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Aim: Tree mortality is increasing world-wide, leading to changes in forest composi-tion and altering global biodiversity. Nonetheless, owing to the multifaceted stochas-tic nature of tree mortality, large-scale spatial patterns of mortality across species ranges and their underlying drivers remain difficult to understand. Our main goal was to describe the geographical patterns and drivers of the occurrence of mortality (presence of a mortality event) and the intensity of tree mortality (amount of mortal-ity related to that mortality event) in Europe. We hypothesized that the occurrence of mortality represents background mortality and is higher in the margin than in core populations, whereas the intensity of mortality could have a more even distribution according to the spatial and temporal stochasticity of die-off events.Location: Europe (Spain, France, Germany, Belgium, Sweden and Finland).Major taxa studied: More than 1.5 million trees belonging to 20 major forest tree species.Methods: We developed binomial and truncated negative binomial models to tease apart the occurrence and intensity of tree mortality in National Forest Inventory plots at the range-wide scale. The occurrence of mortality indicated that at least one tree had died in the plot, whereas the intensity of mortality referred to the number of dead trees per plot.Results: The highest occurrence of mortality was found in peripheral regions and the climatic trailing edge linked with drought, whereas the intensity of mortality was driven by competition, drought and high temperatures and was scattered uniformly across species ranges.Main conclusions: We show that tree background mortality, but not die-off, is gener-ally higher in the trailing-edge populations. It remains to be explored whether other demographic traits, such as growth, reproduction and regeneration, also decrease at the trailing edge of European tree populations., Université de Bordeaux

A new fire policy reinforcing aggressive fire suppression was established in Mediterranean France in response to the devastating wildfires of the 1990s, but to what extent this has changed fire activity yet remains poorly understood. For this purpose, we compared the number and location of ignitions and of burned areas between two 20-year periods (1975-1994 vs. 1995-2014), in parallel to the changes in fuel covering, human activity promoting ignitions, and fire weather. The number of fires decreased almost continuously since 1975, but sharply after 1994, suggesting an effect of better fire prevention due to the new policy. But the major change in fire activity is a considerable reduction in fire size and burned areas after 1994, especially during summer and in the most fire-prone places, in response to massive efforts put into fire suppression. These reductions have occurred while the covering by fuel biomass, the human pressure on ignition, and the fire weather index increased, thus making the study area more hazardous. Our results suggest that a strategy of aggressive fire suppression has great potential for counterbalancing the effects of climate changes and human activities and for controlling fire activity in the short term. However, we discuss whether such a suppression-oriented approach is sustainable in the context of global changes, which cast new fire challenges as demonstrated by the devastative fires of 2003 and 2016. We advocate for a more comprehensive fire policy to come.

AimBackground tree mortality is a complex demographic process that affects forest structure and long-term dynamics. We aimed to test how drought intensity interacts with interspecific and intraspecific competition (or facilitation) in shaping individual mortality patterns across tree species ranges.LocationEuropean latitudinal gradient (Spain to Finland).Time period1985 – 2014.Major taxa studiedScots pine (Pinus sylvestrisL.) and European beech (Fagus sylvaticaL.).MethodsWe performed logistic regression models based on individual tree mortality recorded in five European National Forest Inventories. We computed the relative importance of climatic drought intensity, basal area of conspecific and heterospecific trees (proxy of indirect intra- and interspecific competition or facilitation) and the effects of their interactions on mortality along the latitudinal gradient of both species range.ResultsIncrease in drought intensity over the study period was associated with higher mortality rates in both species. Drought was the most important driver of beech mortality at almost all latitudes while Scots pine mortality was mainly driven by basal area. High conspecific basal area was associated with high mortality rates in both species while high heterospecific basal area was correlated with mortality rates that were high in Scots pine but low in beech.Main conclusionsBeech mortality was directly affected by drought while Scots pine mortality was indirectly affected by drought through interactions with basal area. Despite their different sensitivity to drought and basal area, the highest predicted mortality rates for both species were at the ecotone between Mediterranean and cool temperate biomes, which can be explained by the combined effect of drought and competition. In the context of global warming, which is expected to be particularly strong in the Mediterranean biome, our results suggest that populations at the southern limit of species ranges may experience increased mortality rates in the near future.BIOSKETCHThe authors’ research is focused on functional trait ecology and global change, with special attention to mortality and demography processes. The authors use modelling multidisciplinary approaches to understand complex processes in ecology on a large geographical scale.

