Field studies were conducted over two growing seasons to evaluate the effect of different levels of third instar European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), on whole-plant dry matter in corn grown for silage. Mean (± SEM) whole-plant dry matter was significantly greater by 18.8% in uninfested control plants than in plants with an infestation level of 6 larvae/plant in 2004. Whole-plant dry matter in 2005 was significantly greater by 10.5% in control plants than in plants with an infestation level of 5 larvae/plant. Economic injury levels were calculated for each year using regression equations between whole-plant dry matter and European corn borer infestation level. Plant growth stage and infestation level had no effect on percent acid detergent fiber, neutral detergent fiber, and crude protein values for either year. Greenhouse studies were conducted to examine the relationship between aboveground herbivory by European corn borer and belowground herbivory by root knot nematode, Meloidogyne incognita Chitwood (Tylenchida: Heteroderidae), in corn. Two experiments were conducted to measure belowground herbivory by M. incognita in juvenile penetrations and eggs/root system. In the first experiment, the main effects interaction was not significant for either M. incognita juvenile penetrations or eggs/root system. Overall mean juvenile penetrations/root system across all three growth stages, at infestation levels of 1 and 3 larvae/plant were significantly less than in the non-infested control. In addition, overall mean eggs/root system at an infestation level of 3 larvae/plant were significantly less than in the control. In the second experiment, the main effects interaction was significant for both juvenile penetrations and eggs/root system. At the 8 and 10 leaf growth stages, juvenile penetrations/root system at infestation levels of 1 and 3 larvae/plant were significantly less than in the control. In addition, eggs/root system at an infestation level of 3 larvae/plant were significantly less than in the control, at all growth stages. In the reciprocal study, which examined the effect of different M. incognita inoculation levels on European corn borer stalk tunneling, no significant effect of inoculation level on European corn borer stalk tunneling was found. / Ph. D.

Due to the paucity of long-term harvest impact data, the primary goals of this study were to quantify the long-term effects of different harvest disturbances twenty-four years after harvest on two major wetland functions: stand productivity and C storage. This study evaluated the effects of three harvest types that were originally applied in 1986 to a tupelo (Nyssa aquatic)-cypress (Taxodium distichum) forested wetland in the Mobile-Tensaw River Delta of southwestern Alabama. Treatments were: 1. Helicopter harvest (HELI), 2. Skidder simulation where 50% of the site was rutted to a depth of 30 cm (SKID), and 3. Helicopter harvest followed by glyphosate herbicide removal of all sprouts and seedbank regeneration for two years following harvest (GLYPH). An adjacent mature stand (94 years old) within the same original composition represented mature forest or pre-harvest reference conditions (REF). Above- and belowground plant biomass, belowground woody debris, soil C, and soil CO2 efflux were measured. Twenty-four years after treatments were applied, forest C levels were higher in SKID treatments (206.1 Mg C ha-1) than in HELI treatments (168.7 Mg C ha-1). GLYPH treatments are holding less (144.2 Mg C ha-1) while REF areas hold 332.6 Mg C ha-1. SKID treatments are also holding the most biomass of all treatments with 243.2 Mg ha-1 of overstory biomass. Ecosystem C and biomass patterns indicate HELI and SKID are becoming similar to the original site conditions represented by the REF areas. The resiliency of these highly disturbed stands are explained by the frequent inputs of non-compacted sediments, presence of species well adapted to very poorly drained and aerated conditions, high rates of coppice regeneration, shrink-swell ameliorative properties of the soil and creation of more complex microtopography within SKID treatments. / Master of Science

