Semi-natural grasslands with traditional management are known to be very species-rich, with many plant species strongly associated with the habitat. The last century’s decline of semi-natural grasslands, as a result of land use change and abandonment, has made the remaining semi-natural grassland a high concern for conservation. Since management can be costly and the available resources often are limited, it is important to use the most beneficial management method for preserving and enhancing the biodiversity. One semi-natural grassland type of certain interest around the Baltic region are coastal grasslands. In this study, I investigated vascular plant species occurrence in ten managed coastal grasslands located in the Stockholm archipelago. The effect of recent land-uplift and future sea level rise on the ten coastal grasslands were analyzed as well. A major part of all ten grasslands had been subjected to the process of isostatic land-uplift the past 100 years. Five of the grasslands were currently managed by grazing and the other five were managed by traditional mowing. There was no significant difference in alpha and gamma diversity between the two management types. Though, higher biodiversity values, such as more different species found in total, higher amount of species typically linked to meadows and pastures, less succession species and more red-listed species, were found in the traditionally mowed grasslands. Furthermore, a sea level rise with 40 cm the next 100 years in the region would lead to loss of plant species diversity in the investigated coastal grasslands. One major concern is the limited areas upwards on the coastal grasslands for species to migrate to in the future, which is expected to primarily affect species associated with meadows and pastures. The study highlights the importance of open and managed grassland areas further up the coastal grasslands for local species to migrate to, if the sea level would rise as predicted by future climate scenarios. Albeit weak the results of this study indicate that traditional mowing generates higher biodiversity values compared to grazing, hence it should be the preferred management method in coastal grasslands in the Stockholm archipelago.

Increasing sea levels and anthropogenic disturbances have caused the world’s coastal vegetation to decline 25-50% in the past 50 years. Future sea level rise (SLR) rates are expected to increase, further threatening coastal habitats. In combination with SLR, the Everglades ecosystem has undergone large-scale drainage and restoration changing Florida’s coastal vegetation. Everglades National Park (ENP) has 21 coastal plant species threatened by SLR. My dissertation focuses on three aspects of coastal plant community change related to SLR and dehydration. 1) I assessed the extent and direction coastal communities—three harboring rare plant species—shifted from 1978 to 2011. I created a classified vegetation map and compared it to a 1978 map. I hypothesized coastal communities transitioned from less salt- and inundation-tolerant to more salt- and inundation-tolerant communities. I found communities shifted as hypothesized, suggesting the site became saltier and wetter. Additionally, all three communities harboring rare plants shrunk in size. 2) I evaluated invading halophyte (salt-tolerant) plant influence on soil salinity via a replacement series greenhouse experiment. I used two halophytes and two glycophytes (non-salt-tolerant) to look at soil salinity over time under 26 and 38‰ groundwater. I hypothesized that halophytes increase soil salinity as compared to glycophytes through continued transpiration during dry, highly saline periods. My results supported halophytic influence on soil salinity; however, not from higher transpiration rates. Osmotic or ionic stress likely decreased glycophytic biomass resulting in less overall plant transpiration. 3) I assessed the best plant life-stage to use for on-the-ground plot-based community change monitoring. I tested the effects of increasing salinity (0, 5, 15, 30, and 45‰) on seed germination and seedling establishment of five coastal species, and compared my results to salinity effects on one-year olds and adults of the same species. I hypothesized that seedling establishment was the most vulnerable life-stage to salt stress. The results supported my hypothesis; seedling establishment is the life-stage best monitored for community change. Additionally, I determined the federally endangered plant Chromolaena frustrata’s salinity tolerance. The species was sensitive to salinity >5‰ at all developmental stages suggesting C. frustrata is highly threatened by SLR.