AimTo test whether adaptive and plastic trait responses to climate across species distribution ranges can be untangled using field observations, under the rationale that, in natural forest tree populations, long-term climate shapes local adaptation while recent climate change drives phenotypic plasticity.LocationEurope.Time period1901-2014.TaxaSilver fir (Abies alba Mill.) and sessile oak (Quercus petraea (Matt.) Liebl.).MethodsWe estimated the variation of individual tree height as a function of long-term and short-term climates to tease apart local adaptation, plasticity and their interaction, using mixed-effect models calibrated with National Forest Inventory data (in-situ models). To validate our approach, we tested the ability of in-situ models to predict independently tree height observations in common gardens where local adaptation to climate of populations and their plasticity can be measured and separated. In-situ model predictions of tree height variation among provenances (populations of different geographical origin) and among planting sites were compared to observations in common gardens and to predictions from a similar model calibrated using common garden data (ex-situ model).ResultsIn Q. petraea, we found high correlations between in-situ and ex-situ model predictions of provenance and plasticity effects and their interaction on tree height (r > 0.80). We showed that the in-situ models significantly predicted tree height variation among provenances and sites for Abies alba and Quercus petraea. Spatial predictions of phenotypic plasticity across species distribution ranges indicate decreasing tree height in populations of warmer climates in response to recent anthropogenic climate warming.Main conclusionsOur modelling approach using National Forest Inventory observations provides a new perspective for understanding patterns of intraspecific trait variation across species ranges. Its application is particularly interesting for species for which common garden experiments do not exist or do not cover the entire climatic range of the species.

Raw data can be freely accessed online for French National Forest Inventories (http://inventaire-forestier.ign.fr), Quercus petraea (https://arachne.pierroton.inra.fr/QuercusPortal) and Abies alba common gardens (online repository, in progress). Climate data used for this study are available online (http://gentree.data.inra.fr/climate). R codes used for data analyses can be obtained from the corresponding author upon request.; International audience; Aim To test whether intraspecific trait responses to climate among populations across species distribution ranges can be untangled using field observations, under the rationale that, in natural forest tree populations, long-term climate shapes population responses while recent climate change drives phenotypic plasticity. Location Europe. Time period 1901-2014. Taxa Silver fir (Abies alba Mill.) and sessile oak [Quercus petraea (Matt.) Liebl.]. Methods We estimated the variation of individual tree height as a function of long-term and short-term climates to tease apart provenance effects (variation among populations of different geographical origin), plasticity and their interaction, using mixed-effect models calibrated with national forest inventory data (in-situ models). To validate our approach, we tested the ability of in-situ models to predict independently tree height observations in common gardens experiments where provenance and plastic effects can be measured and separated. In-situ model predictions of tree height variation among provenances and among planting sites were compared to observations in common gardens and to predictions from a similar model calibrated using common garden data (ex-situ model). Results In Q. petraea, we found high correlations between in-situ and ex-situ model predictions of provenance and plasticity effects and their interaction for tree height (r > .80). We showed that the in-situ models significantly predicted tree height variation among provenances and sites for A. alba and Q. petraea. Spatial predictions of phenotypic plasticity across species distribution ranges indicate decreasing tree height in populations of warmer climates in response to recent anthropogenic climate warming. Main conclusions Our modelling approach using national forest inventory observations provides a new perspective for understanding patterns of intraspecific trait variation across species ranges. Its application is particularly interesting for species for which common garden experiments do not exist or do not cover the entire climatic range of the species.

International audience; How populations of long-living species respond to climate change depends on phenotypic plasticity and local adaptation processes. Marginal populations are expected to have lags in adaptation (i.e. differences between the climatic optimum that maximizes population fitness and the local climate) because they receive pre-adapted alleles from core populations preventing them from reaching a local optimum in their climatically marginal habitat. Yet, whether adaptation lags in marginal populations are a common feature across phylogenetically and ecologically different species and how lags can change with climate change remain unexplored. To test for range-wide patterns of phenotypic variation and adaptation lags of populations to climate, we (a) built model ensembles of tree height accounting for the climate of population origin and the climate of the site for 706 populations monitored in 97 common garden experiments covering the range of six European forest tree species; (b) estimated populations' adaptation lags as the differences between the climatic optimum that maximizes tree height and the climate of the origin of each population; (c) identified adaptation lag patterns for populations coming from the warm/dry and cold/wet margins and from the distribution core of each species range. We found that (a) phenotypic variation is driven by either temperature or precipitation; (b) adaptation lags are consistently higher in climatic margin populations (cold/warm, dry/wet) than in core populations; (c) predictions for future warmer climates suggest adaptation lags would decrease in cold margin populations, slightly increasing tree height, while adaptation lags would increase in core and warm margin populations, sharply decreasing tree height. Our results suggest that warm margin populations are the most vulnerable to climate change, but understanding how these populations can cope with future climates depend on whether other fitness-related traits could show similar adaptation lag patterns.