As florestas tropicais exibem elevada biodiversidade e desempenham um importante papel no ciclo global do carbono. Porém, essas florestas têm sido impactadas aceleradamente nos últimos anos. No Brasil, a floresta tropical Atlântica está restrita a aproximadamente 7 % de sua extensão original e seus remanescentes ainda sofrem ameaças. Com isso, informações sobre a dinâmica da vegetação desse bioma são importantes para entender o funcionamento desse ecossistema e servem de subsídio para auxiliar em sua conservação e restauração. No presente estudo, os principais processos que regem a dinâmica de uma floresta (mortalidade, recrutamento e crescimento) foram estimados em duas fisionomias distintas da floresta tropical Atlântica. Além disso, a variação líquida da biomassa acima do solo (BAS) também foi estimada. Para o presente estudo foram utilizadas cinco (5) parcelas permanentes de um (1) hectare inseridas no projeto temático Gradiente Funcional, sendo: quatro (4) na floresta de Terras Baixas e uma (1) na floresta de Restinga. Os inventários foram realizados no momento da implantação das parcelas permanentes (2006) e após dois (2) anos. Os resultados obtidos demonstraram que a Restinga (1635 ind.ha-1) possui maior densidade total de indivíduos vivos com o DAP \'>OU=\'4,8 cm que a Terras Baixas (1221 ind.ha-1). O estoque de biomassa acima do solo (BAS) foi maior na Terras Baixas (212,3 Mg.ha-1) que na Restinga (166,3 Mg.ha-1) (DAP \'>OU=\'4,8 cm). A distribuição dos indivíduos nas classes de diâmetro influenciou essas diferenças estruturais: a Restinga possui maior densidade total de indivíduos na menor classe de diâmetro e a floresta de Terras Baixas possui maior densidade de indivíduos na maior classe. Tal fato também impactou a distribuição do estoque de BAS nas classes de diâmetro: a floresta de Restinga estoca mais biomassa na classe de 10-30 cm e a floresta de Terras Baixas na maior classe de DAP, \'>OU=\'50 cm. A mediana da taxa de incremento diamétrico da comunidade arbórea não diferiu entre os dois tipos de floresta, sendo igual a 1,0 mm.ano-1 na Restinga e 0,8 mm.ano-1 na Terras Baixas (DAP \'>OU=\'4,8 cm). Diferenças na taxa de incremento diamétrico com relação às formas de vida (árvores e palmeiras) foram observadas. A taxa de incremento diamétrico das árvores apresentou relação positiva com o aumento das classes de DAP e o índice de iluminação de copa. O mesmo padrão não foi observado para as palmeiras. A taxa de mortalidade da comunidade arbórea foi semelhante entre os dois tipos de floresta, sendo igual a 2,46 % na Restinga e 2,00 % na Terras Baixas (DAP \'>OU=\'4,8 cm). A taxa de recrutamento também foi semelhante entre as florestas, sendo igual a 1,42 % na Restinga e 1,36 % na floresta de Terras Baixas. A variação líquida da BAS foi maior nas Terras baixas (0,64 Mg.ha-1.ano-1) que na Restinga (0,32 Mg.ha-1.ano-1). Por fim, a floresta tropical Atlântica apresenta maior densidade de indivíduos vivos, menor estoque de BAS, menor taxa de incremento e reposição quando comparada com outras florestas tropicais, possivelmente devido à distribuição da precipitação ao longo do ano / Tropical forests display a biodiversity unmatched by any other vegetation type and play an important role in the global terrestrial carbon cycle. However, tropical forests have been severely impacted in the last years. In Brazil, the tropical Atlantic forest is restricted to approximately 7 % of its original extension and its fragments still remain threatened. Hence, information regarding the vegetation dynamic of this important biome is important to understand the functioning of this ecosystem and support conservation and restoration actions. Here, the principal processes that drive forest dynamics (mortality, recruitment and growth) were estimated for two tropical Atlantic forest types. Additionally, the net aboveground biomass change (AGB) also was estimated. In this study, five one-hectare permanent plots established by the Gradiente Funcional project were adopted: four (4) plots in lowland and one (1) in seasonally flooded Atlantic forest. The forestry inventories were performed in the same time of permanent plot establishment (2006) and after two (2) years (2008). The results showed that the seasonally flooded Atlantic forest has greater total stem density (DBH \'>OU=\'4.8 cm) than lowland Atlantic forest, equal to 1635 ind.ha-1 in seasonally flooded and 1221 ind.ha-1 in lowland forest. The aboveground biomass (AGB) was greater in lowland than in seasonally flooded forest, equal to 166.3 Mg.ha-1 in seasonally flooded and 212.3 Mg.ha-1 in lowland forest (4,8 cm DBH). The distribution of trees among DBH classes influenced these structural differences: the seasonally flooded forest has