural and culturally influenced fire regimes in historical, current, or future climatic settings.

Climate drove the fire cycle and humans influenced fire occurrence in the East European boreal forest

AbstractUnderstanding long‐term forest fire histories of boreal landscapes is instrumental for parameterizing climate–fire interactions and the role of humans affecting natural fire regimes. The eastern sections of the European boreal zone currently lack a network of annually resolved and centuries‐long forest fire histories. To fill in this knowledge gap, we dendrochronologically reconstructed the 600‐year fire history of a middle boreal pine‐dominated landscape of the southern part of the Republic of Komi, Russia. We combined the reconstruction of fire cycle (FC) and fire occurrence with the data on the village establishment and climate proxies and discussed the relative contribution of climate versus human land use in shaping historic fire regimes. Over the 1340–1610 ce period, the territory had a FC of 66 years (with the 90% confidence envelope of 56.8 and 78.6 years). Fire activity increased during the 1620–1730 ce period, with the FC reaching 32 years (31.0–34.7 years). Between 1740–1950, the FC increased to 47 years (41.9–52.0). The most recent period, 1960–2010, marks FC's historic maximum, with the mean of 153 years (102.5–270.3). Establishment of the villages, often as small harbors on the Pechora River, was associated with a non‐significant increase in fire occurrence in the sites nearest the villages (p = 0.07–0.20). We, however, observed a temporal association between village establishment and fire occurrence at the scale of the whole studied landscape. There was no positive association between the former and the FC. In fact, we documented a decline in the area burned, following the wave of village establishment during the second half of the 1600s and the first half of the 1700s. The lack of association between the dynamics of FC and the dates of village establishments, and the significant association between large fire years and the early and latewood pine chronologies, used as historic drought proxy, indirectly suggests that the climate was the primary control of the landscape‐level FCs in the studied forests. Pine‐dominated forests of the Komi Republic may hold a unique position as the ecosystem with the shortest history of human‐related shifts in fire cycles across the European boreal region.

Future increases in lightning ignition efficiency and wildfire occurrence expected from drier fuels in boreal forest ecosystems of western North America

Lightning-induced fire is the primary disturbance agent in boreal forests. Recent large fire years have been linked to anomalously high numbers of lightning-caused fire starts, yet the mechanisms regulating the probability of lightning ignition remain uncertain and limit our ability to project future changes. Here, we investigated the influence of lightning properties, landscape characteristics, and fire weather on lightning ignition efficiency—the likelihood that a lightning strike starts a fire—in Alaska, United States of America, and Northwest Territories, Canada, between 2001 and 2018. We found that short-term fuel drying associated with fire weather was the main driver of lightning ignition efficiency. Lightning was also more likely to ignite a wildfire in denser, evergreen forest areas. Under a high greenhouse gas emissions scenario, we predicted that changes in vegetation and fire weather increase lightning ignition efficiency by 14 ± 9% in Alaska and 31 ± 28% in the Northwest Territories per 1 °C warming by end-of-century. The increases in lightning ignition efficiency, together with a projected doubling of lightning strikes, result in a 39%–65% increase in lightning-caused fire occurrence per 1 °C warming. This implies that years with many fires will occur more frequently in the future, thereby accelerating carbon losses from boreal forest ecosystems.

Short-interval fires increasing in the Alaskan boreal forest as fire self-regulation decays across forest types

Climate drivers are increasingly creating conditions conducive to higher frequency fires. In the coniferous boreal forest, the world’s largest terrestrial biome, fires are historically common but relatively infrequent. Post-fire, regenerating forests are generally resistant to burning (strong fire self-regulation), favoring millennial coniferous resilience. However, short intervals between fires are associated with rapid, threshold-like losses of resilience and changes to broadleaf or shrub communities, impacting carbon content, habitat, and other ecosystem services. Fires burning the same location 2 + times comprise approximately 4% of all Alaskan boreal fire events since 1984, and the fraction of short-interval events (< 20 years between fires) is increasing with time. While there is strong resistance to burning for the first decade after a fire, from 10 to 20 years post-fire resistance appears to decline. Reburning is biased towards coniferous forests and in areas with seasonally variable precipitation, and that proportion appears to be increasing with time, suggesting continued forest shifts as changing climatic drivers overwhelm the resistance of early postfire landscapes to reburning. As area burned in large fire years of ~ 15 years ago begin to mature, there is potential for more widespread shifts, which should be evaluated closely to understand finer grained patterns within this regional trend.