Ecological niche modelling is a method designed to describe and predict the geographic distribution of an organism. This procedure aims to quantify the species-environment relationship by describing the association between the organism’s occurrence records and the environmental characteristics at these points. More simply, these models attempt to capture the ecological niche that a particular organism occupies. A popular application of ecological niche models is to predict the potential distribution of invasive alien species in their introduced range. From a biodiversity conservation perspective, a pro-active approach to the management of invasions would be to predict the potential distribution of the species so that areas susceptible to invasion can be identified. The performance of ecological niche models and the accuracy of the potential range predictions depend on the quality of the data that is used to calibrate and evaluate the models. Three different types of input data can be used to calibrate models when producing potential distribution predictions in the introduced range of an invasive alien species. Models can be calibrated with native range occurrence records, introduced range occurrence records or a combination of records from both ranges. However, native range occurrence records might suffer from geographical bias as a result of biased sampling or incomplete sampling. When occurrence records are geographically biased, the underlying environmental gradients in which a species can persist are unlikely to be fully sampled, which could result in an underestimation of the potential distribution of the species in the introduced range. I investigated the impact of geographical bias in native range occurrence records on the performance of ecological niche models for 19 invasive plant species by simulating two geographical bias scenarios (six different treatments) in the native range occurrence records of the species. The geographical bias simulated in this study was sufficient to result in significant environmental bias across treatments, but despite this I did not find a significant effect on model performance. However, this finding was perhaps influenced by the quality of the testing dataset and therefore one should be wary of the possible effects of geographical bias when calibrating models with native range occurrence records or combinations there of. Secondly, models can be calibrated with records obtained from the introduced range of a species. However, when calibrating models with records from the introduced range, uncertainties in terms of the equilibrium status and introduction history could influence data quality and thus model performance. A species that has recently been introduced to a new region is unlikely to be in equilibrium with the environment as insufficient time will have elapsed to allow it to disperse to suitable areas, therefore the occurrence records available would be unlikely to capture its full environmental niche and therefore underestimate the species’ potential distribution. I compared model performance for seven invasive alien plant species with different simulated introduction histories when calibrated with native range records, introduced range records or a combination of records from both ranges. A single introduction, multiple introduction and well established scenario was simulated from the introduced range records available for a species. Model performance was not significantly different when compared between models that were calibrated with datasets representing these three types of input data under a simulated single introduction or multiple introduction scenario, indicating that these datasets probably described enough of the species environmental niche to be able to make accurate predictions. However, model performance was significantly different for models calibrated with introduced range records and a combination of records from both ranges under the well established scenario. Further research is recommended to fully understand the effects of introduction history on the niche of the species. Copyright / Dissertation (MSc)--University of Pretoria, 2009. / Zoology and Entomology / unrestricted

The aim of my study is to determine the influence of historical management practices on the camposition of vascular species in hilly country forests on the Vysoká stráň hill in the Bohemian Karst Protected Landscape Area in the Czech Republic. My thesis is divided into two parts. The first part of my thesis describes influences of forest management on the herb diversity from different scientific studies. There is some information about the influence of herbivorous grazing, pollarding, litter raking and fire. Secondly provides a case study dealing with the influence of historical management on the vascular species composition in the forest understory. My experimental area was divided into 32 treatments, where three different ways of historical management were used - mowing of the understory vegetation, litter raking and the combination of mowing and litter raking. The fourth area was a control treatment. Change in the herb diversity depending on a time is statistically significant but changes in the herb diversity depending on different ways of historical management are statistically insignificant. We detected a distinct decline in the herb diversity during our experiment, also on the control treatments. The main gradient of vegetation points from mesophilous acidophytes to species characteristic...

Grazing seems to be not only means of conserving species diversity of grasslands but also - in many cases - of its increasing. This thesis tries to find out the mechanisms of grazing that result in increased or decreased species diversity. The experiment was proceeded in Slupenec, a part of Český Krumlov, where horse pastures are situated. In order to compare grazing and mowing I collected phytosociological data from plots 1 x 1 mš. I also collected data about soil moisture, composition and pH in order to find out the main gradients in vegetation. The vegetation data from plots 20 x 20 cmš were collected before and after grazing on three pasturelands. In the chosen locality, higher level of plant species diversity have been proven on pastures in comparison with meadows. There is significantly higher percentage of plots contributing to the higher level of ß diversity on pastures than on meadows. At the same time the variability of vegetation was in most cases explained by the categorial variable pasture / meadow. Concerning selectivity of grazing, it has been proven that horses prefer grazing on species with greater cover abundance. The plots with lower rate of grazed species have got higher ß diversity. In terms of functional traits of plants, it has been proven that higher percentage of no-rosette...