high stem density at smaller DBH size class while the lowland forest has high stem density at larger DBH size class. In addition, that variation in size structure also impacted the AGB distribution among DBH classes: the seasonally flooded Atlantic forest has more AGB in the 10-30 cm DBH class and the lowland Atlantic forest has more AGB in the large size class ( \'>OU=\'50 cm). The median growth rate of tree community did not differ between the two forest types, equal to 1.0 mm.yr-1 in seasonally flooded and 0.8 mm.yr-1 in lowland forest ( \'>OU=\'4.8 cm DBH). Differences in growth rates regarding the life forms (tree and palm) were observed. Tree growth rates showed positive relationship with crown illumination index and DBH classes. The same tendency was not observed for palm life form. The mortality rate of tree community did not differ between the two forest types, equal to 2.46 % in seasonally flooded and 2.00 % in lowland forest ( \'>OU=\'4.8 cm DBH). The recruitment rate also did not differ between the two forest types, equal to 1.42 % in seasonally flooded and 1.36 % in lowland forest (\'>OU=\'4.8 cm DBH). The lowland Atlantic forest AGB net change (0.64 Mg.ha-1.yr-1) was higher than seasonally flooded Atlantic forest (0.64 Mg.ha-1.yr-1). Finally, the tropical Atlantic forest has higher stem density, lower AGB, lower growth and turnover rates than other tropical forests probably due to rainfall distribution among year

As florestas tropicais exibem elevada biodiversidade e desempenham um importante papel no ciclo global do carbono. Porém, essas florestas têm sido impactadas aceleradamente nos últimos anos. No Brasil, a floresta tropical Atlântica está restrita a aproximadamente 7 % de sua extensão original e seus remanescentes ainda sofrem ameaças. Com isso, informações sobre a dinâmica da vegetação desse bioma são importantes para entender o funcionamento desse ecossistema e servem de subsídio para auxiliar em sua conservação e restauração. No presente estudo, os principais processos que regem a dinâmica de uma floresta (mortalidade, recrutamento e crescimento) foram estimados em duas fisionomias distintas da floresta tropical Atlântica. Além disso, a variação líquida da biomassa acima do solo (BAS) também foi estimada. Para o presente estudo foram utilizadas cinco (5) parcelas permanentes de um (1) hectare inseridas no projeto temático Gradiente Funcional, sendo: quatro (4) na floresta de Terras Baixas e uma (1) na floresta de Restinga. Os inventários foram realizados no momento da implantação das parcelas permanentes (2006) e após dois (2) anos. Os resultados obtidos demonstraram que a Restinga (1635 ind.ha-1) possui maior densidade total de indivíduos vivos com o DAP \'>OU=\'4,8 cm que a Terras Baixas (1221 ind.ha-1). O estoque de biomassa acima do solo (BAS) foi maior na Terras Baixas (212,3 Mg.ha-1) que na Restinga (166,3 Mg.ha-1) (DAP \'>OU=\'4,8 cm). A distribuição dos indivíduos nas classes de diâmetro influenciou essas diferenças estruturais: a Restinga possui maior densidade total de indivíduos na menor classe de diâmetro e a floresta de Terras Baixas possui maior densidade de indivíduos na maior classe. Tal fato também impactou a distribuição do estoque de BAS nas classes de diâmetro: a floresta de Restinga estoca mais biomassa na classe de 10-30 cm e a floresta de Terras Baixas na maior classe de DAP, \'>OU=\'50 cm. A mediana da taxa de incremento diamétrico da comunidade arbórea não diferiu entre os dois tipos de floresta, sendo igual a 1,0 mm.ano-1 na Restinga e 0,8 mm.ano-1 na Terras Baixas (DAP \'>OU=\'4,8 cm). Diferenças na taxa de incremento diamétrico com relação às formas de vida (árvores e palmeiras) foram observadas. A taxa de incremento diamétrico das árvores apresentou relação positiva com o aumento das classes de DAP e o índice de iluminação de copa. O mesmo padrão não foi observado para as palmeiras. A taxa de mortalidade da comunidade arbórea foi semelhante entre os dois tipos de floresta, sendo igual a 2,46 % na Restinga e 2,00 % na Terras Baixas (DAP \'>OU=\'4,8 cm). A taxa de recrutamento também foi semelhante entre as florestas, sendo igual a 1,42 % na Restinga e 1,36 % na floresta de Terras Baixas. A variação líquida da BAS foi maior nas Terras baixas (0,64 Mg.ha-1.ano-1) que na Restinga (0,32 Mg.ha-1.ano-1). Por fim, a floresta tropical Atlântica apresenta maior densidade de indivíduos vivos, menor estoque de BAS, menor taxa de incremento e reposição quando