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Does plant flammability differ between leaf and litter bed scale? Role of fuel characteristics and consequences for flammability assessment
Abstract
The increasing concern regarding fire in the wildland-urban interface (WUI) around the world highlights the need to better understand the flammability of WUI fuels. Research on plant flammability is rapidly increasing but commonly only considers a single fuel scale. In some cases, however, different fuel scales (e.g. leaf and litter bed) have greater influence on fire, for instance, when it spreads from the litter bed to the lower canopy. Examining fuel flammability at these different scales is necessary to better know the overall flammability but also provides insights into the drivers of flammability. To investigate if leaf and litter bed flammability differed, laboratory experiments were conducted on 15 species (native or exotic) commonly found in the WUI of south-eastern France. Species were ranked and the association of fuel characteristics with flammability sought at both scales. For most species, leaf and litter bed flammability differed because of strong fuel characteristics (e.g. leaf thickness or litter bulk density), entailing differences in rankings based on fuel scale and potentially leading to a misrepresentation of flammability of the species studied. Favouring species with lower flammability at both scales in the WUI, especially near housing, may help reduce undesired effects during wildfires.
... The risk of fire in the wildland-urban interface (WUI) area is a major concern in fire policy and this transitional zone is crucial for strategic vegetation planning. Plant-trait-based flammability approaches have been shown to provide relevant information on wildland fire behavior that can be implemented at the wildland-urban interface (Simeoni et al., 2012;Ganteaume, 2018) and particularly for species selection for green firebreaks (Krix et al., 2019). Few studies have emphasized the importance of identifying the fire-sensitive species in WUI fire management (Romero et al., 2019) and establishing effective buffer zones based on suitable varieties (Della Rocca et al., 2015). ...
... Its higher values are associated with higher rates of energy and mass exchange, resulting in shorter ignition delays and faster fire spread (Anderson, 1970;Chandler et al., 1983;Dimitrakopoulos and Panov, 2001;Weise et al., 2005;Saura-Mas et al., 2010;Engber and Varner, 2012;Simeoni et al., 2012). Leaf and litter-leaf area, as well as specific leaf area (SLA), were found to be significant predictors of flammability, with broader and wider leaves having a lower time-to-ignition (Murray et al., 2013;Ganteaume, 2018). Also, larger leaf area was found to be significantly related to high spread rate and combustibility (de Magalhães and Schwilk, 2012;Krix et al., 2019) and to have a negative relationship with sustainability (de Magalhães and Schwilk, 2012). ...
... Also, larger leaf area was found to be significantly related to high spread rate and combustibility (de Magalhães and Schwilk, 2012;Krix et al., 2019) and to have a negative relationship with sustainability (de Magalhães and Schwilk, 2012). Leaf thickness, on the other hand, had a negative effect on ignitability (prolonged time-to-ignition) but a positive effect on sustainability (Grootemaat et al., 2017a;Ganteaume, 2018;Romero et al., 2019). Bark thickness was found to be inversely related to burning rate and the mass loss rate (Frejaville et al., 2013) and corkbark thickness was found to be a good predictor of sustainability and consumability (Dehane et al., 2015). ...
The occurrence of large and recurring forest fires has long been associated with fire-prone environments, but this perception has been shifted rapidly in recent decades as Earths' landscapes have become increasingly threatened by severe and unpredictable fires as a result of climate changes. In this regard, the flammability of trees is a topic of great interest for ecology, management, and the development of sustainable restoration and rehabilitation plans. Tree species differ in regard to flammability, and many plant functional traits contribute to flammability at species, community and vegetation level. The relationship between plant traits and flammability at species level is important for a broader understanding of the vegetation-fire dynamic at the local and landscape scales. This review summarizes the current state of knowledge regarding the impact of individual plant traits of tree species on flammability components. By keywords-based searching of academic databases, 85 research papers were collected and analyzed. The literature synthesis shows: i) main issues addressed in studies on plant trait-based tree flammability, ii) general research output and biogeographic regions studied, iii) inventory of tree taxa investigated, iv) relationships between plant traits and flammability components, v) the most relevant plant traits that determine the flammability-related differences between species.
... Furthermore, a flammability index (FI) was proposed by Valette in 1990 [37] for Mediterranean forest species, which considers ignition time, flame duration, and burning time. This index is used in different parts of the world as a simple technique for evaluating flammability parameters and is still widely used by various authors [38][39][40][41][42][43][44][45][46][47] to study and classify tree species according to their flammability in order to find those that are less flammable and useful for forest fire risk management. Simultaneously, there is a need to understand the relationship between forest fuels and their thermochemical parameters, including heating values, to be able to include them in wildfire risk evaluations [48][49][50]. ...
... To evaluate the flammability of the collected leaves, flammability tests were carried out using a methodology developed by various researchers [37][38][39][40][41][42][43][44][45][46]73,74], based on the use of a 500 W epiradiator (model 534 RC2, Quartz Alliance, France). ...
... A mass of 10 g of fresh leaves was weighed on an analytical balance (AS 220.R2, Radwag) and placed in a thermostatic oven (digital drying oven, model JK-DO-9030A, JKI, China) at 110 °C for 24 h to ensure total loss of water. Afterward, samples were weighed and the moisture content (MC), defined as the weight of water as a function of dry weight, was determined according to Equation (5) [6,38,43,45,83,84]: ...
Forest fires are one of the main environmental threats in Chile. Fires in this Mediterranean climate region frequently affect native forests and exotic plantations, including in several cases urban and rural settlements. Considering the scarcity of information regarding the fire response dynamics of tree species that are frequently affected by fires, this study aims to establish a flammability classification according to the evolution of the fire initiation risk presented by the most affected forest species in the Valparaíso region. Three exotic species, Eucalyptus globulus, Pinus radiata, and Acacia dealbata, and two native species, Cryptocarya alba and Quillaja saponaria, were studied. Flam-mability assays indicate that E. globulus, A. dealbata, and C. alba are extremely flammable, whereas P. radiata and Q. saponaria are flammable. Furthermore, E. globulus and A. dealbata have the highest heating values while Q. saponaria has the lowest values. The extreme flammability of E. globulus, A. dealbata, and C. alba indicates a high susceptibility to ignite. Furthermore, the high heat of combustion of E. globulus and A. dealbata can be associated with a high energy release, increasing the risk of fires spreading. In contrast, Q. saponaria has the lowest predisposition to ignite and capacity to release heat. Accordingly, this work shows that all studied tree species contain organic metabolites that are potentially flammable (sesquiterpenes, aliphatic hydrocarbons, alcohol esters, ketones, diterpenes, and triterpenes) and can be considered as drivers of flammability in vegetation. Finally, these preliminary results will aid in the construction of more resilient landscapes in the near future.
... However, it was demonstrated that individual homeowners can reduce the vulnerability of their houses to wildland fire, this include the creation of a defensible space (Bell et al. 2007) and the choice of less flammable species for landscaping. Species differ in their ability to ignite and combust (Randall & Duryea 2011;Ganteaume 2018) as well as to produce firebrands (Manzello et al. 2009) and, consequently, the role of the ornamental vegetation in fire propagation must be well understood for a better fire prevention in these areas. ...
... litter) can propagate surface fire or act as receptor fuel for spot fires; the litter structure and composition also influences flammability, thus must be taken into account in the choice of species. Several works agreed, for instance, that the ornamental species Pittosporum tobira was among the less flammable species (Long et al. 2006b;Ganteaume et al. 2013aGanteaume et al. , 2013bGanteaume 2018). Furthermore, the plant structure (branching pattern, leaf density in the 8 canopy, capacity to retain dead fuel) contributes to the overall flammability. ...
... In contrast to wildland vegetation whose flammability has been extensively studied, a few scientific works investigated the flammability of ornamental vegetation, providing lists of desirable plants based on their fire related characteristics (Doran et al. 2004;Behm et al. 2004;Hansen et al. 2007;Hermansen-Báez 2011;Moritz & Svihra 1998) and ranking species according to their flammability (Frommer & Weise 1995;White et al. 1996;Irby et al. 2000;Beall 2001;White et al. 2002;Moritz 2003;Etlinger & Beall 2004;Behm et al. 2004;Weise et al. 2005;Long et al. 2006b, White & Zipperer 2010, Bartoli et al. 2011Ganteaume 2018). ...
Each year, wildfires spreading in wildland-urban interfaces (WUI) damage or destroy homes and other important assets, threatening residents and firefighters and sometimes resulting in loss of lives. In most cases, the vegetation located in the area comprised between wildland vegetation and housing is responsible for this fire propagation. This ornamental vegetation can be composed of native or exotic species, but also of landscaping dead fuel materials such as mulches that also sustain fire propagation and present a more heterogeneous structure compared to wildland vegetation. Correctly assessing the flammability of these ornamental species in order to properly choose, locate and maintain them when landscaping is necessary to reduce wildfire risk and prevent fire propagation in WUI. Likewise, an adapted modelling of fire propagation in this type of vegetation is needed.
... Instead, bulk density and twig cover were identified as the main drivers of flaming duration. The dominant effect of bulk density on flaming duration is consistent with other studies (Curt et al., 2011;van Altena et al. 2012;Zhao et al., 2019) although Ganteaume (2018) found the proportion of fine particles in the litter bed positively influenced flaming duration, in addition to bulk density. At higher bulk densities, there is more fuel available to burn, leading to longer flaming durations (El Houssami et al. 2018;Ganteaume 2018). ...
... The dominant effect of bulk density on flaming duration is consistent with other studies (Curt et al., 2011;van Altena et al. 2012;Zhao et al., 2019) although Ganteaume (2018) found the proportion of fine particles in the litter bed positively influenced flaming duration, in addition to bulk density. At higher bulk densities, there is more fuel available to burn, leading to longer flaming durations (El Houssami et al. 2018;Ganteaume 2018). In addition, at higher bulk densities radiative heat transfer through the fuel bed is lower, meaning there is less efficient preheating of fuel, leading to longer burning times (Bartoli et al., 2011). ...
... However, their initial dataset suggesting support for this hypothesis was too small to be conclusive, while two other studies suggesting decoupling of decomposability and flammability, considered flammability of individual litter particles (Grootemaat et al., 2015;, which are difficult to interpret in terms of field-relevant flammability of surface litter fuel beds. As oxygen limitation should be the most important driver for flammability when scaling up from single leaf to the surface litter fuel, testing this hypothesis at field-relevant litter bed level is needed (Ganteaume, 2018;Ganteaume et al., 2009Ganteaume et al., , 2014Scarff & Westoby, 2006). ...
... While most previous studies only focused on the effects of leaf size-related traits (e.g. leaf curliness (uncorrected for leaf size), leaf length, leaf volume, specific leaf area) on flammability (Burton et al., 2021;Cornwell et al., 2015;de Magalhães & Schwilk, 2012;Engber & Varner, 2012;Ganteaume, 2018;Kane et al., 2021;Scarff & Westoby, 2006;Schwilk & Caprio, 2011;Zhao et al., 2016), leaf shape (index) was not really included before. Thus, we used shape index as the quantitative trait representing shape independently of litter tissue volume. ...
Biological decomposition and wildfire are two predominant and alternative processes that can mineralize organic C in forest litter. Currently, the relationships between decomposition and fire are still poorly understood.
We provide an empirical test of the hypothesized decoupling of surface litter bed decomposability and flammability, and the underlying traits and trait spectra.
We employed a 41‐species set of gymnosperms of very broad evolutionary and geographic spread, because of the wide range of (absent to frequent) fire regimes they are associated with.
