A derivation of the statistical characteristics of forest fires

The analysis of large data sets concerning fires in various forested areas of the world has pointed out that burned areas can often be described by different power-law distributions for small, medium and large fires and that a scaling law for the time intervals separating successive fires is fulfilled. The attempts of deriving such statistical laws from purely theoretical arguments have not been fully successful so far, most likely because important physical and/or biological factors controlling forest fires were not taken into account. By contrast, the two-layer spatially extended forest model we propose in this paper encapsulates the main characteristics of vegetational growth and fire ignition and propagation, and supports the empirically discovered statistical laws. Since the model is fully deterministic and spatially homogeneous, the emergence of the power and scaling laws does not seem to necessarily require meteorological randomness and geophysical heterogeneity, although these factors certainly amplify the chaoticity of the fires. Moreover, the analysis suggests that the existence of different power-laws for fires of various scale might be due to the two-layer structure of the forest which allows the formation of different kinds of fires, i.e. surface, crown, and mixed fires.     

Death Rides the Forest: Perceptions of Fire, Land Use, and Ecological Restoration of Western Forests

Abstract:  Large wild fires occurring in forests, grasslands, and chaparral in the last few years have aroused much public concern. Many have described these events as "catastrophes" that must be prevented through aggressive increases in forest thinning. Yet the real catastrophes are not the fires themselves but those land uses, in concert with fire-suppression policies that have resulted in dramatic alterations to ecosystem structure and composition. The first step in the restoration of biological diversity (forest health) of western landscapes must be to implement changes in those factors that have caused degradation or are preventing recovery. This includes changes in policies and practices that have resulted in the current state of wildland ecosystems. Restoration entails much more than simple structural modifications achieved though mechanical means. Restoration should be undertaken at landscape scales and must allow for the occurrence of dominant ecosystem processes, such as the natural fire regimes achieved through natural and/or prescribed fires at appropriate temporal and spatial scales.     

Intra-annual time dynamical patterns of fire sequences observed in Patagonia (Argentina)

Temporal dynamical analysis in fire sequences recorded from 1992 to 2005 in a fire-vulnerable area of Patagonia (Argentina) was performed by using the Allan Factor statistics. Three typologies of fires were investigated: forest, steppe and urban. The obtained results show the presence of annual periodicities, superimposed to a time-scaling behaviour, which characterize the point processes of fires as fractal time processes with a rather high degree of time-clusterization of the events.     

Simulating the potential for ecological restoration of dryland forests in Mexico under different disturbance regimes

Examining the potential for ecological restoration is important in areas where anthropogenic disturbance has degraded forest landscapes. However, the conditions under which restoration of degraded tropical dry forests (TDF) might be achieved in practice have not been determined in detail. In this study, we used LANDIS-II, a spatially explicit model of forest dynamics, to assess the potential for passive restoration of TDF through natural regeneration. The model was applied to two Mexican landscapes under six different disturbance regimes, focusing on the impact of fire and cattle grazing on forest cover, structure and composition. Model results identified two main findings. First, tropical dry forests are more resilient to anthropogenic disturbance than expected. Results suggested that under both a scenario of small, infrequent fires and a scenario of large, frequent fires, forest area can increase relatively rapidly. However, forest structure and composition differed markedly between these scenarios. After 400 years, the landscape becomes increasingly occupied by relatively shade-tolerant species under small, infrequent fires, but only species with both relatively high shade tolerance and high fire tolerance can thrive under conditions with large, frequent fires. Second, we demonstrated that different forms of disturbance can interact in unexpected ways. Our projections revealed that when grazing acts in combination with fire, forest cover, structure and composition vary dramatically depending on the frequency and extent of the fires. Results indicated that grazing and fire have a synergistic effect causing a reduction in forest cover greater than the sum of their individual effects. This suggests that passive landscape-scale restoration of TDF is achievable in both Mexican study areas only if grazing is reduced, and fires are carefully managed to reduce their frequency and intensity.     