Trends and patterns in annually burned forest areas and fire weather across the European boreal zone in the 20th and early 21st centuries

Fire remains one of the main natural disturbance factors in the European boreal zone and understanding climatic forcing on fire activity is important for projecting effects of climate change on ecosystem services in this region. We analyzed records of annually burned areas in 16 administrative regions of the European boreal zone (countries or administrative units within countries) and fire weather variability to test for their spatio-temporal patterns over the 1901-2017 period.Over the 1992-2017 period, the region exhibited large variability in forest fire activity with the fire cycles varying from ~1600 (St. Petersburg region) to ~37000 years (Finland). The clustering of administrative units in respect to their burned area, suggested the presence of sub-regions with synchronous annual variability in burned areas. Large fire years (LFYs) in each of the clusters were associated with the development of the high pressure cell over or in immediate proximity of the regions in question in July, indicating climatic forcing of LFYs. Contingency analysis indicated that there was no long-term trend in the synchrony of LFYs observed simultaneously in several administrative units. We documented a trend towards higher values of Monthly Drought Code (MDC) for the months of April and May in the western (April) and northern (April and May) sections. The significant positive correlation between biome-wide fire activity index and June SNAO (Summer North Atlantic Oscillation) (r = 0.53) pointed to the importance of large-scale atmospheric circulation, in particular the summer European blocking pattern, in controlling forest fires across EBZ. The forest fire activity of the European boreal zone remains strongly connected to the annual climate variability. Higher frequency of strongly positive SNAO states in the future will likely synchronize years with a large area burned across the European boreal zone.

Overwintering fires in boreal forests.

Forest fires are usually viewed within the context of a single fire season, in which weather conditions and fuel supply can combine to create conditions favourable for fire ignition-usually by lightning or human activity-and spread1-3. But some fires exhibit 'overwintering' behaviour, in which they smoulder through the non-fire season and flare up in the subsequent spring4,5. In boreal (northern) forests, deep organic soils favourable for smouldering6, along with accelerated climate warming7, may present unusually favourable conditions for overwintering. However, the extent of overwintering in boreal forests and the underlying factors influencing this behaviour remain unclear. Here we show that overwintering fires in boreal forests are associated with hot summers generating large fire years and deep burning into organic soils, conditions that have become more frequent in our study areas in recent decades. Our results are based on an algorithm with which we detect overwintering fires in Alaska, USA, and the Northwest Territories, Canada, using field and remote sensing datasets. Between 2002 and 2018, overwintering fires were responsible for 0.8 per cent of the total burned area; however, in one year this amounted to 38 per cent. The spatiotemporal predictability of overwintering fires could be used by fire management agencies to facilitate early detection, which may result in reduced carbon emissions and firefighting costs.

Historical and future global burned area with changing climate and human demography

Summary Wildfires influence terrestrial carbon cycling and represent a safety risk, and yet a process-based understanding of their frequency and spatial distributions remains elusive. We combine satellite-based observations with an enhanced dynamic global vegetation model to make regionally resolved global assessments of burned area (BA) responses to changing climate, derived from 34 Earth system models and human demographics for 1860–2100. Limited by climate and socioeconomics, recent BA has decreased, especially in central South America and mesic African savannas. However, future simulations predict increasing BA due to changing climate, rapid population density growth, and urbanization. BA increases are especially notable at high latitudes, due to accelerated warming, and over the tropics and subtropics, due to drying and human ignitions. Conversely, rapid urbanization also limits BA via enhanced fire suppression in the immediate vicinity of settlements, offsetting the potential for dramatic future increases, depending on warming extent. Our analysis provides further insight into regional and global BA trends, highlighting the importance of including human demographic change in models for wildfire under changing climate.

