La dégradation du bois dans les processus biogéomorphologique sur versant forestier

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVINInternational audienceIn mountain, forest can play a protective role against natural hazard such as rockfalls, avalanches, shallow landslides, debris flow and erosion. Complementary with the classical researches on the dissipate effect of standing trees, the protective capacity of wood slash are increasingly taken into account in the European protection forest. Slash are the results of logging, and are composed by stumps and logs. Slash can be a real barrier, if they are strategically positioned in the corridors subject to rockfalls or avalanches. Thus, after logging, slash can maintain or increase the roughness on the ground, and slow down or stop the gravitational phenomena. However, wood slash is a living material and it is subjected to decomposition process. After a cut and due to the wood decay, mass movement (e.g. erosion or shallow landslide) can be then reactivated. Loggings have different consequences on the biogeomorphological processes over time. The initial objective of our study is to analyze the decomposition and mechanical resistance loss of slash for the protection against rockfalls or avalanches over time. A chronosequence study has been established for three main species in European Alps: Pinus nigra, Fagus sylvatica and Picea abies. Several methods were used to assess wood density loss as an indicator: classical method for dry density measurement, resistograph, pilodyn, and spectrometry. All the results are presented via the calculation of different indexes of density decay. These results have been compared to crash test realized in laboratory to simulate a rock impact. To do a parallel with other topics as erosion or shallow landslide, we will also present the assessment of sediment production after clear-cutting in the black marls of the southern French Alps

Arbres et chutes de pierres : l'utilisation des produits de coupes comme structures de protection pare-pierres sur versants forestiers

International audienceIn mountainous regions forested slopes play an important protective role against rockfalls. Complementary with the classical researches on the dissipative effect of standing and living trees, this research program studies the protective capacity of tree stumps and log against snow avalanches and rockfalls. In Austria, the felling technique Alpi has been developed. It allows a specialised lumberjack to create small rockfall barriers using one or two tree stems anchored on high tree stumps. Lying tree stems can be then used to increase efficiently the roughness of the soil and so to limit or avoid triggering and propagation falling rocks. But, due to the wood decay, the efficiency of such protective works is decreasing with time. The main objectives of this research program are to quantify the efficiency of rockfall protection structures depending on their characteristics (stump and stems density, position on the slope, tree species, etc.), and to evaluate their resistance over time taking into account the decomposition of wood. This study is part of the European Interreg IV Alpine Space project "MANFRED".En région de montagne, les versants forestiers jouent un rôle de protection contre les chutes de pierres. En complément des recherches traditionnelles sur l'effet dissipatif des arbres sur pieds et vivant, ce programme de recherche étudie la capacité protectrice des souches et des troncs au sol contre les avalanches et les chutes de pierres. En Autriche, la technique de coupe Alpi a été développée, elle permet à un bûcheron spécialisé de créer des barrières pare-pierres à l'aide d'un ou deux troncs d'arbres, ancré sur des souches hautes. Les troncs d'arbres au sol peuvent être utilisés pour augmenter efficacement la rugosité au sol et ainsi limiter ou éviter le déclenchement et la propagation des chutes de pierres. Mais, en raison de la décomposition du bois, l'efficacité de ces structures de protection diminue avec le temps. Les principaux objectifs de ce programme de recherche sont de quantifier l'efficacité de ces structures pare pierres en fonction de leurs caractéristiques (densité des souches et des troncs, position sur la pente, types d'essences, etc.), et d'évaluer leur résistance au cours du temps en tenant compte de la décomposition du bois. Cette étude s'inscrit dans le cadre du projet européen Interreg IV Espace Alpin "Manfred"

Ouvrages de protection en bois contre l'activité de chutes de blocs dans les Alpes françaises du Sud

International audienceRockfall risk is one of the most present risks in mountain regions. A forest management intervention can increase the protection role played by mountain forests. Many structural protection structures making with fresh wood are present in the Alps to protect human against rockfalls activities. Since traditionally forest managers are using dead wood, in particular slash wood. Dead woods used are mainly the timbers produced by thinning or clear-cutting harvesting. These structures have been principally developed in Austria at the end of last century. Today, use of slash as protection is present across all the alpine space

Considération des débris ligneux grossiers comme protection contre les chutes de blocs dans les forêts de montagne

