Two thresholds determine climatic control of forest-fire size in Europe

Abstract. Fire weather indices predict fire extent from meteorological conditions assuming a monotonic function; this approach is frequently used to predict future fire patterns under climate change scenarios using linear extrapolation. However, the relationship between weather and fire extent may potentially depend on the existence of fuel humidity thresholds above which this relationship changes dramatically, challenging this statistical approach. Here we combine the continuous and the threshold approaches to analyze satellite-detected fires in Europe during 2001–2010 in relation to meteorological conditions, showing that fire size response to increasing dryness follows a ramp function, i.e. with two plateaus separated by a phase of monotonic increase. This study confirms that at a continental and a high-resolution temporal scales, large fires are very unlikely to occur under moist conditions, but it also reveals that fire size stops to be controlled by fuel humidity above a given threshold of dryness. Thus, fuel humidity control only applies when fire is not limited by other factors such as fuel load, as large fires are virtually absent in dry regions with less than 500 mm of average annual rainfall, i.e. where fuel amount is insufficient. In regions with sufficient fuel, other factors such as fire suppression or fuel discontinuity can impede large fires even under very dry weather conditions. These findings are relevant under current climatic trends in which the fire season length, in terms of number of days with DC (drought code) values above the observed thresholds (break points), is increasing in many parts of the Mediterranean, while it is decreasing in Eastern Europe and remains unchanged in Central Europe.

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Contrasting large fire activity in the French Mediterranean

Natural Hazards and Earth System Science ◽

10.5194/nhess-19-1055-2019 ◽

2019 ◽

Vol 19 (5) ◽

pp. 1055-1066 ◽

Cited By ~ 5

Author(s):

Anne Ganteaume ◽

Renaud Barbero

Keyword(s):

Fire Suppression ◽

Sharp Decrease ◽

Weather Conditions ◽

Fire Spread ◽

Return Level ◽

Fire Season ◽

Burned Area ◽

The West ◽

Forest Resilience ◽

Large Fires

Abstract. In the French Mediterranean, large fires have significant socioeconomic and environmental impacts. We used a long-term georeferenced fire time series (1958–2017) to analyze both spatial and temporal distributions of large fires (LFs; ≥100 ha). The region was impacted in some locations up to six times by recurrent LFs and 21 % of the total area burned by LFs occurred on a surface that previously burned in the past, with potential impact on forest resilience. We found contrasting patterns between the east and the west of the study area, the former experiencing fewer LFs but of a larger extent compared to the latter, with an average time of occurrence between LFs exceeding 4000 ha < 7 years mostly in the eastern coastal area and > 50 years in the west. This longitudinal gradient in LF return level contrasts with what we would expect from mean fire weather conditions strongly decreasing eastwards during the fire season but is consistent with larger fuel cover in the east, highlighting the strong role of fuel continuity in fire spread. Additionally, our analysis confirms the sharp decrease in both LF frequency and burned area in the early 1990s, due to the efficiency of fire suppression and prevention reinforced at that time, thereby weakening the functional climate–fire relationship across the region.

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The extraordinary 2019/20 Australian bushfire season: contributing processes and environmental impacts

10.5194/egusphere-egu2020-3804 ◽

2020 ◽

Author(s):

Albert van Dijk ◽

Marta Yebra

Keyword(s):

Fire Suppression ◽

Fire Severity ◽

Critical Factor ◽

Fuel Load ◽

Fire Season ◽

Native Forest ◽

Environmental Consequences ◽

Eastern Australia ◽

Ability To Regenerate ◽

History Of

The recent Australian summer witnessed bushfire at a scale that is without historical precedent. We analysed the scale and severity of the fires, the main processes contributing to their scale, and environmental consequences that have already become apparent.&#160; We did this by combining satellite-derived information of vegetation cover, biomass and history, of soil and vegetation moisture content, and of fire extent and severity. More than 80,000 km2 was burnt, much of it native forest. Fire severity varied, but was overall greater than in preceding years. A critical factor contributing to fire conditions was a multi-year drought in Eastern Australia, which culminated in 2019 with the hottest and driest year in more than a century. During the fire season, fire danger conditions were further exacerbated by oceanic modes in the Indian and Southern Oceans, which limited circulation and caused excessive heating of the Australian land mass. Fuel availability in forests was unusually high. Reasons for this were several, including afforestation and regrowth as well as effective fire suppression in preceding years, while a contributing role for CO2 fertilisation is also plausible. Combined with the drought and associated vegetation mortality, this created a high and flammable fuel load. The fires strongly affected Australia&#8217;s total living carbon pool, which was already depleted by several years of below-average rainfall. Greenhouse gas releases associated with drought and bushfires are not considered in official emission accounts, but are of comparable magnitude. The smoke emissions also caused direct health impacts, affecting cities like Sydney, Melbourne and Canberra for prolonged periods. Most of the burnt forests are resilient to fire and will regenerate, assuming rainfall conditions improve. The severity, scale and connectedness of some of the fire complexes suggest ecological recolonization may be very slow, while a number of threatened species may not recover. Perhaps most concerning, some of the forests affected had burnt only years before, whereas other areas contained vegetation communities not experiencing fire for centuries, raising questions about their ability to regenerate and possibly permanent ecological regime shifts.

