The role of disturbance is widely recognized as a fundamental driver of ecological organization from individual species to entire landscapes. Anthropogenic disturbances from military training provide a unique opportunity to examine effects of disturbance on vegetation dynamics, physicochemical soil properties, and ecosystem processes. Additionally, plant functional diversity has been suggested as the key to ecosystem processes such as productivity and nutrient dynamics. I investigated how disturbance and functional composition both singly and in combination affect vegetation dynamics, soil physicochemical properties, and ecosystem processes. I conducted my research at Fort Pickett, Virginia, USA to take advantage of the spatially and temporally predictable disturbance regime. In order to investigate the effect of plant functional composition on ecosystem properties, I used functional groups comprised of species with similar physiology and effects on ecosystem processes (C4 grasses, C3 grasses, legumes, forbs, woody plants). My study showed that two distinct disturbances associated with military training, vehicle maneuvers, and fire; affect functional group abundance, within functional group richness, and total species richness. I found strong effects of vehicle maneuvers on soil physical properties including an increase in bulk density and reduction in soil porosity. Fire also influenced soil physical properties but more indirectly through the reduction of above ground litter inputs. Though many of the measured physicochemical soil properties at Fort Pickett exhibited statistically significant effects of disturbance, the strength of these relationships appears to be modulated by influences of previous land use. I found statistically significant (P < 0.05) effects of disturbance on chlorophyll fluorescence, and effect of functional composition on available soil N- NH4+. In addition, I detected a significant interactive effect of disturbance class and functional composition on soil CO2 flux. The interactive effects of disturbance and functional composition on soil CO2 flux demonstrated how the loss of functional diversity could lead to instability in ecosystem processes in disturbed ecosystems.    In a dynamic ecosystem, I demonstrated that the abundance and diversity of plant functional groups was significantly influenced by disturbance. By experimentally altering the abundance and diversity of these functional groups in a disturbance-mediated ecosystem, I showed that functional groups and presumably species influence key ecosystem processes. / Ph. D.

Carbon sequestration is paramount to reducing climate change. Grasslands, representing 40% of all terrestrial area, can serve as a primary sequestration location if optimal management strategies can be realized. This study used system dynamics modeling to examine the temporal dynamics of carbon stocks and flows in response to grass species composition, grazing intensity, and temperature and precipitation changes at the landscape level. While there are other biogeochemical models in existence, they are either meant to model large areas, including globally, or are meant to be at a farm level and have limited plot sizes, limiting the options for rangeland managers to test management strategies in larger areas. The aims included conducting a field study of the rangeland, create an initial model; evaluate how the model responded to grazing, temperature, and precipitation changes; and compare the model outcomes to prior work to test the behavior of the model as the start of validation. This thesis used four plant functional groups (C3 and C4 grasses, forbs, and legumes) as the base groups for the model. C4 grasses were not found in in the field study but served to test whether the model detected changes in sequestration when grassland composition is changed. The results demonstrated an approach of using functional groups in system dynamics modeling to optimize carbon sequestration while accounting for diverse management strategies, as has been seen in other biogeochemical models. The model was aligned with prior field research in terms of carbon sequestration levels. The model was able to note differences in grazing regimes, temperature, and precipitation changes in terms of carbon sequestration. Grazing scenarios showed that while increased grazing impacted aboveground litter, it had little impact on sequestration; there was only a 4% increase in carbon with no grazing, Changes in temperature, up to 3°C, were predicted to increase carbon sequestration by 16% from 0.442 to 0.514 kg*m-2*day-1 while decreases in precipitation, both alone and in combination with increasing temperatures, was predicted to decrease sequestration up to 44%. This has to do with the grassland composition, ii especially as this was a C3 dominated grassland which grows in the winter and early spring and required more water but lower temperatures for growth. Future research should continue model validation, test additional functional groups like shrubs, implement more soil carbon pools and flows and add a nitrogen component to the model.

The arrival order of species, also known as priority effects, can have big impacts on the future plant community assembly and diversity. Knowledge about priority effects can be of importance in grassland restoration and to counteract invasive species invasion. The aim of this study was to evaluate if and how much plant traits change with arrival order. Here, 15 grassland species, divided into functional groups (forbs, grasses, and legumes) were sown in a greenhouse. The plants were subject to one of three treatments: ‘Control Alone’ (CA), where each plant was grown alone, ‘Control Together’ (CT), where two plants arrived at the same time and ‘Priority effect’ where the first plant (PE.1) arrived three weeks before the second plant (PE.2). For plants grown alone, there was significant differences between the functional groups for the traits mass relative growth rate, height relative growth rate, plant height, plant width and width of stolons. Forbs had significantly largest relative growth rate (RGR), smallest height after three and six weeks of growth as well as largest stolons at both times. Grasses showed significant largest width at three weeks of growth and legumes reached about the same width as grasses after six weeks of growth. The plants that arrived second showed significantly less growth compared to the other treatments which imply that priority effects occurred. The growth strategy for forbs was fast RGR and production of stolons, for grasses it was primarily an increased width and for legumes it was a combination of reaching large height and width.

