Academic literature on the topic 'Foret dense tropicale'

René Catinot, qui fut directeur des recherches forestières au CTFT (Centre Technique Forestier Tropical, qui deviendra le département Forêt du Cirad) et figure historique de la foresterie tropicale française, se posait cette question au début des années 1960. Dès leur arrivée en Afrique Tropicale, nous dit Catinot, « les Forestiers chargés de la gestion de la forêt dense ont cherché à la régénérer ». Les connaissances forestières de milieux tempérés sont insuffisantes face à des forêts qui comportent 200 à 300 espèces, mais qui contiennent nettement moins de bois qu’une forêt aménagée en zone tempérée. Un débat oppose alors les tenants d’une régénération naturelle à ceux de la régénération artificielle. Le problème de l’exploitation sélective tropicale, particulièrement en Afrique, est le faible taux de prélèvement qui limite l’arrivée de lumière au sol et empêche la germination nécessaire au renouvellement des espèces les plus appréciées commercialement, lesquelles sont souvent à tendance héliophile. Catinot est affirmatif « il ne faut pas compter sur la Nature seule pour la régénérer ». Les techniques sylvicoles s’emploient, dès lors, à favoriser ces espèces commerciales, soit en « stimulant la croissance des plants préexistants » (sylviculture utilisant la régénération naturelle), soit en « transplantant dans les forêts épuisées par l’exploitation des plants d’essences nobles préalablement produits en pépinière » (régénération artificielle). Le langage sur la « noblesse » des essences trahit les conceptions de l’époque. Le terme de « diversité biologique » n’apparaitra qu’en 1968 et il faudra encore quelques années pour comprendre que la biodiversité ne se limite pas à la somme des espèces, mais représente l'ensemble des interactions entre les êtres vivants. Que nous dit Catinot, qui exprime bien la pensée des forestiers tropicaux de l’époque ? La sylviculture utilisant la régénération naturelle, « c’est avant tout une destruction lente et prudente du couvert » en utilisant délianage, dégagement et empoisonnement des « espèces gênantes ». Quant à la régénération artificielle, « il faut bien détruire la forêt préexistante pour donner aux plants la lumière indispensable à leur croissance ». Ce langage serait inaudible aujourd’hui, à l’heure où l’on cherche à promouvoir « l’exploitation à faible impact » et où la certification Forest Stewardship Council « de bonne gestion forestière » (FSC) s’interroge sur la compatibilité de l’exploitation, même à faible impact, avec le maintien de « paysages forestiers intacts »...

L’article présente la dernière partie de l’étude sur la sylviculture en forêt dense africaine. L’auteur y décrit une nouvelle méthode, appelée la méthode « Nouvelle croissance » que le Centre technique forestier tropical (CTFT, centre technique maintenant intégré au Cirad) a pu développer au Gabon depuis 1958. L'objectif est de donner le plus de lumière possible aux jeunes arbres plantés tout en maintenant une ambiance forestière acceptable par le soin et l'entretien des nouvelles espèces d'ombrage. Dans la conclusion, il compare les coûts et les rendements des différentes méthodes sylvicoles employées dans les forêts tropicales humides, et il fait le point sur la situation actuelle et les perspectives d’avenir.

Il est devenu habituel de lire dans les médias des articles accusant l’exploitation fores- tière d’être le moteur principal de la dispari- tion des forêts tropicales. Ce raccourci trom- peur est imputable à une méconnaissance des réalités forestières, d’une part, et à un amalgame entre l’exploitation forestière et le défrichement agricole, d’autre part. L’ob- jet de cet article rédigé par un praticien fami- lier du contexte forestier tropical était d’éva- luer l’impact de l’exploitation forestière sur les forêts denses tropicales humides, dans les deux continents africain et améri- cain. Par souci d’exactitude, les phases de l’exploitation ont été distinguées : installa- tion de la base vie, constitution du réseau routier de vidange des bois, réalisation des opérations d’exploitation proprement dites. Des scénarios ont été distingués en fonction de la richesse de la forêt, dont le volume ré- colté varie de 3 à 15 m3/ha. Ces estimations confirment que la destruction du couvert fo- restier demeure si faible qu’à l’exception de situations radicales l’exploitation forestière ne constitue pas une menace pour le main- tien durable des forêts tropicales. La mise en place des réseaux routiers et de pistes de débardage affecte de 4,5 à 5,5 % du couvert forestier. Les surfaces des trouées consé- cutives à l’abattage ne dépassent jamais 4 % du couvert, et restent inférieures à 2 % pour les forêts riches ou de richesse moyenne. L’exploitation forestière n’est donc direc- tement responsable que de la destruction de 5,5 à 8,5 % du couvert forestier. Dans le cas de forêts riches, des exploitations fores- tières successives peuvent certes atteindre 12 à 16 % du couvert forestier. Mais c’est alors sans compter sur la dynamique de reconstitution de la biomasse opérant natu- rellement entre deux séquences d’exploi- tation. En Asie du Sud-Est où l’exploitation est plus intensive en raison d’un potentiel commercial très élevé, la reconstitution du couvert est acquise en 20 ans. Si la valeur économique d’une forêt dense tropicale humide diminue avec l’exploitation, ses va- leurs biologique et écologique restent quant à elles sensiblement intactes.

Cet article représente la première partie d’une étude réalisée par l’auteur sur le développement des méthodes de la sylviculture appliquée aux forêts denses humides d’Afrique. L’étude compare les résultats obtenus par chaque méthode. Les articles suivants exposeront les aboutissements des recherches en foresterie menées ces dernières années, incluant une discussion à la lumière des résultats de ces recherches sur les méthodes utilisées au moment de la rédaction de l’article. L’auteur décrit aussi une nouvelle méthode rendant compte des expériences passées et de nouvelles idées. L’article apporte des éléments essentiels à l’heure où les gouvernements africains promeuvent des programmes de reforestation et de restauration forestière dans les forêts tropicales humides. Cela est désormais primordial en raison de l’augmentation et l’extension de l’exploitation forestière en Afrique, nécessaire pour stimuler les économies africaines, mais épuisant indéniablement le capital forestier.