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Spatial pattern analyses of post-fire residual stands in the black spruce boreal forest of western Quebec

International Journal of Wildland Fire ◽

10.1071/wf10049 ◽

2010 ◽

Vol 19 (8) ◽

pp. 1110 ◽

Cited By ~ 42

Author(s):

Amar Madoui ◽

Alain Leduc ◽

Sylvie Gauthier ◽

Yves Bergeron

Keyword(s):

Black Spruce ◽

Weather Conditions ◽

Spatial Analyses ◽

Fire Size ◽

The West ◽

Forest Patches ◽

Burned Areas ◽

Large Fires ◽

The Many ◽

Landsat Satellite

In this study, we characterised the composition and configuration of post-fire residual habitats belonging to two physiographic zones of the black spruce–moss domain in western Quebec. Thirty-three large fires (2000–52 000 ha) were selected and extracted on classified Landsat satellite imagery. The results show that a minimum of 2% and a maximum of 22% of burned areas escaped fire, with an overall average of 10.4%. The many forest patches that partially or entirely escaped fire formed residual habitats (RHs). It was found that although the area of RHs follows a linear relationship with fire size, their proportion appears relatively constant. Spatial analyses showed that the fires could be separated into two groups depending on the physiographic zones (East-Canadian Shield v. West-Clay Belt Lowlands). Fires in the west zone generate less RHs and appear to be associated with more extreme weather conditions. In most cases there was no association with water or wetlands; in some fires the presence of RHs is associated with the proximity of water bodies. The failure to find an association between RHs and wetlands suggests that this type of environment is part of the fuel. Coniferous woodland with moss appears particularly overrepresented within RHs. Our results suggest that the local and regional physiographic conditions strongly influence the creation of RHs; therefore, it is important to consider those differences when applying ecosystem-based management.

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Factors influencing large wildland fire suppression expenditures

International Journal of Wildland Fire ◽

10.1071/wf07010 ◽

2008 ◽

Vol 17 (5) ◽

pp. 650 ◽

Cited By ~ 41

Author(s):

Jingjing Liang ◽

Dave E. Calkin ◽

Krista M. Gebert ◽

Tyron J. Venn ◽

Robin P. Silverstein

Keyword(s):

Wildland Fire ◽

Fire Suppression ◽

Burned Area ◽

Private Land ◽

Fire Size ◽

Department Of Agriculture ◽

Factors Influencing ◽

The Us ◽

Large Fires ◽

Parsimonious Model

There is an urgent and immediate need to address the excessive cost of large fires. Here, we studied large wildland fire suppression expenditures by the US Department of Agriculture Forest Service. Among 16 potential non-managerial factors, which represented fire size and shape, private properties, public land attributes, forest and fuel conditions, and geographic settings, we found only fire size and private land had a strong effect on suppression expenditures. When both were accounted for, all the other variables had no significant effect. A parsimonious model to predict suppression expenditures was suggested, in which fire size and private land explained 58% of variation in expenditures. Other things being equal, suppression expenditures monotonically increased with fire size. For the average fire size, expenditures first increased with the percentage of private land within burned area, but as the percentage exceeded 20%, expenditures slowly declined until they stabilised when private land reached 50% of burned area. The results suggested that efforts to contain federal suppression expenditures need to focus on the highly complex, politically sensitive topic of wildfires on private land.

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Fire weather index system components for large fires in the Canadian boreal forest

International Journal of Wildland Fire ◽

10.1071/wf03066 ◽

2004 ◽

Vol 13 (4) ◽

pp. 391 ◽

Cited By ~ 72

Author(s):

B. D. Amiro ◽

K. A. Logan ◽

B. M. Wotton ◽

M. D. Flannigan ◽

J. B. Todd ◽

...