comparada com outras florestas tropicais, possivelmente devido à distribuição da precipitação ao longo do ano / Tropical forests display a biodiversity unmatched by any other vegetation type and play an important role in the global terrestrial carbon cycle. However, tropical forests have been severely impacted in the last years. In Brazil, the tropical Atlantic forest is restricted to approximately 7 % of its original extension and its fragments still remain threatened. Hence, information regarding the vegetation dynamic of this important biome is important to understand the functioning of this ecosystem and support conservation and restoration actions. Here, the principal processes that drive forest dynamics (mortality, recruitment and growth) were estimated for two tropical Atlantic forest types. Additionally, the net aboveground biomass change (AGB) also was estimated. In this study, five one-hectare permanent plots established by the Gradiente Funcional project were adopted: four (4) plots in lowland and one (1) in seasonally flooded Atlantic forest. The forestry inventories were performed in the same time of permanent plot establishment (2006) and after two (2) years (2008). The results showed that the seasonally flooded Atlantic forest has greater total stem density (DBH \'>OU=\'4.8 cm) than lowland Atlantic forest, equal to 1635 ind.ha-1 in seasonally flooded and 1221 ind.ha-1 in lowland forest. The aboveground biomass (AGB) was greater in lowland than in seasonally flooded forest, equal to 166.3 Mg.ha-1 in seasonally flooded and 212.3 Mg.ha-1 in lowland forest (4,8 cm DBH). The distribution of trees among DBH classes influenced these structural differences: the seasonally flooded forest has high stem density at smaller DBH size class while the lowland forest has high stem density at larger DBH size class. In addition, that variation in size structure also impacted the AGB distribution among DBH classes: the seasonally flooded Atlantic forest has more AGB in the 10-30 cm DBH class and the lowland Atlantic forest has more AGB in the large size class ( \'>OU=\'50 cm). The median growth rate of tree community did not differ between the two forest types, equal to 1.0 mm.yr-1 in seasonally flooded and 0.8 mm.yr-1 in lowland forest ( \'>OU=\'4.8 cm DBH). Differences in growth rates regarding the life forms (tree and palm) were observed. Tree growth rates showed positive relationship with crown illumination index and DBH classes. The same tendency was not observed for palm life form. The mortality rate of tree community did not differ between the two forest types, equal to 2.46 % in seasonally flooded and 2.00 % in lowland forest ( \'>OU=\'4.8 cm DBH). The recruitment rate also did not differ between the two forest types, equal to 1.42 % in seasonally flooded and 1.36 % in lowland forest (\'>OU=\'4.8 cm DBH). The lowland Atlantic forest AGB net change (0.64 Mg.ha-1.yr-1) was higher than seasonally flooded Atlantic forest (0.64 Mg.ha-1.yr-1). Finally, the tropical Atlantic forest has higher stem density, lower AGB, lower growth and turnover rates than other tropical forests probably due to rainfall distribution among year

O mapeamento da estrutura florestal em escala de paisagem nos permite avaliar como as florestas respondem aos impactos da ação humana e a mudanças nas condições ambientais. Neste contexto, a tese tem como objetivo elaborar modelos de estimativa de biomassa acima do solo e fechamento de dossel utilizando imagens de satélite, em diferentes estágios de sucessão de Mata Atlântica localizada em área com complexidade topográfica. Para alcançar este objetivo geral, temos dois objetivos específicos: (1) avaliar o efeito da geomorfologia na modelagem da biomassa florestal e fechamento do dossel; (2) analisar os resultados das estimativas considerando diferentes estágios de sucessão florestal e testar o efeito da radiação solar direta sobre o fechamento do dossel. Primeiro, sumarizamos os mais frequentes temas ecológicos e métodos utilizados na literatura ligados a modelagem de estrutura florestal por meio do sensoriamento remoto. Subsequentemente, utilizamos dados de campo e imagens de satélite (LANDSAT TM e ALOS AVNIR-2) para estimar biomassa e fechamento do dossel. Utilizamos modelo digital de elevação como fonte de informação geomorfológica. Foram encontradas melhores estimativas de biomassa e fechamento do dossel quando integramos as imagens de satélite com uma variável geomorfométrica