Lightning Ignitions Research Articles

Factors influencing fire severity under moderate burning conditions in the Klamath Mountains, northern California,USA

AbstractTopography, weather, and fuels are known factors driving fire behavior, but the degree to which each contributes to the spatial pattern of fire severity under different conditions remains poorly understood. The variability in severity within the boundaries of the 2006 wildfires that burned in the Klamath Mountains, northern California, along with data on burn conditions and new analytical tools, presented an opportunity to evaluate factors influencing fire severity under burning conditions representative of those where management of wildfire for resource benefit is most likely. Fire severity was estimated as the percent change in canopy cover (0–100%) classified from the Relativized differenced Normalized Burn Ratio (RdNBR), and spatial data layers were compiled to determine strength of associations with topography, weather, and variables directly or indirectly linked to fuels, such as vegetation type, number of previous fires, and time since last fire. Detailed fire progressions were used to estimate weather (e.g., temperature, relative humidity, temperature inversions, and solar radiation) at the time of burning. A generalized additive regression model with random effects and an additional spatial term to account for autocorrelation between adjacent locations was fitted to fire severity. In this fire year characterized by the relative absence of extreme fire weather, topographical complexity most strongly influenced severity. Upper‐ and mid‐slopes tended to burn at higher fire severity than lower‐slopes. East‐ and southeast‐facing aspects tended to burn at higher severity than other aspects. Vegetation type and fire history were also important predictors of fire severity. Shrub vegetation was more likely to burn at higher severity than mixed hardwood/conifer or hardwood vegetation. As expected, fire severity was positively associated with time since previous fire, but the relationship was non‐linear. Of the weather variables analyzed, temperature inversions, common in the complex topography of the Klamath Mountains, showed the strongest association with fire severity. Inversions trapped smoke and had a dampening effect on severity within the landscape underneath the inversion. Understanding the spatial controls on mixed‐severity fires allows managers to better plan for future wildfires and aide in the decision making when managing lightning ignitions for resource benefit might be appropriate.

Controls on interannual variability in lightning-caused fire activity in the western US

Lightning-caused wildfires account for a majority of burned area across the western United States (US), yet lightning remains among the more unpredictable spatiotemporal aspects of the fire environment and a challenge for both modeling and managing fire activity. A data synthesis of cloud-to-ground lightning strikes, climate and fire data across the western US from 1992 to 2013 was conducted to better understand geographic variability in lightning-caused wildfire and the factors that influence interannual variability in lightning-caused wildfire at regional scales. Distinct geographic variability occurred in the proportion of fires and area burned attributed to lightning, with a majority of fires in the interior western US attributed to lightning. Lightning ignition efficiency was highest across the western portion of the region due to the concomitance of peak lightning frequency and annual nadir in fuel moisture in mid-to-late summer. For most regions the number of total and dry lightning strikes exhibited strong interannual correlation with the number of lightning-caused fires, yet were a poor predictor of area burned at regional scales. Commonality in climate–fire relationships for regional annual area burned by lightning- versus human-ignited fires suggests climate conditions, rather than lightning activity, are the predominant control of interannual variability in area burned by lightning-caused fire across much of the western US.

Seasonality of fire weather strongly influences fire regimes in South Florida savanna-grassland landscapes.

Fire seasonality, an important characteristic of fire regimes, commonly is delineated using seasons based on single weather variables (rainfall or temperature). We used nonparametric cluster analyses of a 17-year (1993–2009) data set of weather variables that influence likelihoods and spread of fires (relative humidity, air temperature, solar radiation, wind speed, soil moisture) to explore seasonality of fire in pine savanna-grassland landscapes at the Avon Park Air Force Range in southern Florida. A four-variable, three-season model explained more variation within fire weather variables than models with more seasons. The three-season model also delineated intra-annual timing of fire more accurately than a conventional rainfall-based two-season model. Two seasons coincided roughly with dry and wet seasons based on rainfall. The third season, which we labeled the fire season, occurred between dry and wet seasons and was characterized by fire-promoting conditions present annually: drought, intense solar radiation, low humidity, and warm air temperatures. Fine fuels consisting of variable combinations of pyrogenic pine needles, abundant C4 grasses, and flammable shrubs, coupled with low soil moisture, and lightning ignitions early in the fire season facilitate natural landscape-scale wildfires that burn uplands and across wetlands. We related our three season model to fires with different ignition sources (lightning, military missions, and prescribed fires) over a 13-year period with fire records (1997–2009). Largest wildfires originate from lightning and military ignitions that occur within the early fire season substantially prior to the peak of lightning strikes in the wet season. Prescribed ignitions, in contrast, largely occur outside the fire season. Our delineation of a pronounced fire season provides insight into the extent to which different human-derived fire regimes mimic lightning fire regimes. Delineation of a fire season associated with timing of natural lightning ignitions should be useful as a basis for ecological fire management of humid savanna-grassland landscapes worldwide.