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.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-11-03T13:38:20Z No. of bitstreams: 2 Dissertação - Joicy Martins Morais - 2013.pdf: 662106 bytes, checksum: c4e0fdd3441863cf9479d5c5b0b5be2f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2014-11-04T09:48:31Z (GMT) No. of bitstreams: 2 Dissertação - Joicy Martins Morais - 2013.pdf: 662106 bytes, checksum: c4e0fdd3441863cf9479d5c5b0b5be2f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2014-11-04T09:48:31Z (GMT). No. of bitstreams: 2 Dissertação - Joicy Martins Morais - 2013.pdf: 662106 bytes, checksum: c4e0fdd3441863cf9479d5c5b0b5be2f (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2013-11-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The relationships between organisms and the mechanisms that enable their coexistence are important issues in ecology. Plant removal experiments are useful to elucidate these mechanisms because its cause and effect response. This thesis aims to analyze experiments removal plants. The first paper presents a scientometric review of the literature on plant functional groups removal 1991. We verify aspects such as the number of publications and citations in journals indexed in the Web of Science, which country was made in order to detect bias and geographic gaps, which ecological effects were tested, which functional groups were removed, which removal methods and which environments were made. We found an increase in the number of publications and citations on average during this period, however, was 12 times lower than publications on functional diversity, perhaps due to the difficulty of conducting experiments. The work was mainly carried out in temperate climates where taxonomic knowledge is greater, in addition, there is a prevalence of vegetation that is most commonly removed than trees. Testing interactions, especially competition, was the main reason for removal, perhaps as a matter of historical competition is considered the interaction that influences the co-occurrence of species. The cut at ground was the major method of removing, undisturbing the soil and avoiding undesirable effects of herbicides, and few studies have been made in a greenhouse and in the natural environment at the same time, which could elucidate responses to natural conditions and at the same time responses specific interactions. In the second paper we carried a case study on removal of capim-flecha ( Tristachya leiostachya Ness.), a dominant grass in Emas National Park (ENP), and what the consequences of such removal to the co-occurring grasses. We expected that the absence of capim-flecha allow further growth of grasses, however we found no difference in mean values between treatments indicating that there is no competition influencing the abundance of capim-flecha. The historical context burned in PNE may have influenced this high dominance of capim-flecha. We believe that experiments are useful in testing interactions, especially if they are made in the natural environment and greenhouse at the same time. / As relações entre os organismos e os mecanismos que permitem sua coexistência são importantes questões em ecologia. Experimentos de remoção de plantas são úteis em elucidar esses mecanismos, devido sua capacidade de resposta do tipo causa e efeito. Esta dissertação se propõe a analisar experimentos de remoção de plantas. O primeiro artigo apresenta uma revisão cienciométrica das publicações sobre remoção de grupos funcionais de plantas, desde 1991. Verificamos aspectos como o número de publicações e citações em periódicos indexados no Web of Science, em qual país foi feito, visando detectar vieses e lacunas geográficas, quais efeitos ecológicos testados, quais grupos funcionais removidos, quais os métodos de remoção e em quais ambientes foram feitos. Encontramos um crescimento no número de publicações bem como na média de citações nesse período, contudo foi 12 vezes menor do que publicações sobre diversidade funcional, talvez devido à dificuldade de realizar experimentos. Os trabalhos foram realizados principalmente em climas temperados onde o conhecimento taxonômico é maior, além disso, há um predomínio de vegetação rasteira que é mais comumente removida do que árvores. Testar interações, principalmente competição, foi o principal motivo de remoção, talvez por uma questão histórica de competição ser considerada a interação que mais influencia a co-ocorrência de espécies. O corte rente ao solo foi o principal método de remoção, evitando revolver o solo e efeitos indesejáveis de herbicidas, e poucos trabalhos foram feitos em casa de vegetação e ambiente natural ao mesmo tempo, o que poderia elucidar respostas à condições naturais e ao mesmo tempo respostas específicas de interações. No segundo capítulo fizemos um estudo de caso sobre remoção de capim-flecha (Tristachya leiostachya Ness.), uma gramínea dominante no Parque Nacional das Emas (PNE), e quais as consequências dessa remoção para as gramíneas co-ocorrentes. Esperávamos que a ausência do capim-flecha permitiria um maior crescimento das gramíneas, contudo não encontramos diferença nos valores das médias entre os tratamentos o que indica que não há competição influenciando a abundância do capim-flecha. O contexto histórico queimadas no PNE pode ter influenciado essa alta dominância do capim-flecha. Consideramos que experimentos são úteis em testar interações, sobretudo se forem feitos em ambiente natural e casa de vegetação ao mesmo tempo.

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