We found that the interspecific pattern of mass loss proportions in a “common garden” decomposition experiment was not correlated with any of the flammability parameters and an RDA analysis also showed that the decomposability and flammability of leaf litter in litter layers were decoupled across species. This decoupling originates from the former depending mostly on size and shape spectrum traits and the latter on PES traits and those trait spectra being virtually uncorrelated.
Synthesis : Our results show that, indeed, leaf litter decomposability and flammability parameters are decoupled across species, and this decoupling can be explained by their different drivers in terms of trait spectra: chemical traits for decomposability and size‐shape traits for litter layer flammability.
... The moisture content (MC) of the leaves was determined through the following process: 10 g of fresh leaves was weighed on an analytical balance (AS 220.R2, Radwag, Radom, Poland) and placed in a thermostatic drying oven (digital oven, model JK-DO-9030A, Shanghai Jingke Scientific Instrument Co. Ltd., Shanghai, China) at 60 • C for 48 h to guarantee total dehydration. The samples were then weighed, and MC-which was determined as the weight of water versus dry weight-was determined using Equation (5) [33,[53][54][55][56][57]: ...
... Regarding the MC of E. globulus leaves, it was similar to that reported by other authors [55,57], where for Mediterranean species it ranged from 68% to 253%. It is generally accepted that MC is a flammability determinant, and it was shown that MC delays the IT, as the water molecule slows down the heat transfer to the fuel and is reflected in an increase in the IT variable [57]. ...
Mediterranean climate regions have become more vulnerable to fire due to the extreme weather conditions and numerous Eucalyptus globulus plantation areas. The aim of this study is to analyze the fire hazard related to E. globulus in a forest fire scenario, based on the contrast of thermochemical parameters and their relationship with chemical properties, considering the predominant exotic forest species (E. globulus, Pinus radiata, Acacia dealbata, and Acacia melanoxylon) present in the Valparaiso region, Chile. The results revealed that although all of the studied species were highly flammable, E. globulus was extremely flammable, as its leaves contain high concentrations of essential oils, monoterpenes, and sesquiterpenes, which can generate a flammable atmosphere due to their low flashpoint and the strong negative influence shown between the essential oils, volatile terpenes, and limonene concentration. Moreover, the heat of combustion of E. globulus was positively correlated with its high essential oil contents. Finally, all of the studied species had low flashpoints and high heating values; therefore, they are predisposed to ignite in the presence of a heat source, releasing high amounts of energy during combustion, which contributes to the risk of the formation and spread of canopy fires among these tree formations.
... Plants 2021, 10, 2164 2 of 23 litter in order to increase the flame spread and, therefore, to decrease the fire time of residence [21]. Many plant traits are known to have a high impact on plant flammability such as the fuel moisture content (FMC), the leaves/needles structural characteristics (e.g., thickness, surface-to-volume ratio, etc.), and the fuel structure (e.g., the shoot or fuel bed bulk density) among others [23,[25][26][27]. These traits differ among species that present different fire-adaptive strategies, but they can also differ within the same species according to fire frequency [28,29]. ...
... These flammability components are used in most studies assessing plant flammability [19,20] even though these studies have been criticized recently due to the high correlation between the flammability variables belonging to different components [21] or to their scarce empirical support [22]. Even if the flammability components are well defined, in some cases, plant flammability can differ among scales (e.g., from fuel particle to litter) [23] and among fire-resilient and fire-resistant species according to their flammability strategies [24]. For example, some pine species considered as less flammable, due to their low flammability at the plant level, have highly flammable Addressing the gaps previously highlighted, two common Mediterranean pine species presenting different adaptive strategies to fire (fire-resistant and fire-embracer) were studied in the present work. ...
Flammability is a major factor involved in Mediterranean plant evolution that has led to the diversity of fire-related traits according to fire regimes and fire-adaptive strategies. With on-going climate change, new fire regimes are threatening plant species if they do not adapt or acclimate. Studying flammability and terpene content variation according to the different fire frequencies in the recent fire history represents a great challenge to anticipating the flammability of ecosystems in the near future. The flammability of shoots and litter as well as the needle terpene contents of two pine species with different fire adaptive strategies (Pinus halepensis and Pinus sylvestris) were measured according to two fire modalities (0 vs. 1–2 fire events over the last 60 years). Results showed that, regardless of the species and the fuel type, flammability was higher in populations having undergone at least one past fire event even when factors influencing flammability (e.g., structural traits and hydric content) were considered. The terpene content did not vary in P. sylvestris’ needles according to the fire modality, but that of sesqui- and diterpenes was higher in P. halepensis’ needles sampled in the “Fire” modality. In addition, associations made between flammability and terpene content using random forest analyses indicated that the terpene molecules differed between fire modalities for both species and fuel types. The same results were obtained with significant terpenes driving flammability as were highlighted in the PLS analyses, especially for P. halepensis for which enhanced shoot flammability in the “Fire” modality agreed with the adaptive strategy of this species to fire.
... However, an equivalent examination of patterns at the litter bed-scale has not yet been made. Identifying what drives flammability at different scales is critical as the results from one scale may not be transferable to another scale (Ganteaume, 2018;Grootemaat et al., 2017a). This is particularly important for litter beds which contain senesced, fallen leaves rather than green leaves. ...
... The direct effect of SLA on flame duration is likely related to the influence of SLA on leaf ignitability. Leaves with higher SLA have been found to ignite more quickly owing to the greater surface area exposed to heat and flames (Ganteaume, 2018;Grootemaat et al., 2015;Murray et al., 2013). ...
Fallen plant material such as leaves, needles and branches form litter beds which strongly influence fire ignition and spread. Traits of the dominant species influence litter flammability directly by determining how individual leaves burn and indirectly through the structure of the litter bed. However, we are yet to determine the relative importance of these different drivers across a range of plant species from different biomes.
We undertook a meta‐analysis, combining leaf trait, litter structure and flammability data for 106 species from North America, South America, Europe, Asia and Australia. The dataset encompassed broad‐leaved and coniferous species from seven different experimental studies. Relationships between leaf traits, litter structure and key flammability metrics—sustainability, combustibility and consumability—were analysed using bivariate and piecewise structural equation modelling (SEM).
Traits which characterise the three‐dimensional nature of the leaf and how much space a leaf occupies showed much stronger associations to litter structure and flammability than other morphological traits. Leaf curl, surface area to volume ratio (SAV) and SLA predominately influence litter flammability indirectly via litter structure with SLA being the only leaf trait which had a negative direct effect on flame duration. Packing ratio and bulk density were influenced by different combinations of leaf traits and, in turn, they aligned with different flammability metrics. Bulk density predicted flame spread rate and flame duration whereas packing ratio predicted consumption.
Synthesis . We identified key leaf and litter traits which influence different components of litter bed flammability. Importantly, we show that the effects of these leaf and litter traits are consistent across a wide range of taxa and biomes. Our study represents a significant step towards developing trait‐based models for predicting surface wildfire behaviour. Such models will more flexibly accommodate future shifts in the composition of plant species triggered by altered fire regimes and climate change.
... We used measurements of leaf functional traits, and leaf-and shoot-level flammability taken across 43 common indigenous perennial New Zealand plant species to (a) explore how flammability differs between leaf-and shoot-level measurements for the same species; (b) compare the leaf-level (Mason et al., 2016) and shootlevel (Wyse et al., 2016) flammability rankings with the flammability of the same species in field conditions as determined by expert opinion (Fogarty, 2001); and (c) explore which suites of leaf traits, if any, are correlated with shoot flammability. While a recent study by Ganteaume (2018) compared leaf and litter bed level flammability, to our knowledge, ours is the first study to compare the most commonly used plant flammability measurement (i.e. leaf-level flammability) to an approach (shoot-level flammability) that retains aspects of plant architecture, and which then assesses each approach against independently determined rankings derived from expert opinion of field-based fire behaviour. ...
... The arrangement of leaves and twigs at the shoot level likely reflects the way that fire burns through a plant canopy, with the propagation of fire from twig to twig on a shoot similar to propagation from branch to branch in a canopy. The burning of single leaves may be more relevant to litter fires (Ganteaume, 2018;Varner et al., 2015), albeit without the more realistic representation of fuel bulk density that comes with burning leaf litter mixes. ...
Flammability is an important plant trait, relevant to plant function, wildfire behaviour and plant evolution. However, systematic comparison of plant flammability across ecosystems has proved difficult because of varying methodologies and assessment of different fuels comprising different plant parts. We compared the flammability of plant species at the leaf‐level (most commonly used in flammability studies) and shoot‐level (which retains aspects of plant architecture). Furthermore, we examined relationships between leaf functional traits and flammability to identify key leaf traits determining shoot‐level flammability.
We collated and analysed existing leaf‐ and shoot‐level flammability data from 43 common indigenous perennial New Zealand plant species, along with existing data on leaf morphological and chemical traits.
Shoot‐level flammability was decoupled from leaf‐level flammability. Moreover, leaf‐level rankings of flammability were not correlated with rankings of flammability of plants derived from expert opinion based on field observations, while shoot‐level rankings had a significant positive relationship. Shoot‐level flammability was positively correlated with leaf dry matter content (LDMC), phenolics and lignin, and negatively correlated with leaf thickness.
Synthesis . Our study suggests that shoot‐level measurements of flammability are a useful and easily replicable way of characterizing the flammability of plants, particularly canopy flammability. With many parts of the world becoming more fire‐prone, due to anthropogenic activities, such as land‐use change and global warming, this finding will help forest and fire managers to make informed decisions about fuel management, and improve modelling of fire‐vegetation‐climate feedbacks under global climate change. Additionally, we identified some key, widely measured leaf traits, such as leaf dry matter content (LDMC), that may be useful surrogates for plant flammability in global dynamic vegetation models.
... Previous studies have shown that leaf litter f lammability varies between species (Della Rocca et al., 2018), with leaf morphology being an important inf luencing factor (Burton et al., 2021;Grootemaat et al., 2017). Leaves with lower thickness and higher specific leaf area or surface to volume ratio tend to ignite more easily (Ganteaume, 2018;Popović et al., 2021). Furthermore, leaf morphology has a strong effect on the physical structure of litter layers that inf luences combustion characteristics and fire propagation (Tumino et al., 2019). ...
Temperate forests of Central Europe are exposed to increasing fire risk. However, little is known about combustion properties of leaf litter, which plays an important role in the spread of surface fires. We used cone calorimetry to compare combustion properties of leaf litter samples from seven common tree species of Central European forests by reconstructing a litter layer of original depth in sample holders with a size of 10 cm × 10 cm. In addition to mono-specific leaf litter beds, combustion experiments included mixtures of different litter types, mixtures of litter and bryophytes and one mixture of litter and fine woody debris, totalling to 13 different setups (i.e. litter types). Recorded combustion properties included ignitability, flaming duration and heat release. Differences in combustion properties were analysed using analyses of variance followed by pairwise post-hoc tests. Combustion properties mainly differed between different litter types (broadleaf, pine needle, short needle). Highest total and peak heat release were observed for Scots pine (Pinus sylvestris), while peak heat release rates showed only minor differences for litter of the remaining species. Broadleaf litter was characterized by highest ignitability. For short-needle litter, we observed long flaming duration and incomplete combustion, resulting in the lowest total heat release on a sample mass basis. For litter mixtures of pine and broadleaf litter, we observed lower peak heat release rates in comparison to mono-specific pine litter. Mosses reduced peak heat release rates and increased the proportion of unburned biomass. However, the magnitude of this effect differed between bryophyte species included in the mixtures. The addition of fine woody debris strongly increased total heat release, highlighting the importance of fine woody fuels for fire behaviour. The results of this study provide valuable baseline information on combustion behaviour of leaf litter from Central European forests. Due to the limitations of laboratory combustion experiments to reproduce conditions of real forest fires, there is a need for future field studies investigating fire behaviour under natural conditions.