Temporal variability of forest fires in eastern Amazonia

Widespread occurrence of fires in Amazonian forests is known to be associated with extreme droughts, but historical data on the location and extent of forest fires are fundamental to determining the degree to which climate conditions and droughts have affected fire occurrence in the region. We used remote sensing to derive a 23-year time series of annual landscape-level burn scars in a fragmented forest of the eastern Amazon. Our burn scar data set is based on a new routine developed for the Carnegie Landsat Analysis System (CLAS), called CLAS-BURN, to calculate a physically based burn scar index (BSI) with an overall accuracy of 93% (Kappa coefficient 0.84). This index uses sub-pixel cover fractions of photosynthetic vegetation, non-photosynthetic vegetation, and shade/burn scar spectral end members. From 23 consecutive Landsat images processed with the CLAS-BURN algorithm, we quantified fire frequencies, the variation in fire return intervals, and rates of conversion of burned forest to other land uses in a 32 400 km2 area. From 1983 to 2007, 15% of the forest burned; 38% of these burned forests were subsequently deforested, representing 19% of the area cleared during the period of observation. While 72% of the fire-affected forest burned only once during the 23-year study period, 20% burned twice, 6% burned three times, and 2% burned four or more times, with the maximum of seven times. These frequencies suggest that the current fire return interval is 5-11 times more frequent than the estimated natural fire regime. Our results also quantify the substantial influence of climate and extreme droughts caused by a strong El Ni?o Southern Oscillation (ENSO) on the extent and likelihood of returning forest fires mainly in fragmented landscapes. These results are an important indication of the role of future warmer climate and deforestation in enhancing emissions from more frequently burned forests in the Amazon.     

Reconciling Salvage Logging of Boreal Forests with a Natural-Disturbance Management Model

Abstract:  In North American boreal forests, wildfire is the dominant agent of natural disturbance. A natural-disturbance model has therefore been promoted as an ecologically based approach to forest harvesting in these systems. Given accelerating resource demands, fire competes with harvest for timber, and there is increasing pressure to salvage naturally burned areas. This creates a management paradox: simultaneous promotion of natural disturbance as a guide to sustainability while salvaging forests that have been naturally disturbed. The major drivers of postfire salvage in Canadian boreal forests are societal perceptions, overallocation of forest resources, and economic and policy incentives, and postfire salvage compromises forest sustainability by diminishing the role of fire as a critical, natural process. These factors might be reconciled through consideration of fire in resource allocations and application of active adaptive management. We provide novel treatment of the role of burn severity in mediating biotic response by examining its influence on the amount, type, and distribution of live, postfire residual material, and we highlight the role of fire in shaping spatial and temporal patterns in forest biodiversity. Maintenance of natural postfire forests is a critical component of an ecosystem-based approach to forest management in boreal systems. Nevertheless, present practices focus heavily on expediting removal of timber from burned forests, despite increasing evidence that postfire communities differ markedly from postharvest systems, and there is a mismatch between emerging management models and past management practices. Policies that recognize the critical role of fire in these systems and facilitate enhanced understanding of natural system dynamics in support of development of sustainable management practices are urgently needed.     

Climate effects on fire regimes and tree recruitment in Black Hills ponderosa pine forests