Wildfire severity and postfire salvage harvest effects on long‐term forest regeneration

AbstractFollowing a wildfire, regeneration to forest can take decades to centuries and is no longer assured in many western U.S. environments given escalating wildfire severity and warming trends. After large fire years, managers prioritize where to allocate scarce planting resources, often with limited information on the factors that drive successful forest establishment. Where occurring, long‐term effects of postfire salvage operations can increase uncertainty of establishment. Here, we collected field data on postfire regeneration patterns within 13‐ to 28‐yr‐old burned patches in eastern Washington State. Across 248 plots, we sampled tree stems &lt;4 m height using a factorial design that considered (1) fire severity, moderate vs. high severity; (2) salvage harvesting, salvaged vs. no management; and (3) potential vegetation type (PVT), sample resides in a dry, moist, or cold mixed‐conifer forest environment. We found that regeneration was abundant throughout the study region, with a median of 4414 (IQR 19,618) stems/ha across all plots. Only 15% of plots fell below minimum timber production stocking standards (350 trees/ha), and &lt;2% of plots were unstocked. Densities were generally highest in high‐severity patches and following salvage harvesting, although high variability among plots and across sites led to variable significance for these factors. Post hoc analyses suggested that mild postfire weather conditions may have reduced water stress on tree establishment and early growth, contributing to overall high stem densities. Douglas fir was the most abundant species, particularly in moderate‐severity patches, followed by ponderosa pine, lodgepole pine, western larch, and Engelmann spruce. Generalized additive models (GAMs) revealed species‐level climatic tolerances and seed dispersal limits that portend future challenges to regeneration with expected future climate warming and increased fire activity. Postfire regeneration will occur on sites with adequate seed sources within their climatic tolerances.

Effects of human-related and biotic landscape features on the occurrence and size of modern forest fires in Sweden

The influence of landscape features on the occurrence and size of forest fires in Northern Europe has not been well-studied. In this paper, we analyzed the impact of human-related landscape properties (road and human population density), biotic features (amount of firebreak area and vegetation zone) and fire weather indices (Buildup Index, BUI and Initial Spread Index, ISI) on the occurrence and size of forest fires in Sweden from 1998 to 2017. To analyze the environmental controls of fire occurrence and fire size under different levels of climatological fire hazard, we divided the data into two subsets: (1) large fire years (LFY), defined as the years with the total amount of burned area being higher than the dataset-wide average (2002, 2003, 2006, 2008 and 2014), and (2) the remaining years (nLFY). Our analytical approach was based on spatial models using Integrated Nested Laplace Approximations (INLA). Models built on both LFY and nLFY subsets suggested a strong human influence on fire occurrence: road density, the number of firebreaks, and population density, all were positively associated with fire occurrence, suggesting an important role of human-related ignitions. The southernmost vegetation zones in Sweden (boreo-nemoral and nemoral) exhibited the highest fire occurrence (LFY), a pattern potentially related to a higher population density in combination with weather more conducive for fires in this part of the country. The patterns that emerged from the fire size models pointed to the climate as the main factor controlling fire size, irrespective of the type of years analyzed. Road density, number of firebreaks and population density showed a negative association with fire size, possibly indicating higher efficiency of fire suppression in the areas with higher human presence. Vegetation zones were selected as an informative predictor, indicating that the fire activity varies across the zones, with those in mid-Sweden being the most prone to large events. The ISI correlated strongly and positively with fire size in both subsets (LFY and nLFY), pointing to the role of weather conditions favorable for fire spread, primarily that of surface fires. The BUI showed a weak negative correlation to fire size, indicating that dryness of organic horizon, specifically its deeper layers, is less relevant for predicting fire size. Contemporary fire activity in Sweden is driven by a combination of human-related ignitions, and weather conditions controlling fire spread with a moderate effect of vegetation composition and generally efficient fire suppression. Human-related landscape features (roads and population density) play a major role in shaping ignition patterns, whereas climate (ISI) and vegetation properties appear informative as predictors of fire size, even under a modern fire suppression effort.