... By taking these components into account, plant flammability can be investigated in a variety of ways, either looking at individual or whole plant components or different fuel types (live or dead), with all methods having advantages and disadvantages (Schwilk, 2015). Flammability studies are often used to understand different types of fires, such as ground, surface, or canopy (Ganteaume, 2018). Although all fire types are important to understand, canopy fires are the main types that are attributed to severe wildfires, thus need to be better investigated (Alam et al., 2020). ...
... By taking these components into account, plant flammability can be investigated in a variety of ways, either looking at individual or whole plant components or different fuel types (live or dead), with all methods having advantages and disadvantages (Schwilk, 2015). Flammability studies are often used to understand different types of fires, such as ground, surface, or canopy (Ganteaume, 2018). Although all fire types are important to understand, canopy fires are the main types that are attributed to severe wildfires, thus need to be better investigated (Alam et al., 2020). ...
Canopy fires are increasing globally with anthropogenic climate and land-use changes, even in fire-sensitive rainforest ecosystems. Identifying the ecological drivers that may be aiding canopy fires, such as species or growth form flammability, is crucial to recognising and mitigating fire risks. To address this, we quantified the shoot-flammability of 124 rainforest plant species using an experimental approach. We compared three flammability measures (burnt biomass, total burn time and maximum temperature reached) with plant functional traits across seven different growth forms (i.e., canopy, pioneer, and understory trees; pioneer, understory and invasive shrubs, and vines) and nine common plant families and other higher-level clades, such as conifers, hereafter abbreviated to families. From burning > 600 sun-exposed shoots, we found trees were higher in flammability than shrubs and vines, and the plant families: Sapindaceae, Proteaceae, Fabaceae, and Lauraceae, had especially high flammability, whereas Moraceae was very low. Of the functional traits examined, leaf dry matter content was consistently and significantly positively associated with species flammability. Invasive shrubs as a group were not particularly flammable, although there were exceptions, e.g., wild tobacco (Solanum mauritianum) was highly flammable. This study has two important implications for the management of fire in rainforests. First, we have demonstrated that many tropical rainforest trees may readily burn under severe fire conditions if fire were to reach the rainforest canopy. Second, a large proportion of the > 1 million rainforest trees planted in the Wet Tropics under restoration planting schemes are from our most flammable rainforest plant families, as these families are often recommended for their carbon sequestration potential. Hence, these plantings may be highly vulnerable to fire and if planted along the borders of primary forest they may carry fire into their canopies. Therefore, where fire risk is high, we recommend planting species with low flammability along borders of plantings and forests to act as ‘green firebreaks’ to reduce the risk of fire incursions.
... The traits of individual plants or plant parts (e.g. shoots, leaves, stems, bark, etc.) do not equate directly to community flammability (Ganteaume, 2018;Pérez-Harguindeguy et al., 2013), which is a complex system (Zylstra, 2011). FRaME addresses this by modelling fire behaviour using the Forest Flammability Model (FFM), which is fully detailed and validated elsewhere (Zylstra et al., 2016), but briefly introduced here. ...
Floral fire ecology incorporates a feedback loop in which plants influence fire behaviour and fire behaviour influences the flora. Recent advances in fire behaviour modelling have quantified many plant‐based drivers of fire behaviour, but the consequent ecological effects of this have not yet been adequately modelled mechanistically.
Here, I introduce the Fire Research and Modelling Environment (FRaME) as the open‐source R package frame on GitHub. FRaME calculates the influence of plants on fire behaviour using a biophysical, mechanistic model of fire behaviour, building this into complex simulations. From these, it models heat transfer from flames into surrounding surfaces, calculating its ecological effects on plants and soils. I demonstrate the application of the central analysis functions using a detailed case study, in which I validate predictions of fire behaviour and ecological effects, and derive quantitative measures for the efficacy of different management treatments to mitigate fire risk to a vulnerable ecosystem.
FRaME modelling predicted ecological effects such as the breaking of seed dormancy, scorch and the girdling of different tree strata, consistent with observed effects and providing insights into treatment efficacy that were not captured by existing assumptions. FRaME analyses were able to identify treatments that both increased the likelihood of success in containing fires and minimised fire impacts on a fire‐sensitive ecosystem.
FRaME provides a platform to examine the full role of fire in an ecosystem, from the ways that biota drive flammability to the influence of that flammability on the ecosystem. By mechanistically incorporating the effect of biophysical drivers throughout this feedback, FRaME can provide novel insights and solutions for complex problems, quantify risk and guide effective mitigation measures. The model is extensible, providing a conceptual framework into which emerging work on flammability and fire effects can be incorporated.
... Furthermore, the methodology distinguished which differences between standing grass and litter fuels were due to the inherent nature of the fuels, and which were due to the fuel structure. More generally, our results illustrate that not accounting for fine fuel structure in manipulative experiments may lead to inaccurate extrapolations of the ecological effects of fire (Fernandes and Cruz, 2012;Ganteaume, 2018;Varner et al., 2015). We demonstrated our methodology with cogongrass, a widespread and problematic invader in Southeast US pine forests that can limit the use of prescribed fire as a management tool in heavily invaded areas, and we report the most precise estimates to date of fire behavior from cogongrass-fueled fires. ...
A better understanding of how dominant fuels affect fire behavior can improve predictions and comparisons of the ecological effects of fires in forests and other ecosystems. Current methods for evaluating effects of fuel characteristics on fire behavior, including maximum temperature and heating duration, range from small-scale laboratory to large-scale field experiments. Small-scale experiments often have many replicates and high measurement precision but can lack realism, while field experiments may have few replicates and lower measurement precision, thereby making comparisons across ecosystems difficult. Here, we present a method to experimentally evaluate ecological effects of fire while maintaining realism in fuel structure. We applied the method to investigate fire behavior effects of cogongrass (Imperata cylindrica), an invasive grass with a vertical growth form that is widespread across Southeast US forests. Examining the effect of fuel structure (piled vs. standing) on fire behavior for a range of fuel loads illustrated how more realistic standing fuels produced shorter heating duration (s above 100 °C), taller flame heights, and faster spread rates compared to piled fuels. Average heating duration was ~2–4 times longer and ranged more widely when fuels were piled (80–277 s) compared to standing (41–57 s). Flame heights were ~1.4 times taller when fuels were standing than piled. These differences highlight that maintaining natural fuel structure in experimental fires produces more realistic estimates of fire behavior and effects. Consequently, not maintaining realistic vertical fuel structure could lead to overestimation of potential fire impacts related to temperature (e.g., tissue damage) but underestimate potential impacts related to flame heights, such as total engulfment of tree seedlings and saplings by fire. Altogether, our method effectively maintained fuel structure, enabling assessment of more probable fire behavior and impacts of the invasive grass than if fuels were simply piled. This approach may help further bridge the gap in realism between small-scale experiments and large-scale fires, enabling comparisons of the ecological effects of fires and fire-invasion interactions across forest ecosystems.
... Our study indicates that aggravated drought could imply a shift towards higher litter ignitability through increases of wax cover. Our work also demonstrates a negative relationship between flame sustainability (through flame residence time) and both, leaf area and SLA as previously shown (Scarff and Westoby, 2006;Grootemaat et al., 2017;Ganteaume, 2018). Correlations with SLA theoretically indicate that in denser and/or thicker leaf tissues (low SLA values) flame sustainability is promoted. ...
Several plant chemical traits (cellulose, tannins and terpenes) have been related to plant flammability. Contrastingly, no study has focused on the relationship between plant flammability and physico-chemical leaf litter traits with a focus on cuticular wax concentration. This study focuses on alkane cuticular waxes because of their relatively low flash point and storage in the cuticle of all vascular plant species. The sclerophyllous species Quercus coccifera is the model species since it is the main shrub species in the Mediterranean basin and all previously investigated sclerophyllous species feature a high cuticular wax content. Litter was collected in a Mediterranean garrigue where Q. coccifera grows under natural drought and recurrent aggravated drought (consisting of 5.5 years of rain restriction). These different drought conditions were expected to imply different alkane wax concentrations since one of the major roles of cuticular waxes is evapotranspiration limitation during drought. Litter flammability was assessed through ignition delay, flame residence time and flame height (assessed using an epiradiator) and gross heat of combustion (using an adiabatic bomb calorimeter). Results showed that the higher cuticular alkane concentrations reached under aggravated drought were associated with an increased leaf litter flammability as expected. These results confirm that all potentially flammable organic metabolites (terpenes as previously reported in other studies, and cuticular alkane waxes) are drivers of vegetation flammability. It is suggested that Q. coccifera flammability (considered as low to moderate), could increase under a drier scenario in the Mediterranean area. We hypothesize that fire severity would accordingly be intensified in shrubs dominated by this sclerophyllous species without necessarily increasing vulnerability of Q. coccifera to fire since this is a resprouter species after fire and is one of the main pioneer species during post-fire vegetation succession.
... However, we note that within this study, whilst we have analysed leaf-level flammability as well as small-scale lab-formed litter and branch level flammability, we have not assessed whole tree or community level flammability, due to feasibility and experimental constraints. It is likely that the canopy flammability of the fire embracer species tested scales differently from the leaf, branch and litter flammability [45,46], and that the results may vary between different genera. For instance, fire embracers can be flammable at the tree level but not necessarily at the leaf level [12] (although in the Pinus species tested here this is not the case). ...
Fire is increasingly being recognised as an important evolutionary driver in fire-prone environments. Biochemical traits such as terpene (volatile isoprenoid) concentration are assumed to influence plant flammability but have often been overlooked as fire adaptations. We have measured the leaf-level flammability and terpene content of a selection of Pinus species native to environments with differing fire regimes (crown fire, surface fire and no fire). We demonstrate that this biochemical trait is associated with leaf-level flammability which likely links to fire-proneness and we suggest that this contributes to post-fire seedling survival. We find that surface-fire species have the highest terpene abundance and are intrinsically the most flammable, compared to crown-fire species. We suggest that the biochemical traits of surface fire species may have been under selective pressure to modify the fire environment at the leaf and litter scale to moderate fire spread and intensity. We indicate that litter flammability is driven not only by packing ratios and bulk density, but also by terpene content.
... Their integration would help to predict fire behaviour and contribute to the prevention of wildfires (Keane, 2015). The bulk density and the proportion of fine fuels are also considered the best descriptors of flame duration (Ganteaume, 2018). The fuel bulk density influences the individual shrub flammability, especially the flame spread rate, combustion efficiency and heat release (Rothermel, 1972;Doran et al., 2004;Scarff and Westoby, 2006;Fernandes and Cruz, 2012;Keane, 2015;Schwilk, 2015;Grootemaat et al., 2017;Pausas et al., 2017). ...
In Morocco, efforts have been made to prevent wildfires, although with limited results because the pyric properties of forest fuels have not been adequately quantified. In order to remedy this gap, the bulk densities of litter and individual shrubs and biomass per size class were assessed in five fire-prone forest regions. The study covered a total of 35 sites, on both north and south-facing slopes in the Central Plateau, the Middle Atlas (Western and Eastern), the Western Rif and the Pre-Rif regions. Litter depths ranged from 1.1 (Ononis natrix L.) to 7.5 cm (Daphne laureola L.), and the bulk density of individual shrubs varied from 0.35 (D. laureola) to 4.64 mg/cm3 (Thymelaea tartonraira L.). The lowest fine fuel bulk density was found for D. laureola (0.22 kg/m3), and the highest for T. tartonraira (4.05 kg/m3). As for the bulk density of individual shrubs, no significant intra-species differences were found between the sampled regions, except for Arbutus unedo L., while the effect of the region on fine fuel biomass was not significant in all species. Linear functions were used to adjust the fine fuel biomass for the effect of individual shrub volume. Such adjustments will be very useful for forest managers, since they will make it possible to estimate the fine fuel biomass contained in a shrub using simple, independent measurements of the shrub size. Integrating data on the structural traits of fuels into fire behaviour prediction systems will facilitate estimations of fuel hazards in the regions studied and thus guide decision-makers in their task of protecting both humans and natural resources.
