Fireline Intensity Research Articles

Distribution characteristics of radiant heat flux on tank surfaces exposed to wildland fires

AbstractThe expansion of wildland‐urban interface tends to expose fuel tanks to wildland fires, the frequency and intensity of which have increased in recent years due to climate change and global warming. This study aims to examine the distribution of radiant heat flux on a tank surface exposed to a wildland fire front, with a particular focus on extreme wildland fires. A theoretical framework is proposed, which consists of a flame length correlation, a thermal radiation model, and a correlation of time to failure, to calculate the radiant heat flux received by the tank surface and the potential for tank failure. The horizontal and circumferential distribution characteristics of the radiant heat flux are intensively discussed, in terms of the heat flux profile, heating area, total heat, time to failure versus the fireline intensity, fireline width, spacing, and tank volume. In particular, extreme wildland fires, characterized by large fireline intensities, could potentially destroy the tank within the safety spacing mandated by the current regulations. Therefore, a comparison between the time to failure and the radiant heating duration is suggested to ascertain the necessary safety spacing. The uncertainties in model calculations are also addressed, which arise from the use of different flame length correlations and flame radiative fractions.

Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands.

Climate change is increasing the frequency of droughts and the risk of severe wildfires, which can interact with shrub encroachment and browsing by wild ungulates. Wild ungulate populations are expanding due, among other factors, to favorable habitat changes resulting from land abandonment or land-use changes. Understanding how ungulate browsing interacts with drought to affect woody plant mortality, plant flammability, and fire hazard is especially relevant in the context of climate change and increasing frequency of wildfires. The aim of this study is to explore the combined effects of cumulative drought, shrub encroachment, and ungulate browsing on the fire hazard of Mediterranean oak woodlands in Portugal. In a long-term (18 years) ungulate fencing exclusion experiment that simulated land abandonment and management neglect, we investigated the population dynamics of the native shrub Cistus ladanifer, which naturally dominates the understory of woodlands and is browsed by ungulates, comparing areas with (no fencing) and without (fencing) wild ungulate browsing. We also modeled fire behavior in browsed and unbrowsed plots considering drought and nondrought scenarios. Specifically, we estimated C. ladanifer population density, biomass, and fuel load characteristics, which were used to model fire behavior in drought and nondrought scenarios. Overall, drought increased the proportion of dead C. ladanifer shrub individuals, which was higher in the browsed plots. Drought decreased the ratio of live to dead shrub plant material, increased total fuel loading, shrub stand flammability, and the modeled fire parameters, that is, rate of surface fire spread, fireline intensity, and flame length. However, total fuel load and fire hazard were lower in browsed than unbrowsed plots, both in drought and nondrought scenarios. Browsing also decreased the population density of living shrubs, halting shrub encroachment. Our study provides long-term experimental evidence showing the role of wild ungulates in mitigating drought effects on fire hazard in shrub-encroached Mediterranean oak woodlands. Our results also emphasize that the long-term effects of land abandonment can interact with climate change drivers, affecting wildfire hazard. This is particularly relevant given the increasing incidence of land abandonment.

Applicability analysis of flame height estimation based on Byram’s fireline intensity model under flat and windless conditions

Forest fire have a serious impact on forest ecosystems, the safety of people’s lives and property, and social stability. The height of surface flames, as the main indicator of forest fire behavior, which is an important parameter for forest fire management. The relationship between fireline intensity and flame height proposed by Byram has been widely used in estimating flame height; however, its applicability to the surface fuel of typical forest stands in the Yunnan–Guizhou Plateau of China has not yet been analyzed. In this study, the surface fuel in the area was taken as the research object, and the flame height of different fuel bed characteristics was measured through an indoor burning experiment. The applicability of three methods—the directly used Byram’s model, corrected model, and re-established prediction model—was analyzed to estimate the flame height in the Yunnan–Guizhou Plateau. We found that the flame height of the typical forest stands in the Yunnan–Guizhou Plateau ranged from 0.05 to 1.2 m and was significantly affected by the moisture content, load, and height of the fuel bed. Although the fireline intensity exhibited a significant linear relationship with the flame height, directly using Byram’s method to predict the flame height of surface fires was impractical, as its mean prediction error exceeded 150%. The mean relative errors of the prediction model obtained by modifying Byram’s method and that based on the characteristics of the fuel bed were both within 15%, which is significantly lower than that of the original Byram’s method. Based on the results of this study, we propose two methods that are suitable for predicting the flame height of surface fires in the typical forests of the Yunnan–Guizhou Plateau in China, which is of great significance for further understanding the relationship between flame height, fireline intensity, and scientific forest fire management.