Keyword(s):

Weather Conditions ◽

Fire Season ◽

Fire Weather ◽

Fuel Moisture ◽

Weather Index ◽

Fire Weather Index ◽

System Parameters ◽

System Components ◽

Large Fires ◽

Canadian Boreal Forest

Canadian Fire Weather Index (FWI) System components and head fire intensities were calculated for fires greater than 2 km2 in size for the boreal and taiga ecozones of Canada from 1959 to 1999. The highest noon-hour values were analysed that occurred during the first 21 days of each of 9333 fires. Depending on ecozone, the means of the FWI System parameters ranged from: fine fuel moisture code (FFMC), 90 to 92 (82 to 96 for individual fires); duff moisture code (DMC), 38 to 78 (10 to 140 for individual fires); drought code (DC), 210 to 372 (50 to 600 for individual fires); and fire weather index, 20 to 33 (5 to 60 for individual fires). Fine fuel moisture code decreased, DMC had a mid-season peak, and DC increased through the fire season. Mean head fire intensities ranged from 10 to 28 MW m−1 in the boreal spruce fuel type, showing that most large fires exhibit crown fire behaviour. Intensities of individual fires can exceed 60 MW m−1. Most FWI System parameters did not show trends over the 41-year period because of large inter-annual variability. A changing climate is expected to create future weather conditions more conducive to fire throughout much of Canada but clear changes have not yet occurred.

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Fire weather thresholds and burnt area in Portugal

10.5194/egusphere-egu21-14916 ◽

2021 ◽

Author(s):

Tomás Calheiros ◽

Akli Benali ◽

João Neves Silva ◽

Mário Pereira ◽

João Pedro Nunes

Keyword(s):

Land Use ◽

Cluster Analysis ◽

Fire Suppression ◽

Cumulative Effect ◽

Weather Conditions ◽

Weather Data ◽

Fire Weather ◽

Burnt Area ◽

Fire Weather Index ◽

Large Fires

Fire strongly depends on the weather, especially in Mediterranean climate regions with rainy winters but dry and hot summers, as in Portugal. Fire weather indices are commonly used to assess the current and/or cumulative effect of weather conditions on fuel moisture and fire behaviour. The Daily Severity Rating (DSR) is a numeric rating of the difficulty of controlling fires, based on the Canadian Fire Weather Index (FWI), developed to accurately assess the expected efforts required for fire suppression. Recently, the 90th percentile of DSR (90pDSR) was identified as a good indicator of extreme fire weather and well related to the burnt area in some regions of the Iberian Peninsula. The purposes of this work were: 1) to verify if this threshold is adequate for all continental Portugal; 2) to identify and characterize local variations of this threshold, at a higher spatial resolution; and, 3) to analyse other variables that can explain this spatial heterogeneity.We used fire data from the Portuguese Institute for the Conservation of Nature and Forests and weather data from ERA5, for the 2001&#160;&#8211;&#160;2019 study period. We also used the Land Use and Occupation Charter for 2018 (COS2018), provided by the Directorate-General for Territory, to assess land use and cover in Portugal. The meteorological variables to compute the DSR are air temperature, relative humidity, wind speed and daily accumulated precipitation, at 12 UTC. DSR percentiles (DSRp) were computed for summer period (between 15th May and 31st October) and combined with large (>100 ha) burnt areas (BA), with the purpose to identify which DSRp value is responsible of a large amount of BA (80 or 90%). Cluster analysis was performed using the relation between DSRp and BA, in each municipality of Continental Portugal.Results reveal that the 90pDSR is an adequate threshold for the entire territory. However, at the municipalities&#8217; level, some important differences appear between DSRp thresholds that explain 90 and 80% of the total BA. Cluster analysis shows that these differences justified the existence of several statistically significant clusters. Generally, municipalities where large fires take place in high or very high DSRp are located in north and central coastal areas, Serra da Estrela, Serra de Montejunto and Algarve. In contrast, clusters where large fires where registered with low DSRp appear in northern and central hinterland. COS2018 data was assessed to analyse if and how the vegetation cover type influences the clusters&#8217; distribution and affects the relationship between DSRp and total BA. Preliminary results expose a possible vegetation influence, especially between forests and shrublands.