secundária do relevo (Fator de iluminação), que é baseada no ângulo de incidência da radiação solar sobre faces de morros. O índice \"solo/vegetação\", elaborado no presente estudo, apresentou melhores estimativas de fechamento do dossel quando comparado com a performance de diferentes índices de vegetação. A biomassa estimada pelas imagens possibilitou a diferenciação entre diferentes estágios de sucessão florestal / Mapping forest structure in landscape scale enables the evaluation of how forested areas respond to human impact and environmental conditions. In this context, the thesis aims to evaluate modeling approaches to estimate forest aboveground biomass and canopy closure with satellite images in different successional forest stands located at a rugged terrain region. Towards that goal, the specific objectives are: (1) to evaluate the effect of topographic features in the remotely sensed biomass and canopy closures estimations; (2) to analyze the modeled data over different successional stands of Atlantic Rainforest and test the effect of the annual direct sunlight in the forest canopy closure. First, we summarize the most frequent ecological inferences discussed in the literature and the methods used about forest structure modeling by using remote sensing data. Afterward, ground biophysical forest data and satellite images (LANDSAT TM and ALOS AVNIR-2) were used to estimate biomass and canopy closure. The modeling approach includes topographic features derived from digital elevation model. Our results show improved biomass and canopy closure estimates when the modeling includes satellite data interacting with a secondary geomorphometric variable (the Illumination Factor), that is based on direct solar beam angle. The soil/vegetation index, suggested in the present study, showed a better performance when compared with other vegetation index to estimate canopy closure. The modeled biomass shows evident biophysical distinction among different forest succession stages

In the southern Great Plains, the encroachment of grassland ecosystems by mesquite (Prosopis glandulosa), is widespread, and prescribed fire is commonly used in its control. Despite this, substantial quantitative information concerning their influences on the community composition, functional dynamics, and soil organic carbon (SOC) storage potential of grassland ecosystems is lacking. The objectives of this study were to: a) quantify the effects of seasonal prescribed fire treatments and mesquite encroachment on aboveground net primary productivity (ANPP) and herbaceous community composition; b) characterize SOC pool sizes, turnover, and storage potential relative to vegetation type and fire treatment; c) evaluate the structure and diversity of soil microbial communities relative to vegetation type; and d) characterize the functional diversity of these same microbes using the GeoChip functional gene microarray. Repeated winter and summer fires led to increased ANPP rates (average, 434 and 313 g m-2 y-1, respectively), relative to unburned controls (average, 238 g m-2 y-1), altered herbaceous community composition, and increased the storage of resistant forms of SOC, but did not affect overall SOC storage. Herbaceous ANPP rates did not differ significantly as a result of mesquite encroachment, but herbaceous community composition and SOC storage did. Mesquite soils contained significantly more total, slow-turnover, and resistant forms of SOC than those that occurred beneath C3 or C4 grasses. Similarity among the soil bacterial and fungal communities associated with the major vegetation types in this system was low to moderate. Significant differences were detected among soil fungi, with the mesquite-associated fungi harboring significant differences in community structure relative to the fungal communities associated with each of the other vegetation types examined. Despite this result, few significant differences were detected with respect to the functional diversity of these communities, suggesting either a high degree of functional redundancy, or that the functional differences harbored by these communities are beyond the scope of the GeoChip. The results of this study demonstrate that both fire and mesquite encroachment have the potential to alter ecosystem components and processes significantly, providing new insight regarding the effects of these widespread land use and land cover changes on ecosystem structure and function.

