The general aim of my dissertation was to investigate the role of plant interactions in driving population dynamics. Both theoretical and empirical approaches were employed. All my studies were conducted on the basis of metabolic scaling theory (MST), because the complex, spatially and temporally varying structures and dynamics of ecological systems are considered to be largely consequences of biological metabolism. However, MST did not consider the important role of plant interactions and was found to be invalid in some environmental conditions. Integrating the effects of plant interactions and environmental conditions into MST may be essential for reconciling MST with observed variations in nature. Such integration will improve the development of theory, and will help us to understand the relationship between individual level process and system level dynamics. As a first step, I derived a general ontogenetic growth model for plants which is based on energy conservation and physiological processes of individual plant. Taking the mechanistic growth model as basis, I developed three individual-based models (IBMs) to investigate different topics related to plant population dynamics: 1. I investigated the role of different modes of competition in altering the prediction of MST on plant self-thinning trajectories. A spatially-explicit individual-based zone-of-influence (ZOI) model was developed to investigate the hypothesis that MST may be compatible with the observed variation in plant self-thinning trajectories if different modes of competition and different resource availabilities are considered. The simulation results supported my hypothesis that (i) symmetric competition (e.g. belowground competition) will lead to significantly shallower self-thinning trajectories than asymmetric competition as predicted by MST; and (ii) individual-level metabolic processes can predict population-level patterns when surviving plants are barely affected by local competition, which is more likely to be in the case of asymmetric competition. 2. Recent studies implied that not only plant interactions but also the plastic biomass allocation to roots or shoots of plants may affect mass-density relationship. To investigate the relative roles of competition and plastic biomass allocation in altering the mass-density relationship of plant population, a two-layer ZOI model was used which considers allometric biomass allocation to shoots or roots and represents both above- and belowground competition simultaneously via independent ZOIs. In addition, I also performed greenhouse experiment to evaluate the model predictions. Both theoretical model and experiment demonstrated that: plants are able to adjust their biomass allocation in response to environmental factors, and such adaptive behaviours of individual plants, however, can alter the relative importance of above- or belowground competition, thereby affecting plant mass-density relationships at the population level. Invalid predictions of MST are likely to occur where competition occurs belowground (symmetric) rather than aboveground (asymmetric). 3. I introduced the new concept of modes of facilitation, i.e. symmetric versus asymmetric facilitation, and developed an individual-based model to explore how the interplay between different modes of competition and facilitation changes spatial pattern formation in plant populations. The study shows that facilitation by itself can play an important role in promoting plant aggregation independent of other ecological factors (e.g. seed dispersal, recruitment, and environmental heterogeneity). In the last part of my study, I went from population level to community level and explored the possibility of combining MST and unified neutral theory of biodiversity (UNT). The analysis of extensive data confirms that most plant populations examined are nearly neutral in the sense of demographic trade-offs, which can mostly be explained by a simple allometric scaling rule based on MST. This demographic equivalence regarding birth-death trade-offs between different species and functional groups is consistent with the assumptions of neutral theory but allows functional differences between species. My initial study reconciles the debate about whether niche or neutral mechanisms structure natural communities: the real question should be when and why one of these factors dominates. A synthesis of existing theories will strengthen future ecology in theory and application. All the studies presented in my dissertation showed that the approaches of individual-based and pattern-oriented modelling are promising to achieve the synthesis.

Dans cette étude, la composition taxonomique, la structure spatiale et les ressources végétales naturelles prélevées dans les forêts de Kigwena et de Rumonge ont été analysées. Les deux forêts sont situées au Sud-Ouest du Burundi, dans une région de forte densité de population. L’objectif global de l’étude était d’élucider des éléments de l’écologie de la forêt dense de Kigwena et de la forêt claire de Rumonge en vue de leur conservation optimale. Les résultats montrent que les deux forêts sont floristiquement différentes malgré leur proximité géographique. Les types biologiques sont dominés par les phanérophytes, les chaméphytes et les thérophytes ;ce qui traduit des adaptations des espèces aux conditions du milieu de vie forestier caractérisé par un stress hydrique et par d’importantes perturbations d’origine anthropique. Le spectre phytogéographique est en concordance avec la localisation des forêts étudiées dans le territoire phytogéographique du District du Mosso/Malagarazi, caractérisé par la présence des espèces à distribution soudano-zambézienne et des espèces de liaison soudano-zambézienne et guinéo-congolaise. La forêt dense de Kigwena et la forêt claire de Rumonge sont soumises aux mêmes influences phytogéographiques et n’ont donc pas connu d’isolement géographique. Dans la forêt dense de Kigwena, l’hétérogénéité spatiale de la végétation, reflétée par l’existence de quatre groupements végétaux distincts, obéit aux gradients d’humidité et d’anthropisation. Par contre, dans la forêt claire de Rumonge, l’hétérogénéité spatiale de la végétation se traduit par des variations stationnelles dans un même fond floristique. L’analyse de la distribution spatiale a montré que les arbres sont répartis selon le modèle spatial agrégé. Les ressources végétales prélevées dans les deux forêts par les populations riveraines servent en grande partie à des fins médicinales, matérielles, alimentaires, énergétiques et/ou socio-culturelles. Les prélèvements de ces ressources, l’agriculture intensive, l’exploitation des carrières, l’aménagement des infrastructures, les feux de forêt et les chablis ont été identifiés comme étant des facteurs de perturbation qui amplifiaient la dégradation et le risque de disparition des forêts de Kigwena et de Rumonge. En définitive, cette étude a montré que les forêts étudiées ont une richesse taxonomique et une valeur conservatoire considérables. Mais ces forêts sont menacées sous l’action d’une anthropisation croissante. Pour