4 research outputs found
DĂ©veloppement dâun modĂšle de dynamique forestiĂšre Ă grande Ă©chelle pour simuler les forĂȘts françaises dans un contexte non-stationnaire
Context. Since the industrial revolution, European forests have shown expansion of their area and growing stock. This expansion, together with climate change, drive changes in the processes of forest dynamic. The emergence of a European bioeconomy strategy suggests new developments of forest management strategies at European and national levels. Simulating future forest resources and their management with large-scale models is therefore essential to provide strategic planning support tools. In France, forest resources show high diversity as compared with other European countries' forests. The MARGOT forest dynamic model (MAtrix model of forest Resource Growth and dynamics On the Territory scale), was developed by the national forest inventory (IFN) in 1993 to simulate French forest resources from data of this inventory, but has been the subject of restricted developments, and simulations remain limited to a time horizon shorter than 30 years, under âbusiness as usualâ management scenarios, and not taking into account non-stationary forest and environmental contexts.Aims. The general ambition of this thesis was to consent a significant development effort on MARGOT model, in order to tackle current forestry issues. The specific objectives were: i) to assess the capacity of MARGOT to describe French forest expansion over a long retrospective period (1971-2016), ii) to take into account the heterogeneity of forests at large-scale in a holistic way, iii) to account for the impacts of forest densification in demographic dynamic processes, iv) to encompass external climatic forcing in forest growth, v) in a very uncertain context, to be able to quantify NFI sampling uncertainty in model parameters and simulations with respect to the magnitude of other trends considered. The development of forest management scenarios remained outside the scope of this work.Main results. A generic method for forest partitioning according to their geographic and compositional heterogeneity has been implemented. This method is intended to be applied to other European forest contexts. A method of propagating sampling uncertainty to model parameters and simulations has been developed from data resampling and error modelling approaches. An original approach to integrating density-dependence in demographic processes has been developed, based on a density metric and the reintroduction of forest stand entities adapted to the model. A strategy for integrating climate forcing of model demographic parameters was developed based on an input-output coupling approach with the process-based model CASTANEA, for a subset of French forests including oak, beech, Norway spruce, and Scots pine forests. All of these developments significantly reduced the prediction bias of the initial model.Conclusions. These developments make MARGOT a much more reliable forest resource assessment tool, and are based on an original modeling approach that is unique in Europe. The use of ancient forest statistics will make it possible to evaluate the model and simulate the carbon stock of French forests over a longer time horizon (over 100 years). Intensive simulations to assess the performance of this new model must be done.Contexte. Depuis la rĂ©volution industrielle, les forĂȘts europĂ©ennes connaissent une dynamique dâexpansion de leur surface et de leur stock de bois. Cette expansion, conjuguĂ©e au changement climatique, entraĂźne des modifications des processus de dynamique forestiĂšre. LâĂ©mergence de la bioĂ©conomie europĂ©enne augure dans ce contexte dâĂ©volutions des stratĂ©gies de gestion forestiĂšre Ă lâĂ©chelle europĂ©enne et nationale. La simulation des ressources forestiĂšres futures et de leur pilotage par des modĂšles Ă grande Ă©chelle spatiale est donc indispensable pour fournir des outils de planification