Divergent responses of fire to recent warming and drying across south‐eastern Australia

The response of fire to climate change may vary across fuel types characteristic of differing vegetation types (i.e. litter vs. grass). Models of fire under climatic change capture these differing potential responses to varying degrees. Across south-eastern Australia, an elevation in the severity of weather conditions conducive to fire has been measured in recent decades. We examined trends in area burned (1975-2009) to determine if a corresponding increase in fire had occurred across the diverse range of ecosystems found in this part of the continent. We predicted that an increase in fire, due to climatic warming and drying, was more likely to have occurred in moist, temperate forests near the coast than in arid and semiarid woodlands of the interior, due to inherent contrasts in the respective dominant fuel types (woody litter vs. herbaceous fuels). Significant warming (i.e. increased temperature and number of hot days) and drying (i.e. negative precipitation anomaly, number of days with low humidity) occurred across most of the 32 Bioregions examined. The results were mostly consistent with predictions, with an increase in area burned in seven of eight forest Bioregions, whereas area burned either declined (two) or did not change significantly (nine) in drier woodland Bioregions. In 12 woodland Bioregions, data were insufficient for analysis of temporal trends in fire. Increases in fire attributable mostly to warming or drying were confined to three Bioregions. In the remainder, such increases were mostly unrelated to warming or drying trends and therefore may be due to other climate effects not explored (e.g. lightning ignitions) or possible anthropogenic influences. Projections of future fire must therefore not only account for responses of different fuel systems to climatic change but also the wider range of ecological and human effects on interactions between fire and vegetation.

Modelling the determinants of ignition in the Sydney Basin, Australia: implications for future management

Variations in area burnt by fire are governed by four processes: biomass growth, availability to burn, fire weather and ignitions. Insight into these how these processes are shaped by biophysical and human influences is required to underpin the development of effective management strategies. Patterns of natural and arson ignitions were examined within the densely populated Sydney region of south-eastern Australia to determine the extent to which management can alter the risk of ignition. Arson ignitions were more likely on ridges in association with human infrastructure, i.e. roads and houses. Lightning ignitions also occurred more frequently on ridges, but at greater distances from human infrastructure. These patterns are consistent with those reported in studies from forested regions in the northern hemisphere. Fuel age had a variable effect with lightning more likely in older fuels (>25 years) and arson more likely in younger fuels (<10 years). Probability of both ignition types increased under more severe fire weather. Climate change is predicted to increase the severity of fire weather and is therefore likely to result in an increase in ignition frequency in the Sydney Basin. Urban expansion is also likely to have significant effects on ignitions and resultant risks to people and property via an increase in the probability of arson ignitions.

Three centuries of fire in montane pine‐oak stands on a temperate forest landscape

AbstractQuestion: What was the role of fire in montane pine‐oak (Pinus‐Quercus) stands under changing human land uses on a temperate forest landscape in eastern North America?Location: Mill Mountain in the central Appalachian Mountains, Virginia, US.Methods: A dendroecological reconstruction of fire history was generated for four stands dominated by xerophytic pine and oak species. The fire chronology began under presettlement conditions following aboriginal depopulation. Subsequent land uses included European settlement, iron mining, logging, and US Forest Service acquisition and fire protection.Results: Fires occurred approximately every 5 years until 1930 without any evidence of a temporal trend in fire frequency. Burning ceased after 1930. Area‐wide fires affecting multiple pine stands were common, occurring at intervals of approximately 16 years. Most living pines became established during the late 1800s and early 1900s. Dead pines indicated that an older cohort established ca. 1730. Most hardwoods were established between the 1920s and 1940s.Conclusions: Except for fire protection, changes in land use had no discernible influence on fire frequency. Lightning ignitions and/or large fire extent may have been important for maintaining frequent burning in the 1700s, while fuel recovery may have constrained fire frequency during later periods. The disturbance regime appears to be characterized by frequent surface fires and occasional severe fires, insect outbreaks or other disturbances followed by pine recruitment episodes. Industrial disturbances appear to have had little influence on the pine stands. The greatest impact of industrial society is fire exclusion, which permitted hardwood establishment.