Climate influences forest structure through effects on both species demography (recruitment and mortality) and disturbance regimes. Here, I compare multi-century chronologies of regional fire years and tree recruitment from ponderosa pine forests in the Black Hills of southwestern South Dakota and northeastern Wyoming to reconstructions of precipitation and global circulation indices. Regional fire years were affected by droughts and variations in both Pacific and Atlantic sea surface temperatures. Fires were synchronous with La Ni?as, cool phases of the Pacific Decadal Oscillation (PDO), and warm phases of the Atlantic Multidecadal Oscillation (AMO). These quasi-periodic circulation features are associated with drought conditions over much of the western United States. The opposite pattern (El Ni?o, warm PDO, cool AMO) was associated with fewer fires than expected. Regional tree recruitment largely occurred during wet periods in precipitation reconstructions, with the most abundant recruitment coeval with an extended pluvial from the late 1700s to early 1800s. Widespread even-aged cohorts likely were not the result of large crown fires causing overstory mortality, but rather were caused by optimal climate conditions that contributed to synchronous regional recruitment and longer intervals between surface fires. Synchronous recruitment driven by climate is an example of the Moran effect. The presence of abundant fire-scarred trees in multi-aged stands supports a prevailing historical model for ponderosa pine forests in which recurrent surface fires affected heterogenous forest structure, although the Black Hills apparently had a greater range of fire behavior and resulting forest structure over multi-decadal time scales than ponderosa pine forests of the Southwest that burned more often.     

Restoration of Landscape Structure Altered by Fire Suppression

There is increasing interest in applying landscape ecological research to the management of wildlands, particularly regarding the negative effects of fragmentation and the benefits of corridors. Patch-producing large disturbances, such as fires and floods, produce a spatial mosaic structure in landscapes to which many species are sensitive. Management of the spatial structure of the patch mosaic has seldom been an explicit concern, however, in part because of insufficient knowledge about bow this spatial structure is affected by alterations in the disturbance regime. Yet the patch mosaic structure of many landscapes has been altered by disturbance control (such as fire suppression), and there is substantial interest in restoring natural disturbance regimes in some wildland landscapes. It has been proposed that, in landscapes subjected to decades of fire suppression, simple reinstatement of the natural fire regime may lead to adverse effects because fuel buildup during fire suppression may result in unusually large fires. It has also been proposed that the use of small prescribed fires may be an effective approach to restoration of landscapes subjected to fire suppression. Here I use a spatial GIS-based simulation model to analyze the effects of reinstating a natural fire regime in the Boundary Waters Canoe Area, Minnesota, after 82 years of fire suppression. The simulation experiment suggests that suppression can be expected to significantly alter landscape structure, but landscape structure can generally be restored within 50–75 years by reinstating the natural fire regime. Unusually large fires would probably hasten the restoration of landscape structure, while small prescribed fires will not restore the landscape but instead will produce further alteration.     

Modelling forest–savanna mosaic dynamics in man-influenced environments: effects of fire,climate and soil heterogeneity

Forests and savannas are the major ecotypes in humid tropical regions. Under present climatic conditions, forest is in a phase of natural expansion over savanna, but traditional human activities, especially fires, have strongly influenced the succession. We here present a new model, FORSAT, dedicated to the forest–savanna mosaic on a landscape scale and based on stochastic modelling of key processes (fire and succession cycle) and consistent with common field data. The model is validated by comparison between the qualitative emergent behaviour of the model and results of biogeographical field studies. Three types of forest succession are shown: progression of the forest edge, formation and coalescence of clumps in savanna and global afforestation of savanna. The parameters (frequency of savanna fires, climate and soil fertility) appear to have comparable effects and there is a sharp threshold between a forest edge progression scenario and the cluster formation one. Moreover, pioneer seed dispersal pattern and recruitment are determinant: peaked curves near a seed source and far dispersal combine to increase the fitness of the pioneers.     

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.     

Analysis of time-scaling properties in forest-fire sequence observed in Italy

The investigation of the temporal fluctuations in forest fires is an important topic in the context of environmental sciences. Time-scaling scale-invariant methodologies have been used to characterize the temporal properties of forest-fire sequences detected in the Italian territory. Our findings reveal that the fire sequences can be considered as fractal processes with a high degree of time-clusterization of the events. The time-clustering phenomenon is clearly visible from timescales of order of days. Furthermore the fire sequences observed with decreasing latitude tend to be more time-clusterized.     