Lightning as a major driver of recent large fire years in North American boreal forests

Changes in climate and fire regimes are transforming the boreal forest, the world’s largest biome. Boreal North America recently experienced two years with large burned area: 2014 in the Northwest Territories and 2015 in Alaska. Here we use climate, lightning, fire and vegetation data sets to assess the mechanisms contributing to large fire years. We find that lightning ignitions have increased since 1975, and that the 2014 and 2015 events coincided with a record number of lightning ignitions and exceptionally high levels of burning near the northern treeline. Lightning ignition explained more than 55% of the interannual variability in burned area, and was correlated with temperature and precipitation, which are projected to increase by mid-century. The analysis shows that lightning drives interannual and long-term ignition and burned area dynamics in boreal North America, and implies future ignition increases may increase carbon loss while accelerating the northward expansion of boreal forest. The boreal forest is being transformed by changes in its climate–fire regime. Analysis now shows that lightning drives year-to-year and long-term ignition and burned area trends in boreal North America.

Does wildfire likelihood increase following insect outbreaks in conifer forests?

Although there is acute concern that insect‐caused tree mortality increases the likelihood or severity of subsequent wildfire, previous studies have been mixed, with findings typically based on stand‐scale simulations or individual events. This study investigates landscape‐ and regional‐scale wildfire likelihood following outbreaks of the two most prevalent native insect pests in the US Pacific Northwest (PNW): mountain pine beetle (MPB;Dendroctonus ponderosae) and western spruce budworm (WSB;Choristoneura freemani). We leverage seamless census data across numerous insect and fire events to (1) summarize the interannual dynamics of insects (1970–2012) and wildfires (1984–2012) across forested ecoregions of the PNW; (2) identify potential linked disturbance interactions with an empirical wildfire likelihood index; (3) quantify this insect‐fire likelihood across different insect agents, time lags, ecoregions, and fire sizes. All three disturbance agents have occurred primarily in the drier, interior conifer forests east of the Cascade Range. In general, WSB extent exceeds MPB extent, which in turn exceeds wildfire extent, and each disturbance typically affects less than 2% annually of a given ecoregion. In recent decades across the PNW, wildfire likelihood does not consistently increase or decrease following insect outbreaks. There is evidence, however, of linked interactions that vary across insect agent (MPB, WSB), space (ecoregion), and time (interval since insect onset). Specifically, in most cases following MPB activity, fire likelihood is neither higher nor lower than in non‐MPB‐affected forests. In contrast, fire likelihood is lower following WSB activity across multiple ecoregions and time lags. In addition, insect‐fire likelihood is not consistently associated with interannual fire extent, suggesting that other factors (e.g., climate) control the disproportionately large fire years accounting for regional fire dynamics. Thus, although both bark beetles and defoliators alter fuels and associated fire potential, the windows of opportunity for increased or decreased fire likelihood are too narrow—or the phenomena themselves too rare—for a consistent signal to emerge across PNW conifer forests. These findings suggest that strategic plans should recognize (1) the relative rarity of insect‐fire interactions and (2) the potential ecosystem restoration benefits of native insect outbreaks, when they do occur.