Climate, lightning ignitions, and fire severity in Yosemite National Park, California, USA

Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focussed on large fires and have left the issues of fire severity and ignition frequency unaddressed. Lightning ignitions, any of which could burn a large area given appropriate conditions for fire spread, could be the first indication of more frequent fire. We examined the relationship between snowpack and the ignition and size of fires that occurred in Yosemite National Park, California (area 3027 km2), between 1984 and 2005. During this period, 1870 fires burned 77 718 ha. Decreased spring snowpack exponentially increased the number of lightning-ignited fires. Snowpack mediated lightning-ignited fires by decreasing the proportion of lightning strikes that caused lightning-ignited fires and through fewer lightning strikes in years with deep snowpack. We also quantified fire severity for the 103 fires >40 ha with satellite fire-severity indices using 23 years of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities and the complexity of higher-severity burn patches increased with the log10 of annual area burned. Using one snowpack forecast, we project that the number of lightning-ignited fires will increase 19.1% by 2020 to 2049 and the annual area burned at high severity will increase 21.9%. Climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions may be understated – fires may become more frequent and more severe.

Characterizing wildfire regimes in the United States

Wildfires statistics for the conterminous United States (U.S.) are examined in a spatially and temporally explicit manner. We use a high-resolution data set consisting of 88,916 U.S. Department of Agriculture Forest Service wildfires over the time period 1970-2000 and consider wildfire occurrence as a function of ecoregion (land units classified by climate, vegetation, and topography), ignition source (anthropogenic vs. lightning), and decade. For the conterminous U.S., we (i) find that wildfires exhibit robust frequency-area power-law behavior in 18 different ecoregions; (ii) use normalized power-law exponents to compare the scaling of wildfire-burned areas between ecoregions, finding a systematic change from east to west; (iii) find that wildfires in the eastern third of the U.S. have higher power-law exponents for anthropogenic vs. lightning ignition sources; and (iv) calculate recurrence intervals for wildfires of a given burned area or larger for each ecoregion, allowing for the classification of wildfire regimes for probabilistic hazard estimation in the same vein as is now used for earthquakes.

Understanding a flammable planet – climate, fire and global vegetation patterns

Understanding a flammable planet – climate, fire and global vegetation patterns

Fire history of the San Francisco East Bay region and implications for landscape patterns

The San Francisco East Bay landscape is a rich mosaic of grasslands, shrublands and woodlands that is experiencing losses of grassland due to colonization by shrubs and succession towards woodland associations. The instability of these grasslands is apparently due to their disturbance-dependent nature coupled with 20th century changes in fire and grazing activity. This study uses fire history records to determine the potential for fire in this region and for evidence of changes in the second half of the 20th century that would account for shrubland expansion. This region has a largely anthropogenic fire regime with no lightning-ignited fires in most years. Fire suppression policy has not excluded fire from this region; however, it has been effective at maintaining roughly similar burning levels in the face of increasing anthropogenic fires, and effective at decreasing the size of fires. Fire frequency parallels increasing population growth until the latter part of the 20th century, when it reached a plateau. Fire does not appear to have been a major factor in the shrub colonization of grasslands, and cessation of grazing is a more likely immediate cause. Because grasslands are not under strong edaphic control, rather their distribution appears to be disturbance-dependent, and natural lightning ignitions are rare in the region, I hypothesize that, before the entrance of people into the region, grasslands were of limited extent. Native Americans played a major role in creation of grasslands through repeated burning and these disturbance-dependent grasslands were maintained by early European settlers through overstocking of these range lands with cattle and sheep. Twentieth century reduction in grazing, coupled with a lack of natural fires and effective suppression of anthropogenic fires, have acted in concert to favor shrubland expansion.

Human vs. Lightning Ignition of Presettlement Surface Fires in Coastal Pine Forests of the Upper Great Lakes

To recover direct evidence of surface fires before European settlement, we sectioned fire-scarred logging-era stumps and trees in 39 small, physically isolated sand patches along the Great Lakes coast of northern Michigan and northern Wisconsin. While much information was lost to postharvest fire and stump deterioration, 147 fire-free intervals revealed in cross-sections from 29 coastal sand patches document numerous close interval surface fires before 1910; only one post-1910 fire was documented. Cross-sections from the 10 patches with records spanning >150 yr suggest local fire occurrence rates before 1910 ca. 10 times the present rate of lightning-caused fire. Since fire spread between or into coastal sand patches is rare, and seasonal use of the patches by Native people before 1910 is well documented, both historically and ethnographically, ignition by humans probably accounts for more than half of the pre-1910 fires recorded in cross-sections.

Prediction of Areas Prone to Lightning Ignition

A method for handling lightning-caused ignitions as part of a comprehensive fire hazard assessment procedure is presented. The locations at which lightning ignitions tend to occur have always been difficult to predict, and do not correlate with usual environmental factors. The model described here uses geographic information system techniques to remove large-scale trends in elevation. The meso-scale residual can be used to predict sites that are prone to lightning ignitions. The model is compared to historic records of lightning ignitions for the ACT, and is found to work very well. A range of other uses for the technique in fire management are discussed.