Effect of prior disturbances on the extent and severity of wildfire in Colorado subalpine forests

Disturbances are important in creating spatial heterogeneity of vegetation patterns that in turn may affect the spread and severity of subsequent disturbances. Between 1997 and 2002 extensive areas of subalpine forests in northwestern Colorado were affected by a blowdown of trees, bark beetle outbreaks, and salvage logging. Some of these stands were also affected by severe fires in the late 19th century. During a severe drought in 2002, fires affected extensive areas of these subalpine forests. We evaluated and modeled the extent and severity of the 2002 fires in relation to these disturbances that occurred over the five years prior to the fires and in relation to late 19th century stand-replacing fires. Occurrence of disturbances prior to 2002 was reconstructed using a combination of tree-ring methods, aerial photograph interpretation, field surveys, and geographic information systems (GIS). The extent and severity of the 2002 fires were based on the normalized difference burn ratio (NDBR) derived from satellite imagery. GIS and classification trees were used to analyze the effects of prefire conditions on the 2002 fires. Previous disturbance history had a significant influence on the severity of the 2002 fires. Stands that were severely blown down (> 66% trees down) in 1997 burned more severely than other stands, and young (approximately 120 year old) postfire stands burned less severely than older stands. In contrast, prefire disturbances were poor predictors of fire extent, except that young (approximately 120 years old) postfire stands were less extensively burned than older stands. Salvage logging and bark beetle outbreaks that followed the 1997 blowdown (within the blowdown as well as in adjacent forest that was not blown down) did not appear to affect fire extent or severity. Conclusions regarding the influence of the beetle outbreaks on fire extent and severity are limited, however, by spatial and temporal limitations associated with aerial detection surveys of beetle activity. Thus, fire extent in these forests is largely independent of prefire disturbance history and vegetation conditions. In contrast, fire severity, even during extreme fire weather and in conjunction with a multiyear drought, is influenced by prefire stand conditions, including the history of previous disturbances.     

Conservation Threats Due to Human-Caused Increases in Fire Frequency in Mediterranean-Climate Ecosystems

Periodic wildfire is an important natural process in Mediterranean-climate ecosystems, but increasing fire recurrence threatens the fragile ecology of these regions. Because most fires are human-caused, we investigated how human population patterns affect fire frequency. Prior research in California suggests the relationship between population density and fire frequency is not linear. There are few human ignitions in areas with low population density, so fire frequency is low. As population density increases, human ignitions and fire frequency also increase, but beyond a density threshold, the relationship becomes negative as fuels become sparser and fire suppression resources are concentrated. We tested whether this hypothesis also applies to the other Mediterranean-climate ecosystems of the world. We used global satellite databases of population, fire activity, and land cover to evaluate the spatial relationship between humans and fire in the world's five Mediterranean-climate ecosystems. Both the mean and median population densities were consistently and substantially higher in areas with than without fire, but fire again peaked at intermediate population densities, which suggests that the spatial relationship is complex and nonlinear. Some land-cover types burned more frequently than expected, but no systematic differences were observed across the five regions. The consistent association between higher population densities and fire suggests that regardless of differences between land-cover types, natural fire regimes, or overall population, the presence of people in Mediterranean-climate regions strongly affects the frequency of fires; thus, population growth in areas now sparsely settled presents a conservation concern. Considering the sensitivity of plant species to repeated burning and the global conservation significance of Mediterranean-climate ecosystems, conservation planning needs to consider the human influence on fire frequency. Fine-scale spatial analysis of relationships between people and fire may help identify areas where increases in fire frequency will threaten ecologically valuable areas.     