A 700-year record of large fire years in northern Scandinavia shows large variability and increased frequency during the 1800 s

Years with climatically mediated increases in boreal forest fire activity, referred to as large fire years (LFYs), contribute to a disproportionally large portion of the burned area over centuries, and are important drivers of ecosystem processes by affecting forest structure, biodiversity, and carbon balance at regional and continental scales. We analysed changes in LFY return intervals in northern Sweden (the area above 60 °N) over 1273–1960 using a network of 29 sites with dendrochronologically reconstructed fires, complemented by documentary records of fires available from forestry statistics. We observed large variability in return intervals of LFYs, an increase in LFY frequency during the 1800 s, and consistent associations between LFY occurrence and 500-hPa pressure anomalies over the European sub-continent over 1800xps2#1960. An increase in LFY frequency during the 1800 s might be climatically driven, and would thus long precede the period of likely human-induced climatic changes of the 1900 s. Long-term variability in climatically driven LFYs may present a challenge in partitioning the effects of human-related and human-independent components of climatic forcing upon forest fire activity.

Resistance of the boreal forest to high burn rates.

Boreal ecosystems and their large carbon stocks are strongly shaped by extensive wildfires. Coupling climate projections with records of area burned during the last 3 decades across the North American boreal zone suggests that area burned will increase by 30-500% by the end of the 21st century, with a cascading effect on ecosystem dynamics and on the boreal carbon balance. Fire size and the frequency of large-fire years are both expected to increase. However, how fire size and time since previous fire will influence future burn rates is poorly understood, mostly because of incomplete records of past fire overlaps. Here, we reconstruct the length of overlapping fires along a 190-km-long transect during the last 200 y in one of the most fire-prone boreal regions of North America to document how fire size and time since previous fire will influence future fire recurrence. We provide direct field evidence that extreme burn rates can be sustained by a few occasional droughts triggering immense fires. However, we also show that the most fire-prone areas of the North American boreal forest are resistant to high burn rates because of overabundant young forest stands, thereby creating a fuel-mediated negative feedback on fire activity. These findings will help refine projections of fire effect on boreal ecosystems and their large carbon stocks.

Multi‐century reconstruction of fire activity in Northern European boreal forest suggests differences in regional fire regimes and their sensitivity to climate

Summary Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large‐scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n = 62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5–67.0° N and 9.3–20.5° E) over AD 1400–1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60° N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40 years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region. Over 1420–1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well‐defined zones with characteristic fire activity, with the geographical division at approximately 60° N. The northern and mid‐boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

Three large fire years threaten resilience of closed crown black spruce forests in eastern Canada

An emerging paradigm regarding vegetation response to climate warming is that the interaction of weather extremes and disturbance will trigger abrupt changes in ecosystem types by overcoming resilience of dominant species. Black spruce (Picea mariana (Mill.)) ecosystems are widespread across the North American boreal forest, because of ecophysiological adaptations that allowed these communities to thrive in fire‐prone areas. We investigated resilience of spruce‐moss forests to weather‐disturbance interaction after a 3‐year period (2005 to 2007) of major fire activity caused by extreme fire weather in eastern Canada. Pre‐ and post‐fire conifer densities and environmental parameters related to seed rain, post‐fire seedbeds, microclimate, and post‐fire weather were measured in 133 burned stands throughout the closed‐crown forest of Quebec. Critically low black spruce (BS) regeneration was observed in almost all of the stands, leading to a decrease in stand density and a shift of species dominance from BS to jack pine (Pinus banksiana (Lamb.), JP). The studied sites were characterized by thick residual organic matter, resulting in a predominance of charred duff, a seedbed associated with low water retention and high variation in temperature. While high levels of JP seedling establishment were reported on this seedbed, it was unfavorable to BS germination and survival in the context of warm and dry weather that prevailed in post‐fire summers. In these ecosystems, early vegetation establishment patterns are generally reliable predictors of future stand composition and the exclusion of BS will presumably be maintained through succession. During large fire years, high proportions of the landscape are subjected to the interaction of fire regime and weather that creates unsuitable conditions for BS regeneration. Hence, vegetation change is susceptible to happen at a broad scale. Therefore, the frequency of major fire years could have a decisive influence on the rate of vegetation response to climate change in this biome.