Les espèces végétales envahissantes sont l'un des principaux facteurs de changement de la biodiversité dans les écosystèmes terrestres. Une détection précise et précoce des espèces exotiques est donc cruciale pour surveiller les invasions en cours et pour prévenir leur propagation. Présentement, les méthodes de surveillance des invasions biologiques permettent de suivre la propagation des envahisseurs à travers les aires de répartition géographique, mais une attention moindre a été accordée à la surveillance des espèces envahissantes à travers le temps. Les plates-formes de télédétection, capables de fournir des informations détaillées sur les variations des traits foliaires dans le temps et l'espace, sont particulièrement bien placées pour surveiller les plantes envahissantes en temps réel. Les changements temporels des traits fonctionnels sont exprimés dans la signature spectrale des espèces par des caractéristiques d'absorption spécifiques de la lumière associés aux pigments photosynthétiques et aux constituants chimiques tous deux liés à la phénologie. Ainsi, les variations temporelles dans la réponse spectrale des plantes peuvent être utilisées afin de mieux identifier des espèces individuelles. L'un des envahisseurs les plus problématiques au Canada est le roseau commun, Phragmites australis (Cav.) Trin. ex Steudel sous-espèce australis, dont la propagation menace la biodiversité des écosystèmes de zones humides en Amérique du Nord. Déterminer la période de l'année où cet envahisseur se distingue d’avantage, du point de vue spectral et fonctionnel, des autres plantes de la communauté serait centrale dans une meilleure gestion du roseau commun. Pour ce faire, nous avons utilisé des traits fonctionnels et une série temporelle de données spectrales foliaires à haute résolution au cours d'une saison de croissance à Boucherville, Québec, Canada, afin de déterminer la séparabilité spectrale de l'envahisseur par rapport aux espèces co-occurrentes et comment cette dernière varie à travers le temps. Nos résultats ont révélé que la spectroscopie foliaire a permis de distinguer le phragmite des espèces co-occurrentes avec une précision de plus de 95% tout au long de la saison de croissance – un résultat prometteur pour le futur de la télédétection des espèces végétales envahissantes. / Invasive plant species are one of the main drivers of biodiversity change in terrestrial ecosystems. Accurate detection of exotic species is critical to monitor on-going invasions and early detection of incipient invasions is necessary to prevent further spread. At present, surveillance methods of biological invasions allow to track the spread of invaders across geographic ranges, but less attention has been given to invasive species monitoring across time. Remote sensing platforms, capable of providing detailed information on foliar trait variations across time and space, are uniquely positioned for monitoring invasive plants in real time. Temporal changes in foliar traits are expressed in a species spectral profile through specific absorption features related to variation in photosynthetic pigments and chemical constituents driven by phenology. Thus, variations in a plant’s spectral response can be used to improve the identification of individual species. One of Canada’s most problematic invaders is the common reed, Phragmites australis (Cav.) Trin. ex Steudel subspecies australis, whose spread threatens biodiversity in wetland ecosystems in North America. Determining the time of year when the invader is spectrally and functionally more distinct from other plants in the community would be central to better management of common reed. To do so, we collected a time-series of foliar traits and high-resolution leaf spectral data over the course of a growing season at Boucherville, Quebec, Canada, to determine the spectral separability of the invader from co-occurring species and how its detection varies over time. Our results revealed that leaf-level spectroscopy distinguished Phragmites and co-occurring species with > 95% accuracy throughout the growing season – a promising result for the future remote detection of invasive plant species.

No description available.

Die andauernde Armut, Nahrungsmittel- und Ernährungsunsicherheit im Subsahara-Afrika (SSA) verweist auf die Notwendigkeit für einen Paradigmenwechsel in Ernährungssystemen, der auf die Diversifizierung von Nahrungsmittelketten wie z.B. durch unzureichend genutzten Pflanzenarten (UPS) abzielt. In diesen Pflanzenarten steckt das Potenzial, SSA mit ertragreichen, ernährungssensiblen und resilienten Nahrungsmittelsystemen zu versorgen. Jüngste Studien belegen, dass UPS zur Ernährungssicherheit beitragen, indem sie Grundnahrungsmittel ergänzen, einen Puffer in Zeiten von Knappheit darstellen und die Nahrungsqualität erhöhen. Sie können somit als ein Hebel zur Armutslinderung verstanden werden. Trotz der Bedeutsamkeit von UPS, fokussiert sich die bestehende Literatur vorwiegend auf