M. Guijarro

Fuel bulk density and fuel moisture content effects on fire rate of spread: a comparison between FIRETEC model predictions and experimental results in shrub fuels

Fuel bulk density and fuel moisture content effects on fire rate of spread were assessed in shrub fuels, comparing experimental data observed in outdoor wind tunnel burns and predictions from the physically-based model FIRETEC. Statistical models for the combined effects of bulk density and fuel moisture content were fitted to both the experimental and the simulated rate of spread values using non-linear regression techniques. Results confirmed a significant decreasing effect of bulk density on rate of spread in a power law in both laboratory burns and simulations. However, experimental data showed a lesser effect than simulations, suggesting a difference in the effective drag. Fuel moisture content effect was highly consistent, showing a similar exponential relationship with rate of spread in laboratory and in simulations. FIRETEC simulations showed similar orders of magnitude with predictions of two field-based empirical models, finding a significant correlation between rate of spread values. The study confirms the efficacy of the combined approach through experimental data and simulations to study fire behaviour.

Laboratory characterization of firebrands involved in spot fires

Abstract• IntroductionWildfires 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.• ObjectivesTo 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.• ResultsAll 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).

Smouldering fire-induced changes in a Mediterranean soil (SE Spain): effects on germination, survival and morphological traits of 3-year-old Pinus pinaster Ait.

In the present study, a smouldering fire was reproduced in a substrate from a Pinus pinaster forest in the southeastern Iberian Peninsula. Experiments were carried out, in laboratory, using soil monoliths to assess the short-term fire-induced effects on germination, survival and morphological traits in young (3-year-old) specimens of Pinus pinaster Ait. The fire caused a severe reduction in the litter and humus layer relative to a control (unburnt) soil. A lower percentage of accumulated germination (29% in the burnt soil compared with 71% in the control soil) reduced final seedling density, and a lower seedling height was observed in burnt soil. Furthermore, the amount of biomass fixed per unit of leaf area and the concentration of foliar nutrients were lower in the seedlings grown in the burnt soil. However, the amount of biomass fixed per individual seedling was significantly higher in the burnt soil than in the control soil. The results confirm the observed lesser P. pinaster recruitment in burnt stands in southeastern Spain.

Characterizing potential wildland fire fuel in live vegetation in the Mediterranean region

Key messageFuel moisture and chemical content affecting live plant flammability can be measured through laboratory and field techniques, or remotely assessed. Standardization of methodologies and a better understanding of plant attributes and phenological status can improve models for fire management.ContextWildland fire management is subject to manifold sources of uncertainty. Beyond the difficulties of predicting accurately the fire behavior, uncertainty stems from incomplete understanding of ecological susceptibility to fire.AimsWe aimed at reviewing current knowledge of (i) plant attributes and flammability: fuel moisture and chemical content in leaves; (ii) experimental evaluation of flammability in the laboratory and in the field; and (iii) proxy evaluation of flammability: vegetation cover assessment at large scale, fuel seasonality, and biomass distribution using remote sensing and Light Detection and Ranging (LiDAR) techniques.MethodsWe conducted a review of scientific literature from the last two decades on the three selected issues, with a specific focus on the Mediterranean region.ResultsWe have evidenced important knowledge gaps: (i) developing standardized methodologies for laboratory- and field-scale assessment of vegetation flammability; (ii) introducing reliable approaches to test the impact of biogenic volatile organic compounds on fire spread; (iii) improving the analysis of spatiotemporal changes in vegetation dynamics, acknowledging distinctive vegetation phenological status as a relevant driver affecting leaf biomass and moisture contents; and (iv) further exploring the processes that shape fuel dynamics to understand how fuel characteristics change over time and space.ConclusionWe propose some improvements in the current knowledge of vegetation science and wildland fire ecology, aiming to generate more realistic models and effective planning in support of fire management in the Mediterranean basin.

Possible land management uses of common cypress to reduce wildfire initiation risk: a laboratory study

Accurate determination of flammability is required in order to improve knowledge about vegetation fire risk. Study of the flammability of different plant species is essential for the Mediterranean area, where most ecosystems are adapted to natural fire but vulnerable to recurrent human-induced fires, which are the main cause of forest degradation. However, the methods used to evaluate vegetation flammability have not yet been standardized. Cupressus sempervirens is a native or naturalized forest tree species in the Mediterranean area that is able to tolerate prolonged drought and high temperatures. The aim of this study was to characterize the flammability of C.xa0sempervirens var. horizontalis at particle level by using different bench-scale calorimetry techniques (mass loss calorimeter, epiradiator and oxygen bomb) to determine the main flammability descriptors (ignitability, sustainability, combustibility and consumability) in live crown and litter samples. Our findings indicate that this variety of cypress is relatively resistant to ignition because of the high ash content, the high critical heat flux, the high time to ignition displayed by both crown and litter samples and the ability of the leaves to maintain a high water content during the summer. We also discuss the possibility of exploiting some morphological, functional and ecological traits of the species to construct a barrier system (with selected varieties of cypress) as a promising complementary land management tool to reduce the fire spread and intensity in a Mediterranean context.