L’article présente la troisième partie de l’étude sur la sylviculture en forêt dense africaine. Dans les deux premières parties, l’auteur discutait des principales méthodes de foresterie développées jusque-là. Dans cette partie, il examine les causes d’échec. Il considère que le facteur déterminant pour le succès des plantations est la lumière du jour, que les jeunes plantules reçoivent généralement insuffisamment. L’auteur décrit ensuite le programme de recherche entrepris par le Centre technique forestier tropical (CTFT, centre technique maintenant intégré au Cirad) sur ce thème afin de déterminer la quantité de lumière délivrée par chaque système sylvicole, la quantité optimale de lumière demandée par chaque espèce étudiée et l’effet de l’illumination latérale. Ces études utilisent des calculs graphiques, des mesures aux instruments, et des dispositifs expérimentaux particuliers.

L’article présente la quatrième partie de l’étude sur la sylviculture en forêt dense africaine. Dans cet article, l’auteur compare les différentes méthodes employées dans les forêts tropicales humides à la lumière des résultats de recherche exposés dans le précédent article. Il conclut que, excepté pour les méthodes « Limba » et « Okoumé », le principal facteur limitant le succès des plantations semble être le manque de lumière arrivant jusqu’au niveau du sol. Sur la base de cette observation fondamentale et d’autres considérations techniques, il propose des changements aux méthodes conventionnelles, avec la perspective d’améliorer leurs chances de succès.

Developing and enhancing strategies to characterize actual forests structure is a timely challenge, particularly for tropical forests. P-band synthetic aperture radar (SAR) tomography (TomoSAR) has previously been demonstrated as a powerful tool for characterizing the 3-D vertical structure of tropical forests, and its capability and potential to retrieve tropical forest structure has been discussed and assessed. On the other hand, the abilities of L-band TomoSAR are still in the early stages of development. Here, we aim to provide a better understanding of L-band TomoSAR capabilities for retrieving the 3-D structure of tropical forests and estimating the top height in dense forests. We carried out tomographic analysis using L-band UAVSAR data from the AfriSAR campaign conducted over Gabon Lopé Park in February 2016. First, it was found that L-band TomoSAR was able to penetrate into and through the canopy down to the ground, and thus the canopy and ground layers were detected correctly. The resulting TomoSAR vertical profiles were validated with a digital terrain model and canopy height model extracted from small-footprint Lidar (SFL) data. Second, there was a strong correlation between the L-band Capon beam forming profile in HH and HV polarizations with Land Vegetation Ice Sensor (LVIS) Level 1B waveform Lidar over different kinds of forest in Gabon Lopé National Park. Finally, forest top height from the L-band data was estimated and validated with SFL data, resulting in a root mean square error of 3 m and coefficient of determination of 0.92. The results demonstrate that L-band TomoSAR is capable of characterizing 3-D structure of tropical forests.

Tropical forests have exceptional floristic diversity, but their characterization remains incomplete, in part due to the resource intensity of in-situ assessments. Remote sensing technologies can provide valuable, cost-effective, large-scale insights. This study investigates the combined use of airborne LiDAR and imaging spectroscopy to map tree species at landscape scale in French Guiana. Binary classifiers were developed for each of 20 species using linear discriminant analysis (LDA), regularized discriminant analysis (RDA) and logistic regression (LR). Complementing visible and near infrared (VNIR) spectral bands with short wave infrared (SWIR) bands improved the mean average classification accuracy of the target species from 56.1% to 79.6%. Increasing the number of non-focal species decreased the success rate of target species identification. Classification performance was not significantly affected by impurity rates (confusion between assigned classes) in the non-focal class (up to 5% of bias), provided that an adequate criterion was used for adjusting threshold probability assignment. A limited number of crowns (30 crowns) in each species class was sufficient to retrieve correct labels effectively. Overall canopy area of target species was strongly correlated to their basal area over 118 ha at 1.5 ha resolution, indicating that operational application of the method is a realistic prospect (R2 = 0.75 for six major commercial tree species).

Tropical forests are a key component of the global carbon cycle and climate change mitigation. Field- or LiDAR-based approaches enable reliable measurements of the structure and above-ground biomass (AGB) of tropical forests. Data derived from digital aerial photogrammetry (DAP) on the unmanned aerial vehicle (UAV) platform offer several advantages over field- and LiDAR-based approaches in terms of scale and efficiency, and DAP has been presented as a viable and economical alternative in boreal or deciduous forests. However, detecting with DAP the ground in dense tropical forests, which is required for the estimation of canopy height, is currently considered highly challenging. To address this issue, we present a generally applicable method that is based on machine learning methods to identify the forest floor in DAP-derived point clouds of dense tropical forests. We capitalize on the DAP-derived high-resolution vertical forest structure to inform ground detection. We conducted UAV-DAP surveys combined with field inventories in the tropical forest of the Congo Basin. Using airborne LiDAR (ALS) for ground truthing, we present a canopy height model (CHM) generation workflow that constitutes the detection, classification and interpolation of ground points using a combination of local minima filters, supervised machine learning algorithms and TIN densification for classifying ground points using spectral and geometrical features from the UAV-based 3D data. We demonstrate that our DAP-based method provides estimates of tree heights that are identical to LiDAR-based approaches (conservatively estimated NSE = 0.88, RMSE = 1.6 m). An external validation shows that our method is capable of providing accurate and precise estimates of tree heights and AGB in dense tropical forests (DAP vs. field inventories of old forest: r2 = 0.913, RMSE = 31.93 Mg ha−1). Overall, this study demonstrates that the application of cheap and easily deployable UAV-DAP platforms can be deployed without expert knowledge to generate biophysical information and advance the study and monitoring of dense tropical forests.