Correlations between forest fires in British Columbia, Canada, and sea surface temperature of the Pacific Ocean

Correlations and cross-correlations between forest fires in the province of British Columbia, Canada, and sea surface temperatures in the Pacific Ocean were evaluated. British Columbia has a long Pacific Ocean coastline; given that there may be teleconnections between the province's forest fires and climate variability over the ocean, significant correlations may exist between forest fires and the sea surface temperature of the Pacific Ocean. Fire occurrences and areas burned through lightning-caused and human-caused fires were analyzed against individual 1° × 1° grid cells of anomalies in the sea surface temperature to determine correlations for the period 1950-2006. Significant correlations (p < 0.05) for vast areas of the ocean were found between occurrences of lightning-caused fires and sea surface temperature anomalies for time lags of 1 and 2 years, whereas significant correlations between occurrences of human-caused fires and sea surface temperature anomalies occurred extensively for many time lags. To support the results of this approach, correlations between fire data and the Niño 3.4, Pacific Decadal Oscillation, and Arctic Oscillation indices were tested for the same period. Significant correlations were found between fire occurrences and these indices at certain time lags. Overall, fire occurrence appeared to be more extensively correlated with sea surface temperature anomalies than was area burned. These results support the hypothesis that teleconnections exist between fire activity in British Columbia and sea surface temperatures in the Pacific Ocean, and the correlations suggest that linear regression models or other regression techniques may be appropriate for predicting fire severity from the sea surface temperatures of one or more previous years.     

Conservation Threat of Increasing Fire Frequencies in the Western Ghats, India

Abstract:  The acceleration of processes such as forest fragmentation and forest fires in landscapes under intense human pressures makes it imperative to quantify and understand the effects of these processes on the conservation of biodiversity in these landscapes. We combined information from remote-sensing imagery and ground maps of all fires in the Mudumalai Wildlife Sanctuary (MWLS) in the Western Ghats of India over 14 years (1989–2002). These spatial data on fire occurrence were integrated with maps of vegetation types found in the MWLS to examine fire conditions in each. We calculated the average fire-return interval for each of the vegetation types individually and for the MWLS as a whole. Using vegetation data from the larger Nilgiri Biosphere Reserve and the entire Western Ghats region, we conservatively estimated fire-frequency information for these larger regions. Because the MWLS does not contain tropical evergreen or montane forests, we were unable to estimate fire conditions in these forest types, which represent 31% of all Western Ghats vegetation cover. For the MWLS, all vegetation types had average fire-return intervals of <7 years, and the sanctuary as a whole had a fire-return interval of 3.3 years. Compared with a 13-year MWLS fire data set from 1909–1921, this represents a threefold increase in fire frequency over the last 80 years. We estimated average fire-return intervals of roughly 5 years for both the larger Nilgiri Biosphere Reserve and the entire Western Ghats region. Given other recent reports, the estimated fire frequencies for the Western Ghats forests outside protected reserves are conservative. We conclude that the current fire regime of the Western Ghats poses a severe and persistent conservation threat to forests both within and outside protected reserves.     

Fire history and tree recruitment in the Colorado Front Range upper montane zone: implications for forest restoration

Forests experiencing moderate- or mixed-severity fire regimes are presumed to be widespread across the western United States, but few studies have characterized these complex disturbance regimes and their effects on contemporary forest structure. Restoration of pre-fire-suppression open-forest structure to reduce the risk of uncharacteristic stand-replacing fires is a guiding principle in forest management policy, but identifying which forests are clear candidates for restoration remains a challenge. We conducted dendroecological reconstructions of fire history and stand structure at 40 sites in the upper montane zone of the Colorado Front Range (2400-2800 m), sampled in proportion to the distribution of forest types in that zone (50% dominated by ponderosa pine, 28% by lodgepole pine, 12% by aspen, 10% by Douglas-fir). We characterized past fire severity based on remnant criteria at each site in order to assess the effect of fire history on tree establishment patterns, and we also evaluated the influence of fire suppression and climate. We found that 62% of the sites experienced predominantly moderate-severity fire, 38% burned at high severity, and no sites burned exclusively at low severity. The proportion of total tree and sapling establishment was significantly different among equal time periods based on a chi-square test, with highest tree and sapling establishment during the pre-fire-suppression period (1835-1919). Superposed epoch analysis revealed that fires burned during years of extreme drought (95% CI). The major pulse of tree establishment in the upper montane zone occurred during a multidecadal period of extreme drought conditions in the Colorado Front Range (1850-1889), during which 53% of the fires from the 1750-1989 period burned. In the upper montane zone of the Colorado Front Range, historical evidence suggests that these forests are resilient to prolonged periods of severe drought and associated severe fires.     