stratĂ©gique. En France, les ressources forestiĂšres se caractĂ©risent par une diversitĂ© marquĂ©e par rapport Ă dâautres pays europĂ©ens. Le modĂšle de dynamique forestiĂšre MARGOT (MAtrix model of forest Resource Growth and dynamics On the Territory scale), a Ă©tĂ© mis en place par lâinventaire forestier national (IFN) en 1993 pour simuler les ressources forestiĂšres françaises Ă partir des donnĂ©es de cet inventaire, mais nâa Ă©tĂ© lâobjet que de travaux de recherche restreints depuis son origine. Ses simulations restent limitĂ©es Ă un horizon temporel de moyen terme (infĂ©rieur Ă 30 ans), sous des scĂ©narios de gestion de type business as usual, et ne tenant pas compte des contextes forestiers et environnementaux non-stationnaires.Objectifs. Cette thĂšse a pour ambition gĂ©nĂ©rale de consacrer un effort de recherche de rupture sur le modĂšle MARGOT, afin dâaborder les enjeux forestiers actuels. Les objectifs prĂ©cis sont : i) de dĂ©terminer la capacitĂ© du modĂšle MARGOT Ă restituer lâexpansion forestiĂšre française sur une pĂ©riode rĂ©trospective longue (1971-2016), ii) de prendre en compte de façon synthĂ©tique de lâhĂ©tĂ©rogĂ©nĂ©itĂ© des forĂȘts Ă grande Ă©chelle, iii) de prendre en compte le phĂ©nomĂšne de densification des forĂȘts dans la dynamique dĂ©mographique, iv) dâinclure les forçages climatiques externes dans la dynamique de croissance des forĂȘts, v) dans un contexte devenu trĂšs incertain, de pouvoir mesurer le niveau dâincertitude des simulations rĂ©sultant de lâerreur dâĂ©chantillonnage de lâinventaire forestier au regard des Ă©volutions tendancielles considĂ©rĂ©es. Le dĂ©veloppement de scĂ©narios de gestion forestiĂšre reste hors du champ de ce travail. Principaux rĂ©sultats. Une mĂ©thode gĂ©nĂ©rique de partition des forĂȘts selon leur hĂ©tĂ©rogĂ©nĂ©itĂ© gĂ©ographique et compositionnelle a Ă©tĂ© mise en place, avec une vocation applicative Ă dâautres contextes forestiers europĂ©ens. Une mĂ©thode de propagation de lâincertitude dâĂ©chantillonnage aux paramĂštres du modĂšle, puis aux simulations, a Ă©tĂ© dĂ©veloppĂ©e Ă partir dâapproches de rĂ©-Ă©chantillonnage de donnĂ©es et de modĂ©lisation dâerreurs. Une approche originale dâintĂ©gration des phĂ©nomĂšnes de densitĂ©-dĂ©pendance dĂ©mographique, fondĂ©e sur une mĂ©trique de densitĂ© et la rĂ©introduction dâun concept de « peuplement forestier » adaptĂ© Ă ce modĂšle, a Ă©tĂ© dĂ©veloppĂ©e. Une stratĂ©gie dâintĂ©gration des forçages climatiques des paramĂštres dĂ©mographiques du modĂšle a Ă©tĂ© dĂ©veloppĂ©e Ă partir dâune approche dâhybridation entrĂ©es-sorties avec le modĂšle fonctionnel CASTANEA pour un sous-ensemble de la forĂȘt française incluant les espĂšces de chĂȘnes, de hĂȘtre, dâĂ©picĂ©a commun, et de pin sylvestre. Lâensemble de ces dĂ©veloppements a permis de rĂ©duire trĂšs notablement le biais de prĂ©diction du modĂšle initial. Conclusions. Les dĂ©veloppements consentis font du modĂšle MARGOT un outil dâexploration et de planification plus fiable des ressources forestiĂšres, et reposant sur une approche de modĂ©lisation originale et unique en Europe. Lâutilisation de statistiques forestiĂšres anciennes permettra dâĂ©valuer le modĂšle et de simuler le stock de carbone de la forĂȘt française sur un horizon temporel plus importante (de plus de 100 ans). Une Ă©valuation approfondie des performances de ce nouveau modĂšle par des simulations intensives doit ĂȘtre conduite
Modeling and propagating inventoryâbased sampling uncertainty in the largeâscale forest demographic model âMARGOTâ