L'etat des forets tropicales est alarmant, leur surface regresse rapidement depuis quelques decennies parallelement, les pays en voie de developpement, en general, et l'afrique centrale, en particulier, ont subi d'importantes mutations politiques, economiques et sociales devant l'acceleration recente de cette tendance, les organismes internationaux ont reagi par le biais d'un concept nouveau, celui du developpement durable la mise en ouvre d'une politique fondee sur ce principe se heurte, toutefois, a une difficulte majeure qui est le manque d'informations scientifiques sur lesquelles fonder la preservation et la mise en valeur de ces ressources. Il ne s'agit pas simplement d'etudier les mecanismes de la deforestation, mais de proposer une approche mettant en evidence les contraintes et processus tant ecologiques que sociaux relatifs a la dynamique observee. C'est dans ce cadre d'investigation que s'insere la problematique de notre travail elle concerne, en particulier, les interactions ville-foret dans la region de pointe-noire (r du congo), zone qui a connu une tres forte urbanisation depuis une cinquantaine d'annees. Deux phenomenes a priori antagonistes peuvent etre observes dans cette region : une progression naturelle de la foret, opposee a des defrichements de plus en plus consequents dans ce contexte, nous avons pu mettre en evidence la dynamique forestiere : celle-ci est a la fois positive (avancee de la foret) et negative (recul de la foret) la premiere releve d'une tendance ancienne a la progression forestiere dans un climax forestier. On distingue deux types deprogression une reconquete forestiere dont la vitesse moyenne annuelle est comprise entre 9 m et 2 m selon les surfaces considerees, et une conquete forestiere dont la vitesse moyenne annuelle est de 2 m a 0 5 metres la seconde dynamique releve de l'action anthropique en effet, elle est la consequence directe de deux mecanismes, qui sont l'urbanisation (croissance demographique et migrations) et le contexte politico-economique (crise caracterisee par un mouvement migratoire vers les campagnes) cette action se traduit par une augmentation des surfaces defrichees au profit de l'agriculture et de l'exploitation du bois comme combustible. Cette action se localise preferentiellement dans un rayon de 50 km autour de pointe- noire et le long des principaux axes de communication. Le bilan global d

Un modele de dynamique forestiere, considere comme un outil d'aide a l'amenagement forestier, peut reposer sur differents niveaux de description du peuplement : arbre, distribution ou peuplement. Une approche est recherchee pour construire un modele a un niveau de description intermediaire entre l'arbre et le peuplement, en s'appuyant sur les donnees du dispositif sylvicole de paracou en guyane francaise. Dans un premier temps la possibilite de construire un modele de trouees est evaluee sous deux angles : d'une part en regardant si la variable d'interaction des modeles de trouees permet d'expliquer les accroissements observes a paracou, d'autre part en testant si le peuplement peut etre decoupe en placettes independantes. Dans les deux cas les resultats n'encouragent pas a la construction d'un modele de trouees. Neanmoins un modele arbre dependant des distances, qui ne tient compte que de la competition pour la lumiere est obtenu ; ce modele permet de reproduire les distributions diametriques et en hauteur ainsi que la repartition spatiale a grande echelle (< 10 m) observees a paracou. Dans un deuxieme temps des methodes pour changer de niveau de description sont etablies, en suivant deux approches : la premiere est une approche methodologique permettant de transposer les equations d'un modele arbre en un modele de distribution en utilisant des techniques issues de la physique (theoreme de liouville, approximation du champs moyen, methode des moments). Cette methodologie est appliquee au modele arbre construit dans la premiere partie. Il en ressort que l'espace joue dans ce modele un role qui ne peut etre neglige. Dans une deuxieme approche un modele qui melange le niveau individuel et le niveau distribution est construit, la transition de la composante distribution a la composante individuelle s'effectuant au diametre de 40 cm. Ce modele met en evidence la variabilite stochastique liee a la demographie des arbres de grande taille.

Orientador: Marcos Silveira Buckeridge
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Biologia
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Biologia Vegetal
Doutora em Biologia Vegetal