Field-based generic empirical flame length–fireline intensity relationships for wildland surface fires

Background Fireline intensity (If) quantifies the power of the fireline and is used for various purposes. If and flame length (Lf) are relatable to each other using an empirical power function, which has been considered fuel-specific. Aims The aim of this study was to develop generic Lf − If relationships based on a robust set of field head fires from the literature (n = 797) conducted worldwide in forest, shrubland and grassland. Methods Lf was determined from the base of the fuel bed for comparability across fires in different fuel heights, and the effect of vegetation type was examined. Key results Although If could be approximately described using the same function in forest and shrubland, fires in grassland required different fitted coefficients; we speculate that fuel particles’ surface area-to-mass ratio is the main fuel metric influencing flame structure. Conclusions Fuel-generic relationships for If are reasonably accurate and encompass the high end of surface fire If. Previous studies suggested their unviability, most likely because of limitations in the number of observations and data ranges, difficulty in objectively measuring Lf and variation in Lf definition. Implications The generic relationships presented in this work will be of interest for research and management purposes when specific models for If are non-existent.

Characterizing Forest Fuel Properties and Potential Wildfire Dynamics in Xiuwu, Henan, China

As global climate change and human activities increasingly influence our world, forest fires have become more frequent, inflicting significant damage to ecosystems. This study conducted measurements of combustible materials (moisture content ratio, ignition point, and calorific value) across 14 representative sites. We employed Pearson correlation analysis to ascertain the significant differences in combustible properties and utilized entropy methods to evaluate the fire resistance of materials at these sites. Cluster analysis led to the development of four combustible models. Using BehavePlus software, we simulated their fire behaviors and investigated the effects of wind speed and slope on these behaviors through sensitivity analysis. The results revealed notable differences in the moisture content ratios among different types of combustibles, especially in sites 2, 3, 8, 9, and 13, indicating higher fire risks. It was also found that while humus has a higher ignition point and lower calorific value, making it less prone to ignite, the resultant fires could be highly damaging. The Pearson analysis underscored significant variations in the moisture content ratios among different combustibles, while the differences in ignition points and calorific values were not significant. Sites 5 and 6 demonstrated stronger fire resistance. The simulations indicated that fire-spread speed, fireline intensity, and flame length correlate with, and increase with, wind speed and slope. Sensitivity analysis confirmed the significant influence of these two environmental factors on fire behavior. This study provides critical insights into forest fire behavior, enhancing the capability to predict and manage forest fires. Our findings offer theoretical support for forest fire prediction and a scientific basis for fire management decision-making.

Developing customized fuel models for shrub and bracken communities in Galicia (NW Spain)

Geospatial fire behaviour and fire hazard simulators, fire effects models and smoke emission software commonly use standard fuel models in order to simplify data collection and the inclusion of complex fuel scenarios. These fuel models are often mapped using remotely sensed data. However, given the great complexity of fuelbeds, with properties that vary widely in both time and space, the use of these standard fuel models can greatly limit accurate fuel mapping. This affects fuel hazard assessment, fuel reduction treatment plans, fire management decision-making and evaluation of the environmental impact of wildfire. In this study, we developed unique customized fire behaviour fuel models for shrub and bracken communities, by using k-medoids clustering analysis based on both fuel structural characteristics and potential fire behaviour. We used an original database of 722 destructive sample plots in nine different shrub and bracken communities covering the entire distribution area in Galicia (NW Spain), one of the regions in Europe most affected by forest fires. Measurements of cover, height and fuel fractions loads differentiated by size and vegetative state (live or dead) were used to estimate the potential rate of fire spread with five different models including fireline intensity, heat per unit area and the flame length for each sampling site and considering extreme environmental conditions. The optimal number of clusters was established by combining practical knowledge about the shrubland communities under study and their associated fire behaviour, with maximization of the mean value of the silhouette variable and minimization of the within-cluster sum of squares. The structural characteristics of the medoids derived from the analysis were associated with each of the proposed customized fuel models. Finally, a simple dichotomous classification based only on shrub height was developed to enable construction of spatially explicit fuel model maps based on remotely sensed data. Thus, the methodology applied allows generation of a more realistic representation of fuel distribution in the landscape, based on fuel structure measurements of natural regional ecosystems rather than on the use of standard models. We believe that the proposed methodology is generally applicable to communities composed of other shrub and fern species in different biogeographical regions.