International audienceModels based on national forest inventory (NFI) data intend to project forests under management and policy scenarios. This study aimed at quantifying the influence of NFI sampling uncertainty on parameters and simulations of the demographic model MARGOT. Parameter varianceâcovariance structure was estimated from bootstrap sampling of NFI field plots. Parameter variances and distributions were further modeled to serve as a plugâin option to any inventoryâ based initial condition. Fortyâyear time series of observed forest growing stock were compared with model simulations to balance model uncertainty and bias. Variance models showed high accuracies. The Gamma distribution best fitted the distributions of transition, mortality and felling rates, while the Gaussian distribution best fitted tree recruitment fluxes. Simulation uncertainty amounted to 12% of the model bias at the country scale. Parameter covariance structure increased simulation uncertainty by 5.5% in this 12%. This uncertainty appraisal allows targeting model bias as a modeling priority
The Centuries-Long Expansion of French Forests, Driven Prevalently by Increased Growing Stock, Shows no sign of Saturation
Lâexpansion en surface et en stock sur pied de la forĂȘt française est Ă©tudiĂ©e Ă partir des donnĂ©es dâinventaire forestier national et de la statistique DaubrĂ©e (1908). Son hĂ©tĂ©rogĂ©nĂ©itĂ© gĂ©ographique, selon la propriĂ©tĂ© (privĂ©e, domaniale, autre forĂȘt publique soumise), et la composition ligneuse (feuillus/rĂ©sineux) a Ă©tĂ© examinĂ©e. Entre 1908 et 2010, lâaugmentation de surface est de 5,1 millions dâhectares (+ 50 %), dâintensitĂ© maximale dans le Massif central, et deux fois plus forte dans les feuillus que dans les rĂ©sineux, pour une proportion globale inchangĂ©e. Entre 1975 et 2010, le stock sur pied (+ 59 %, + 930 millions de mĂštres cubes) a Ă©voluĂ© trois fois plus rapidement que la surface, avec une gĂ©ographie diffĂ©renciĂ©e : plus marquĂ©e dans le sud du pays et la Bretagne pour les surfaces, et dans le Massif central pour le stock, indiquant une certaine continuitĂ© avec lâexpansion antĂ©rieure en surface. Sur la mĂȘme pĂ©riode, les forĂȘts privĂ©es et les autres forĂȘts publiques soumises prĂ©sentent des variations relatives de surfaces similaires (+ 20 %), mais les premiĂšres ont connu une capitalisation deux fois plus forte de leur stock (+ 80 %). Entre 1987 et 1994, la forĂȘt privĂ©e feuillue prĂ©sente les plus fortes progressions (+ 280 000 ha et + 105 millions de mĂštres cubes), suivie par la forĂȘt privĂ©e rĂ©sineuse (+ 60 000 ha et + 63 millions de mĂštres cubes). Sur la dĂ©cennie rĂ©cente (2006-2015), lâaugmentation des surfaces et des stocks se maintient au rythme de 120 000 ha/an et 44 millions de mĂštres cubes par an, ce qui suggĂšre sa poursuite au cours des prochaines dĂ©cennies.The expansion of French forests both in surface area and growing stock is explored based on National Forest Inventory data and DaubrĂ©eâs statistics (1908). The author studied their geographic variability under various ownership schemes (private, state or other regulated public forests) and depending on their composition (hardwood/softwood). Between 1908 and 2010, the increase in surface area was 5.1 million hectares (+ 50 %) with the fastest expansion rate in the Massif Central, and hardwoods gaining ground twice as fast as softwoods, although their overall proportions remained unchanged. Between 1975 and 2010, growing stock (+ 59 %, + 930 million cubic metres) developed three times more quickly than surface area but varied considerably between geographic locations: with area taking the lead in southern France and Brittany while in the Massif Central growing stock was preeminent, pointing to a certain continuity in relation to previous expansion of forest area. Over the same period, private forests and other regulated public forests experienced similar relative surface area variations (+ 20 %), but private forests accumulated twice as much growing stock (+ 80 %) as public forests. Between 1987 and 1994, private deciduous forests increased the most (+ 280 000 ha and + 105 million cubic metres), followed by private coniferous forests (+ 60 000 ha and + 63 million cubic metres). In the recent decade (2006-2015), increases in areas and stocks have continued at a pace of respectively 120 000 ha/year and 44 million cubic metres suggesting that this trend will continue over coming decades