Sur la base d'observations effectuées en Côte-d'Ivoire et dont les résultats déjà publiés figurent en annexe, nous abordons ici la régénération naturelle de la forêt au sens large de dynamique cicatricielle ou de reconstitution après destruction partielle, anthropique ou non. Vu la complexité du milieu et des mécanismes, une modélisation ou simplification a été nécessaire. Nous nous sommes limités aux espèces que nous avons qualifiées de structurales, c'est-à-dire aux arbres qui dominent physionomiquement et écologiquement les formations auxquelles ils appartiennent. Ceci exclut notamment les lianes et les espèces véritablement sciaphiles. Dans la lignée de la tradition forestière et conformément aux idées de MANGENOT et de VAN STEENIS, nous distinguons trois groupes stratégiques d'arbres illustrés chacun par un exemple. Le groupe des "forestières sciaphiles" est illustré par Turraeanthus africana, celui des "géantes anémochores" par Khaya ivorensis et celui des "pionnières" par Trema guineensis. La comparaison des caractères vitaux de ces trois espèces avec ceux de dix autres espèces montre le caractère "naturel" de ces groupes, leur validité quant à l'étude de la régénération naturelle. Les caractères les plus marquants de ces groupes se rapportent à l'écophysiologie des graines qui présentent une dormance secondaire chez les pionnières et n'en présentent pas chez les deux autres groupes. Les graines des géantes anémochores se distinguent en effet par leur aptitude à supporter la déshydratation, alors que celles des "forestières" y sont, au contraire, très sensibles. Au chapitre 2, nous introduisons une simplification supplémentaire, en privilégiant une vision de la dynamique forestière marquée par la phase d'ouverture et le rôle de la "composition floristique initiale". Ceci est justifié par le fait que nous ne considérons que des arbres plus ou moins héliophiles et celui que nous considérons uniquement des ouvertures qui n'affectent pas les qualités physico-chimiques du sol (au contraire de leurs qualités biologiques). Cette simplification permet de tirer parti d'une notion classique que nous remettons à l'honneur : celle de "potentiel floristique". Elle permet d'intégrer, dans le schéma explicatif de la régénération naturelle, les conditions stationnelles et historiques. En accord avec d'autres auteurs anciens ou actuels, nous distinguons trois formes principales de potentiel : le potentiel végétatif, le potentiel séminal édaphique et le potentiel extérieur ou advectif. Ces trois formes de potentiel s'expriment quand elles en trouvent la possibilité (existence +survie+ stimulation), selon l'ordre de priorité du premier arrivé, c'est-à-dire : potentiel végétatif puis potentiel séminal édaphique puis potentiel advectif. Leur résistance au choc de l'ouverture croit dans le même ordre. On met enfin et surtout en évidence une concordance entre forme de potentiel et stratégie. Cet ensemble de considérations permet de montrer qu'en fonction de l'intensité des perturbations, de leurs répétitions, la régénération naturelle conduit à des formations dominées par des plantes issues d'un potentiel floristique ou d'un autre, écologiquement bien différents. On aboutit à des changements floristiques et dynamiques profonds. Nous donnons quelques exemples d'évolution de la végétation interprétés selon notre modèle : trouée –persistance – stimulation - expression, le premier à l'échelle de la trouée de chablis, le deuxième à l'échelle du champ sur brûlis, le troisième à l'échelle du paysage. Le modèle permet de montrer qu'il est possible d'orienter la régénération naturelle pour favoriser le développement de formations dominées par des espèces de tel ou tel groupe stratégique. Dans la majorité des cas, il importe surtout d'éviter le développement explosif et quasi irréversible des espèces les plus envahissantes. La lenteur des processus d'évolution vers un hypothétique climax est telle qu'il importe de garder la plus extrême prudence quant à l'aménagement des formations plus ou moins primaires encore existantes et dont le "potentiel" biologique est inestimable
Ecological cycle of three canopy forming species are described from Ivory Coast. They exemplify three different strategies or groups which encompass most of the largest trees. These are the pioneer or belukar strategy of which Trema guineensis is the type, the large wind-born group with khaya ivorensis as an example and shade forest species group with Turraeanthus africana. The seed ecophysiology is what distinguishes those groups at first step. Seeds of Trema exhibit a photoblastic dormancy and are very long lived. Seeds of Khaya can endure high evaporative conditions and the dry seed stay alive for a rather long time, meanwhile Turraenthus seed is very rapidly destroyed by dessication and cannot stay alive without ongoing germination. All the other traits of these species are linked with seed type, mainly: dispersal, growth rate, longevity, height, etc. All this canopy species highly benefit from high light level and need gaps to reach maturity. That is why gaps are so important for their succession, and their reproduction can be described as triggered by the cover opening. Past story and surrounding vegetation determine available species for regeneration. This floristical potential can be divided into three main classes: vegetative potential, seed bank, and dispersal potential. As a rule of thumb, they exhibit the same order of priority in their expression. Each potential has to exist, resist and be stimulated to get in the run till the opening space is saturated back. Identity between strategies and potentials, to say shade species and vegetative potential, pioneer and seed bank, large wind-born species and dispersal potential is demonstrated and used to predict natural regeneration after different degrees of perturbations. Some examples of application are given and some stumbling points are discussed

O presente trabalho teve por objetivo realizar a caracterização silvigênica de um trecho de Floresta Ombrófila Densa Sub Montana em conjunto com o estabelecimento de possíveis relações entre as alterações espaciais do mosaico silvático e os fatores abióticos (solo e topografia). O método utilizado foi o de interceptação de linhas de inventário, com identificação das ecounidades descrito por Torquebiau (1986). Foram dispostas linhas paralelas entre si e distantes 10 m uma da outra. Todos os indivíduos dominantes (mais altos naquele ponto), cujas projeções horizontais das copas interceptaram as linhas, foram amostrados na caracterização silvigênica. Foram tomadas medidas, de no mínimo quatro pontos, da projeção horizontal da copa destes indivíduos até as linhas de inventário, em um sistema de eixos ortogonais (coordenadas x e y). Cada árvore marcada no campo foi classificada, quanto à sua arquitetura, em: árvores do futuro, árvores do presente e árvores do passado (OLDEMAN,1987). As áreas de clareira que interceptaram as linhas também foram amostradas, medidas e mapeadas. O estabelecimento das diversas ecounidades em cada trecho amostrado é feito a partir da união das copas de árvores de mesma categoria. O desenho do mosaico e o cálculo das áreas das ecounidades foram feitos por meio do programa TNTmips, a partir das coordenadas das copas dentro das linhas de inventário. Este trabalho resultou na representação gráfica da cobertura vegetal da área estudada e a sua correlação com os fatores abióticos. Para avaliar o papel dos fatores abióticos na composição espacial do mosaico vegetacional, foram analisadas e combinadas as diversas informações em um Sistema de Informações Geográficas (SIG). Para tal, cada \"classe\" de informação constitui um plano de informação ou um \"layer\" dentro do SIG. De acordo com os resultados pode-se concluir que a caracterização silvigênica indicou que a área estudada representa uma floresta em fase de pré-maturidade por apresentar sinais de perturbações recentes, traduzidas nas altas proporções de ecounidades 1A e em reorganização observadas. Conclui-se ainda quepode ser estabelecida uma relação entre a distribuição das ecounidades e os fatores abióticos estudados.
This study aimed to realize the silvigenic characterization of a Dense Rain Forest in according to the establishment of possible relationships between spatial changing\'s on the silvatic standards and some abiotic factors, such as soil and topography. The method applied was the inventory line interception, identifying the ecounits described by Torquebiau (1986). There were set parallel lines in every 10 meters. All dominant trees (the highest in that point), whose horizontal canopy projections intercepted the inventory line, were sampled in the silvigenic characterization. To measure the canopy projection, there were used at least four points on the inventory line as an orthogonal axis system (X and Y coordinates). Each sampled tree was classified based on its architecture features as: trees of the future, trees of the present or trees of the past (OLDEMAN, 1987). The gap surface crossing inventory lines were also measured and mapped. The ecounit establishment is created by the connection of canopies from the same category (future, present or past). The ecounits design were mapped and its surface measured using the TNTmips software, based on all canopies coordinates over the inventory lines. The study resulted in the graphic representation of vegetation coverage and its correlation with abiotic factors. To evaluate the contribution of the abiotic factors on the vegetation mosaic`s spatial composition, a Geographic Information System (GIS) was settled to combine and analyze all data. Different information classes were overlapped as layers on the GIS environment. According to the results it`s possible to conclude that silvigenic characterization indicates that the studied area represents a pre mature forest, based on recent disturbances sings, confirmed on high rates of ecounits as 1A or reorganization types. It is concluded that the silvigenic mapping represented the architectural behavior of the species related to the soil classification.