Controls on variations in MODIS fire radiative power in Alaskan boreal forests: Implications for fire severity conditions

Fire activity in the Alaskan boreal forest, though episodic at annual and intra-annual time scales, has experienced an increase over the last several decades. Increases in burned area and fire severity are not only releasing more carbon to the atmosphere, but likely shifting vegetation composition in the region towards greater deciduous dominance and a reduction in coniferous stands. While some recent studies have addressed qualitative differences between large and small fire years in the Alaskan boreal forest, the ecological effects of a greater proportion of burning occurring during large fire years and during late season fires have not yet been examined.Some characteristics of wildfires that can be detected remotely are related to fire severity and can provide new information on spatial and temporal patterns of burning. This analysis focused on boreal wildfire intensity (fire radiative power, or FRP) contained in the Moderate Resolution Imaging Spectroradiometer (MODIS) daily active fire product from 2003 to 2010. We found that differences in FRP resulted from seasonality and intra-annual variability in fire activity levels, vegetation composition, latitudinal variation, and fire spread behavior.Our studies determined two general categories of active fire detections: new detections associated with the spread of the fire front and residual pixels in areas that had already experienced front burning. Residual pixels had a lower average FRP than front pixels, but represented a high percentage of all pixels during periods of high fire activity (large fire years, late season burning, and seasonal periods of high fire activity). As a result, the FRP from periods of high fire activity was less intense than those from periods of low fire activity. Differences related to latitude were greater than expected, with higher latitudes burning later in the season and at a higher intensity than lower latitudes. Differences in vegetation type indicate that coniferous vegetation is the most fire prone, but deciduous vegetation is not particularly fire resistant, as the proportion of active fire detections in deciduous stands is roughly the same as the fraction of deciduous vegetation in the region.Qualitative differences between periods of high and low fire activity are likely to reflect important differences in fire severity. Large fire years are likely to be more severe, characterized by more late season fires and a greater proportion of residual burning. Given the potential for severe fires to effect changes in vegetation cover, the shift toward a greater proportion of area burning during large fire years may influence vegetation patterns in the region over the medium to long term.

Controls on carbon consumption during Alaskan wildland fires

AbstractA method was developed to estimate carbon consumed during wildland fires in interior Alaska based on medium‐spatial scale data (60 m cell size) generated on a daily basis. Carbon consumption estimates were developed for 41 fire events in the large fire year of 2004 and 34 fire events from the small fire years of 2006–2008. Total carbon consumed during the large fire year (2.72 × 106 ha burned) was 64.7 Tg C, and the average carbon consumption during the small fire years (0.09 × 106 ha burned) was 1.3 Tg C. Uncertainties for the annual carbon emissions ranged from 13% to 21%. Carbon consumed from burning of black spruce forests represented 76% of the total during large fire years and 57% during small fire years. This was the result of the widespread distribution of black spruce forests across the landscape and the deep burning of the surface organic layers common to these ecosystems. Average carbon consumed was 3.01 kg m−2 during the large fire year and 1.69 kg m−2 during the small fire years. Most of the carbon consumption was from burning of ground layer fuels (85% in the large fire year and 78% in small fire years). Most of the difference in average carbon consumption between large and small fire years was in the consumption of ground layer fuels (2.60 vs. 1.31 kg m−2 during large and small fire years, respectively). There was great variation in average fuel consumption between individual fire events (0.56–5.06 kg m−2) controlled by variations in fuel types and topography, timing of the fires during the fire season, and variations in fuel moisture at the time of burning.