O mapeamento da estrutura florestal em escala de paisagem nos permite avaliar como as florestas respondem aos impactos da ação humana e a mudanças nas condições ambientais. Neste contexto, a tese tem como objetivo elaborar modelos de estimativa de biomassa acima do solo e fechamento de dossel utilizando imagens de satélite, em diferentes estágios de sucessão de Mata Atlântica localizada em área com complexidade topográfica. Para alcançar este objetivo geral, temos dois objetivos específicos: (1) avaliar o efeito da geomorfologia na modelagem da biomassa florestal e fechamento do dossel; (2) analisar os resultados das estimativas considerando diferentes estágios de sucessão florestal e testar o efeito da radiação solar direta sobre o fechamento do dossel. Primeiro, sumarizamos os mais frequentes temas ecológicos e métodos utilizados na literatura ligados a modelagem de estrutura florestal por meio do sensoriamento remoto. Subsequentemente, utilizamos dados de campo e imagens de satélite (LANDSAT TM e ALOS AVNIR-2) para estimar biomassa e fechamento do dossel. Utilizamos modelo digital de elevação como fonte de informação geomorfológica. Foram encontradas melhores estimativas de biomassa e fechamento do dossel quando integramos as imagens de satélite com uma variável geomorfométrica secundária do relevo (Fator de iluminação), que é baseada no ângulo de incidência da radiação solar sobre faces de morros. O índice \"solo/vegetação\", elaborado no presente estudo, apresentou melhores estimativas de fechamento do dossel quando comparado com a performance de diferentes índices de vegetação. A biomassa estimada pelas imagens possibilitou a diferenciação entre diferentes estágios de sucessão florestal / Mapping forest structure in landscape scale enables the evaluation of how forested areas respond to human impact and environmental conditions. In this context, the thesis aims to evaluate modeling approaches to estimate forest aboveground biomass and canopy closure with satellite images in different successional forest stands located at a rugged terrain region. Towards that goal, the specific objectives are: (1) to evaluate the effect of topographic features in the remotely sensed biomass and canopy closures estimations; (2) to analyze the modeled data over different successional stands of Atlantic Rainforest and test the effect of the annual direct sunlight in the forest canopy closure. First, we summarize the most frequent ecological inferences discussed in the literature and the methods used about forest structure modeling by using remote sensing data. Afterward, ground biophysical forest data and satellite images (LANDSAT TM and ALOS AVNIR-2) were used to estimate biomass and canopy closure. The modeling approach includes topographic features derived from digital elevation model. Our results show improved biomass and canopy closure estimates when the modeling includes satellite data interacting with a secondary geomorphometric variable (the Illumination Factor), that is based on direct solar beam angle. The soil/vegetation index, suggested in the present study, showed a better performance when compared with other vegetation index to estimate canopy closure. The modeled biomass shows evident biophysical distinction among different forest succession stages

No description available.