Un simulateur de la dynamique forestière, basé sur un modèle d'arbre dépendant des distances, est élaboré, calibré et évalué à partir des données accumulées sur le dispositif sylvicole expérimental de Paracou en Guyane française. Il met en oeuvre des arbres, localisés au sein d'un peuplement et dotés de règles de vie décrites par des sous-modèles de croissance, de mortalité et de recrutement en fonction d'un pas de temps de trois ans.

Le modèle de croissance est dérivé d'un type « potentiel x réducteur » adapté aux spécifités de la forêt dense tropicale humide de Guyane. Il prédit l'accroissement d'un arbre en fonction de son diamètre à l'instant t et de son environnement immédiat décrit par deux indices de compétition : l'un rend compte de la pression subie par l'arbre en début de période de croissance et l'autre de l'évolution de cette pression dans un passé proche. La prise en compte de quinze groupes d'espèces, homogènes du point de vue de la croissance, améliore considérablement son efficacité. Quatre modèles de mortalité prédisent la mort sur pied et la mort par chablis primaire, secondaire ou complexe d'un arbre, et un modèle de recrutement permet de gérer l'apparition de nouveaux individus sur des placeaux de 100m2 en fonction de la place disponible dans le peuplement.

Le simulateur, programmé en langage SmallTalk-80 selon la technique des systèmes multi-agents, est doté d'une interface conviviale permettant à l'utilisateur : (i) de suivre l'évolution virtuelle dans le temps d'un peuplement et de chacun des individus qui le constituent par le biais de cartes et de graphiques ; (ii) de réaliser à tout moment une gamme très variée d'interventions de manière à tester des scénarios sylvicoles. Les différents essais réalisés montrent que le peuplement que nous avons généré présente un comportement satisfaisant quelle que soit l'intensité des perturbations imposées, mais que sa réactivité est faible au regard d'un modèle matriciel calibré à partir des mêmes données. Nous proposons différentes voies d'amélioration et soulignons l'intérêt que peut présenter ce type de modèle pour la communauté scientifique et, à plus long terme, le gestionnaire forestier.

Une typologie des structures est réalisée à partir d'un échantillon systématique de 3,12 ha et de cinq placeaux de 2 400 à 4 000 m2, représentant les principaux types d'organisation du peuplement. Cette analyse met en évidence des variations des structures spatiales (verticales et horizontales) et des structures floristiques, qui dépendent pour partie de la situation topographique au sein du dispositif, mais également de l'histoire locale du peuplement. L'analyse détaillée des modèles de répartition spatiale des individus et des processus (ponctuels) spatiaux qui permettent d'en rendre compte, montre qu'il convient alors d'envisager, pour les situations exemptes de perturbations majeures depuis longtemps, des mécanismes dynamiques alternatifs à la régénération dans les chablis. Il s'agit de phénomènes de substitution au sens large, qui ne font pas intervenir le potentiel germinatif ; le comblement de trouées de petite dimension, occasionnées par la mort sur pied d'un (ou de quelques) individu(s) de la voûte, est réalisé par les individus prééxistants. Les modalités de la substitution diffèrent en fonction de la situation topographique, en versants ou sur les crêtes. L'ensemble des résultats indique, que seule la prise en compte de l'hétérogénéité spatio-temporelle des mécanismes de renouvellement permettra de concevoir un modèle fiable de la dynamique de fonctionnement des forêts tropicales.

Cette étude est réalisée en forêt dense humide du Cameroun. Elle étudie : i) la capacité d’adaptation des espèces plantées selon deux méthodes dans les dispositifs sylvicoles abandonnés de Mangombé et ii) compare la biodiversité floristique de la forêt de Mangombé à celles de Bidou et Campo moins perturbées. Dans les plantations, les espèces qui se sont adaptées au site et presentant les meilleures performances de croissance diamétrique sont : Gmelina arborea (Verbenaceae), Dipterocarpus alatus (Dipterocarpaceae) et Aucoumea klaineana (Burseraceae). La régénération naturelle en plantation est diversifiée. Les forêts naturelles sont diversifiées comme l’indique l’indice de Shannon, le coefficient de diversité générique et la richesse spécifique qui est élevée à Mangombé (38 familles et 91 espèces), intermédiaire à Bidou (32 familles et 88 espèces) et faibles à Campo (29 familles et 75 espèces). La zoochorie concerne plus de 71% des espèces recensées. L’importance de la flore à affinité forestière consolide la possibilité pour la régénération naturelle de reconstituer à long terme la biodiversité végétale en plantation
This study is carried in rainforest of Cameroon. It presents: i) the adaptability of species, planted with two sylviculturals methods in Mangombe’s plantation and ii) compare the biodiversity of Mangombe’s forest to those of Bidou and Campo less disturbed. The adapted species with few mortality and best diameter increasement are : Gmelina arborea (Verbenaceae), Dipterocarpus alatus (Dipterocarpaceae) and Aucoumea klaineana (Burseraceae). The natural regeneration under canopy in plantation is heterogenic and diversed. The vegetation indices show a high diversity in all the sites : Shannon index, generic diversity and specific richness which is higher in Mangombe (38 families and 91 species), intermediary in Bidou (32 families and 88 species) and lower in Campo (29 families and 75 species). Zoochory concerns more than 71% of the species, and suggests a major role of animals in the regeneration process. The abondance of species familiar to non disturbed natural forest confirms the possibility for the natural regeneration to reconstitute in long term the biodiversity in plantation