Experimental Analysis on the Behaviors of a Laboratory Surface Fire Spreading across a Firebreak with Different Winds

In this work, a series of laboratory surface fire experiments were performed over a pine needle fuel bed to investigate the effectiveness of a firebreak and the behaviors of a surface fire across a firebreak. Seven wind velocities of 0~3.0 m/s and six firebreak widths of 10~35 cm are varied. The behaviors of a surface fire across the firebreak, the heat flux received by fuel surface and fuel temperature before and after the firebreak are analyzed and compared simultaneously. The main conclusions are as follows: the behaviors of a surface fire spreading across a firebreak under different wind velocities are classified into three categories—no ignition, ignition by flame contact and ignition by spot fires. When the wind velocity is not more than 1.0 m/s, the surface fire cannot successfully cross the firebreak; as wind velocity changes from 1.5 m/s to 2.5 m/s, the fuel after the firebreak can be ignited by flame contact for relatively narrow firebreak conditions; when the wind velocity increases to 3.0 m/s, the burning fuel can be blown away along the fuel bed, and the fuel behind the firebreak will be ignited by spot fire. A linear relationship between the threshold of firebreak width and the fireline intensity is obtained, and the linear fitting coefficient in this paper is larger than the results reported by Wilson (0.36). For no ignition conditions, the fuel temperature and the heat flux received by the fuel after firebreak are significantly lower than those before the firebreak, whereas their variations over time are similar to those before the firebreak for ignition conditions. Moreover, for no ignition conditions, the maximum fuel temperature and the heat flux after the firebreak increase with wind velocity, but decrease with firebreak width. Additionally, when the fuel temperature (253 °C) and the heat flux received by the fuel considering the radiation and convection (43 kW/m2) after firebreak exceed a threshold value, the surface fire can successfully cross the firebreak.

The effects of junction fire development on thermal behaviour at the field scale

Junction fires, a type of dynamic fire behaviour, involve the merging of two active fire fronts. They have been known to result in extreme increases in rates of fire spread and intensity over short periods of time. When these phenomena occur during wildfires, they pose significant risks to emergency management and communities. Few studies have investigated rates of spread and fireline intensity of junction fires using field scale experiments. This study captured data from 16 junction fires in mallee-heathlands utilising a drone-mounted thermal camera. Mean rates of spread and fireline intensity were found to increase toward the 75 % stage of merging before decreasing, similar to those found in other studies. Although, normalised temperature areas increased continuously for the whole duration of merging. Additionally, a custom-built radiative heat flux device was used to measure temperature and radiant heat at the fireline within the junction fires. Peak radiative heat flux was also found to be highest at the 75 % stage of merging, following a gradual decrease as propagation progressed. Heat flux measurements were found to be similar to those of previous studies conducted in similar vegetation. Due to the many complex interactions between weather, fuel and fire behaviour, further studies need to be conducted to appropriately determine the effects of merging on rates of spread and fire intensity.

A forest fire hazard model and map for a wildland urban interface not meteorologically prone to forest fires

Fire Hazard (FH) modelling is a relevant fire prevention/assessment tool. This work proposes a FH model and generates a FH map for a wildland urban interface area in Germany that is not prone to extreme fires. The main input data include weather, topography, fuel, and anthropic-related potential ignition points. The main steps include (1) identification/description of weather scenarios, and for each scenario (2) analysis of potential fire ignition through simulation of Fire Probability (FP), (3) modelling the potential fire behaviour through simulation of Fireline Intensity (FLI), (4) generation of a FH map by combining FP and FLI maps, and (5) integration of all maps into a final FH map. Extreme ignition and propagation conditions were considered: (1) a fuel model that describes the fire performance in an extreme drought scenario, (2) the human influence through mechanistic ignitions, and (3) the worst case of all scenarios. As results, four weather scenarios were identified and described. FP maps, FLI maps, and FH maps were created for each of them, and finally an integrated FH map (IFHM) was derived.