In the southwest United States, the current prolonged warm drought is similar to the predicted future climate change scenarios for the region. This study aimed to determine patterns in vegetation response to the early 21st century drought across multiple biomes. We hypothesized that different biomes (forests, shrublands, and grasslands) would have different relative sensitivities to both climate drivers (precipitation and temperature) and legacy effects (previous-year's productivity). We tested this hypothesis at eight Ameriflux sites in various Southwest biomes using NASA Moderate-resolution Imaging Spectroradiometer Enhanced Vegetation Index (EVI) from 2001 to 2013. All sites experienced prolonged dry conditions during the study period. The impact of combined precipitation and temperature on Southwest ecosystems at both annual and sub-annual timescales was tested using Standardized Precipitation Evapotranspiration Index (SPEI). All biomes studied had critical sub-annual climate periods during which precipitation and temperature influenced production. In forests, annual peak greenness (EVImax) was best predicted by 9-month SPEI calculated in July (i.e., January-July). In shrublands and grasslands, EVImax was best predicted by SPEI in July through September, with little effect of the previous year's EVImax. Daily gross ecosystem production (GEP) derived from flux tower data yielded further insights into the complex interplay between precipitation and temperature. In forests, GEP was driven by cool-season precipitation and constrained by warm-season maximum temperature. GEP in both shrublands and grasslands was driven by summer precipitation and constrained by high daily summer maximum temperatures. In grasslands, there was a negative relationship between temperature and GEP in July, but no relationship in August and September. Consideration of sub-annual climate conditions and the inclusion of the effect of temperature on the water balance allowed us to generalize the functional responses of vegetation to predicted future climate conditions. We conclude that across biomes, drought conditions during critical sub-annual climate periods could have a strong negative impact on vegetation production in the southwestern United States.

En Europe, la prairie occupe près de 40% de la surface agricole utile et fournit un ensemble de services environnementaux et agricoles, tout en constituant un réservoir de diversité végétale et animale. Cet écosystème herbacé, plurispécifique et multifonctionnel est un système biologique complexe qui fait interagir l’atmosphère, la végétation et le sol, via les cycles biogéochimiques, notamment ceux du carbone et de l’azote. Motivées par le maintien des biens et services des prairies face aux changements climatiques et atmosphériques, les recherches actuelles sur l’écosystème prairial s’attachent à étudier l’évolution des processus clés du système prairial (i .e. production, échanges gazeux, changements d’espèce) sous changement climatique complexe. Ce projet de thèse a pour objectif d’étudier in situ les impacts des principales composantes du changement climatique (température de l’air, précipitations, concentration atmosphérique en gaz carbonique) sur des prairies extensives de moyenne montagne. Nous cherchons à mettre en évidence les changements de structure et de fonctionnement de l’écosystème prairial sous l’influence d’un scénario de changement climatique prévu à l’horizon 2080 pour le centre de la France. Ce scénario (ACCACIA A2) prévoit une augmentation de 3.5°C des températures de l’air, une augmentation des concentrations atmosphériques en CO2 de 200 ppm et une réduction des précipitations estivales de 20 %. Nos résultats indiquent qu’à moyen terme (trois ans de traitements expérimentaux) le réchauffement a des effets néfastes sur la production annuelle du couvert végétal. L’effet bénéfique d’une élévation des teneurs en CO2 sur la production aérienne n’apparaît qu’à partir de la troisième année. La richesse spécifique (nombre d’espèces) et les indices de diversité taxonomique n’ont pas montré de variations significatives sous changement climatique. Cependant après trois années de réchauffement, l’abondance des graminées semble être altérée. Contrairement à la production, les traits sont plus affectés par la concentration en CO2 élevée que par le réchauffement. Après trois ans de traitements, des mesures d’échanges gazeux (CO2) à l’échelle du couvert végétal pendant la saison de croissance ont montré un effet négatif du réchauffement sur l’activité photosynthétique du couvert et une acclimatation de la photosynthèse au