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Neste estudo foram avaliadas a eficiência técnica e os custos da aplicação do tratamento silvicultural de desbaste de liberação seletivo, com anelamento e aplicação de herbicida (Tordon, 50% de diluição), em uma Floresta Ombrófila Densa no Estado de Roraima, manejada de acordo com os preceitos de exploração de impac o reduzido e legislação vigente. Para analisar o efeito das características das árvores na desvitalização, foi ajustado o modelo logístico binário, indicando uma menor velocidade de desvitalização para espécies secundárias. Foram aneladas 21,6 arv/ha para favorecer a liberação de 12 arv/ha, em uma relação de 1,8 competidores para cada Árvore Comercial potencial para Colheita Futura (ACCF) liberada. O resultado da aplicação do tratamento silvicultural demonstrou uma boa efetividade da técnica, alcançando 100% de mortalidade das árvores competidoras no período de 12 meses após o anelamento, com 69% das árvores desvitalizadas dentro do período de 6 meses. O custo por hectare atingiu valor total de R$ 77,88/ha, sendo o custo por árvore anelada de R$ 3,59 e o custo por árvore liberada de R$ 5,56. Ao analisar a distribuição dos custos, observou-se que 50% deveu-se ao insumo (no caso específico a aquisição do herbicida), 47% decorreu do custo da mão de obra, 2% deveu-se à aquisição de equipamento de proteção individual e 1% de materiais. O rendimento operacional alcançou 0,76 homem/dia.ha-1, abrangendo as atividades de localização das ACCF, identificação das competidoras, corte de cipós, anelamento e aplicação de herbicida.
This study presents the results of the analysis of technical efficiency and costs of silvicultural treatments carried out in a lowland Amazon rainforest logged under reduced impact logging guidelines. The silvicultural treatments consisted in the postharvest girdling and herbicide treatment (using the product Tordon in 50% solution) of remnant trees competing with future crop trees (FCTs). In order to analyze the effect of girdling on tree mortality, a binary logistic model was utilized. The model indicated a slower mortality process for secondary tree species. On average 21,6 competing trees per hectare were girdled in order to benefit 12 FCTs per hectare (on average 1,8 competitors for every FCT). Results demonstrate the high efficiency of the silvicultural treatments tested: mortality of competitors attained 69% within six months after girdling and 100% within twelve months after girdling. The total cost of girdling amounted to BRL 77,88 per ha, BRL 3,59 per girdled tree and BRL 5,56 per FCT. Of the total cost, 50% arose from production factors, especially herbicide acquisition, while 47% were labor costs. Only 2% of the total costs were generated from work safety equipment and 1% of other materials. The operative productivity attained 0,76 man days per hectare which included the activities competitor identification, climber cutting, girdling and herbicide application.

I. An original methodological discussion is proposed on the problem of the typology of tropical rain forest’s plant communities, based on the study of forest types across gradients of continentality and elevation, within Atlantic central Africa. These investigations were based on the statement that the main problems in forest typology are related to the non-zonal or zonal character of the different vegetation types and to non considering the relations and differences between forest strata.

II. Field data consisted in phytosociological homogeneous sample plots localized within different recognized phytogeographical entities, in a region of tropical Africa where these entities are known to be well conserved. A total of 37 such plots were inventoried in the region extending from the littoral forests of Ndoté, Equatorial Guinea, which are wet evergreen forests, to the continental forests of the Dja, Cameroon, known as evergreen seasonal forests. The studied region also included the oriental Atlantic forests of Equatorial Guinea, known as moist evergreen forests or caesalp forests. In various parts of this continentality gradient, some plots were localized within climax non-zonal formations, namely the submontane rain forests. The emphasis was put on the vegetation of the Monte Alén National Park.

The sampling methodology was willing to be as "complete ", including all strata, "quantitative ", enumerating all individuals, and "representative ", within each stratum, as possible. These multi-layers plots were realised using nested sub-plots, with a sampling size of 100 individuals for every ligneous stratum recognized (dominant trees, dominated trees and shrubs) and a sampling size of 200m² for the herbaceous and suffrutex stratum.

Forest types were defined independently for each stratum and the differences were analysed. A method was proposed for the simultaneous analysis of all floristic data, converting and standardizing the values from ligneous strata, on the one hand, and from understorey strata, on the other hand.

III. Ten forest types were described using IndVal and discussed in the general context of the guineo-congolian region, from a syntaxonomic view point (agglomerative classification) and from a phytogeographical view point (divisive classification). Homologies between these two approaches are described. The proposed phytogeographical system is based on an "open " conception of hierarchical classifications, combining advantages of agglomerative and divisive classifications. In concrete terms, the non-zonal criteria, for example the submontane variants, are categorised separately and in analogy with the zonal criteria, related to the usual phytochoria.