International audienceEach year in mountains, natural hazards generate extensive damage and even sometimes casualties. For rockfalls, forests can play an important role of passive protection by increasing the energy dissipation of falling rocks. Up to now, many studies have shown the efficiency of forests standing against rockfalls, but these studies are only considering living trees. In Austria and Switzerland, managers are using stumps, logs and other timber products to create natural dams against rockfalls since twenty years ago. Today, French foresters have adopted these techniques, but there is still a lack of knowledge on the efficiency and the lifetime of such protective structures. The main objectives of this research are to quantify the mechanical resistance loss according to the wood decay kinematics. The materials studied include logs, stumps and roots. Preliminary works have been done in order to select experimental plots in the Alpine Space. The species studied are: silver fir, norway spruce, european beech and pinus nigra. The wood decay rates are quantified by using a chronosequence approach. Different methods are used to quantify the wood density loss. A specific protocol, based on both non-destructive and destructive tests, has been built for analyzing the loss of mechanical resistance of these woody elements. The final objective is to integrate, in rockfall trajectory simulation models in 3D, the actions of this biological engineering protective works taking into account their efficiency evolution within their lifetime

L'utilisation des cernes de croissance des arbres pour l'étude des événements et des changements morphologiques : intérêts, méthodes et apports des recherches alpines à la dendrogéomorphologie

National audienceDendrogeomorphology relies on the capacity of ligneous vegetation to react to external stresses induced by changes in the environment and to register these impacts in the form of either characteristic annual rings or morphological anomalies. By analyzing the radial growth of trees (in the form of one or several abnormally narrow or wide tree rings), their morphology (scars, resumption of apical growth, change of growth axis, tilting, adventitious roots) and their associated anomalies (formation of reaction wood, tangential rows of traumatic resin ducts, modification of the cell structure), past events such as debris flows, landslides, rockfalls, snow avalanches and fluvial dynamics can be dated with yearly or even monthly precision. In turn, dating these events allows for the assessment of environmental changes and geomorphic processes on a spatial scale. This document presents the main dendrogeomorphic methods, starting with the Event-Change/Impact/Response concept, and examines the model’s evolution, especially in the Alps. The article also discusses the model’s relevance for the study of geomorphic processes with respect to the knowledge and management of natural hazards and risks.La dendrogéomorphologie repose sur la capacité des végétaux ligneux à réagir à des stress exogènes, liés aux perturbations ou aux changements qui affectent son milieu de croissance, et sur l'enregistrement de ces réponses dans les caractéristiques des cernes annuels de bois ou dans des anomalies morphologiques visibles. L'analyse de la croissance radiale des arbres (cerne ou série de cernes anormalement étroits ou larges), de leur morphologie (cicatrice, reprise de croissance apicale, changement d’axe, courbure, racine adventive) et de leur anatomie (formation de bois de réaction, de rangées tangentielles de canaux résinifères traumatiques, modifications de la structure cellulaire) permet de dater les événements passés (lave torrentielle, glissement de terrain, chute de pierres, avalanche, crue, tempête, intervention humaine, etc.) avec une résolution temporelle annuelle à mensuelle, de reconstituer les changements environnementaux et de spatialiser et identifier les processus géomorphologiques. Ce texte présente les principes de l'approche basée sur le concept d'"Événement-Changement/Impact/Réponse", ses développements récents, tout particulièrement dans les Alpes, et son intérêt pour l'étude des processus géomorphologiques en vue de la connaissance et de la gestion des risques naturels

Optimisation de la gestion des forêts à fonction de protection contre les chutes de pierres par l’utilisation des rémanents

National audienceIn mountainous regions, forests play an important protective role against rockfalls. Complementary with the classical researches on the dissipative effect of standing and living trees, this research studies the protection efficiency given by wood slash against snow avalanches and rockfalls. Stumps and lying stems can be used to increase efficiently the soil roughness and so limit or avoid triggering and propagation of falling rocks. However, due to the wood decay, the efficiency of such protective works is decreasing with time. The main experiments to estimate this protection are presented.En région de montagne, les forêts sur les versants jouent un rôle de protection contre les chutes de pierres. En complément des recherches traditionnelles sur l’effet dissipatif des arbres sur pied et vivants, cette recherche étudie l’efficacité de protection offerte par les rémanents contre les avalanches et les chutes de pierres. Les souches et les troncs d’arbres au sol peuvent être utilisés pour augmenter efficacement la rugosité au sol et ainsi limiter ou éviter le déclenchement et la propagation des chutes de pierres. Mais, en raison de la décomposition du bois, l’efficacité de ces structures de protection diminue avec le temps. Les principales expérimentations pour évaluer cette protection sont présentées

Réponse des processus processus biogéomorphologique sur les versants de montagne : deux exemples d'applications de coupes forestières dans les Alpes du Sud française