Forest fires are expected to be more frequent and more intense with climate change, including in temperate and mountain forest ecosystems. In the Alps, forest vulnerability to fire resulting from interactions between climate, fuel types, vegetation structure and tree resistance to fire is little understood. This paper aims at identifying trends in the vulnerability of Alpine forest ecosystems to fire at different scales (tree species, stand level and biogeographic level) and according to three different climatic conditions (cold season, average summer and extremely dry summer). To explore Alpine forest vulnerability to fire, we used surface fuel measurements, forest inventory and fire weather data to simulate fire behaviour and ultimately post-fire tree mortality across 4438 forest plots in the French Alps. The results showed that cold season fires (about 50% of the fires in the French Alps) have a limited impact except on low-elevation forests of the Southern Alps (mainly Oak, Scots pine). In average summer conditions, mixed and broadleaved forests of low elevations suffer the highest mortality rates (up to 75% in coppices). Finally, summer fires occurring in extremely dry conditions promote high mortality across all forest communities. Lowest mortality rates were observed in high forest stands composed of tree species presenting adaptation to surface fires (e.g. thick bark, high canopy) such as Larch forests of the internal Alps. This study provides insights on the vulnerability of the main tree species and forest ecosystems of the French Alps useful for the adaptation of forest management practices to climate changes.

International audience; Aim: Potential fire intensity (PFI) is among the main drivers of fire–vegetation inter- actions, but how it varies across species ranges is unknown. We test whether PFI and crown fire likelihood (CFL) increase preferentially towards the warm/dry range margins of mountain trees. Location: The western Alps. Methods: We analysed PFI and CFL patterns across the environmental niches of major mountain forest tree species, using field observation data of surface fuels with fire simulations. Empirical relationships between PFI and climate and vegetation structure were identified in communities sampled for surface fuels, and then used to predict PFI in national forest inventory plots. Results: Simulations indicate that autumn drought, spring precipitation, tree cover and tree basal area drove the variation in understorey fuels among communities, leading to different PFI among and within the environmental ranges of mountain tree species. PFI was consistently higher in the open-canopy, dry margins of domi- nant tree species. The highest PFI values were found in the sub-Mediterranean and subalpine forests in conjunction with higher CFL. Main conclusions: Mountain trees are exposed to higher PFI and CFL at their dry range margins, suggesting higher potential fire impacts at their rear distribution edges. Moist montane species (Fagus sylvatica, Abies alba, Picea abies) are exposed to surface fires of lower intensities than sub-Mediterranean (Quercus pubescens, Pinus nigra, P. sylvestris) and subalpine species (Larix decidua, P. uncinata, P. cembra) which may both experience higher likelihood of crown fires at their dry and flam- mable margins.

International audience; Fire resistance traits drive tree species composition in surface‐fire ecosystems, but how they covary at different scales of variation and with the environment is not well documented. We assessed the covariation of bark thickness (BT), tree height, and crown base‐to‐height ratio across Alpine forests, after accounting for the effects of tree diameter and competition for light on individual trait variation. Traits consistently correlated across individuals and communities, although the variance of BT mainly occurred among species, whereas crown elevation traits varied mainly within species. Aridity, temperature, and competition contributed to explain the variation of fire resistance traits among and within species, driving a trade‐off between fire resistance and the ability to compete for light. Thick‐barked species (fire‐tolerant) that self‐prune their lower branches (flame‐avoiders) dominated the most fire‐prone and flammable communities in sub‐Mediterranean southern Alps, whereas thin‐barked tree species that grow tall (competition for light) dominated the least fire‐prone communities in the northern Alps. Our findings suggest a long‐term interaction between mountain tree species and fire regime. Higher allocation to trunk elongation occurs in moist and shade environments, while higher allocation to thicken the bark and distancing the crown base from surface fuels occurs in open‐canopy, dry forests where fire spreads with higher intensity.