Analysis of ecological relationships for the 10 communities showed that the main variables related to the floristic variability in our mainland rain forests are elevation, rainfall, hygrometry (estimated using bryophytes cover levels) and distance to the ocean. The two extremes on the vertical microclimatic gradient, dominant trees stratum and herbaceous stratum, give similar typologies, however canonical analysis showed that for the herbaceous layer, non-zonal variables (hygrometry and elevation) were gaining more importance when the influence of the two zonal variables was attenuated. In every case, spatial autocorrelation was less important than the environment in explaining floristic variability but its role increased in the spatial arrangement of understorey species, whose dispersal capacity is generally lower than canopy trees. The phytosociological, phytogeographical and ecological description of forest types is accompanied by a physiognomical description using biological types spectrum, as well as architectural models, leaf sizes, etc.

With regard to diversity, we have demonstrated that species richness was higher from upper to lower strata because of the accumulation in lower strata of species from various strata. On the other hand, the proper stratum diversity, i.e. the structural set, decreased from dominant trees to shrubs. The proper diversity of the herb layer showed relatively high figures mainly due to the higher individual density in relation to the existence of microstrata. Within the 37 sample plots, 1,050 taxa have been identified to species or morpho-species levels, for a total of 25,750 individuals. These taxa represent 442 genus among 104 families. The richest forest type is found on the foothills of the Niefang range, on the windward side. This forest type is also characterised by a high number of oligotypic genus and by species belonging to functional types indicators of glacial refuges. These functional types are defined on the basis of the dispersal capacity and on kind of stand needed for effective germination. We formulated the hypothesis that this kind of "foothills refuge ", characterised by his zonal nature, could have been one of the rare refuges for species from mainland rain forests, while montane and fluvial refuges would mainly have preserved species from non-zonal forest types: (sub)montane and riverine.

Based on indicator species of submontane forests, a potential distribution map of this forest type has been realised at the Atlantic central African scale. More than 400 submontane forest localities have been mapped. These forests begin at 400m of altitude near the ocean, and progressively at higher altitude for increasing distance to the ocean. Many lowland localities also comprised submontane species, which could indicate the existence of ecological transgressions. These transgressions would allow migratory tracks for submontane species between isolated mountain ranges, not only during glacial periods, through heights at the northern and southern borders of the congo basin, but also contemporarily through the lowland riverine forest network, in the centre of this basin. Finally, a special attention has been attributed to littoral forests and to some cases of choroecological transgressions, coupled to the ecological equalization phenomenon.

Doctorat en sciences agronomiques et ingénierie biologique
info:eu-repo/semantics/nonPublished

Friedrich Wöhler was referring to the field of organic chemistry during the early 1800s when he wrote the above but his comments would not be out of place in the context of embarking upon a global study of past and present human relationships with tropical forests. Dense vegetation, difficulty of navigation, issues of preservation, political and health concerns, poisonous plants, animals, and insects, and the prospect of carrying out sampling or excavation in high humidity have all meant that our knowledge of human history and prehistory in these environments is under-developed relative to temperate, arid, or even polar habitats. There have been theoretical questions as to what kind of human activity one would even expect to find in tropical forest environments, which seem hostile to human foraging (Hart and Hart, 1986; Bailey et al., 1989) let alone thriving agricultural or urban settlements (Meggers, 1971, 1977, 1987). This has, until relatively recently, left the state of archaeological tropical forest research in a similar position to popular conceptions of these environments—untouched, primeval wilderness. Public ideas of an archaeologist investigating a tropical forest are probably synonymous with someone in a shabby-looking leather hat being chased, if not by a large stone boulder then by a group of Indigenous people with blowpipes, as they wade through dense undergrowth and vines while clutching a golden discovery that has been lost to the western world for thousands of years (Spielberg, 1981). The more recent development of the best-selling Uncharted video game series has done little to change these ideas amongst the next generation of media consumers, with players taking on the role of Francis Drake’s mythical ancestor in search of long lost treasure, frequently hidden within caves and ruins surrounded by vines and dense canopies (Naughty Dog et al., 2016). The idea of treasure hidden within tropical forest is also not a modern conception. The long-term myth of El Dorado, a city covered in gold, fuelled exploration of the tropical forests of South America by renowned individuals, including Sir Walter Raleigh, from the sixteenth to the nineteenth centuries (Nicholl, 1995).

On the basis of the plethora of evidence for tropical forest agricultural practices and urban networks in Chapters 5 and 6 it seems somewhat surprising that these environments, and their human occupants, could ever come to be seen as static, primordial, or ‘elusive’ (Biesbrouck et al., 1999). Yet, today, Indigenous peoples living in tropical forests are often depicted as being isolated from the outside world, and as equally, passively threatened by external agricultural, economic, and political forces as the habitats in which they reside. As I showed in Chapter 2, the hunting and foraging practices of these groups can provide useful insights into how our prehistoric ancestors may have made a diverse living in environments that have frequently been considered too poor in crucial resources for long-term human occupation. Somehow, however, these parallels and comparisons have also seen these groups framed as relics of some of the earliest members of our species. This has been encouraged by claims that some of these groups genetically represent ‘archaic’ lineages of Homo sapiens that survived in dense forest habitats in different regions (Endicott et al., 2003; Ranaweera et al., 2014; Ranasinghe et al., 2015).Whether this is the case or not, it is now clear from historical and ethnographic information that tropical forest foragers, the world over, have been involved in complex economic and political networks to varying extents at different points in time. Moreover, the present cultural and subsistence systems of many of these groups have been significantly affected by the infiltration of colonial and imperial regimes from the seventeenth century onwards, as well as the more recent, disruptive effects of global capitalism. This chapter is an attempt to document how Eurocentric concerns with ‘exploration’, developments in literature, modern conservation movements, and the ‘pristine’ hunter-gatherer debate have contributed to the removal of tropical forest societies from history and their placement into isolated, primeval conceptions of tropical forest environments. In response to this, I review evidence for historical and ethnographic connections of tropical forest hunter-gatherers, and agriculturalists, with societies in neighbouring territories.

