Global warming reshapes European pyroregions

Wildland fire is expected to increase in response to global warming, yet little is known about future changes to fire regimes in Europe. Here, we developed a pyrogeography based on statistical fire models to better understand how global warming reshapes fire regimes across the continent. We identified five large-scale pyroregions with different levels of area burned, fire frequency, intensity, length of fire period, size distribution, and seasonality. All other things being equal, global warming was found to alter the distribution of these pyroregions, with an expansion of the most fire prone pyroregions ranging respectively from 50% to 130% under 2° and 4°C global warming scenarios. Our estimates indicate a strong amplification of fire across parts of southern Europe and a subsequent shift toward new fire regimes, implying substantial socio-ecological impacts in the absence of mitigation or adaptation measures

Les effets du passage d’un feu dans un peuplement arboré : synthèse des connaissances et applications pour le gestionnaire forestier méditerranéen

L’incendie de forêt constitue la première perturbation des écosystèmes forestiers méditerranéens. Il existe de fortes interactions entre l’écosystème, les pratiques sylvicoles et les régimes de feu associés. Cet article constitue une synthèse des connaissances disponibles et les appliquent aux problématiques rencontrées par le gestionnaire forestier méditerranéen

Simulation numérique du couplage surface/canopée lors de la propagation d'un feu de cime en forêt boréale

Cette étude concerne la modélisation numérique de l'ignition et de la propagation d'un feu de cime dans une forêt de type boréale. La formulation mathématique est basée sur une approche multi-phasique, construite à partir des équations de conservation (masse, quantité de mouvement, énergie) gouvernant l'évolution du système couplé formé par la végétation et l'atmosphère ambiant. Le modèle a préalablement été testé à petite échelle, sur des feux de litière homogène et à plus grande échelle, sur des feux de prairie et en garrigue Méditerranéenne. Les résultats ont été comparés avec des données expérimentales de la littérature, et les prédictions des modèles empiriques opérationnels développés en Australie (MkV) et aux Etats Unis (BEHAVE). Le modèle a été étendu à un profil de végétation composé d'une couche arbustive et d'une canopée. Les résultats mettent en évidence le rôle joué par le combustible de surface sur la dynamique générale du feu de cime. Les résultats numériques ont été confrontés aux données collectées à l'occasion de campagnes expérimentales internationales réalisées au Canada

SurEau-Ecos v2.0: a trait-based plant hydraulics model for simulations of plant water status and drought-induced mortality at the ecosystem level

A widespread increase in tree mortality has been observed around the globe, and this trend is likely to continue because of ongoing climate-induced increases in drought frequency and intensity. This raises the need to identify regions and ecosystems that are likely to experience the most frequent and significant damage. We present SurEau-Ecos, a trait-based, plant hydraulic model designed to predict tree desiccation and mortality at scales from stand to region. SurEau-Ecos draws on the general principles of the SurEau model but introduces a simplified representation of plant architecture and alternative numerical schemes. Both additions were made to facilitate model parameterization and large-scale applications. In SurEau-Ecos, the water fluxes from the soil to the atmosphere are represented through two plant organs (a leaf and a stem, which includes the volume of the trunk, roots and branches) as the product of an interface conductance and the difference between water potentials. Each organ is described by its symplasmic and apoplasmic compartments. The dynamics of a plant's water status beyond the point of stomatal closure are explicitly represented via residual transpiration flow, plant cavitation and solicitation of plants' water reservoirs. In addition to the “explicit” numerical scheme of SurEau, we implemented a “semi-implicit” and “implicit” scheme. Both schemes led to a substantial gain in computing time compared to the explicit scheme (&gt;10 000 times), and the implicit scheme was the most accurate. We also observed similar plant water dynamics between SurEau-Ecos and SurEau but slight disparities in infra-daily variations of plant water potentials, which we attributed to the differences in the representation of plant architecture between models. A global model's sensitivity analysis revealed that factors controlling plant desiccation rates differ depending on whether leaf water potential is below or above the point of stomatal closure. Total available water for the plant, leaf area index and the leaf water potential at 50 % stomatal closure mostly drove the time needed to reach stomatal closure. Once stomata are closed, resistance to cavitation, residual cuticular transpiration and plant water stocks mostly determined the time to hydraulic failure. Finally, we illustrated the potential of SurEau-Ecos to simulate regional drought-induced mortality over France. SurEau-Ecos is a promising tool to perform regional-scale predictions of drought-induced hydraulic failure, determine the most vulnerable areas and ecosystems to drying conditions, and assess the dynamics of forest flammability.</p

Effets de la récurrence des incendies sur le comportement du feu dans des suberaies (Quercus suber L.) et maquis méditerranéens sur les cinquante dernières années