International audience; Wildfire containment is often very challenging for firefighters, especially for large and rapidly spreading fires where the risk of firefighter entrapment is high. However, the conditions leading to these 'dangerous' fires are poorly understood in Mediterranean Europe. Here, we analyse reports and interviews of firefighters over the last 40 years in four regions of south-eastern France and investigate the weather conditions that induce large fires, fast-growing fires and fires that are conducive to entrapment. We adopt a quantile regression model to test the effect of weather conditions across different fire sizes and growth rates. The results show that strong winds drive the largest fires everywhere except in Corsica, the southernmost region, where high temperature is the main driver. Strong winds also drive entrapments whereas high temperatures induce rapidly spreading fires. This emphasises that wind-driven fire is the dominant pattern of dangerous fires in France, but it reveals that large 'convective' fires can also present considerable danger. Beyond that, the Fire Weather Index appears to be a good predictor of large fires and fires conducive to entrapments. Identifying weather conditions that drive 'dangerous' wildfires will provide useful information for fire agencies to better prepare for adverse fire behaviours.

Subalpine ecosystems are mainly affected by surface fires of low to medium intensity. Charcoal production from trees is limited by the duration and the intensity of fires and, thus, by the wood combustibility. Because the combustibility properties of species differ, the functional traits that determine the wood combustibility could explain in part the taxonomic differences of charcoal abundance within the paleo-soil charcoal (pedoanthracological) assemblages. Eight subalpine broadleaf and needleleaf tree species were considered. Wood combustibility was studied by experimental burning on barked-wood samples exposed to constant heat. Two combustibility parameters are estimated (i) the rate of mass loss rate (MLR) and, (ii) the burning rate (BR). While BR describes the proportion of burned biomass per time unit assessing the potential conversion of wood biomass into charcoal, MLR reports the speed of fuel combustion that estimates the potential amount of charcoal produced. A hierarchy of species-specific potential production of charcoal by trees is developed by the use of biplot distribution of MLR and BR. Species were ranked from the worst to the best biomass for the charcoal conversion: Larix decidua , Abies alba , Pinus uncinata , Picea abies , Betula pendula , Salix caprea , Pinus cembra , Sorbus aucuparia . With thin smooth bark and dense wood, broadleaf trees are expected to produce more charcoal than needleleafs. Bark thickness and wood density explain half of combustibility variance. Thus, the production of charcoal tends to vary among species and among individuals of different diameter according to bark thickness. The amount of charcoal is a proxy of both (i) the abundance of understorey-tree (sapling) due to the high combustion susceptibility of small diameter stems (thin and smooth bark), and (ii) the abundance of woody debris on the ground produced by the canopy trees. The results are discussed in light of simulated data from Holocene soil charcoal assemblages for the western subalpine Alps. Among needleleafs, L. decidua is the least productive in terms of charcoal abundance due to its low MLR and BR values, whereas P. cembra and broadleafs should be better represented in the soil charcoal assemblages. This study provides rational elements for the quantitative calibration of soil charcoal through species-specific charcoal mass coefficients.

International audience; A good knowledge of the spatiotemporal patterns of the causes of wildfire ignition is crucial to an efficient fire policy. However, little is known about the situation in southeastern France because the fire database contains unreliable data. We used data for cases with well-established causes from 1973 to 2013 to determine the location of spatial hotspots, the seasonal distribution, the underlying anthropogenic and environmental drivers, and the tendency of five main causes to generate large fires. Anthropogenic ignitions were predominant (88%) near human settlements and infrastructures in the lowlands, whilst lightning-induced fires were more common in the coastal mountains. In densely-populated, urban areas, small summer fires predominate because of the negligence of private individuals around their homes, or accidental ignitions near infrastructures. In rural hinterlands, ignitions due to negligence by professionals generate many medium-sized fires from fall to spring. Intentional and accidental ignitions contribute the most to the total burned area and to large fires. We conclude that socioeconomic factors partially control the fire regime, influencing the timing, spatial distribution and potential size of fires. This improved understanding of why, where and when ignitions occur provides the opportunity for controlling certain causes of ignitions, and adapting French policy to global changes