... As such, laboratory-based plant flammability research has been criticised for having limited applicability to field-scale flammability [31]. Recent studies have involved cross-scale comparisons between the flammability of individual leaves and litter beds [50,55] but the challenge of translating the results of laboratory studies to landscape scales remains [56,57]. Given the potential magnitude of the effect of species on field-scale flammability, being able to understand how plant-level traits link to field-scale fire behaviour is critical to understanding fire at larger scales. ...
Vegetation is a key determinant of wildfire behaviour at field scales as it functions as fuel. Past studies in the laboratory show that plant flammability, the ability of plants to ignite and maintain combustion, is a function of their traits. However, the way the traits of individual plants combine in a vegetation community to affect field flammability has received little attention. This study aims to bridge the gap between the laboratory and field by linking plant traits to metrics of field-scale flammability. Across three prescribed burns, in Eucalyptus-dominated damp and dry forest, we measured pre-burn plant species abundance and post-burn field flammability metrics (percentage area burnt, char and scorch height). For understory species with dominant cover-abundance, we measured nine traits that had been demonstrated to influence flammability in the laboratory. We used fourth-corner ordination to evaluate covariation between the plant traits, species abundance and flammability. We found that several traits covaried at the species level. In some instances, these traits (e.g. specific leaf area and bulk density) could have cumulative effects on the flammability of a species while in other instances (e.g. moisture and specific leaf area) they may have counteractive effects, assuming trait effects on flammability are akin to previous research. At field scales, species with similar traits tended to co-occur, suggesting that the effects of individual traits accumulate within a plant community. Fourth-corner analyses found the trait-field flammability relationship to be statistically significant. Traits significantly associated with increasing field flammability metrics were: bulk density (negatively associated) and hydrocarbon quantity, specific leaf area and surface area to volume ratio (all positively associated). Our study demonstrates that some traits known to influence flammability in the laboratory can be associated with field-scale flammability metrics. Further research is needed to isolate the contributions of individual traits to understand how species composition drives forest flammability.
... Unlike for testing building materials, 58 flammability of vegetation can be subject to debate [16] and some authors developed 59 alternative frameworks [17][18][19][20]. No standardized procedure exists for evaluating the four 60 components of flammability for natural fuels [9] and different metrics can be used to quantify The plant geometry, structure and composition (leaves, twigs of various diameters) were 95 already identified as primary parameters determining its flammability [16,35,[63][64][65][66]. ...
Structure of vegetation significantly influences its flammability and resulting fire spread. Despite considerable amount of laboratory studies, experimental works carried out with full plant specimens, representative of field conditions, are still limited. Present study aims to collect meaningful experimental data on structure and flammability of shrub of rockrose and evaluate the predictions of a fire model (WFDS) against this dataset. Spatial distribution of fuel elements, sorted according to their characteristic thickness, was established from destructive measurements. 28 fire tests were conducted with full plants under a calorimeter. Foliar moisture content was in the range of 4–18% on dry basis. Radiant panels were used as source of ignition. Flammability was investigated using ignitability, sustainability, combustibility and consumability. Comparison to previous studies highlighted the necessity of standardization among test procedures. Principal component analysis revealed four flammability regimes depending on proportion of thin fuel elements within the crown, position of ignition and duration of preheating. Finally, combustion dynamics of a shrub was numerically investigated with WFDS. A bulk density model was developed from the characterization study and used as input data for the numerical code. Predicted HRR was in good agreement with experiments, although simulation results need improvement in initiation phase of burning.
Introduction
A variety of plant traits, from architectural to the cellular level, have been connected to flammability, but intraspecific variability of plant traits (ITV) and components of flammability is poorly studied. The lack of knowledge about ITV of plant traits related to flammability appears to be a major shortcoming in further interpreting species flammability and fire behavior and incorporating the data into models.
Methods
Morpho-ecophysiological traits (width, length, thickness, weight, area, volume, moisture content, flatness, specific leaf area, density of leaf tissue, ratio of area to volume) and time-to-ignition of fresh foliage were measured in seven populations of Picea omorika .
Results
All leaf traits are presented along with their correlations to the flammability trait. The seven populations differed in terms of fresh leaves’ time-to-ignition. Differences among populations in morpho-ecophysiological traits were also significant but not consistent among populations. PCA classified 49 elements into three different groups, where three populations were clustered by higher leaf area-related traits, other three populations were clustered by higher leaf length, volume, thickness, time-to-ignition, density index, moisture content, width, weight, and one population was classified between the two main groups. The first two principal components accounted for 87% of the total variance: variability in leaf area- and leaf weight-derived parameters (specific leaf area and density index) and time-to-ignition primarily defined the formation of the first axis, while variability in leaf flatness (based on leaf weight and thickness) primarily contributed to the formation of the second axis.
Discussion
Results suggest high ITV in natural populations of P. omorika regardless of site fire history.
Background In most wildland–urban interface (WUI) fires, damage to buildings results from poor surrounding vegetation management. No simulation had been conducted yet on historical WUI fires with Computational Fluid Dynamics modelling. Aims It was interesting to check the feasibility of this modelling in simulating past fire cases for different scenarios of vegetation management and fire propagation. Methods We studied three cases of WUI dwellings surrounded by gardens (subject to French regulations on fuel reduction) adjacent to forest affected by a past fire. The 3D fire propagation was assessed using the Fire Dynamic Simulator model (FDS) and taking into account accurate fire environment (fine vegetation distribution, terrain, etc.). Key results Results showed that, in the current model state, brush-clearing mitigated fire intensity and propagation and damage to ornamental vegetation. However, it sometimes highlighted that this measure could be strengthened when the effects of topography and wind were combined. Conclusions FDS modelling at the WUI scale using accurate vegetation distribution proved to be functionally satisfactory, exhibiting realistic fire behaviour. Implications Once validated, this modelling will ultimately help to assess when fuel reduction is efficient in fire mitigation and to pinpoint possible limitations.
Background Few studies have focused on the integral assessment of live fuel flammability in the boreal forest. Aims We aimed to examine the flammability of living needles of jack pine (Pinus banksiana) as characterised by their form, moisture and chemical content at different ages and times of the year. Methods With a calorimeter and open flame, we estimated needle ignitability, consumption rate, maximum speed and amount of energy released. We measured their form, moisture and chemical content. Key results Needle form has a major effect on ignitability, whereas chemical composition primarily influences the amount and rate of energy release. Needles <1 year old are the least flammable; they are rounder and voluminous, with higher moisture and nitrogen content. Needles ≥1 year old are more flammable; they are drier, more curved, thinner, longer, and contain more carbon and terpenes. Needles release more energy during the early growing season, when starch and lipids are at their peak concentrations. Conclusions Moisture content is not the major factor affecting the flammability of jack pine live needles; physicochemical changes specific to age and month of collection are the most influential factors. Implications Assessing the multi-faceted properties of live fuels flammability will help to comprehend stand- and landscape-scale fire behaviour.
• Leaf flammability is a functional trait that can vary widely among plant species. At present, however, the effects that increasing radiant heat flux have on variation in leaf flammability among species are not well understood. Yet, such effects could have important implications for wildfire models that take into account species’ differences in flammability.
• We examined how five leaf flammability attributes spanning ignitibility (times to incandescence and flaming), sustainability (incandescence and flame durations) and combustibility (proportion of leaves entering flaming combustion) responded to increasing radiant heat fluxes (29.6 to 96.6 kWm⁻²) in 10 species of fire-prone woodlands.
• As radiant heat flux increased, times to incandescence and flaming became significantly faster and proportions of leaves entering flaming combustion became significantly higher. In contrast, incandescence duration became significantly shorter at high radiant heat flux. Differences among species in these flammability attributes decreased with increasing radiant heat flux, with species becoming significantly more similar to each other. Differences among species in flame duration, however, were not significantly affected by increasing radiant heat flux, with leaf flaming durations in each species remaining relatively fixed across the radiant heat flux gradient.
• Our findings show that leaf flammability is significantly affected by increasing radiant heat flux. We suggest that of the flammability attributes assessed in our study, flame duration is the most informative to include in wildfire models which explicitly consider species’ flammability, given that differences among species in flame duration are maintained across a radiant heat flux gradient.
In flammability assessment, the terpene effect is usually studied using their total or subgroup content, missing, therefore, the information that could be provided by the molecules themselves. In this study, the specific role of terpenes on leaf flammability was sought comparing different levels of terpene identification—total, subgroup (i.e. mono-, sesqui-, and diterpene), and single compound—as well as their interactions with fuel moisture content (FMC) in four species common in Mediterranean Wildland–Urban Interfaces (Pinus halepensis, Cupressocyparis leylandii, Hesperocyparis arizonica, Cupressus sempervirens). Pinus halepensis was the most flammable species (low FMC and higher sesquiterpene content but low terpene diversity) while Cupressocyparis leylandii presented the highest terpene diversity and total terpene content (higher mono- and diterpene content). Flammability was differently affected according to the terpene identification level used in the models. The effects ranged from non-significant for most species studied, using subgroup or total terpene content, to mostly significant, using single compound content. Regarding the former, the lack of significant results could be due to opposite effects of different single compounds within a terpene subgroup. For the latter, terpene molecules driving flammability and their effects (positive or negative) differed among species. A cumulative effect with FMC was also highlighted in some cases but terpenes mostly remained the main flammability drivers regardless of the species. Using the refined terpene level in modelling allowed a better understanding of the compounds’ role on flammability, which is useful in the identification of plant traits linked to flammability.
Wildfires in the Valparaı´so region (Chile) account for one of the main threats to local biodiversity, ecosystem
services and infrastructure. This study focused on producing an initial record of thermo- and physicochemical properties of
local forest species. For this purpose, leaf samples of species found in the Pen˜uelas Lake National Reserve, namely Pinus
radiata, Eucalyptus globulus, Acacia dealbata, Quillaja saponaria and Cryptocarya alba, were collected and analysed.
Higher and lower heating value, flash point, density and moisture content tests were performed for each sample. Overall
results showed that lower heating values measured for both native and exotic species could indicate a high energy release
source in wildfires. However, differences in the flash point between species indicated that C. alba and E. globulus had a
lower ignition resistance than other species tested, possibly due to a lower flash point. In contrast, Q. saponaria and
A. dealbata had the highest flash point for native and exotic species respectively. Finally, all presented data and procedures
were aimed at establishing a foundation for a national database of critical forest species properties to be used in wildfire
simulation tools. This database will enhance forest fire management effectiveness in Chile.
Among plant characteristics promoting flammability, terpenes have received little attention, especially regarding the vegetation surrounding housing. Here, mono-, sesqui- and diterpenes were screened in live and dead leaves of ornamental species found in wildland–urban interfaces (WUIs) of south-eastern France. Terpene content and composition were compared among species and between fuel types. Their influence on flammability was assessed through several variables and compared with that of leaf thickness and moisture content. Six of the 17 species examined contained terpenes. Terpene diversity and content differed among species but not between fuel types. Mono-, sesqui- and diterpenes (especially the highly concentrated compounds) were involved to varying degrees in both leaf and litter flammability. Their effects could be opposite according to the flammability variable and the fuel type considered. Leaf sesquiterpene content and litter total terpene content had the strongest influence on maximum combustion temperature; the former also mainly drove leaf flaming duration. The other flammability variables were more strongly associated with either moisture content or leaf thickness. Our findings highlight the idea that fire management in the WUI must also acknowledge the potential for ornamental species containing terpenes, such as Pinus halepensis, to affect fire behaviour.