Forest-Dwelling Vertebrate Faunas and Natural Fire Regimes in British Columbia: Patterns and Implications for Conservation

Structure of native vertebrate faunas within 12 different forest types were related to features of the natural fire regime. Relations between faunal structure and fire regime followed patterns expected if faunas were adapted to fire regimes. Proportions of species breeding early in succession tended to increase with increasing fire size or burn rate (ha/year; p = 0.03); those breeding late in succession tended to decrease ( p = 0.04). As fire size increased, proportions of species breeding in cavities decreased ( p < 0.01). Proportions of species using downed wood to breed increased as the interval between fires increased and downed wood accumulated ( p < 0.01). Forestry practices to maintain biodiversity should mimic natural disturbance patterns, which differ across forest types. Implications for management to maintain vertebrate diversity are summarized in terms of the silvicultural system employed, the size of patches logged, the rate of timber removal, and the appropriate degree of connectivity among unlogged patches.     

Fire Regime Changes in La Michilía Biosphere Reserve, Durango, Mexico

Abstract: The ability of reserves to maintain natural ecosystem processes such as fire disturbance regimes is central to long-term conservation. Fire-scarred tree samples were used to reconstruct fire regimes at five study sites totaling approximately 230 ha in pine (   Pinus spp.) and oak ( Quercus spp.) forests of La Michilía Biosphere Reserve on the dry east slope of the Sierra Madre Occidental, Durango, Mexico. Study sites covered a 20-km environmental gradient of elevation, topography, and human land uses. Plant communities ranged from oak-pine to mixed conifer forests. Fires were frequent at all sites prior to 1930, when large-scale grazing of domestic livestock was initiated. Widespread fires have been excluded from four of the five sites since 1945, with an essentially uninterrupted regime of frequent fires continuing only in the reserve core. Xeric sites had many, smaller fires, whereas mesic sites had fewer but larger fires. On a reserve-wide scale, a fire burned on at least one site nearly every year, usually in the dry spring or early summer season, but fire years were rarely synchronous among the sites. Fire occurrence was weakly related to the Southern Oscillation climate pattern; major reserve-wide fire years almost never coincided with wet Southern Oscillation extremes but only occasionally matched dry extremes. Maintenance of the long-term frequent-fire regime in the reserve core is one indicator that the biosphere reserve model has been successful in conserving natural processes, but the protected area is small ( 7000 ha). Because of the key role of frequent-fire regimes in regulating ecosystem structure and function, restoration of the ecological role of fire disturbance is a desirable conservation strategy.     

青藏高原东部亚高山森林草甸植被地理格局的成因探讨

森林和草地在亚高山带的阴阳坡上形成的交错分布现象,呈一弧型分布在青藏高原东部本文在综合分析了造成这种现象的生理生态因素之后,重点从火的作用、放牧作用和历史时期人类活动的影响等方面,讨论了青藏高原东部亚高山带植被地理格局的形成原因结果表明,自然生态因素和人类活动的影响在植被地理格局的形成过程中起着同等重要的作用,其中火灾的作用以及气候变化造成的人口迁移压力作用十分明显．在这一弧形区域内,林线以下的亚高山灌丛或草甸多属次生类型,人类有目的的烧荒和长期放牧阻滞了植被的顺向演替,维持了亚高山（灌丛）草甸的偏途顶级状态．