angebaute Nahrungsmittel und Nutzpflanzen. Diese Dissertation trägt dazu bei, die Faktoren Geschlecht, kollektives Handeln und die Auswahl von Vermarktungskanälen unter den Sammler:innen von UPS zu verstehen. Sie besteht aus drei Aufsätzen, die auf unterschiedliche Weise einen Beitrag zur existierenden Literatur zu UPS leisten. Alle drei Aufsätze basieren auf Haushaltsbefragungen von 864 Baobabsammler:innen (Baobab ist ein Beispiel für UPS) in Malawi, im Jahr 2021. Im ersten Aufsatz analysieren wir die geschlechtsspezifischen Unterschiede bei der Sammlung und Vermarktung von Baobab. Insbesondere evaluieren wir die Unterschiede zwischen weiblichen Baobabmanagerinnen (FBMs) und männlichen Baobabmanagern (MBMs) in Bezug auf die gesammelte Menge Baobab. Wir erweitern unsere Analyse und betrachten auch die Unterschiede zwischen FBMs in Bezug auf ihren Familienstand (verheiratete FBMs vs. unverheiratete FBMs). Bestehende Literatur über Geschlecht und Agrarwirtschaft hat sich überwiegend auf die geschlechtsspezifischen Unterschiede in Bezug auf agrarwirtschaftliche Ertragfähigkeit und die Einführung von Technologien fokussiert, insbesondere im Hinblick auf den Hauptpflanzenbestand in Afrika. Dahingegen existieren begrenzte empirische Untersuchungen über die geschlechtsspezifischen Unterschiede zwischen den UPS-Geschlechtern, besonders in Bezug auf das Sammeln und die Vermarktung von Baobabprodukten. Wir wenden ein exogenes Switching-Regressionsmodell in einem kontrafaktischen Rahmen an. Diese Methode erlaubt uns, Schätzungen über die Kausaleffekte des Geschlechts auf das Sammeln und die Vermarktung von Baobabprodukten anzustellen. Die Ergebnisse zeigen, dass weibliche Baobabmanagerinnen und unverheiratete weibliche Baobabmanagerinnen wahrscheinlicher weniger sammeln und verkaufen als männlichen Baobabmanager und verheiratete weibliche Baobabmanagerinnen. Die empirischen Ergebnissen zeigen, dass die geschlechtsspezifischen Unterschiede auch unter Kontrolle sozioökonomischer Faktoren (z.B. Alter, Bildung, Familienstand) bestehen bleiben. Dementsprechend deuten diese Ergebnisse darauf hin, dass es unbeobachtete Faktoren wie z.B. Normen und Führungsqualitäten gibt, die geschlechtsspezifische Unterschiede beeinflussen. In dem zweiten Aufsatz untersuchen wir die Wirkung kollektiven Handelns auf das Wohlergehen der Haushalte von UPS-Sammler:innen. Die Anzahl von Studien über kollektives Handeln und seine Wirkung auf das Wohlergehen von Haushalten in Entwicklungsländern wächst. Jedoch sind die empirischen Ergebnisse und Aussagen gemischt und nicht aussagekräftig. Ein Großteil dieser Wirkungsstudien konzentrieren sich auf Grundnahrungsmittel und hochwertige Nutzpflanzen, mit einem geringem Anteil an Studien über UPS. Dennoch fehlen empirische Befunde und Aussagen über das südliche Afrika, insbesondere Malawi, und in Bezug auf Baobab. Einige Studien über UPS berücksichtigen den Selfselektionsbias nicht. Des Weiteren geben diese Studien keinen Auskunft über die Heterogenität der Wirkungen, insbesondere in Bezug auf das Geschlecht Wir verwenden den inversen wahrscheinlichkeitsgewichteten Regressionsanpassungsschätzer (IPWRA), um die durchschnittlichen und heterogenen Verfahrenseffekte kollektiven Handelns auf Baobab-Einkommen und Ernährungssicherung zu schätzen. Wir stellen fest, dass die Mitgliedschaft in Genossenschaften das Baobab-Einkommen, den Vielfaltswert in der Haushaltsernährung, und den Nahrungsmittelkonsumwert jeweils um 3,57%, 11% und 5,6% erhöht. Die Wirkung der Genossenschaftsmitgliedschaft ist bei männlichen Baobabmanagern höher. Dahingegen weisen Haushalte mit unverheirateten, weiblichen Managerinnen bessere Wirkungen auf das Gemeinwohl auf. Die empirischen Befunde beleuchten daher die Notwendigkeit, kollektives Handeln im unzureichend genutzten Pflanzensektor durch Genossenschaften zu unterstützen, um das Gemeinwohl der Haushalte zu verbessern. In der dritten Aufsatz evaluieren wir die durchschnittlichen Verteilungseffekte der Auswahl von Vermarktungskanälen in Bezug auf das Einkommen und die Ernährungssicherheit von Baobabsammler:innen. Sie beteiligen sich hauptsächlich in drei Vertriebskanälen: Im landwirtschaftlichen Betrieb, in ländlichen Märkten und urbanen Märkten. Wir zeigen, dass die Beteiligung an Vertriebskanälen außerhalb der Landwirtschaft, also Off-Farm Marktkanälen, (ländliche und urbane Märkte) im Verhältnis zu landwirtschaftlichen Vertriebskanälen höhere Baobab-Einkommen und Ernährungssicherheit zur Folge hat. Wir wenden den Quantilbehandlungseffekt an, um zu zeigen, dass der durchschnittliche Behandlungseffekt in der Umstellung von landwirtschaftlichen Vertriebskanal zu Vertriebskanälen außerhalb der Landwirtschaft, also Off-Farm Vertriebskanälen, sich je nach Quantil des Baobab-Einkommens und der Ernährungssicherheit unterscheidet. Die Baobab-Einkommenseffekte von Vertriebskanälen außerhalb der Landwirtschaft, also Off-Farm Marktkanäle, im Vergleich zu landwirtschaftlichen Vertriebskanäle, also