Leaf flammability is a multidimensional plant functional trait with emerging importance for wildfire risk management. Understanding relationships among leaf flammability attributes not only provides information about the properties of leaves as fuels in the wildland-urban interface (WUI), it can also offer an effective way to identify low-leaf-flammability species. We examined relationships between leaf ignitibility, sustainability and combustibility among 60 plant species of the WUI of eastern Australia. We found that leaf ignitibility and sustainability worked in opposition to each other as dimensions of flammability. Species with leaves that were slow to ignite were those with leaves that sustained burning for the longest, whereas species with leaves that were fast to ignite had leaves that burned for the shortest periods of time. Low leaf combustibility was related to short leaf burning sustainability but not to ignitibility. We created an overall leaf flammability index (OLFI) to rank species on emergent properties of ignitibility, sustainability and combustibility attributes in combination. We found that low-leaf-flammability species with low OLFI values had small leaf area, high leaf mass per area and high leaf water content. Our findings have implications for species selection for green firebreaks in the WUI.
South-eastern France is strongly affected by wildfires mostly occurring in the wildland-urban interfaces (WUIs). A WUI fire is often initiated in dead surface fuel, then can propagate to shrubs and trees when the lower canopy is close to (or touches) the ground. Whereas a previous study assessed the fire propagation from the fuel bed to the lower canopy of different species used as ornamental vegetation in this region, the objectives of the current work consisted of checking if the modelling of this fire propagation was possible using WFDS (Wildland-Urban Interface Fire Dynamical Simulator) in comparing experimental and modelling results. Experimental and modelling constraints (i.e. branch geometric definition, branch motion due to convection) showed differences in some of the recorded data (such as time to ignition, ignition temperature, mass loss and maximum temperature), but comparisons of variation in mass loss and temperature over time showed that modelling the fire propagation at the scale of a branch was possible if the branch fuel-moisture content remained lower than 25%. For both experiments and modelling, the ranking of species according to their branch flammability highlighted identical groups of species.
In the laboratory, different types of litter samples (constructed v. intact) can be used in flammability experiments but the sampling method of these litters could affect litter flammability results. To assess this effect, samples of litters were collected in South-eastern France, according to two different methods previously used in other studies, one keeping intact the structure of the litter layers (non-constructed litter) and the other requiring the construction of the litter, using mainly the surface litter layer (constructed litter). The comparison of flammability results showed that the sampling method had a significant effect on litter bulk-density, rate of spread and rate of consumption, intact litter being more flammable than reconstructed litter that was artificially compacted. The type of vegetation had a significant effect on litter depth, ignitability, sustainability, consumability and combustibility (except on rate of spread) and the litter composition could explain in part this fire behaviour. The effect of the construction of litters on flammability parameters and its magnitude also differed according to vegetation types. Intact litter structure appeared to be an important driver of its flammability, especially of combustibility and consumability. The assessment of these flammability components will differ when using constructed litter samples instead of intact litter samples, especially according to vegetation types. Future research on litter flammability should take into account the bias due to the litter sampling method when the litter is constructed.
Fire behavior and effects in forests and woodlands are influenced by surface fuels and senesced leaf litter in particular. We have known that species exhibit differential flammability for some time, but isolated efforts have often attributed differences to disparate mechanisms. Recent research has expanded the diversity of species evaluated, clarified patterns at the fuelbed level, and provided evidence that the physical and chemical traits of litter or fuelbeds drive flammability. To date, little effort has focused on uniting methods, clarifying the awkward terminology, or, perhaps most importantly, comparing laboratory findings to field observations of fire behavior. Here, we review recent literature and synthesize findings on what we know about the flammability of litter and propose future research directions.
The objectives of this paper are to assess in laboratory conditions the flammability of undisturbed litter sampled beneath plants of seven species that are among those most frequently planted in hedges in Provence (south-eastern France). The variability in litter flammability recorded during burning experiments was partly explained by the proportions of the different litter components of each species. Phyllostachys sp. and Nerium oleander litters were the quickest to ignite whereas Prunus laurocerasus litter had the lowest bulk density and long time-to-ignition, but high flame-propagation. Photinia fraseri litter ignited frequently and had a high flame spread whereas Pittosporum tobira litter ignited the least frequently and for the shortest duration. Cupressus sempervirens litter had the highest bulk density and the longest flaming duration but the lowest flame propagation. Pyracantha coccinea litter was the slowest to ignite and flame propagation was low but lasted a long time. Co-inertia analysis identified species with the same flammability characteristics according to the composition of their litter. Hierarchical cluster analysis ranked the seven species in four distinct clusters from the most flammable (Photinia fraseri and Prunus laurocerasus) to the least flammable (Pittosporum tobira), the other species displaying two groups of intermediate flammability. These latter species should not be used in hedges planted in wildland-urban interfaces in south-eastern France.
In fireprone ecosystems, two important alternative fates for leaves are burning in a wildfire (when alive or as litter) or they get consumed (as litter) by decomposers. The influence of leaf traits on litter decomposition rate is reasonably well understood. In contrast, less is known about the influence of leaf traits on leaf and litter flammability. The aim of this study was twofold: (i) to determine which morphological and chemical leaf traits drive flammability and (ii) to determine whether different (combinations of) morphological and chemical leaf traits drive interspecific variation in decomposition and litter flammability and, in turn, help us understand the relationship between decomposability and flammability.
To explore the relationships between leaf traits and flammability of individual leaves, we used 32 evergreen perennial plant species from eastern Australia in standardized experimental burns on three types of leaf material (i.e. fresh, dried and senesced). Next, we compared these trait–flammability relationships to trait–decomposability relationships as obtained from a previous decomposition experiment (focusing on senesced leaves only).
Within the three parameters of leaf flammability that we measured, interspecific variation in time to ignition was mainly explained by specific leaf area and moisture content. Flame duration and smoulder duration were mostly explained by leaf dry mass and to a lesser degree by leaf chemistry, namely, nitrogen, phosphorus and tannin concentrations.
The variation in the decomposition constant across species was unrelated to our measures of flammability. Moreover, different combinations of morphological and chemical leaf properties underpinned the interspecific variation in decomposability and flammability. In contrast to litter flammability, decomposability was driven by lignin and phosphorus concentrations.
The decoupling of flammability and decomposability leads to three possible scenarios for species’ influence on litter fates: (i) fast‐decomposing species for which flammability is irrelevant because there will not be enough litter to support a fire; ( ii ) species with slow‐decomposing leaves and a high flammability; and ( iii ) species with slow‐decomposing leaves and a low flammability. We see potential for making use of the decoupled trait–decomposition–flammability relationships when modelling carbon and nutrient fluxes. Including information on leaf traits in models can improve the prediction of fire behaviour. We note that herbivory is another key fate for leaves, but this study was focused on fire and decomposition.
Fire is nearly ubiquitous in the terrestrial biosphere, with profound effects on earth surface carbon storage, climate, and forest functions. Fuel quality is an important parameter determining forest fire behavior, which differs among both tree species and organs. Fuel quality is not static: when dead plant material decomposes, its structural, chemical, and water dynamic properties change, with implications for fuel flammability. However, the interactions between decomposition and flammability are poorly understood. This study aimed to determine decomposition’s effects on fuel quality and how this directly and indirectly affects wood flammability. We did controlled experiments on water dynamics and fire using twigs of four temperate tree species. We found considerable direct and indirect effects of decomposition on twig flammability, particularly on ignitability and burning time, which are important variables for fire spread. More decomposed twigs ignite and burn faster at given water content. Moreover, decomposed twigs dry out faster than fresh twigs, which make them flammable sooner when drying out after rain. Decomposed fine woody litters may promote horizontal fire spread as ground fuels and act as a fuel ladder when staying attached to trees. Our results add an important, previously poorly studied dynamic to our understanding of forest fire spread.
Over 2.5 million people and 1 million structures risk destruction from wildland fires in California. One way to mitigate this risk is the manipulation and/or selection of landscape vegetation. In this study, six species were studied for their intrinsic characteristics and tested with a newly-developed laboratory fire protocol at 150 kW to determine heat release rate (HRR). The plants were 2-3 years old and obtained from a local nursery. Whole plants were subjected to desiccation in a dry kiln at 50°C, which was found to be much more effective than simulating fire weather in a greenhouse. This is apparently the first study that has measured plant variables, burned them in a natural vertical position, and related HRR to the plant characteristics. Multiple regression showed the overwhelming importance of foliage and moisture content to peak HRR. Chemical variables were not significant and high moisture contents were found to obscure other plant characteristics.
Eight dominant Mediterranean species were classified into similar groups
according to their expected flammability, by applying multivariate statistical
methods (Hierarchical Cluster Analysis and Canonical Discriminant Analysis) on
the values of their most significant pyric properties (heat content, total and
mineral ash content, surface area-to-volume ratio, particle density). Based on
the statistical classification, meaningful explanations of the flammability
differences among individual species were deduced. The results were in good
agreement with similar rankings based on laboratory tests. Further validation
may render the method widely applicable for the assessment of species
potential flammability without laboratory flammability tests.
The flammability of plant leaves influences the spread of fire through vegetation. Exotic plants invading native vegetation may increase the spread of bushfires if their leaves are more flammable than native leaves. We compared fresh-leaf and dry-leaf flammability (time to ignition) between 52 native and 27 exotic plant species inhabiting dry sclerophyll forest. We found that mean time to ignition was significantly faster in dry exotic leaves than in dry native leaves. There was no significant native-exotic difference in mean time to ignition for fresh leaves. The significantly higher fresh-leaf water content that was found in exotics, lost in the conversion from a fresh to dry state, suggests that leaf water provides an important buffering effect that leads to equivalent mean time to ignition in fresh exotic and native leaves. Exotic leaves were also significantly wider, longer and broader in area with significantly higher specific leaf area-but not thicker-than native leaves. We examined scaling relationships between leaf flammability and leaf size (leaf width, length, area, specific leaf area and thickness). While exotics occupied the comparatively larger and more flammable end of the leaf size-flammability spectrum in general, leaf flammability was significantly correlated with all measures of leaf size except leaf thickness in both native and exotic species such that larger leaves were faster to ignite. Our findings for increased flammability linked with larger leaf size in exotics demonstrate that exotic plant species have the potential to increase the spread of bushfires in dry sclerophyll forest.
The present study proposes a new method in order to evaluate the flammability of live plant parts at bench-scale. Flammability parameters were estimated by the use of a mass loss calorimeter, and fuel moisture content was evaluated by the use of moisture analyser. Forest fuels (Pinus pinaster, Cistus laurifolius, Lavandula stoechas and Daphne gnidium) were monitored under field conditions to detect the changes in the fuel moisture contents during the fire risk season. The combination of two different bench-scale devices (moisture analyser and mass loss calorimeter) guarantees fixed conditions for carrying out laboratory tests (constant bulk density and constant sample dry mass), thus resolving problems detected with other devices and methodologies.
The autoignition delay times of some important Mediterranean forest species: Arbutus adrachne, Abies cephallonica, Pinus brutia, Pinus halepensis, Pistacia lentiscus, Cupressus semprevirens, Olea europaea, Cistus incanus were determined in the range 460–600°C under precisely controlled temperature and airflow conditions. Based on these data the forest fuels examined were classified into two groups: the least and the most flammable species. The autoignition delay data were related to the thermal analysis measurements. The themogravimetric analysis in an inert (nitrogen) atmosphere showed that the thermal decomposition of cellulose in the range of 300–400°C as well as the mass residue at 600°C are directly related to the ignition behavior.