Most of South America lies within the tropics, and lowland tropical ecosystems make up the majority of its landscapes. Although there is great concern for the Amazon ecosystem, the largest of the world’s tropical forests, there are many other fascinating and in some cases more endangered types of lowland forest. Such forests may be defined as lying below 1,000 m above sea level, although it is difficult to set arbitrary limits (Hartshorn, 2001). The two main lowland moist evergreen forests are the Hylea (a term coined by Alexander von Humboldt to denote rain forests of the Amazon Basin) and the much smaller Chocó forest on the Pacific coast between Panama and Ecuador. Two related yet distinctive types of forest are the Mata Atlântica or Atlantic moist evergreen forest and the Mata Decidua or dry deciduous forest, including the caatinga woodland, which is both deciduous and xerophytic (Rizzini et al., 1988). The latter two formations are among the most threatened of all South American forests. Lowland forests vary from dense and multilayered to open and single-layered, from evergreen to deciduous, and from flooded or semi-aquatic to near-arid. Tree heights range from 30 to 40 m with emergent trees reaching over 50 m, to forests where the tallest trees barely attain 20 m (Harcourt and Sayer, 1996; Solorzano, 2001). However, because of its extent and importance, Amazonia will form the principal focus of this chapter. Amazonia covers a vast area (>6 × 106 km2) and contains some 60% of the world’s remaining tropical forest. The Amazon and Orinoco basins influence not only regional climates and air masses, but also atmospheric circulation patterns both north and south of the Equator. The sheer size and diversity of Amazonia exhausts a normal repertoire of grandiose adjectives. The Amazon may or may not be the longest river in the world but it is by far the greatest in terms of discharge, sending around one fifth of the world’s fresh water carried by rivers to the oceans(see chapter 5; Eden, 1990; Sioli, 1984). The drainage basin is twice as large as any other of the world’s catchments.

The above quote from a recent Hollywood film presentation of Colonel Percival Fawcett’s obsessive early twentieth-century search for the remains of the Lost City of Z (Gray, 2016) highlights the effort that it has taken to convince the academic world and the public alike that large urban forms can be developed in tropical forest settings. While the film, and the book by David Grann (2009) upon which it was based, grossly overplay the exploration credentials, respect for Indigenous peoples, and scientific abilities of Colonel Fawcett (Hemming, 2017), this quote encapsulates the difficult working conditions and environmental determinism in western thought that have led to perceptions of ‘impossibility’ of extensive settlements and social complexity in tropical forests. Beyond searches for debated ‘lost’ cities, even where the clear ruins of ancient urban sites have been found in tropical forests, as with the Classic Maya in North and Central America and the Khmer Empire in Southeast Asia, their collapse has been seen as almost inevitable given necessary forest clearance, soil erosion, and population pressure on these delicate environments (Webster, 2002; Diamond, 2005; Chen et al., 2014; Lentz et al., 2014). In particular, the intensive agriculture seen as necessary to fuel the ‘urban revolution’ (Childe, 1950) and the development of cities and elite structures familiar to most archaeological definitions of cities (Adams, 1981; Postgate, 1992), has been considered impossible on the fragile, low nutrient soils of tropical forest habitats (Meggers, 1954, 1971, 1977, 1987). Other, less-discussed threats include natural disasters, such as mudslides and mass-flooding, that continue to trouble tropical regions prone to high annual or seasonal rainfall (Larsen, 2017). Nevertheless, new methodologies and theoretical shifts are highlighting the clear emergence of social complexity and extensive human populations prior to the arrival of European settlers in many of the world’s tropical forest settings. Here, I review the growing dataset of past ‘urban’ forms in tropical forests. As with ‘the origins of agriculture’ in Chapter 5, tropical forests have been crucial in demonstrating that traditional ideas of ‘urbanism’ in archaeology–namely ‘compact’, bounded, and dense populations documented in early Mesopotamia and the Mediterranean, and that dominate European thought—do not capture the whole wealth of ‘urban’ diversity and settlement networks that began to develop from the Middle Holocene.

The central grassland region of North America (Fig. 1.1) is the largest contiguous grassland environment on earth. Prior to European settlement, it was a vast, treeless area characterized by dense head-high grasses in the wet eastern portion, and very short sparse grasses in the dry west. As settlers swept across the area, they replaced native grasslands with croplands, most intensively in the east, and less so in the west (Fig. 1.2). The most drought-prone and least productive areas have survived as native grasslands, and the shortgrass steppe occupies the warmest, driest, least productive locations. James Michener (1974) provided an apt description of the harshness of the shortgrass region in his book Centennial:… It is not a hospitable land, like that farther east in Kansas or back near the Appalachians. It is mean and gravelly and hard to work. It lacks an adequate topsoil for plowing. It is devoid of trees or easy shelter. A family could wander for weeks and never 4 nd enough wood to build a house. (p. 64)… The objective of this chapter is to introduce the shortgrass steppe (Fig. 1.3) and its record of ecological research. First we present an ecological history of the shortgrass steppe since the Tertiary, and provide the geographic and climatic context for the region. Second we describe the major research sites, and the history of the three major entities or programs that have shaped much of the science done in the shortgrass steppe: the U.S. Department of Agriculture (USDA)–Agricultural Research Service (ARS), the International Biological Programme (IBP), and the Long-Term Ecological Research (LTER) Program. Grasses have been an important component of the shortgrass steppe of North America since the Miocene (5–24 million years ago) (Axelrod, 1985; Stebbins, 1981). Before that, during the Paleocene and Eocene (34–65 million years ago), the vegetation was a mixture of temperate and tropical mesophytic forests. Two causes have been proposed as explanations for this ancient change from forest to grassland. First, global temperatures decreased rapidly during the Oligocene (24–34 million years ago), creating conditions for a drier climate. These drier conditions, combined with a renewal of the uplift of the Rocky Mountains that had begun during the Paleocene, left the Great Plains in a rain shadow.