International audience; Little is known about the understorey flammability of European mountain forests. The aim of this study was to determine the relative effects of climate, vegetation structure and composition on the fuel-driven variation in fire spread and intensity. Fire spread and intensity were simulated under constant moisture and weather conditions for a wide range of understorey fuel parameters measured in the litter, grass and shrub layers. Simulation outputs were used to compare understorey flammability between different forest ecosystem types (FET). The FETs were characterized by using a co-inertia analysis between composition and the environment (vegetation structure and climate). The relationships between these factors, fuel properties and understorey flammability were then tested using partial regression analyses.Results The most flammable forests displayed an open canopy (dry-subalpine and open-mediterranean) and grew in areas with dry autumns and wet and cold springs. Fire spread and intensity were controlled by the trade-off between tree cover and dead (litter) and live (grass and shrub) biomass load. Fire intensity also increased as a result of seasonal precipitation patterns (differential distribution between the seasons): rainy springs enhanced biomass growth, whereas dry climates, especially in autumn, promoted shrub biomass and stimulated litter accumulation and residence (higher litterfall and lower decomposition rates). Interestingly, we found a positive relationship between fire intensity and the proportion of conifers that disappeared after accounting for tree cover, indicating that, in the Alps, the open canopy structure of needle-leaved forests makes them potentially more flammable than broad-leaved forests because of the higher load and continuity of surface fuels. Main conclusions Inter-relationships between tree cover, precipitation seasonality and species composition govern the understorey flammability of mountain forests. We also found evidence that tree cover strongly constrains fire spread by driving the amount and type of surface fuel, which suggests that land use change can have a strong influence on flammability patterns .; On connaît peu l'inflammabilité du sous-étage dans les forêts de montagne européennes. Le but de cette étude était de déterminer les effets relatifs du climat, de la structure de la végétation et de la composition du combustible sur la propagation et l'intensité du feu. La propagation et l'intensité des incendies ont été simulés sous humidité constante et les conditions météorologiques pour une large gamme de paramètres de combustible (litière, herbe et arbustes). Les sorties de simulation ont été utilisés pour comparer entre les sous-bois d'inflammabilité types d'écosystèmes forestiers différents (FET). Les FET ont été caractérisés en utilisant une analyse de co-inertie entre la composition et l'environnement (structure de la végétation et le climat). Les relations entre ces facteurs, les propriétés du carburant et sous-bois inflammabilité ont ensuite été testées en utilisant des analyses de régression partielle. Les forêts les plus inflammables affichent un sous-bois clair et poussent dans les zones avec des automnes secs et humides et des printemps frais. La propagation et l'intensité des incendies ont été contrôlés par le compromis entre couvert d'arbres et de morts (litière) et en direct (herbe et arbuste) charge de la biomasse. L'intensité du feu a augmenté en raison de la configuration des précipitations saisonnières (répartition différentielle entre les saisons): printemps pluvieux amélioré la croissance de la biomasse, tandis que les climats secs, surtout en automne, favorisent la biomasse d'arbustes, l'accumulation de la litière et sont temps de résidence (litière plus élevée et la décomposition inférieure les taux). Nous avons trouvé une relation positive entre l'intensité du feu et la proportion de conifères qui a disparu après comptabilisation de la couverture forestière, ce qui indique que, dans les Alpes, la structure des forêts conifères les rend potentiellement plus inflammables que les forêts feuillues en raison de la charge plus élevée et la continuité des combustibles de surface. Les inter-relations entre le couvert forestier, les précipitations saisonnières et la composition des espèces régissent l'inflammabilité du sous-bois des forêts de montagne. Nous avons également constaté que la couverture forestière contrôle fortement le feu par la quantité et le type de combustible de surface, ce qui donne à penser que l'utilisation des terres peut avoir une forte influence sur les modèles d'inflammabilité.