Factors affecting ignition thresholds of the litter layer of shrubland vegetation were investigated using reconstructed litter beds in a laboratory. The factors investigated were fuel moisture content (FMC), litter type (primarily species), pilot ignition source, and wind. Litter beds made from 11 different litter types were ignited with point ignition sources. Litter from Allocasuarina nana (Sieber ex Spreng.) L.A.S. Johnson was used as the standard type across all experiments. Successful ignition was defined as fire spreading a fixed distance from the ignition point. Ignition success was modelled as a logistic function of FMC. Litter type had a major effect on ignitibility. The bulk density of the litter bed and the surface area of litter per volume of litter bed provided reasonably good predictors of the effect of litter type on ignition success. Low-density litter beds ignited at higher FMCs than dense litter beds. The two densest litter beds failed to ignite with the procedures used here. The ignition sources tested had significantly different effects on ignition success. Larger ignition sources were able to ignite wetter fuels than smaller sources. The presence of wind was found to have a different effect on ignition success depending on the location of the ignition source with respect to the litter bed. Wind decreased ignition success when the ignition source was located on top of the litter bed, but aided ignition when the ignition source was located within the litter bed.
Assessment of the flammability of ornamental vegetation (particularly hedges) planted around houses is necessary in light of the increasing urbanization of the wildland-urban interfaces (WUIs) and the high fire occurrence in such areas. The structure and flammability of seven of the species most frequently planted as hedges in Provence (southeastern France) were studied at particle level. Spatial repartition of the different types of fuel particles within plants was assessed by means of the cube method. The leaf flammability was assessed using an epiradiator as a burning device, and measurements of foliar physical characteristics and gross heat of combustion (GHC) helped to explain the results of burning experiments. Co-inertia analysis revealed that species with thin leaves were quick to ignite (Pyracantha coccinea, Phyllostachys sp.) and species with high leaf GHC burned the longest (Pittosporum tobira, Nerium oleander). Species presenting high ignitability (Photinia fraseri, Phyllostachys sp. and Pyracantha coccinea) were characterized by high foliar surface area-to-volume ratio, and species presenting lower ignitability were characterized by high GHC (Pittosporum tobira, Nerium oleander, Cupressus sempervirens). Hierarchical cluster analysis of the flammability variables (ignition frequency, time-to-ignition and flaming duration) categorized the relative flammability of the seven species (including dead Cupressus sempervirens) in five clusters of species from poorly flammable (Pittosporum tobira) to extremely flammable (dead Cupressus sempervirens).This study provides useful information for reducing fire risk in WUIs in the study area.
Methods were developed to predict the moisture content of the elevated dead fine fuel layer in gorse (Ulex europaeus L.) shrub fuels. This layer has been observed to be important for fire development and spread in these fuels. The accuracy of the Fine Fuel Moisture Code (FFMC) of the Canadian Fire Weather Index System to predict the moisture content of this layer was evaluated. An existing model was used to determine the response time and equilibrium moisture content from field data. This response time was incorporated into a bookkeeping model, combining the FFMC and this response time-equilibrium moisture content model. The FFMC poorly predicted the elevated dead fuel moisture content in gorse fuels, and attempts to improve its accuracy through regression modelling were unsuccessful. The response time of the elevated dead fine fuel layer was very fast (38-77 min) and has important implications for fire danger rating. The bookkeeping approach was the most promising method to predict elevated dead fuel moisture content. A limitation was the inability to model fuel-level meteorology. However, this model warrants further validation and extension to other shrub fuels and could he incorporated into existing fire danger rating systems that can utilize hourly weather data.
Fire is one of the most important processes driving plant community composition and structure. Fire regimes are largely governed by climate, vegetation structure, and individual plant traits that influence flammability. We assessed the mechanistic drivers of flammability for a diverse group of 18 California Quercus and allied Chrysolepis and Notholithocarpus species, addressing variation in leaf physical traits, growth form (tree or shrub), phylogeny (Quercus subgenera), and fire regime (low, mixed, or high severity). Differences in flammability were not strongly driven by leaf habit, leaf margin type, or surface area to volume ratio; simple measures of leaf size accounted for most of the observed variation. Further, leaf size was tightly linked to fuelbed depth, a known driver of fire behavior. Litter from trees was generally more flammable than litter from shrubs, primarily a function of differences in leaf size. A hierarchical clustering analysis on the flammability data set divided the oaks into three clusters of low, intermediate, and high flammability, corresponding closely to high-, mixed-, and low-severity fire regimes, respectively. The link between plant flammability traits and fire regime provides further evidence that individual species affect ecosystem processes.
Wildland fire is becoming a concern for residents in many eastern states as fuel loads, weather patterns, and population growth increase risk at the wildland-urban interface. Some messages about reducing risk, however, are based on western wildfire information and are seen as inappropriate by wildland fire communicators in Florida. This case study describes the process of reaching agreement on landscape modifications that reduce the risk of wildland fire for interface residents in the Southeast. The melding of various perspectives through a negotiated process helped create a product that meets a need in this fire-prone state.
The flammability of living vegetation is influenced by a variety of factors, including moisture content. physical structure and chemical composition. The relative flammability of ornamental vegetation is of interest to homeowners seeking to make their homes 'fire safe'. The relative importance of the factors influencing fire behaviour characteristics, such as flammability, is unknown. In the present study, oxygen consumption calorimetry was used to obtain selected combustion characteristics of ornamental vegetation. Peak heat release rate, mass loss rate, time to ignition and effective heat of combustion of 100 × 100-mm samples of foliage and small branches were measured using a bench-scale cone calorimeter. Green and oven-dry samples of 10 species were collected and tested seasonally for a period of 1 year. Similar measurements were made on whole shrubs in an intermediate-scale calorimeter. The range of cone calorimeter peak heat release rates for green and oven-dry samples was 1-176 and 49-331kW m -2 , respectively. Moisture content significantly reduced heat release rates and increased time to ignition. Peak heat release rates for Olea europea and Adenostoma fasciculatum were consistently highest over the year of testing; Aloe sp. consistently had the lowest heat release rate. The correlation of peak heat release rater measured by the cone calorimeter and an intermediate-scale calorimeter was statistically significant yet low (0.5 1) The use of the cone calorimeter as a tool to establish the relative flammability rating for landscape vegetation requires additional investigation.
Six understory species from five pine flatwood sites and six understory species from five hardwood hammock sites were harvested for biomass analyses to compare potential flammability between two ecosystems in the south-eastern coastal plain of the United States. Plant components were separated into live and dead foliage, accumulated litter on and under the plant, and small and large stems. Foliar biomass was further analysed for moisture content, volatile solid content, and energy content. Statistical analyses revealed differences among species and between ecosystems. Serenoa repens plants present a wildfire hazard because they contain greater biomass than other species studied. Ilex glabra and Lyonia ferruginea are also hazardous to wildland–urban interface (WUI) structures because they have greater foliar energy content than other species studied. Callicarpa americana plants present the least wildfire hazard to WUI structures. We conclude that differences in flammability among species exist, but the causes of flammability are different among species. In addition, species in the same genus do not always have the same flammability. Based on measured characteristics, understory plants in pine flatwoods have greater ignitability, sustainability and combustibility than understory plants in hardwood hammocks. However, the measurements for consumability were similar between ecosystems.
Many studies have assumed that plant terpenes favor fire due to their enormous-flammability. However, only a few of them, all performed on green leaves, have demonstrated this. In the present work we investigated the question of whether litter terpene content can be used to estimate flammability and temperatures reached during fire. Epiradiator and burn table tests were used to compare flammability of leaf litter of P. pinaster, P. halepensis, P. pinea, C albidus, C ladanifer, C laurifolius and the mixture of litter of P. pinaster with that of the other five species (e.g. P. pinaster + P. halepensis). Tests with burn table showed increasing spread rates and shorter combustion times under higher terpene contents. Flame height was triggered both with higher a terpene content and bed thickness, whereas the percentage of burned biomass was only significantly correlated to bed height. Epiradiator tests indicated that terpene concentration in leaf litter was positively correlated to flame height and negatively correlated to both flame residence time and ignition delay. Flammability was high for P. pinaster, A halepensis, and hence for P. pinaster + P. halepensis, intermediate for C albidus, P. pinea and P. pinaster combined with each of these species, and low for C laurifolius, C ladanifer and P. pinaster combined with them. Accordingly, their terpene content was high, intermediate and low. We concluded that plants might influence fire intensity, by having stored terpenes in their dead leaves, in addition to having developed traits to survive fire. Thus, a correct management of dead aboveground fuels rich in terpene concentrations, such as those of P. pinaster and P. halepensis, could prove helpful in reducing the hazard of fire.
The time-to-ignition of various dominant Mediterranean forest fuels was measured during laboratory tests, in order to develop a relative flammability classification and determine the moisture of extinction of these fuels. The tests were performed with an ignition apparatus manufactured according to the ISO standards (ISO 5657-1986E) and under a wide gradient of fuel moisture contents, ranging from air-dry to fresh foliage. Moisture content was the single most significant factor that affected fuel flammability. Regression models were developed between the time-to-ignition and the moisture content values of all the fuels tested, and subsequently, were used for the relative flammability ranking of these fuels. Fuel moisture of extinction was assessed to have a threshold value ranging from 40% to more than 140% o.d.w. for the species tested. The flammability ranking of natural fuels can be useful in fuel hazard assessment and fire danger rating, thus facilitating the judicial fire management planning in wildlands and at the rural-urban interface.
Diversity effects on many aspects of ecosystem function have been well documented. However, fire is an exception: fire experiments have mainly included single species, bulk litter, or vegetation, and, as such, the role of diversity as a determinant of flammability, a crucial aspect of ecosystem function, is poorly understood. This study is the first to experimentally test whether flammability characteristics of two-species mixtures are non-additive, i.e., differ from expected flammability based on the component species in monospecific fuel. In standardized fire experiments on ground fuels, including monospecific fuels and mixtures of five contrasting subarctic plant fuel types in a controlled laboratory environment, we measured flame speed, flame duration, and maximum temperature. Broadly half of the mixture combinations showed non-additive effects for these flammability indicators; these were mainly enhanced dominance effects for temporal dynamics - fire speed and duration. Fuel types with the more flammable value for a characteristic determined the rate of fire speed and duration of the whole mixture; in contrast, maximum temperature of the fire was determined by the biomass-weighted mean of the mixture. These results suggest that ecological invasions by highly flammable species may have effects on ground-fire dynamics well out of proportion to their biomass.
Worldwide decomposition rates depend both on climate and the legacy of plant functional traits as litter quality. To quantify the degree to which functional differentiation among species affects their litter decomposition rates, we brought together leaf trait and litter mass loss data for 818 species from 66 decomposition experiments on six continents. We show that: (i) the magnitude of species-driven differences is much larger than previously thought and greater than climate-driven variation; (ii) the decomposability of a species' litter is consistently correlated with that species' ecological strategy within different ecosystems globally, representing a new connection between whole plant carbon strategy and biogeochemical cycling. This connection between plant strategies and decomposability is crucial for both understanding vegetation-soil feedbacks, and for improving forecasts of the global carbon cycle.