Potential shifts in dominant forest cover in interior Alaska driven by variations in fire severity

Large fire years in which >1% of the landscape burns are becoming more frequent in the Alaskan (USA) interior, with four large fire years in the past 10 years, and 79 000 km2 (17% of the region) burned since 2000. We modeled fire severity conditions for the entire area burned in large fires during a large fire year (2004) to determine the factors that are most important in estimating severity and to identify areas affected by deep-burning fires. In addition to standard methods of assessing severity using spectral information, we incorporated information regarding topography, spatial pattern of burning, and instantaneous characteristics such as fire weather and fire radiative power. Ensemble techniques using regression trees as a base learner were able to determine fire severity successfully using spectral data in concert with other relevant geospatial data. This method was successful in estimating average conditions, but it underestimated the range of severity. This new approach was used to identify black spruce stands that experienced intermediate- to high-severity fires in 2004 and are therefore susceptible to a shift in regrowth toward deciduous dominance or mixed dominance. Based on the output of the severity model, we estimate that 39% (approximately 4000 km2) of all burned black spruce stands in 2004 had <10 cm of residual organic layer and may be susceptible a postfire shift in plant functional type dominance, as well as permafrost loss. If the fraction of area susceptible to deciduous regeneration is constant for large fire years, the effect of such years in the most recent decade has been to reduce black spruce stands by 4.2% and to increase areas dominated or co-dominated by deciduous forest stands by 20%. Such disturbance-driven modifications have the potential to affect the carbon cycle and climate system at regional to global scales.

Wildfires in Russia in 2000–2008: estimates of burnt areas using the satellite MODIS MCD45 data

Fire activity in Russia in 2000–2008 was analysed on the basis of the newest satellite MODIS MCD45 Burned Area data. The estimated annual burned areas exhibited strong variability in magnitude, geographical location and affected vegetation, varying in total from 5.4 Mha in 2000 to 33.0 Mha in 2008. The total area burned in 2000–2008 in forests and woodlands (the areas with percent tree canopy cover >40%) was 43.4 Mha, of which about 90% was burned in the eastern Siberia and the Far East, and 53% was burned during the large fire years 2003 and 2008. The forest burned areas were mainly localized within 50–55°N, approximately along the southern edge of the area of boreal forests. The reliability of the resultant estimates was tested by comparison with the known early published results. Some applications of the results in problems of atmospheric composition and air quality are also discussed.

Spatial and temporal corroboration of a fire‐scar‐based fire history in a frequently burned ponderosa pine forest

Fire scars are used widely to reconstruct historical fire regime parameters in forests around the world. Because fire scars provide incomplete records of past fire occurrence at discrete points in space, inferences must be made to reconstruct fire frequency and extent across landscapes using spatial networks of fire-scar samples. Assessing the relative accuracy of fire-scar fire history reconstructions has been hampered due to a lack of empirical comparisons with independent fire history data sources. We carried out such a comparison in a 2780-ha ponderosa pine forest on Mica Mountain in southern Arizona (USA) for the time period 1937-2000. Using documentary records of fire perimeter maps and ignition locations, we compared reconstructions of key spatial and temporal fire regime parameters developed from documentary fire maps and independently collected fire-scar data (n = 60 plots). We found that fire-scar data provided spatially representative and complete inventories of all major fire years (> 100 ha) in the study area but failed to detect most small fires. There was a strong linear relationship between the percentage of samples recording fire scars in a given year (i.e., fire-scar synchrony) and total area burned for that year (y = 0.0003x + 0.0087, r2 = 0.96). There was also strong spatial coherence between cumulative fire frequency maps interpolated from fire-scar data and ground-mapped fire perimeters. Widely reported fire frequency summary statistics varied little between fire history data sets: fire-scar natural fire rotations (NFR) differed by < 3 yr from documentary records (29.6 yr); mean fire return intervals (MFI) for large-fire years (i.e., > or = 25% of study area burned) were identical between data sets (25.5 yr); fire-scar MFIs for all fire years differed by 1.2 yr from documentary records. The known seasonal timing of past fires based on documentary records was furthermore reconstructed accurately by observing intra-annual ring position of fire scars and using knowledge of tree-ring growth phenology in the Southwest. Our results demonstrate clearly that representative landscape-scale fire histories can be reconstructed accurately from spatially distributed fire-scar samples.