cours de la saison de croissance sous CO2 élevé. Ces tendances ont aussi été trouvées sur la photosynthèse foliaire d’une des espèces dominantes du couvert (Festuca arundinacea). L’effet négatif direct du réchauffement à l’échelle foliaire semble être associé à une diminution des sucres dans les limbes. L’acclimatation à l’enrichissement enCO2 à l’échelle foliaire, quant à elle, semble être indirectement dépendante du statu hydrique du sol. Notre étude a aussi porté sur l’analyse des échanges gazeux sol-atmosphère d’un des principaux gaz à effet de serre trace des prairies, l’oxyde nitreux (N2O). Malgré une forte variabilité inter- et intra- annuelle, les flux de N2O semblent être favorisés sous réchauffement. L’augmentation de la température affecte aussi positivement les taux de nitrification et leur pool microbien associé (AOB), et les rejets de N2O via dénitrification. De plus, les flux de N2O mesurés aux champs ont montré une corrélation plus forte à la taille des populations microbiennes (nitrifiantes et dénitrifiantes) en traitement réchauffé qu’en traitement témoin. En conclusion, la température semble être le facteur principal dans les réponses de cette prairie aux changements climatiques futurs. De plus, nos résultats suggèrent que le fonctionnement (production, émissions de N2O) des prairies extensives de moyenne montagne est plus vulnérable aux changements climatiques que la structure de la communauté végétale. / In France, the grassland ecosystem represents an important part of the total of agricultural landscape and provides important economic and ecological services. This multifunctional ecosystem is a complex biological system where atmosphere, plants and soil interact together,via the biogeochemical cycles (particularly carbon and nitrogen cycles). In order to maintain goods and services from grasslands in changing environmental conditions, current research on the grassland ecosystem focus on the evolution of key grassland processes (i.e. production,gaseous exchanges, biodiversity) under multiple and simultaneous climate change.This thesis addresses the impacts of the three main climate change drivers (air temperature, precipitation and atmospheric carbon dioxide concentrations) on an extensively-managed upland grassland in situ. We investigated changes in ecosystem function and structure under the influence of a projected climate scenario for 2080 for central France. This scenario (ACCACIA A2) comprises : air warming of 3.5°C, 20 % reduction of the summer precipitation and an increase of 200 ppm in atmospheric carbon dioxide (CO2).Our results indicate that in the medium term (after three years of experimental treatments), warming had negative effects on the annual aboveground production. Elevated CO2 had no significant effects on aboveground production initially, but positive effects on biomass from the third year onwards. Species richness and the indices of species diversity did not show significant differences in response to climate change, but warming was associated with a decline in grass abundance after three years. Contrary to biomass production, plant traits showed a stronger response to elevated CO2 than to warming. After three years of study, canopy-level photosynthesis showed a negative effect of warming but an acclimation to elevated CO2 during the growing season. This pattern was also found for leaf-level photosynthetic rates measured on a dominant grass species (Festuca arundinacea). For Festuca, the direct negative effect of warming was associated with a decrease in leaf fructan metabolism. In contrast, the photosynthetic acclimation under elevated CO2 observed in Festuca seemed closely linked to the indirect effect of soil water content. Our study also examined effects of climate change on one of the main trace greenhouse gases in grasslands, nitrous oxide (N2O). During our study, N2O fluxes showed significant inter-and intra-annual variability. Nevertheless, mean annual N2O fluxes increased in response to warming. Warming had a positive effect on nitrification rates, denitrification rates and the population size of nitrifying bacteria (AOB). Furthermore, field N2O fluxes showed a stronger correlation with the microbial population size in the warmed compared with the control treatment. Overall, warming seems to be the main factor driving ecosystem responses to projected climate change conditions for this cool, upland grassland. In addition, our results suggest that grassland function (aboveground production, N2O emissions) are more vulnerable to complex climate change than grassland community structure for our study system.