Knowledge of how species differ in their flammability characteristics is needed to develop more reliable lists of plants recommended for landscaping homes in the wildland-urban interface (WUI). As indicated by conflicting advice in such lists, such characterisation is not without difficulties and disagreements. The flammability of vegetation is often described as having four components (ignitability, combustibility, sustainability and consumability). No standards or generally recognised test procedures exist for evaluating these components in plants. Some measurements of flammability include times for ignition, rate of flame spread, flame height and heat release rate. Often, the fire behaviour characteristics of a plant are derived from its physical and chemical characteristics. Thermogravimetric analysis and other thermal analyses of ground samples have long been used to characterise the thermal degradation of vegetation. More recently, researchers have used the oxygen consumption methodology to measure the heat released due to combustion of the vegetation. Although oxygen consumption calorimetry is an improvement in characterising plant flammability, translation of laboratory results to field conditions can be problematic and tests can be expensive.
A series of tests were conducted under laboratory conditions to assess, on the one hand, the capacity of several fuel beds to be ignited by firebrands and to sustain a fire and, on the other hand, the capability of different types of firebrands to ignite fuel beds,. Fuel beds and firebrands were selected amongst the most common in Southern Europe. Regarding fuel bed flammability, results show that grasses are more flammable than litters and, amongst litters, Pinus species are the most flammable. The increase of bulk density and FMC involves an increase of the time-to-ignition, and a decrease of the other flammability parameters. The capability of firebrands to ignite fuel beds is higher when the firebrands drop in flaming phase and with no air flow than in glowing phase with air flow. Logistic regression models to predict fuel bed ignition probability were developed. As a whole, results show a relationship between ignition probability of fuel bed and type or weight of firebrands. Pinus pinaster cone scale, Pinus halepensis cone scale, Eucalyptus globulus leaf and bark can have ignition probabilities at least twice higher than bark of Pines when fallen in flaming combustion.
Document interne;* INRA, Centre de Recherche d'Avignon, Station de Sylviculture (FRA) Diffusion du document : INRA, Centre de Recherche d'Avignon, Station de Sylviculture (FRA)
Les incendies de forêt affectent de grandes surfaces et causent d'importants dommages qui peuvent avoir de lourdes conséquences écologiques, sociales et économiques. Plus de 50 000 feux brûlent environ 500 000 hectares de végétation chaque année dans les pays du bassin méditerranéen européen (JRC, 2006; Lampin-Maillet, 2008). Les interfaces habitat-forêt sont directement concernées par ces incendies : 90% des départs de feux sont liés à l'activité humaine en Europe Méditerranéenne (Eufirelab, 2004), et chaque année de nombreux morts sont à déplorer à cause de ces incendies de forêt, notamment parmi les habitants des interfaces habitat-forêt. Dans le contexte d'une forte pression d'urbanisation et d'une accumulation de biomasse combustible, les interfaces habitat-forêt représentent une véritable préoccupation pour la gestion du risque d'incendie (Davis, 1990; Velez, 1997; Cohen, 2000), particulièrement au regard des deux composantes du risque : l'aléa en terme de départs de feu causés par les activités humaines, et la vulnérabilité, en termes de surfaces brûlées menaçant les zones habitées et aussi de dégâts sur les habitations (Hardy, 2005; Jappiot et al, 2009). La thèse, qui relève des sciences de la géographie et de la cyndinique, montre toute la pertinence de l'utilisation de l'objet géographique « interface » dans le contexte de l'évaluation du risque d'incendie. Cet objet « interface » a été appliqué dans la thèse au cas particulier de l'interface habitat-forêt. Après avoir défini, caractérisé et cartographié l'interface habitat-forêt, la thèse a alors examiné la valorisation possible de cette entrée par l'interface habitat-forêt dans le cadre d'une démarche d'analyse spatiale et de cartographie du risque d'incendie sur le territoire. Ainsi l'interface habitat-forêt a été définie de façon précise dans le contexte du risque d'incendie. Une typologie d'interfaces a été créée, fondée sur la combinaison de deux critères jugés pertinents pour le risque d'incendie, traduisant des caractères prégnants des milieux humain, avec la structure de l'habitat résidentiel, et naturel, avec la structure de la végétation. Les types d'habitat résidentiel : habitat isolé, diffus, groupé dense et groupé très dense ont été définis en posant des principes de distances entre bâtis et de dénombrement des bâtis. Ils ont également été caractérisés en termes de densité de bâtis, de surfaces à débroussailler et de périmètre à défendre. La structure de la végétation a été traduite en termes de continuité horizontale avec un indice d'agrégation emprunté à l'écologie du paysage. L'indice est fondé sur des cartes de végétation, au format raster, d'une résolution maximale de 10 m avec un rayon de 20 m pour la fenêtre de calcul. Un seuil à 95 % discrimine une agrégation faible (végétation éparse) d'une agrégation forte (végétation continue). La méthode de caractérisation et de cartographie des interfaces habitat-forêt, développée dans la thèse, est applicable sur de grandes surfaces et à une grande échelle dans les départements du sud de la France. Elle a permis pour la première fois en France de quantifier l'importance des interfaces habitat-forêt sur un territoire. Puis la carte des interfaces a contribué à produire une nouvelle carte du territoire, alors compartimenté en espaces dits « interfacés » (types d'interfaces habitat-forêt), et en espaces dits « non interfacés » (espaces bâtis hors interfaces et le reste du territoire). Une première relation, forte, entre les types d'interface habitat-forêt et l'importance des départs de feu et des taux de surfaces brûlées a été mise en évidence. Une méthode d'évaluation du risque d'incendie, innovante, a alors été développée dans la thèse. Elle s'appuie sur une analyse spatiale et statistique du territoire, fondée sur une nouvelle cartographie de types de territoire déduite de la cartographie des interfaces habitat-forêt. L'analyse a consisté à croiser les types de territoire et les caractéristiques environnementales, topographiques et socio-économiques avec l'historique des feux à travers la distribution spatiale des départs de feu, celle des surfaces brûlées et la fréquence de passage des incendies. Elle a permis de mettre en évidence l'importance de certaines variables pour leur contribution positive (interfaces habitat-forêt en habitat isolé, espaces naturels autres que forestiers, garrigue, exposition très chaudes, zones de végétation éparse, densité de chemins) ou négative (interfaces habitat groupé dense et très dense, densité de bâtis et de routes, espaces urbains et agricoles, végétation résineuse) à l'explication de trois indicateurs de risque définis comme densité d'éclosion, densité d'incendie et taux de surfaces brûlées. La modélisation de ces indicateurs a contribué à la construction d'un indice global de risque et à sa cartographie qui permet de déduire facilement, et de manière assez directe, l'information synthétique sur les niveaux de risque à l'échelle du territoire. L'approche par les « interfaces habitat-forêt », intrinsèquement porteuses de l'information synthétique aléa/enjeux/vulnérabilité, a servi de clé d'entrée pour une évaluation directe et globale du risque, fondée sur l'observation et la description des territoires d'une part, et en particulier des interfaces habitat-forêt, et sur une analyse spatiale et statistique de ces territoires. Elle permet également de tirer des enseignements d'une meilleure connaissance du territoire et du risque d'incendie associé en termes de prévention. Elle est particulièrement bien adaptée à la mise en évidence, quantifiée, d'une dynamique de territoire lu à travers les interfaces habitat-forêt. Cette dynamique de territoire peut alors être facilement associée à une dynamique du risque dont l'étude est particulièrement intéressante dans le contexte du changement global : dynamique de végétation, dynamique d'urbanisation. Elle offre des perspectives encourageantes en matière de géogouvernance et développement durable dans le cadre d'une prévention contre le risque d'incendie toujours plus efficace et adaptée.
Abstract
Fire is an important ecological disturbance in vegetated ecosystems across the globe, and also has considerable impacts on human infrastructure. Vegetation flammability is a key bottom-up control on fire regimes, and on the nature of individual fires. Although New Zealand (NZ) historically had low fire frequencies, anthropogenic fires have considerably impacted indigenous vegetation as humans used fire extensively to clear forests. Few studies of vegetation flammability have been undertaken in NZ, and only one has compared the flammability of indigenous plants; this was a qualitative assessment derived from expert opinion. We addressed this knowledge gap by measuring the flammability of terminal shoots from a range of trees and shrubs found in NZ. We quantified shoot flammability of 60 indigenous and exotic species, and compared our experimentally derived ranking with expert opinion. The most flammable species was the invasive exotic shrub Ulex europaeus, followed by Eucalyptus viminalis, Pomaderris kumeraho, Dacrydium cupressinum, and Lophozonia menziesii. Our experimentally derived ranking was strongly correlated with expert opinion, lending support to both methods. Our results are useful to ecologists seeking to understand how fires have and will influence NZâs ecosystems, and for fire managers identifying high-risk landscapes, and low flammability species for âgreen firebreaksâ.
WF15047 Accepted 08 December 2015
This paper presents a study to assess the influence of pine needle layer characteristics on combustion for three pine species of the Mediterranean region of France. It identifies the key parameters that explain the combustion of this fuel bed component. A relationship between permeability of the litter layer, fuel bed porosity and needle geometrical properties is presented. Although permeability was found to influence the rate of heat release from the combustion of litter independent of litter species, this was not the case for litter layers with similar mass and porosity. This study also stresses the important role of particle properties on their time to piloted ignition. The surface-to-volume ratio (SVR) of the species is the essential parameter driving the time to ignition as it defines the thermal thickness of single needles. This parameter also influences the combustion dynamics of litters under forced convection. In that case, the heat release rate of pine needle litters with the same permeability increases with the SVR of the species.
Fire is important to climate, element cycles and plant communities, with many fires spreading via surface litter. The influence of species on the spread of surface fire is mediated by their traits which, after senescence and abscission, have 'afterlife' effects on litter flammability. We hypothesized that differences in litter flammability among gymnosperms are determined by litter particle size effects on litterbed packing. We performed a mesocosm fire experiment comparing 39 phylogenetically wide-ranging gymnosperms, followed by litter size and shape manipulations on two chemically contrasting species, to isolate the underlying mechanism. The first-order control on litter flammability was, indeed, litter particle size in both experiments. Most gymnosperms were highly flammable, but a prominent exception was the non-Pinus Pinaceae, in which small leaves abscised singly produced dense, non-flammable litterbeds. There are two important implications: first, ecosystems dominated by gymnosperms that drop small leaves separately will develop dense litter layers, which will be less prone to and inhibit the spread of surface litter fire. Second, some of the needle-leaved species previously considered to be flammable in single-leaf experiments were among the least flammable in litter fuel beds, highlighting the role of the litter traits of species in affecting surface fire regimes.
© 2015 The Authors New Phytologist © 2015 New Phytologist Trust.
Fuel moisture content (FMC) estimation is a critical part of any fire danger rating system, since fuel water status is determinant in fire ignition and fire propagation. However, FMC alone does not provide a comprehensive as- sessment of fire danger, since other factors related to fire ignition (lightning, human factors) or propagation (wind, slope) also need to be taken into account. The problem,in integrating all these factors is finding a common,scale of danger rating that will make,it possible to derive synthetic indices. This paper reviews the importance,of FMC in fire ignition and fire propagation, as well as the most common methods of estimating FMC values. A simple method to convert FMC values to danger ratings is proposed, based on computing ignition potential from thresholds of moisture of extinction adapted to each fuel. The method has been tested for the Madrid region (central Spain), where a fire dan- ger assessment,system,has been built. All the variables related to fire danger were integrated into a dedicated geo- graphic information system,and information provided,to fire managers,through a web,mapping,server. Résumé : L’estimation de la teneur en humidité des combustibles,est un élément critique de tout système d’évaluation
Changes in heat values and in flammability with the seasons of the year for the different species which make up the woodland map of two zones, Sada (coastal area) and Santiago (hillside and plateau area), situated in La Coruña (Galicia, Spain), are reported. These data were evaluated as a help for fighting forest fires, which have been very frequent in this region during the last decade. The species studied are the most abundant in the areas: Eucalyptus globulus Labill., Pinus pinaster Aiton, Ulex europaeus L., Rubus fructicosus L., Pteridium aquilinum L., Sarothamnus scoparius (L.) Link, Quercus robur L., Castanea sativa Miller and Acer pseudoplatanus L. The calorific values were measured by static bomb calorimetry. These data were complemented with flammability determinations and completed with chemical analyses of the different species.