Estimation of Peak Heat Release Rate of a Forest Fuel Bed in Outdoor Laboratory Conditions

Experimental tests were carried out with an adapted bench-scale mass loss calorimeter (MLC) and also in an outdoor wind tunnel to estimate the heat release rate (HRR) of a forest fuel bed. The MLC apparatus uses a calibrated thermopile to quantify the HRR, as an alternative to the classical measurement of oxygen consumption due to combustion. Additional calibration of thermocouples to measure gas temperatures enabled estimation of HRR in experimental burnings conducted in the wind tunnel. The results showed a reasonable agreement between peak HRR (PHRR) values obtained in the MLC and in the wind tunnel, and also demonstrated that PHRR was significantly affected by the wind speed and the rate of spread. The proposed procedure is potentially useful as an initial step in monitoring HRR in outdoor forest fire experiments.

Determinación de la relación superficie/volumen de las acículas muertas

Se ha puesto a punto un metodo para la determinacion de la relacion superficie/volumen de las aciculas muertas. El metodo se basa en la simplificacion propuesta por Brown (1970), pero obteniendo los valores requeridos (superficie y perimetro de las secciones), en cortes micrometricos, mediante un analizador de imagen. El metodo propuesto auna las condiciones de precision y rapidez necesarios. La relacion superficie/volumen (σ) esta intimamente relacionada con el comportamieneto del fuego en los diferentes tipos de combustible forestal. En consecuencia, esta caracteristica fisica se determina para particulas de combustible fino muerto que intervienen, frecuentemente, en los incendios forestales. Los valores obtenidos para Pinus pinaster Ait., P. pinea L. y P. halepensis Mill. son, respectivamente, de 48,24, 57,80 y 79,93 cm-1. Los resultados ponen de manifiesto las variaciones existentes entre especies. Sin embargo, no se encuentran diferencias derivadas de los distintos lugares de muestreo.

Flammability of Two Mediterranean Mixed Forests: Study of the Non-additive Effect of Fuel Mixtures in Laboratory

In the Mediterranean region, wildfires are a major disturbance, determined by ecosystem and forest species characteristics. Both the flammability and resistance to fire of a mixed forest may vary from those of the individual species. Two mixed Mediterranean woodlands, a Cupressus sempervirens and Quercus ilex stand in Italy; and a Juniperus thurifera and Quercus faginea stand in Spain were investigated. Laboratory flammability tests were conducted on live foliage, litter samples and on litter beds from individual and mixed species to evaluate: (i) the flammability traits of the mixtures of live foliage and litter samples; (ii) whether the flammability of the two-species mixtures are non-additive, i.e., differ from expected flammability based on arithmetic sum of the single effects of each components species in monospecific fuel; (iii) the ignition success and initial fire propagation in litter beds. Flammability tests were also conducted on bark samples to estimate the resistance of the tree species to fire. The ignitibility of live foliage was lower and the combustibility was higher in Cupressaceae than in Quercus. Non-additive effects were observed in some flammability components of live foliage and litter, especially in the mixtures of C. sempervirens and Q. ilex. Ignitability and combustibility were higher and lower than expected, respectively, and tended to be driven by Quercus), while the consumability was lowered more than expected by both Cupressaceae. The ignition success in the litter beds was low, especially for the presence of Cupressaceae that increase the bulk density of the mixtures. Cupressaceae, which have a thinner bark, suffered more damage to the cambium after shorter exposure to the heat source than Quercus species. In all the species studied, time to reach lethal temperatures in the cambium was dependent on thickness rather than on flammability of the bark. The study findings revealed that tree species may influence flammability of mixed fuels disproportionately to their load. The studied species showed to exert a contrasted effect on flammability of the mixtures, increasing ignitability and decreasing combustibility and consumability well out of their proportion in the mixture. This may potentially influence fire dynamics in mixed forests.

Influencia de la pendiente y de la carga de acículas muertas de Pinus halepensis en el comportamiento del fuego: modelos desarrollados en laboratorio

Se estudia en laboratorio el comportamiento del fuego en una cubierta de aciculas muertas de Pinus halepensis, considerando siete pendientes (-30o, -20o, -10o, 0o, +10o, +20o, +30o) y tres cargas del combustible (0,4, 0,8 y 1,2 kg · m-2). Los ensayos se realizan sobre un combustible homogeneo de aciculas secas, pero no anhidras. El regimen del fuego es estacionario para todos los valores de la carga entre -30o y +10o y a +20o solo para la carga de 0,4 kg · m-2. Para las cargas de 0,4 y 0,8 kg · m-2, la velocidad de combustion esta relacionada lineal y significativamente con la velocidad de propagacion. Esta relacion es menos estrecha para la carga de 1,2 kg · m-2. La tasa de residuos tiende a aumentar entre -30o y +20o y disminuye al aumentar la carga. Los otros parametros (temperaturas maximas y areas limitadas por las curvas de variacion de la temperatura y la isoterma de 60o C) dependen de la pendiente y aumentan con la carga. Este trabajo se inscribe en un estudio mas global del INIA y del INRA para la elaboracion de modelos de prediccion del comportamiento del fuego.