Edible fire buffers: Mitigation of wildfire with multifunctional landscapes.

Wildfires ravage lands in seasonally dry regions, imposing high costs on infrastructure maintenance and human habitation at the wildland-urban interface. Current fire mitigation approaches present upfront costs with uncertain long-term payoffs. We show that a new landscape intervention on human-managed wildlands-buffers of a low-flammability crop species such as banana irrigated using recycled water-can mitigate wildfires and produce food profitably. This new intervention can complement existing fire mitigation approaches. Recreating a recent, major fire in simulation, we find that a medium-sized (633 m) banana buffer decreases fireline intensity by 96%, similar to the combination of prescribed burns and mechanical thinning, and delays the fire by 316 min, enabling safer and more effective firefighting. We find that under climate change, despite worsened fires, banana buffers will still have a protective effect. We also find that banana buffers with average yield could produce a profit of $56k USD/hectare through fruit sales, in addition to fire mitigation.

Examining the Effectiveness of Aerial Firefighting with the Components of Firebreak Requirements and Footprint Geometry—Critics of the Present Practice

The negative impact of climate change is increasingly evident in the severity of forest fires. Fires are becoming more intense and can often only be controlled by aerial means. Aerial firefighting is known as a very effective method—in some cases, it is the only option—of suppressing fire, but it is a very expensive solution. Recently, the effectiveness of this method has received a lot of criticism, with some studies showing a loss of between 60 and 95%, so it is worth approaching this issue in a different way. The aim of this study is to estimate losses using a new method that has not been used before. For this purpose, this study focuses on two components: the requirements of the firebreak and the geometry of the footprint. For the first, the rules of thumb of the practice were applied depending on the fireline intensity. One is the required coverage level of the surface with suppressant, and the other is the required wetted bandwidth, which is the firebreak. In practice, the firebreak should be 2–2.5 times wider than the length of the flame. For the footprint geometry, the author used the results of previous studies dealing with footprint formation. At the end, the design of the required firebreak and the simplified design of the footprint, which is an ellipsoid, were compared to each other. The results show that, in the case of a fireline intensity of 3 MWm−1 and a coverage level of 2.4 kgm−2, the loss is approximately 36.4–44.6% for the ellipsoidal footprint alone and 86–87.8% for the total amount of extinguishing agent. The conclusion is that future work should focus not on a more accurate description and understanding of emissions but on developing a technology that can change the shape of the footprint from an elliptical to a rectangular shape.

Decadal impacts of wildfire fuel reduction treatments on ecosystem characteristics and fire behavior in alaskan boreal forests

Wildfire activity is increasing in boreal forests as climate warms and dries, increasing risks to rural and urban communities. In black spruce forests of Interior Alaska, fuel reduction treatments are used to create a defensible space for fire suppression and slow fire spread. These treatments introduce novel disturbance characteristics, making longer-term outcomes on ecosystem structure and wildfire risk reduction uncertain. We remeasured a network of sites where fuels were reduced through hand thinning or mechanical shearblading in Interior Alaska to assess how successional trajectories of tree dominance, understory composition, and permafrost change over ∼ 20 years after treatment. We also assessed if these fuel reduction treatments reduce modeled surface rate of fire spread (ROS), flame length, and fireline intensity relative to an untreated black spruce stand, and if surface fire behavior changes over time. In thinned areas, soil organic layer (SOL) disturbance promoted tree seedling recruitment but did not change over time. In shearbladed sites, by contrast, both conifer and broad-leaved deciduous seedling density increased over time and deciduous seedlings were 20 times more abundant than spruce. Thaw depth increased over time in both treatments and was greatest in shearbladed sites with a thin SOL. Understory composition was not altered by thinning but in shearbladed treatments shifted from forbs and horsetail to tall deciduous shrubs and grasses over time. Modeled surface fire behavior was constant in shearbladed sites. This finding is inconsistent with expert opinion, highlighting the need for additional fuels-specific data to capture the changing vegetation structure. Treatment effectiveness at reducing modeled surface ROS, flame length, and fireline intensity depended on the fuel model used for an untreated black spruce stand, pointing to uncertainties about the efficacy of these treatments at mitigating surface fire behavior. Overall, we show that fuel reduction treatments can promote low flammability, deciduous tree dominated successional trajectories, and that shearblading has strong effects on understory composition and permafrost degradation that persist for nearly two decades after disturbance. Such factors need to be considered to enhance the design, management, and predictions of fire behavior in these treatments.