Standardized fuel samples cut from the leaves of 32 plant species and from sheets of filter paper were test-burned in a muffle furnace at 750°C. Spontaneous ignition delay times, a measure of fuel ignitibility, were recorded. Ignition times varied directly with sample thickness and inversely with the logarithm of the surface area-to-volume ratio. Because of its relationship to ignitibility, leaf thickness may be a factor contributing to fire retardance in some plants. Forest. Sci. 17:475-478.
In southeastern pine–oak ecosystems, ecological restoration targets oaks for removal by chemical,
mechanical, burning, or combinations of treatments. Managers often pursue oak removal indiscriminately
despite the poorly understood historical structure, cover, and ecological function within these
ecosystems. Restoration treatments often cite the impediment that oak litter represents to prescribed fire
spread and effectiveness.We evaluated the burning characteristics of eight southeastern Quercus spp. by
burning collected litter under controlled conditions in a combustion chamber. Replicated burns consisted
of 15 g of litter on a 35 cm � 35 cm grid of xylene-soaked cotton strings. Burning characteristics
measured included maximum flame height (cm), flaming duration (s), smoldering duration (s), residual
ash, and mass loss rate (g s�1).Wecompared all 8 oaks using ANCOVA, with litterbed depth as a covariate.
The oaks differed for all burning characteristics measured (P < 0.001). Rank comparisons placed Quercus
stellata and Quercus laevis as the species with greatest fire intensity, sustainability, and consumability,
equivalent in many measures to longleaf pine and other fire resisters. Quercus virginiana and Quercus
hemisphaerica burned with the least intensity, sustainability, and consumability, burning similarly to
sand pine and other fire evaders. These results show that oaks common to southeastern United States
ecosystems have litter properties, similar to pines, which vary in their ability to sustain fire.
Understanding the pyric properties of oak species also suggests that managers prioritize removal of
species that hinder prescribed fire effectiveness for restoration of southeastern USA pine–oak
ecosystems.
To assess the effect of vegetation types and of fire regime on the flammability of dead fuels, samples of litter were collected undisturbed (i.e. keeping the structure of litter layers) in 29 study sites spread over the limestone-derived soils of Provence. The sampling plan comprised the most representative ecosystems of the study area: pure Pinus halepensis stands, mixed pine-oak stands and shrublands. Three classes of litter depth were studied (low, medium and high) to account for the variability existing in the field. Sampling also included the number of fires in each site since 1960 (no fire, 1 or 2 fires and ≥3 fires) and the time interval since the last fire (≤15 years, 15–45 years and >45 years). Flammability experiments were carried out in laboratory using a glowing firebrand and a 10 km h−1 wind. The main variables recorded were: ignition frequency, time-to-ignition, flaming duration, flame rate of spread, flame propagation, mean flame temperature, maximum flame height and rate of consumption.
Results showed that the ignition frequency was higher in mixed stands than in pure pine stands whereas the time-to ignition, flaming duration and rate of consumption were the highest in pure pine stands. The maximum flame height and the flame propagation decreased with the increase of the number of fires and the time-to-ignition was the highest when the interval since the last fire was the shortest. Increases in litter depth resulted in increased mean flame temperature, maximum flame height and flame propagation. These results can be explained, in part, by the proportions of the different litter components.
1. Although species differ in flammability, identifying the traits that influence flammability and linking them to other axes of trait variation has yet to be accomplished. Leaf length may be a key trait influencing the flammability of leaf litter. 2. Differences in species composition across a landscape or changes in composition through time may alter fire behaviour. Forests in the Sierra Nevada of CA, USA, have experienced changes in species composition that have modified the distribution of leaf litter traits. 3. Across three independent data sets, at scales from a single watershed to multiple watersheds and elevations, we tested if mean community leaf length in patterns of fire severity. We used structural equation models to disentangle direct effects of site characteristics from the contribution of species composition. 4. Fire severity was greater at sites inhabited by species with longer leaves than at sites containing short-leaved species, probably as a result of lower litter density. The effect cannot be explained merely by the joint influence of site characteristics on both fire behaviour and species composition. 5. A significant portion of this pattern is driven by shifts in the abundance of Pinus species. In this system, pines are among the longest-leaved species and this makes it difficult to separate leaf-length effects from other possible flammability-enhancing characteristics of pines. Evidence from one data set, however, suggests that the pattern cannot be entirely explained by proportion of pines alone. 6. Synthesis. We demonstrate that a simple integration of a species trait predicts fire severity at landscape scales. This provides a link between the two scales at which most previous work has occurred: species-specific measurements of traits and landscape-level characterisation of fuel loads. Investigations of trait effects on fire behaviour are important because climate change may lead to novel climates and no-analogue species assemblages. In this ecosystem, shorter-leaved species, which have increased in density during the period of fire exclusion, may act as a positive feedback by reducing fire severity and thereby favouring their own establishment. Conversely, restoration of fire to these forests, by increasing the dominance of long-leaved species, may increase flammable fuels.
1. Although it is recognized that plant species vary in their flammability, we currently lack a mechanistic understanding of how plant traits influence fire and how litter mixtures behave in a fire. As modified fire regimes and climate change shift the species composition of communities, a mechanistic perspective is especially important to understand and predict fire in potentially novel plant communities. This work addresses three questions: (i) How do eight species common in Sierra Nevada mixed-conifer forest differ in their litter flammability? (ii) What leaf traits are associated with various flammability components? and (iii) Do individual species measurements predict multi-species combinations, or are there non-additive effects?
2. Leaf litter was collected in Sequoia and Kings Canyon National Parks, California, from eight dominant tree species in mixed-conifer forest. Controlled flammability tests were performed on reconstructed monospecific litter beds and on mixed litter beds, using litter from three species. We tested for non-additive effects in multi-species mixtures using the weighted mean of single-species measures for each flammability component as a null expectation for each mixture; departures from this null indicated non-additive effects.
3. Most flammability components fell within two major axes of variation, one relating to total heat release and another to fire intensity. The eight species differed significantly in all flammability components, with large-leaved species creating litter that burned with higher intensity.
4. Non-additive species mixture effects are common in this system. Flammability tends to be driven by the most flammable component of the mixture.
5. Synthesis. We have demonstrated positive non-additive effects in mixtures of leaf litter. The most flammable constituent species of a mixture has disproportionate effects on the fire environment faced by the entire community. This could potentially influence community assembly and alter the selective environment faced by co-occurring species.
What is flammability? The author suggests that it consists of ignitibility, sustainability, and combustibility. He defines
ignitibility in terms of fuel properties and heat source intensity. Similar methods for determining sustainability and combustibility
have not yet been formulated.
• Introduction
Wildfires are considered the most important disturbance in the Mediterranean Basin, and some are propagated over long distances due to lift-off and ignition of firebrands.
• Objectives
To improve our knowledge of firebrands involved in spotting fires, flammability characteristics of eight types of firebrands commonly generated by wildfires in Southern Europe were determined under laboratory conditions.
• Results
All the firebrands tested showed 100% ignition frequency but with a wide range of time to ignition and flaming duration. Weight loss during combustion was exponentially related to time, and there was a decrease in the ratio of the weight at temperature T to the initial weight with increasing temperatures. In our experimental conditions, there was a significant effect of fuel moisture content on time to ignition, flaming duration, combustion and thermal decomposition. On the basis of the characteristics analysed, three firebrand groups have been identified in relation to spotting: heavy firebrands with ability to sustain flames, efficient for long-distance spotting (pine cones); light firebrands with high surface-to-volume ratio, efficient for short-distance spotting (leaves and thin barks); and light firebrands with low surface-to-volume ratio, efficient for short and, occasionally, long-distance spotting (all the other types of firebands).
The rapid growth of housing in and near the wildland–urban interface (WUI) increases wildfire risk to lives and structures. To reduce fire risk, it is necessary to identify WUI housing areas that are more susceptible to wildfire. This is challenging, because wildfire patterns depend on fire behavior and spread, which in turn depend on ignition locations, weather conditions, the spatial arrangement of fuels, and topography. The goal of our study was to assess wildfire risk to a 60,000 ha WUI area in northwestern Wisconsin while accounting for all of these factors. We conducted 6000 simulations with two dynamic fire models: Fire Area Simulator (FARSITE) and Minimum Travel Time (MTT) in order to map the spatial pattern of burn probabilities. Simulations were run under normal and extreme weather conditions to assess the effect of weather on fire spread, burn probability, and risk to structures. The resulting burn probability maps were intersected with maps of structure locations and land cover types. The simulations revealed clear hotspots of wildfire activity and a large range of wildfire risk to structures in the study area. As expected, the extreme weather conditions yielded higher burn probabilities over the entire landscape, as well as to different land cover classes and individual structures. Moreover, the spatial pattern of risk was significantly different between extreme and normal weather conditions. The results highlight the fact that extreme weather conditions not only produce higher fire risk than normal weather conditions, but also change the fine-scale locations of high risk areas in the landscape, which is of great importance for fire management in WUI areas. In addition, the choice of weather data may limit the potential for comparisons of risk maps for different areas and for extrapolating risk maps to future scenarios where weather conditions are unknown. Our approach to modeling wildfire risk to structures can aid fire risk reduction management activities by identifying areas with elevated wildfire risk and those most vulnerable under extreme weather conditions.
1. Wildfires strongly influence the biotic composition and carbon cycle of many ecosystems. Plant species provide the fuel for wildfires, but vary widely in their flammability. This study aimed to determine what plant characteristics control leaf litter flammability and to clarify how they are related to other functional traits.
2. Litter flammability varied across 14 tree species occurring in a mosaic of five floristic associations.
3. Differences in heat-release rate between species were driven by leaf size, from small-leaved conifers, casuarinas and acacias to large-leaved eucalypts and Brachychiton
4. Large leaves created an open litter-bed structure that burned more rapidly because it was better ventilated. The results on heat-release rate were partitioned according to fundamental principles for the transport of oxygen through a packed fuel bed, showing that heat-release rate scaled linearly with estimated gas-flow rates, as expected in ventilation-controlled fires.
5. Species that were able to resprout after fire had litter that burned more intensely and was more likely to sustain a spreading fire than litter from obligate seeders, and were correspondingly larger-leaved.
6. Many fire-prone wooded ecosystems in the region consist of large-leaved resprouting tree species co-occurring with small-leaved obligate seeders.
13 ref.;Espaces Forestiers et incendies
Firescape -landscaping to reduce fire hazard
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Effects of vegetation 646 types and fire regime on flammability of non-constructed litters in south-eastern France
- A Ganteaume
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- Curt T Borgniet
Ganteaume A, Jappiot M, Lampin-Maillet C, Curt T, Borgniet L (2011a) Effects of vegetation
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types and fire regime on flammability of non-constructed litters in south-eastern France.
Flammability of Christmas trees and other 724 vegetation
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Defensible space landscaping in the urban/wildland interface: a 673 compilation of fire performance ratings of residential landscape plants
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Lubin DM, Shelly JR (1997) Defensible space landscaping in the urban/wildland interface: a
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compilation of fire performance ratings of residential landscape plants. Internal Report No.
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36.01.137, University of California, Richmond, CA.