Farm-Gate Marktkanälen, sind nur für Baobabsammler:innen im oberen Quantil der Einkommensverteilung positiv. In Bezug auf die Ernährungssicherheit ist der positive prozentuale Effekt für Haushalte mit einem niedrigeren Maß an Ernährungssicherheit höher. Basierend auf den drei Aufsätzen dieser Dissertation, ziehen wir folgende Schlussfolgerungen: Aus dem ersten Aufsatz kommen wir zum dem Schluss, dass es geschlechtsspezifische Unterschiede bei der Sammlung und Vermarktung von Baobab gibt. Die Ergebnisse bestätigen auch, dass diese Unterschiede variieren wenn das Geschlecht mit dem Familienstand interagiert. Bemühungen zur Verminderung geschlechtsspezifischer Unterschiede in der Produktion und Vermarktung sollten auf die Befähigung von Frauen / weiblichen Personen sowie auf die zugrundeliegenden Ursachen von Geschlechterungleichheiten ausgerichtet sein. Aus dem zweiten Aufsatz schlussfolgern wir, dass kollektives Handeln zur Verbesserung der Einkommen und Ernährungssicherheit von Baobabsammler:innen beitragen kann. Die wichtigste Auswirkung für die Politikausrichtung ist, dass Bündnisse zwischen den Akteuren im UPS-Sektor ein wichtiges Instrument zur Verbesserung des Gemeinwohls der Haushalte von UPS-Sammler:innen sind. Wir betonen jedoch, dass dieser positive Effekt ungleichmäßig auf alle Genossenschaftsmitglieder verteilt ist. Wiederum kommen wir aus dem dritten Aufsatz zu der Schlussfolgerung, dass die Beteiligung an Vertriebskanäle außerhalb der landwirtschaftlichen Marktkanäle, also Off-Farm Marktkanälen, im Verhältnis zu landwirschaftlichen Vertriebskanälen, also Farm-Gate Marktkanälen, zu einem verbesserten Baobab-Einkommen und einer besseren Ernährungssicherheit unter den Baobabsammler:innen beiträgt. Daher sollte der Zugang zu Verkaufsstellen außerhalb der Landwirtschaft für Baobabsammler:innen verbessert werden. / The persistence of poverty and food and nutrition insecurity in Sub Saharan Africa (SSA), highlights the need for a paradigm shift in food systems that focuses on diversification into other food chains such as underutilised plant species (UPS). These crops have the potential to provide SSA with a productive, nutrition-sensitive, and resilient food system. Recent evidence concludes that UPS contribute to food security by supplementing staples, acting as a buffer in time of shortages, and increasing diet quality, and can be viewed as a lever towards poverty alleviation. Despite the significance of UPS, previous literature has extensively focussed on food and cash crops. This dissertation contributes to understanding of the aspects of gender, collective action and market channel choices among collectors of UPS. It is composed of three essays that contribute to the literature on UPS in different ways. All the three essays are based on a household survey of 864 baobab (an example of UPS) collectors in Malawi in the year 2021. The first essay analyses the gender gaps in the collection and marketing of baobab. In particular, I evaluate the differences in the quantity of baobab collected between male baobab managers (MBMs) and female baobab managers (FBMs). We further extend our analysis and also evaluate differences among FBMs relative to their marital status (i.e. married FBMs vs unmarried FBMs). The literature on gender and agriculture has mainly focused on gender gaps in agricultural productivity and technology adoption, especially for major crops in Africa. However, there is limited empirical evidence on gender gaps in UPS, especially in terms of collection and marketing. We employ an exogenous switching regression model in a counterfactual framework, as this method allows us to estimate the causal effects of gender in the collection and marketing of baobab products. We find that female baobab managers and unmarried female managers are more likely to collect and sell less than male baobab managers and married female managers, respectively. The results demonstrate that the gender gap persists even after controlling for observed socio economic factors (for example, age, education, marital status). These findings mean that there are unobserved factors such as norms and managerial ability that influence the gender gaps. In the second essay, we examine the impact of collective action on household welfare of collectors of UPS. The number of impact studies on collective action and their effect on household welfare in developing countries is growing, yet the findings are mixed and the evidence is inconclusive. Most of these impact studies focus on staple and high-value crops with a few studies on UPS. However, there is no empirical evidence from Southern Africa, particularly Malawi and specific to baobab. In the case of UPS, some studies fail to account for self-selection bias. Further, these studies do not provide an account of heterogeneity in impacts, particularly with respect to gender. We use the Inverse Probability Weight Regression Adjustment estimator (IPWRA) to estimate average and heterogeneous