Vertical and Horizontal Crown Fuel Continuity Influences Group-Scale Ignition and Fuel Consumption

A deeper understanding of the influence of fine-scale fuel patterns on fire behavior is essential to the design of forest treatments that aim to reduce fire hazard, enhance structural complexity, and increase ecosystem function and resilience. Of particular relevance is the impact of horizontal and vertical forest structure on potential tree torching and large-tree mortality. It may be the case that fire behavior in spatially complex stands differs from predictions based on stand-level descriptors of the fuel distribution and structure. In this work, we used a spatially explicit fire behavior model to evaluate how the vertical and horizontal distribution of fuels influences the potential for fire to travel from the surface into overstory tree crowns. Our results support the understanding that crown fuels (e.g., needles and small-diameter branchwood) close to the surface can aid in this transition; however, we add important nuance by showing the interactive effect of overstory horizontal fuel connectivity. The influence of fuels low in the canopy space was overridden by the effect of horizontal connectivity at surface fire-line intensities greater than 1415 kW/m. For example, tree groups with vertically continuous fuels and limited horizontal connectivity sustained less large-tree consumption than tree groups with a significant vertical gap between the surface and canopy but high-canopy horizontal connectivity. This effect was likely the result of reduced net vertical heat transfer as well as decreased horizontal heat transfer, or crown-to-crown spread, in the upper canopy. These results suggest that the crown fire hazard represented by vertically complex tree groups is strongly mediated by the density, or horizontal connectivity, of the tree crowns within the group, and therefore, managers may be able to mitigate some of the torching hazard associated with vertically heterogenous tree groups.

Management of invasive shrubs to mitigate wildfire through fuel pellet production in central Chile

The use of pellets as a replacement for firewood has been promoted in Chile to mitigate atmospheric pollution. However, their high demand has generated stock shortages, which has motivated the search for alternative sources of feedstock. Furthermore, invasive shrubs are a highly available biomass source for bioenergy production in central-southern Chile and may be a significant factor contributing to the spread and increasing virulence observed in wildfires across the region. This study aimed to determine the change in wildfire indicators related to the removal of invasive shrubs in selected zones in the Biobío region and to assess the physicochemical properties of the extracted biomass to develop a pellet formulation to produce a material conforming to ISO standards. The biomass management of Teline monspessulana, Ulex europaeus, and Rubus ulmifolius was evaluated using a fire simulation tool in three areas with contrasting physio-climatic conditions. Our simulation results demonstrated the effectiveness of shrub management on three critical wildfire indicators. Namely, significant decreases were observed in fireline intensity (kW/m) 58–75%, flame length (m) 0–40%, and heat per unit area (kW/m2) 86%. Furthermore, a biomass quality index (BQI) was developed based on the physicochemical parameters of the three shrubs assessed. Based on this BQI, T. monspessulana was selected as the most promising shrub biomass and was consequently used in a pilot shrub-pinewood blending to produce pellets. A blending of 20:80%m/m exhibited properties close to the ISO standard. Our results show that the management of invasive shrubs has the potential to minimize the virulence of wildfires, while the physicochemical characteristics and availability of one of the shrubs analyzed (T. monspessulana) make it a viable alternative biomass source for pellet production in the region.

Mapping fireline intensity and flame height of prescribed gorse wildland fires

Wildfires currently represent an imminent danger to communities around the world due to their increasing severity and frequency. It is critical to expand current knowledge on wildfire behaviour to improve risk management practices, firefighting operation and engineering design in the Wildland Urban Interface (WUI). This paper reports 2D high resolution measurements of Byram's fireline intensity generated during a 150 × 200 m2 gorse shrub burn generated from visual and infrared video footage of the prescribed fire, and Light-Detection-and-Ranging (LiDAR) data of the vegetation canopy, both acquired using Unmanned Aerial Vehicles (UAV) technology. The results show that the fireline intensity was highly variable across the plot, reaching localized peak values over 40,000 kW/m and an averaged intensity of 14,300 kW/m approximately. An empirical correlation for estimating the average flame height was derived from the fireline intensity maps and from experimental measurements carried out during the prescribed burn. The correlation was used to produce a 2D detailed map of the average flame height of the prescribed fire, achieving a 11% of error compared to in-fire measurements. The findings from this research suggests that non-linear variabilities of fireline intensity should be carefully considered to adequately describe wildifire behavior of shrubland fires.