International audienceRockfall risk is one of the most present risks in mountain regions. A forest management intervention can increase the protection role played by mountain forests (Bigot et al., 2009). Traditionally forest managers are using dead wood, in particular slash wood, against rockfall in mountain forest. Dead wood is an important structural and functional component of forest ecosystem (Harmon, 1986). The using materials are high stumps and logs. The “dead woods” used are mainly the timbers produced by thinning or clear-cutting harvesting. These structures have been principally developed in Austria at the end of last century. Today, use of slash as protection is present across all the alpine space. However, up to now there is no scientific knowledge on the efficiency of protection in time. Wood is a living material and it is subject to decomposition of these physical and chemical components. For dead wood, annual temperature is a main driver of decomposition. Initial wood density and diameter are also important drivers. Decomposition of dead wood is generally expressed as a decay constant k which is made up of annual mass or density losses due to respiration, leaching, and fragmentation(Garrett et al., 2007). Climatic factors and forest stands may vary greatly in mountain. Thus, the decomposition of wood is a complex process to study. A chronosequence study was established to evaluate this efficiency of protection in space and time. The main objective is to develop a first approach to study the loss of mechanical properties of slashes according to their decay rates. The experimental areas have been selected based on knowledge of harvesting years. For sampling, slashes were selected with different time since death (0, 5, 10, 15, etc., N-1years). The study areas are conducted in France, Switzerland and Austria. The mains species presented in the Alpine Arc are being sampled: Spruce, European beech, pine for Northern Alps and black pine for Southern Alps. Changes in the mass of logs and density of roots and stumps are used to estimate the decay rate constants. Multiple approaches are used to determine the decomposition of wood as an index. (i) Wood density is defined as the oven dry weight determined on disc sample or disc subsamples of the large sample, where samples are highly decomposed. (ii) Wood chemical properties loss (lignin and cellulose analysis) is determined by NIRS (Near Infrared Spectroscopy) after grinding of drying samples. (iii) Relative density is measured by drilling resistance and use of resistograph on the fields. (iiii) Penetration depth is determined directly by two penetrometers (pilodyn and nail gun)on the field. Later, these decay indexes will be validated and correlate by mechanic and destructive test within field works and laboratory. (i) Winching test to quantify root anchorage of stump. (ii) Real size experiments of rockfall to quantify the energy due for stumps and logs failures at experimental area Vaujany, France. (iii) Impact bending test with Charpy method to measure the resistance of wood material. The different approaches will be presented with the best ones retained for foresters’ practical uses and fieldworks. For the study areas, the predominant factor of decomposition of wood will be identified. Decay rates will be correlated with density and dbh (diameter at breast high) of each species. NIRS will conduct to investigate relationships between decomposition and chemical composition. A first correlation will be established between decay rate index and mechanical resistance of slash wood. Finally, we will present the results on the lifetime per species of these protective structures. This paper proposed to introduce practical methods for field works dedicated to the assessment of rockfall protection structures made with slash woods. This research program is part of the European Interreg IV Alpine Space project “MANFRED”

Dégradation du bois du pin noir d'Autriche après coupe dans les Alpes su sud française

International audienceForest management has different impacts on mountain slope. Forests play an important role of protection against natural hazards such as rockfalls, avalanches, soil erosion and floods. Timber harvesting, whether intended or not, can have dramatic geomorphic or ecological consequences. The result is some slash or dead wood on the slope surface. From slashings, protection structures against rockfalls or avalanches can be built. However, slashings decayed in time and the protection decreased. At the same time, on forested slopes prone to erosion or shallow landslide, wood decay and erosion or mass movement increase. This paper is focused on the wood decay process. The main objective is to determine decay rate of Austria black pine (Pinus nigra) under Mediterranean mountain climate, unknown today. A chronosequence has been established in the Saint-Genis forest in the Drôme valley, Southern French Prealps. A sum of 93 stumps and 134 coarse woody roots (> 2 cm in diameter) has been sampled within the study sites on many stumps cut between 1992 and 2011. Three methods of sampling have been employed to characterize decay rate indicators: (i) basic dry density obtained by a classical method, (ii) drilling resistance measured with a resistograph, and (iii) penetrating resistance obtained with a pilodyn. Data showed an exponential curve with a decay rate of 0.073 for stump, and 0.033 for roots. The time necessary for loosing 50% and 90% of the initial density for stumps and roots were 9 and 21 years, and 41 and 91 years, respectively. Logistic fits gave better statistical fits for both drilling and penetrating resistance data. Two practical applications of these results are presented. The first is to show the rockfall bioprotection structures life time efficiency through the example of slashings (high stumps) decay. The second application concerns the timing of the slope erosion response to planned deforestation, a practice used in the Drôme River basin to replenish incised river channels with sediment. Regarding the decay rate, the life time of the protection structures would be optimum between 5 to 10 years. The remobilization of sediment would be optimum in the first 5 years following the deforestation. Then, the sediment production tends to stabilize because of spontaneous regeneration. This research based on wood decay allow us to improve the knowledge on decomposition of Pinus nigra in Southern Alps, and confirm effects of logging on protection forest and sediment supply from small ravines over time

Detailed chronology of mid-altitude fluvial system response to changing climate and societies at the end of the Little Ice Age (Southwestern Alps and Cévennes, France)

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