treatment effects of collective action on baobab income and food security. We find that the cooperative membership increases baobab income, household dietary diversity score, and food consumption score by 3.57%, 11% and 5.6%, respectively. However, the impact of cooperative membership was higher in male baobab managers. Households with unmarried female managers have better welfare outcomes. The results, therefore, highlight the need to promote collective action through cooperatives in the underutilised crop sector to enhance household welfare. In the third essay, we evaluate the average and distributional effects of market channel choices on income and food security of baobab collectors. Baobab collectors mainly participate in three market channels; farmgate, rural market, and urban market. We show that participation in off-farm market channels (rural and urban market) relative to farmgate, was associated with increases in baobab income and food security. We use quantile treatment effect to show that the average treatment effect of moving from farm gate market channels to off-farm market channels differed across quantiles of baobab income and food security. For baobab income, the effects of the off-farm market channels compared to the farm gate market are positive only for baobab collectors in the upper quantile of the income distribution. In terms of food security, the positive percentage effect is higher for households with low levels of food security. Based on the three essays in this dissertation, we draw the following conclusions. From the first essay we conclude that there are gender gaps in the collection and marketing of baobab. The results also confirm that these gaps vary as gender interacts with marital status. Efforts to reduce gender differences in production and marketing should focus on empowering women and also addressing the underlying causes of gender inequality. From the second essay, we conclude that collective action can contribute to better incomes and food security among baobab collectors. The important policy implication is that organization among actors in the UPS sector is a major mechanism of improving household welfare of collectors of UPS. But we stress that this positive effect is not distributed uniformly across all cooperative members. From the third essay, we conclude that participation in off-farm market channels relative to farmgate contributes to improved baobab income and food security among baobab collectors. Hence baobab collectors’ access to off-farm selling outlets should be enhanced.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Ecosystems currently face widespread biodiversity losses and other environmental disturbances, such as climate warming, related to increased anthropogenic activities. Within this context, scientists consider the effects of such changes on the biodiversity, and hence on the activity, of soil microorganisms. Indeed, soil microorganisms mediate a wide range of soil processes. Currently, knowledge on soil microbial diversity is still limited, partially due to technical limitations. The advent of molecular-based analyses now allows studying the soil microbial diversity. These advances in the study of soil microbial communities have lead to a growing evidence of the critical role played by the microbial community in ecosystem functioning. This relationship is supposed to be relevant for narrow processes, regulated by a restricted group of microorganisms, such as the nitrification process. This PhD thesis aimed at studying ammonia oxidizing bacteria (AOB) community structure and richness as an integrated part of soil functioning. This research aimed at investigating the effect of aboveground plant diversity on ammonia oxidizing bacteria diversity and function in forest and grassland soils with focus on the influence of (a) functional group identity of grassland plants (legumes, grasses, forbs), (b) grassland plant species richness and (c) tree species, on AOB diversity and function. Another objective of this research was to study the effect of a 3°C increase in air temperature on AOB diversity and function. The link between AOB diversity and function (potential nitrification) is also investigated. For grassland ecosystems, a microcosm experiment was realized. An experimental platform containing 288 assembled grassland communities was established in Wilrijk (Belgium). Grassland species were grown in 12 sunlit, climate controlled chambers. Each chamber contained 24 communities of variable species richness (S) (9 S=1, 9 S=3 and 6 S=9).The grassland species belonged to three functional groups: three species of each grasses (Dactylis glomerata L., Festuca arundinacea SCHREB., Lolium perenne L.), forbs (non-N-fixing dicots; Bellis perennis L., Rumex acetosa L., Plantagolanceolata L.), and legumes (N-fixing dicots; Trifolium repens L., Medicago sativa L., Lotus corniculatus L.). Half of these chambers were exposed to ambient temperature and the other half were