A Case Study on the Effects of Weather Conditions on Forest Fire Propagation Parameters in the Malekroud Forest in Guilan, Iran

The present study investigates the effect of climatic parameters, such as air relative humidity and wind speed, on fire spread propagation indexes in the Malekroud Forest, Iran using the FARSITE simulator based on Rothermel’s original fire spread equation. Standard fuel models are used to calibrate the vegetation cover. Sorensen (SC) and kappa (κ) coefficients, as well as the Overestimation Index (OI), are used to estimate the simulation’s accuracy. The results confirm that using both ambient condition data and appropriate fuel models is crucial to reaching reasonable results in fire propagation simulations. The values of the Rate of Fire Spread (ROS), Flame Length (FML), and Fire Line Intensity (FLI) are reported for each particular scenario. The simulation results show that the Sorensen and Kappa coefficient for situations most similar to the real fire reached 0.82 and 0.80, respectively. The investigated fire’s severity is categorized as low-condition fire behavior. The simulation shows that fire propagation falls harshly in the case of air relative humidity by more than 72%, and we will not witness natural fire propagation on a large scale.

Midstory removal of encroaching species has minimal impacts on fuels and fire behavior regardless of burn season in a degraded pine-oak mixture

Anthropogenic fire exclusion has contributed to forest structural and compositional shifts, resulting in the encroachment of shade-tolerant, often fire-sensitive and/or opportunistic species throughout forest ecosystems. In the central and eastern U.S., encroaching species often exhibit fire-suppressing leaf litter and crown traits, reducing prescribed fire efficacy to restore and maintain fire-dependent mixed pine (Pinus spp.) and oak (Quercus spp.) forests. Despite fire’s natural role in these forests, management using prescribed fire alone is unlikely to reverse encroachment impacts due to the degree of forest change since fire exclusion. Consequently, additional management techniques (e.g., chemical, mechanical) are often combined with fire to ensure persistence of pine-oak mixtures and their associated ecosystem services. In current-day degraded pine-oak mixtures in the southeastern U.S., where no one species represents > 75% dominance by stand basal area, encroaching species (e.g., Liquidambar styraciflua, Quercus nigra) commonly form a dense midstory and occupy overstory positions. To quantify how encroaching species in degraded mixed pine-oak forests impact fuels, canopy cover, and seasonal fire behavior, we thinned the midstory (<20 cm DBH) of all non-pyrophytic species in 12 0.10-ha blocks within a mixed pine-oak stand at the Mary Olive Thomas Demonstration Forest (Auburn, Alabama, USA) in July 2021. Midstory thinning reduced basal area by 6.7 m2 ha−1 and canopy cover by 9.1%. In general, midstory thinning reduced shrub and fine woody debris fuel loads while having no impact on herbaceous, leaf litter, and duff fuel loads. When compared to pre-treatment levels, midstory thinning increased the relative proportion of pine leaf litter contribution to the fuelbed by 10.9% while reducing that of encroaching species by 11.4%. Despite changes to residual basal area, canopy cover, and leaf litter contribution, midstory thinning had no statistical impact on fire behavior, regardless of burn season. Early (January) and late dormant season (April) experimental fires had higher fireline intensities, consumed more surface fuels, had faster rates of spread, and higher average maximum temperatures compared to growing season (September) fires. We also noted an increase in these fire parameters with increased pine litter contribution to the fuelbed. In degraded pine-oak mixtures where encroaching species now occupy overstory canopy positions, we recommend investigating the efficacy of more intense thinning treatments that remove overstory individuals with known fire-suppressing traits to have a larger impact on canopy openness, fuels, and fire behavior.