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Quantifying the influence of fuel age and weather on the annual extent of unplanned fires in the Sydney region of Australia

International Journal of Wildland Fire ◽

10.1071/wf10016 ◽

2011 ◽

Vol 20 (1) ◽

pp. 142 ◽

Cited By ~ 56

Author(s):

Owen F. Price ◽

Ross A. Bradstock

Keyword(s):

Negative Relationship ◽

Fold Increase ◽

Influential Factor ◽

Fire Season ◽

Annual Rainfall ◽

Fire Area ◽

The Mean ◽

Mixed Modelling ◽

The Relationship ◽

Fire Extent

Planned fire is used globally to minimise the risk of unplanned fire, but it is important to measure the return for effort in terms of the reduction of risk per unit area of planned fire. Here, we use 30 years of fire mapping from four subregions of the Sydney region to compare the annual extent of unplanned fire with previous planned and unplanned fire. Using linear mixed modelling, we were able to discriminate the relative influence of previous fire, seasonal rainfall and weather during the peak fire season. The mean annual area burnt over the period was 4.11%, comprising 0.53% planned and 3.58% unplanned. We found that weather during the fire season was the most influential factor. Annual rainfall had a modest negative relationship with unplanned fire area. Past fire had some influence, but the relationship implied that approximately three units of planned fire are required to reduce the unplanned fire area by one unit. Managers would need to burn 5.4% per year to halve unplanned fire extent, a ten-fold increase on recent levels. This would increase the total area burnt, and have other effects that need to be considered (from smoke and greenhouse gas emissions, and changes to biodiversity).

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A patch mosaic burning system for conservation areas in southern African savannas

International Journal of Wildland Fire ◽

10.1071/wf01024 ◽

2001 ◽

Vol 10 (2) ◽

pp. 169 ◽

Cited By ~ 84

Author(s):

B.H. Brockett ◽

H.C. Biggs ◽

B.W. van Wilgen

Keyword(s):

Prescribed Burning ◽

Fire Regime ◽

Weather Conditions ◽

Dry Season ◽

Fuel Load ◽

Fire Season ◽

Fire Intensity ◽

Target Area ◽

Conservation Areas ◽

Percentage Area

Fire-prone savanna ecosystems in southern African conservation areas are managed by prescribed burning in order to conserve biodiversity. A prescribed burning system designed to maximise the benefits of a diverse fire regime in savanna conservation areas is described. The area burnt per year is a function of the grass fuel load, and the number of fires per year is a function of the percentage area burnt. Fires are point-ignited, under a range of fuel and weather conditions, and allowed to burn out by themselves. The seasonal distribution of planned fires over a year is dependent on the number of fires. Early dry season fires (May–June) tend to be small because fuels have not yet fully cured, while late season fires (August–November) are larger. More fires are ignited in the early dry season, with fewer in the late dry season. The seasonality, area burnt, and fire intensity are spatially and temporally varied across a landscape. This should result in the creation of mosaics, which should vary in extent and existence in time. Envelopes for the accumulated percentage to be burnt per month, over the specified fire season, together with upper and lower buffers to the target area are proposed. The system was formalised after 8 years of development and testing in Pilanesberg National Park, South Africa. The spatial heterogeneity of fire patterns increased over the latter years of implementation. This fire management system is recommended for savanna conservation areas of >20 000 ha in size.

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Is Arson Associated with Severe Fire Weather in Southern California?

International Journal of Wildland Fire ◽

10.1071/wf9910097 ◽

1991 ◽

Vol 1 (2) ◽

pp. 97 ◽

Cited By ~ 9

Author(s):

R Mees

Keyword(s):

Southern California ◽

Wildland Fire ◽

Fire Suppression ◽

Weather Conditions ◽

Fire Danger ◽

Weather Data ◽

Fire Weather ◽

The Public ◽

Severe Fire ◽

Large Fires

Under severe fire weather conditions arson is believed to be the primary cause of large wildland fires in southern California. Wildland fire suppression personnel and the public use the the expression "This weather brings out the arsonists" to indicate their awareness of the high potential for large arson-caused fires under these conditions. To determine the accuracy of this statement, fire occurrence and weather data were analyzed for four southern California National Forests for a 10-year period (1975–1984). The results showed that the proportion of arson and non-arson person-caused fires remained the same under most fire-danger conditions; however, a much higher percentage of arson fires became large fires when fire danger was severe. Furthermore, the timing of the arsonist contributed to the frequent occurrence of large arson fires. The data presented here refute the idea that most arson fires occur under severe weather conditions and at the same time-validate the utility of maintaining arson prevention programs during most weather conditions.

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