International audience; The impacts of climate change on fires are expected to be highly variable spatially and temporally. In heavily anthropized landscapes, the great number of factors affecting fire regimes further limits our ability to predict future fire activity caused by climate. To address this, we develop a new framework for analysing regional changes in fire regimes from specific spatio-temporal patterns of fires and climate, so-called pyroclimates. We aim to test the trends of fire activity and climate (1973–2009) across the Mediterranean and mountain ecosystems of south-eastern France, and to define the spatial distribution of pyroclimates. Stepwise-PCA and cluster analyses reveal that three pyroclimates capture the spatio-temporal patterns associated with fire regime and climatic conditions. Trend tests indicate a high significant increase in spring temperature and fire weather severity for most of the study area. In contrast, a general decreasing pattern of fire activity is observed since the early 1990s, specifically during summer in historically burned regions. However, winter and spring fires are becoming more frequent and extensive in less fire-prone mountains. Cross-correlation analyses indicate that inter-annual variations in extreme fire weather and fire activity were highly correlated. However, the intensity of relationships is pyroclimate-dependent. Our findings reveal that fire-climate relationships changed rapidly over space and time, presumably according to regional changes in land-use and fire policy. Assessing pyroclimates offers new perspectives for fire management and policy by delineating homogeneous zones with respect to fire, climate and their recent trends, and by revealing geographic contrasts in the underlying fire drivers.

Assessment of biodiversity in a changing world is a key issue and studies on the processes and factors influencing its history at relevant time scales are needed. In this study, we analyzed temporal trends of plant diversity using fossil pollen records from the North American boreal forest-taiga biome (NABT). We selected 205 pollen records spanning the last 15,500 years. Diversity was decomposed into α and γ richness, and β diversity, using Shannon entropy indices. We investigated temporal and spatial patterns of β diversity by decomposing it into independent turnover (variation in taxonomic composition due to species replacements) and nestedness (variation due to species loss) components. The palynological diversity of the NABT biome experienced major rearrangements during the Lateglacial and early Holocene in response to major climatic shifts. The β nestedness likely reflected plant immigration processes and generally peaked before the β turnover value, which mirrors spatial and temporal community sorting related to environmental conditions and specific habitat constraints. Palynological diversity was generally maximal during the Lateglacial and the early Holocene and decreased progressively during the Holocene. These results are discussed according to macro-ecological processes, such as immigration, disturbances, and environmental fluctuations, with climate most notably as the main ecological driver at millennial scales.

Res_eng:Fire risk is expected to increase in the Mediterranean Basin like in many areas worldwide. Climate is likely the main driver of fire activity by conditioning fuel dryness and fire weather. However fire-climate relationships are conditioned by other environmental dimensions like fuel structure (composition and spatial arrangement of flammable vegetation) and human activities. Therefore assessing how climate controls fire activity in heavily anthropized landscapes like European Mediterranean regions requires to design analyses in appropriate geographic units to encompass the dominant fire drivers. We aimed to assess how spatiotemporal patterns of both fire activity and climate structured south-eastern France into homogeneous geographic unit which we defined as ‘pyroclimatic’ regions. We performed a pyroclimatic classification of Mediterranean and mountain areas of south-eastern France at 2 km resolution from the national fire database and daily atmospheric parameters over 1973-2009. This classification was based on multidimensional and clustering analyses. South-eastern France is characterized by three main ‘pyroclimatic’ regions and ten sub-regions from high fire-prone maritime mountains to moderate fire-prone hot lowlands and low fire-prone inner moist mountains. These geographic units are discriminated from each other by fire activity, fire seasonality, fire weather and their recent evolution. We demonstrated that fire activity and fire weather of south-eastern France are highly dynamic in space and time. Characterizing pyroclimatic regions offers new regional perspectives of fire management and policy because two areas having similar fire weather, fire regime and recent temporal trends would benefit for specific attention.

International audience; Relationships between the flammability properties of a given plant and its chances of survival after a fire still remain unknown. We hypothesize that the bark flammability of a tree reduces the potential for tree survival following surface fires, and that if tree resistance to fire is provided by a thick insulating bark, the latter must be few flammable. We tests, on subalpine tree species, the relationship between the flammability of bark and its insulating ability, identifies the biological traits that determine bark flammability, and assesses their relative susceptibility to surface fires from their bark properties. The experimental set of burning properties was analysed by Principal Component Analysis to assess the bark flammability. Bark insulating ability was expressed by the critical time to cambium kill computed from bark thickness. Log-linear regressions indicated that bark flammability varies with the bark thickness and the density of wood under bark and that the most flammable barks have poor insulating ability. Susceptibility to surface fires increases from gymnosperm to angiosperm subalpine trees. The co-dominant subalpine species Larix decidua (Mill.) and Pinus cembra (L.) exhibit large differences in both flammability and insulating ability of the bark that should partly explain their contrasted responses to fires in the past