The effect of broadleaf forests in wildfire mitigation in the WUI – A simulation study

The increasing occurrence of large wildfires in Southern European countries calls for the adoption of more effective measures of fire prevention, to protect people and infrastructures, namely in the Wildland-Urban Interface (WUI). Previous research suggests that broadleaf forests could mitigate the effects of these wildfires due to their lower flammability. However, few attempts have been made to investigate the possibility of using broadleaf-based green firebreaks to protect infrastructures, as an alternative to current less ecologically and economically sustainable fuel reduction techniques. Here, we assess the relevance of a broadleaves fuel model in reducing fire hazard in the WUI, by analyzing a set of six Industrial Zones (IZs) in Central Portugal, severely affected by wildfires during the catastrophic fire season of 2017. We developed alternative scenarios for the spatial simulation of fire behavior, using a factorial combination of weather conditions (standard and extreme), buffer around each IZ (distances of 100 m and 500 m) and land cover (current and broadleaves fuel model). The simulations were grounded on real-world data obtained from reconstructed fire front isochrones and fieldwork. Our results suggest that replacing the flammable vegetation present in the WUI with broadleaf forests could reduce fireline intensity by up to five times, even under extreme weather conditions. This reduction occurs subtly as the broadleaf cover interface is expanded from 100 m to 500 m. Our results show that fires that exceed the suppression capacity in pine and eucalypt forests (>4 m flame length) can be effectively suppressed in broadleaf forests under extreme fire weather (1.4 m flame length) and easily suppressed in broadleaf forests in standard weather (0.8 m flame length). Due to significant changes in land use and extreme weather events, future large wildfires could occur again in Portugal. In this regard, our results corroborate the urgent need to discuss forest management in the country, which has already proven to be insufficient to prevent fire disasters in the WUI.

Prescribed Fire Causes Wounding and Minor Tree Quality Degradation in Oak Forests

Despite the adaptation of many oak (Quercus) species to repeated surface fire, many public land managers in eastern North America resist using prescribed fire as a regeneration tool because of fire’s perceived negative impacts on timber values through the wounding of overstory trees. We retrospectively quantified fire-associated wounds in 139 oak-dominated stands across four national forests, each with a history of zero to six prescribed fires within the last 30 years. For trees &gt; 25.4 cm dbh (n = 8093), fire-associated wounds within the first 3.67 m of height were categorized by type, measured for defect size and graded both accounting for and then ignoring the fire-associated wounds. Most fire-associated wounds (n = 3403) were catfaces (32.5%), seams (30.5%) or bark slough (30.1%), although catfaces had 2.1–6.4 times the average volume loss of any other wound type (9.90 ± 0.72 bd ft). Among the 2160 wounded trees sampled, 741 had multiple (≥2) wounds. Although 29.1% of all trees had at least one wound associated with prescribed fire, only 7.0% of those trees exhibited a reduction in tree grade. The likelihood of wounding was greater in stands receiving more prescribed burns, but unaffected by tree diameter for either thin- or thick-barked species. Considering both the likelihoods of wounding and grade reduction, white oak (Q. alba), chestnut oak (Q. montana), hickory (Carya sp.), shortleaf pine (Pinus echinata) and yellow-poplar (Liriodendron tulipifera) trees were more resistant to prescribed fire damage than other species. While our findings cannot be related directly to individual fire parameters, such as fireline intensity or fire duration, these results do provide estimates of the cumulative effects of multiple management-based prescribed fires that can be incorporated into fire effects models.

Effect of flame zone depth on the correlation of flame length with fireline intensity

Background Previously established correlations of flame length L with fireline intensity IB are based on theory and data which showed that flame zone depth D of a line fire could be neglected if L was much greater than D. Aims We evaluated this correlation for wildland fires where D is typically a non-negligible proportion of L (i.e. roughly L/D &lt; ~2). Methods Experiments were conducted to measure flame length L from line-source fires using a gas burner where IB and D were controlled independently (0.014 ≤ L/D ≤ 13.6). Key results The resulting correlation showed D significantly reduced L for a given IB over the entire range of observations and was in accord with independent data from spreading fires. Flame length is reduced because the horizontal extent of deep flame zones entrains more air for combustion than assumed by theory involving only the vertical flame profile. Conclusions Analysis suggested that the noted variability among published correlations of L with IB may be partly explained by varying L/D ratios typical of wildland fires. Implications Fire behaviour modelling that relies on correlations of L with IB for scaling of heat transfer processes would likely benefit by including the effects of D.