International audiencePast fire recurrence impacts the vegetation structure, and it is consequently hypothesized to alter its future fire behaviour. We examined the fire behaviour in shrubland-forest mosaics of southeastern France, which were organized along a range of fire frequency (0 to 3–4 fires along the past 50 years) and had different time intervals between fires. The mosaic was dominated by Quercus suber L. and Erica–Cistus shrubland communities. We described the vegetation structure through measurements of tree height, base of tree crown or shrub layer, mean diameter, cover, plant water content and bulk density. We used the physical model Firetec to simulate the fire behaviour. Fire intensity, fire spread, plant water content and biomass loss varied significantly according to fire recurrence and vegetation structure, mainly linked to the time since the last fire, then the number of fires. These results confirm that past fire recurrence affects future fire behaviour, with multi-layered vegetation (particularly high shrublands) producing more intense fires, contrary to submature Quercus woodlands that have not burnt since 1959 and that are unlikely to reburn. Further simulations, with more vegetation scenes according to shrub and canopy covers, will complete this study in order to discuss the fire propagation risk in heterogeneous vegetation, particularly in the Mediterranean area, with a view to a local management of these ecosystems.La combustibilité de 51 placettes correspondant à différentes récurrences d’incendies depuis 1959 (0 à 4 feux) a été analysée en Provence siliceuse. Les écosystèmes étudiés sont des suberaies dominées par Quercus suber L., et des maquis à Erica arborea L. et à cistes. Les données descriptives de la végétation combustible (composition, biomasse, recouvrement, hauteur) ont été entrées dans un modèle physique du feu (Firetec) afin d’obtenir des variables descriptives du comportement de feu. Les pertes en masse et en eau de la végétation, la vitesse de propagation et l’intensité du feu augmentent de façon significative avec la continuité verticale du combustible, qui dépend principalement du temps depuis le dernier feu, puis du nombre de feux. Les résultats confirment l’hypothèse selon laquelle le feu est particulièrement intense et rapide dans des peuplements multi-stratifiés à forte charge en éléments fins et forte connexion spatiale entre les individus, tels que les maquis hauts développés après un seul feu intense en cinquante ans. Au contraire, le feu ne se propage pas dans les forêts de chênes submatures n’ayant pas brûlé depuis au moins cinquante ans et caractérisées par une faible connectivité verticale entre la canopée et le sous-étage, favorisant ainsi un effet « auto-protecteur » contre les incendies futurs. Ces informations permettent de discuter de la gestion de la végétation, en particulier la surveillance ou le débroussaillement local des maquis hauts grandement combustibles, pour diminuer le risque de propagation des feux à l’échelle de la parcelle

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. © The Author(s) 2012

A Bioeconomic Projection of Climate‐Induced Wildfire Risk in the Forest Sector

International audienceUnder the influence of climate change, wildfire regimes are expected to intensify and expand to new areas, increasing threats to natural and socioeconomic assets. We explore the environmental and economic implications for the forest sector of climate-induced changes in wildfire regimes. To retain genericity while considering local determinants, we focus on the regional level and take Mediterranean France as an example. Coupling a bioeconomic forest sector model and a model of wildfire activity, we perform spatially explicit simulations under various levels of radiative forcing. By using a probabilistic framework, we also assess the propagation of several sources of uncertainty to the forest sector, considering both climate-induced uncertainty and the intrinsic stochasticity of the fire process. By the end of the century, summer burned areas increase by up to 55%, causing moderate losses of merchantable timber and forest carbon stocks, with cascading impacts for industrial activities and climate mitigation in the forest sector. Implications for industries remain limited, but we observe price increases, especially for softwoods, as well as spatially differentiated changes in producer welfare. Inter-annual fluctuations explain most of uncertainty in wildfire activity, but their impacts on the forest sector are quickly dampened. Over time, owing to the cumulative nature of wildfire impacts on forest resources, uncertainty related to climate warming, climate models’ response and stochasticity intrinsic to the wildfire phenomenon strongly increase in relative importance. Results reassert the need to consider multiple futures in prospective assessments, including uncertainty inherent to natural processes, often omitted in large-scale economic assessments.maint

Projections of fire danger under climate change over France: where do the greatest uncertainties lie?

International audienceGlobal warming is expected to increase droughts and heatwaves, and consequently fire danger in southern Europe in the forthcoming decades. However, an assessment of the uncertainties associated with this general trend at regional scales, relevant to decision-making, is still missing. This study aims at assessing potential climate change impacts on fire danger over France through the projection of the widely used Fire Weather Index (FWI) and at quantifying the different sources of climate-driven uncertainty associated with these projections. We used daily climate experiments covering the 1995-2098 period under two scenarios (RCP4.5 and RCP8.5) provided by the EURO-CORDEX initiative. Our results show an overall increase in FWI throughout the century, with the largest absolute increases in the Mediterranean area. Model uncertainty was very high in western France, previously identified as a potential fire-prone region under future climate. In contrast, large increases in FWI in the Mediterranean area showed low uncertainty across models. Besides, analyzing the natural variability of FWI revealed that extreme years under present-day climate could become much more frequent by the end of the century. The FWI is projected to emerge from the background of natural variability by mid-twenty-first century with a summer elevated fire danger three times more likely when summer temperature anomaly exceeds + 2 degrees C