remédier à cette situation, des mesures et des actions de conservation ont été proposées pour un court terme. /The taxonomic composition, the spatial pattern and the botanical resources used by Man around the Kigwena and Rumonge forests were studied. Both forests are located in the south-western part of Burundi, in a zone characterized by a high population density. The overall objective of this study was to understand the ecology of the Kigwena and Rumonge forests in order to support their conservation. Results show that the flora of the two forests are different although their geographical proximity. The life forms are dominated by phanerophytes, chamaephytes and therophytes; this observation corresponds to an adaptation of the floras to water stress and anthropogenic disturbances. The phytogeographical spectrum is directly related to the presence of the forests in the Mosso/Malagarazi District. It is characterized by the presence of species with a soudano-zambezian distribution and by species of the soudano-zambezian/guineo-congolese connexion. No geographical isolation was evidenced which could have altered the composition of the Kigwena and Rumonge forests. In the Kigwena forest, the spatial floristic heterogeneity is shown by four different plant communities which occur according to anthropogenic disturbance and soil moisture gradients. But, in the Rumonge forest, the spatial floristic heterogeneity occurs with more common species, in the same flora. The analysis of the spatial pattern of the trees revealed a general tendency towards a clumped distribution. Many natural plant resources are removed from the Kigwena and Rumonge forests by neighbouring populations for medicinal use, material use, food use, energy use and cultural use. Removal of natural plant resources, intensive agriculture, quarry exploitation, infrastructure development, forest fires and windfall of trees have been identified as disturbance factors which amplify the degradation and the risk of disappearance of the forests of Kigwena and Rumonge. Ultimately, this study shows that the Kigwena and Rumonge forests are characterized by a high taxonomic richness as well as by their importance for conservation. Nevertheless, these forests are threatened by increasing anthropogenic activities. Consequently, short-term measures and actions for conservation are proposed. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The general aim of my dissertation was to investigate the role of plant interactions in driving population dynamics. Both theoretical and empirical approaches were employed. All my studies were conducted on the basis of metabolic scaling theory (MST), because the complex, spatially and temporally varying structures and dynamics of ecological systems are considered to be largely consequences of biological metabolism. However, MST did not consider the important role of plant interactions and was found to be invalid in some environmental conditions. Integrating the effects of plant interactions and environmental conditions into MST may be essential for reconciling MST with observed variations in nature. Such integration will improve the development of theory, and will help us to understand the relationship between individual level process and system level dynamics. As a first step, I derived a general ontogenetic growth model for plants which is based on energy conservation and physiological processes of individual plant. Taking the mechanistic growth model as basis, I developed three individual-based models (IBMs) to investigate different topics related to plant population dynamics: 1. I investigated the role of different modes of competition in altering the prediction of MST on plant self-thinning trajectories. A spatially-explicit individual-based zone-of-influence (ZOI) model was developed to investigate the hypothesis that MST may be compatible with the observed variation in plant self-thinning trajectories if different modes of competition and different resource availabilities are considered. The simulation results supported my hypothesis that (i) symmetric competition (e.g. belowground competition) will lead to significantly shallower self-thinning trajectories than asymmetric competition as predicted by MST; and (ii) individual-level metabolic processes can predict population-level patterns when surviving plants are barely affected by local competition, which is more likely to be in the case of asymmetric competition. 2. Recent studies implied that not only plant interactions but also the plastic biomass allocation to roots or shoots of plants may affect mass-density relationship. To investigate the relative roles of competition and plastic biomass allocation in altering the mass-density relationship of plant population, a two-layer ZOI model was used which considers allometric biomass allocation to shoots or roots and represents both above- and belowground competition simultaneously via independent ZOIs. In addition, I also performed greenhouse experiment to evaluate the model predictions. Both theoretical model and experiment demonstrated that: plants are able to adjust their biomass allocation in response to environmental factors, and such adaptive behaviours of individual plants, however, can alter the relative importance of above- or belowground competition, thereby affecting plant mass-density relationships at the population level. Invalid predictions of MST are likely to occur where competition occurs belowground (symmetric) rather than aboveground (asymmetric). 3. I introduced the new concept of modes of facilitation, i.e. symmetric versus asymmetric facilitation, and developed an individual-based model to explore how the interplay between different modes of competition and facilitation changes spatial pattern formation in plant populations. The study shows that facilitation by itself can play an important role in promoting plant aggregation independent of other ecological factors (e.g. seed dispersal, recruitment, and environmental heterogeneity). In the last part of my study, I went from population level to community level and explored the possibility of combining MST and unified neutral theory of biodiversity (UNT). The analysis of extensive data confirms that most plant populations examined are nearly neutral in the sense of demographic trade-offs, which can mostly be explained by a simple allometric scaling rule based on MST. This demographic equivalence regarding birth-death trade-offs between different species and functional groups is consistent with the assumptions of neutral theory but allows functional differences between species. My initial study reconciles the debate about whether niche or neutral mechanisms structure natural communities: the real question should be when and why one of these factors dominates. A synthesis of existing theories will strengthen future ecology in theory and application. All the studies presented in my dissertation showed that the approaches of individual-based and pattern-oriented modelling are promising to achieve the synthesis.

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