exposed to (ambient +3°C) temperature. One ambient and one (ambient+3°C) chambers were destructively harvested 4, 16 and 28 months after the start of the experiment. The influence of plant functional group identity on the nitrification process and on AOB community structure and richness (AOB diversity) was assessed in soils collected from the first two destructive amplings (chapter 2). The effect of plant species richness on AOB diversity and function was considered for soils sampled after 16 and 28 months (chapter 3). AOB function was determined by potential nitrification. AOB community structure and richness were assessed by polymerase chain reaction followed by denaturing gradient gel electrophoresis (DGGE) and sequencing of excised DGGE bands. I found that functional group identity can affect AOB community structure. In particular, the presence of legumes, both in monoculture or in mixture with forbs and grasses, lead to AOB community composition changes towards AOB clusters tolerating higher ammonium concentrations. This change in AOB community structure was only linked to increased potential nitrification under monocultures of legumes, when ammonium was supposed to be not limiting. This study revealed that physiological attributes of AOB and resource availability may be important factors in controlling the nitrification process. This research showed that the impact of plant species richness on the nitrification process could be mediated by the interactions between plants and AOB, through competition for substrate. A 3°C increase in air temperature did not affect AOB community structure, richness or function. In forest ecosystems, we studied the effect of tree species in forest sites located in Belgian and in the Grand-Duchy of Luxembourg covered each by several deciduous or coniferous tree species (Fagus sylvatica L., Quercus petraea (Mattuschka) Lieblein, Picea abies (L.) Karst, Pseudotsuga menziesii (Mirbel) Franco). We investigated the influence of these tree species on microbial processes (chapter 5) related to C and N cycling, particularly with emphasize on the nitrification process and on the diversity of AOB (chapter 6). The results showed that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Influence of tree species on nitrification (potential and relative) might be related to the presence of ground vegetation through its influence on soil ammonium and labile C availability. AOB community structure was more site-specific than tree specific. However, within sites, AOB community structure under broadleaved trees differed from the one under coniferous trees. The effect on tree species on AOB was likely to be driven by the influence of tree species on net N mineralization, which regulates the substrate availability for AOB. The results also demonstrated that the relationship between AOB diversity and function might be related both to AOB abundance and AOB community structure and richness. This thesis showed no clear relationship between AOB community structure or richness and AOB function. However, we revealed that aboveground grassland plant richness, grassland plant functional groups and tree species influence AOB community structure and richness. Actuellement, les écosystèmes sont soumis à dimportantes pressions anthropiques et environnementales, pouvant aboutir à des pertes massives de biodiversité. Les scientifiques sinterrogent sur limpact de ces perturbations sur la diversité et, par conséquent, sur lactivité des microorganismes du sol. En effet, ceux-ci régulent de nombreux processus du sol. Actuellement, de nombreuses lacunes subsistent dans la connaissance de la diversité microbienne du sol. Celles-ci peuvent être partiellement attribuées aux difficultés méthodologiques associées à l'étude des micro-organismes du sol. Lavènement des techniques moléculaires nous permet de combler ces lacunes. Les avancées réalisées dans l'étude des communautés microbiennes du sol ont mis en évidence le rôle crucial joué par les communautés microbiennes dans le fonctionnement des écosystèmes. De plus, il semblerait que les processus régulés par un groupe restreint dorganismes, tel le processus de nitrification, soient plus sensibles à toute altération de la communauté. Lobjectif de cette thèse était détudier la structure de la communauté ainsi que la richesse (nombre de bandes DGGE) des bactéries oxydant lammoniac (AOB) comme une partie intégrante du fonctionnement des sols. Notre étude se focalisait sur linfluence de (a) différents groupes fonctionnels de plantes (graminées, légumineuses, dicotylédones), (b) communautés de plantes présentant une richesse spécifique croissante et (c) différentes essences forestières, sur la diversité (structure de la communauté et richesse des AOB) et la fonction des AOB. Cette recherche étudiait également limpact dune augmentation de température de 3°C sur ces paramètres. Létablissement dun lien éventuel entre la diversité et la fonction (nitrification potentielle) des AOB a aussi été envisagé. Concernant les écosystèmes prairiaux, nous avons réalisé une étude en microcosmes. Une plateforme expérimentale comprenant 288 communautés artificielles de plantes a été établie à Wilrijk (Belgique). Cette plateforme consistait en 12 chambres, dont une moitié était à température ambiante et la seconde était à température ambiante augmentée de 3°C. Chaque chambre contient 24 communautés de plantes de richesse spécifique variable (9 S=1, 9 S=3 et 6 S=9). Les communautés de plantes sont créées avec 9 espèces de plantes appartenant à trois groupes fonctionnels : 3 espèces de graminées (Dactylis glomerata L., Festuca arundinacea SCHREB., Lolium perenne L.), de légumineuses (dicotylédones fixatrices dazote ;Trifolim repens L., Medicago sativa L., Lotus corniculatus L.), et de dicotylédones non fixatrices dazote (Bellis perennis L., Rumex acetosa L., Plantago lanceolata L.). Les sols issus dune chambre à température ambiante et dune chambre à température ambiante augmentée de 3°C ont été échantillonnés, respectivement, 4, 16 et 28 mois après le début de lexpérimentation. Linfluence des groupes fonctionnels de plantes sur le processus de nitrification ainsi que sur la structure de la communauté et la richesse des AOB a été mesuré sur les sols issus des deux premiers échantillonnages (chapitre 2). Nous avons mesuré leffet de la richesse croissante en plantes sur la diversité et lactivité des AOB sur les sols échantillonnés après 16 et 28 mois dexpérimentation (chapitre 3). La structure de la communauté ainsi que la richesse des AOB ont été évaluées à laide dune amplification spécifique par réaction de polymérisation en chaîne (PCR) de lADN génomique extrait du sol suivie par une séparation par électrophorèse sur gel dacrylamide en présence dun gradient dénaturant (DGGE). Nous avons identifié les différentes AOB présentes par séquençage des bandes DGGE excisées. Nos résultats ont montré que les différents groupes fonctionnels peuvent affecter la structure de la communauté des AOB. En particulier, la présence de légumineuses, aussi bien en monoculture quen mélange avec des graminées ou des dicotylédones non fixatrices dazote, provoque des changements au sein de la structure de la communauté des AOB, favorisant la présence de clusters tolérants des concentrations en ammonium plus élevées. Ces changements de la structure de la communauté des AOB sont liés à des augmentations de la production potentielle de nitrates (nitrification potentielle) quand lammonium est supposé être non limitant. Cette étude révèle que la physiologie des AOB ainsi que la disponibilité en substrat peuvent être des facteurs majeurs intervenant dans le contrôle du processus de nitrification. Cette recherche montre que linfluence de la richesse spécifique des plantes sur la nitrification pourrait dépendre des interactions entre les plantes et les AOB via la compétition pour le substrat. Une augmentation de la température de lair de 3°C na pas influencé les richesse, structure de la communauté ou les fonctions des AOB. Pour les écosystèmes forestiers, nous aborderons leffet de différentes essences forestières (Picea abies (L.) KARST, Fagus sylvatica L., Quercus petraea LIEBLEIN ; Pseudotsuga menziezii (MIRB.) FRANCO) dans différents peuplements au Grand Duché du Luxembourg et en Belgique. Nous avons étudié l'influence de ces essences forestières sur les processus microbiens (chapitre 5) liés aux cycles du carbone et de lazote, en particulier leur effet sur le processus de nitrification et la diversité des AOB (chapitre 6). Notre étude révèle que linfluence des essences forestières sur la minéralisation nette de lazote pourrait être attribuable à leur effet sur la biomasse microbienne, reflétant ainsi leur effet sur la teneur en matière organique et la disponibilité en carbone. Limpact des essences forestières sur la nitrification (à la fois sur la nitrification relative et sur la nitrification potentielle) serait conditionné par la présence de végétation au sol, en raison de linfluence de celle-ci sur la disponibilité en ammonium et en carbone labile. Nous avons observé que la structure de la communauté des AOB était plus spécifique aux sites quaux essences forestières. Cependant, au sein dun site, elle différait sous feuillus et sous conifères. Les essences forestières influenceraient la structure de la communauté des AOB au travers de limpact quelles ont sur la minéralisation nette de lazote qui régule, quant à elle, la disponibilité en ammonium. Cette recherche démontre que le lien observé entre la diversité et la fonction dépendrait la fois de labondance, de la structure de la communauté et de la richesse des AOB. Cette thèse na révélé aucune relation claire entre la structure de la communauté ou la richesse des AOB et leur fonction. Par contre, nous avons observé que la richesse spécifique et les groupes fonctionnels de plantes prairiales et les essences forestières affectent la structure de la communauté et la richesse des AOB.