L’étude du cycle du carbone (C) dans les écosystèmes terrestres est fondamentale afin de comprendre leur rôle dans le cycle du carbone ainsi que les répercussions du changement climatique sur ces écosystèmes. L'objectif général de la thèse qui est de faire un suivi intra-annuel de l’allocation du C au sein de différents écosystème terrestre (forêt et prairie), se place dans ce contexte. Dans un premier temps les efficiences de production de biomasse aérienne (aBPE, rapport entre les quantités de carbone stockées dans la biomasse aérienne et absorbées) sont calculées pour des intervalles de temps réduits, du mois à la semaine selon l’écosystème (forêt et prairie). Dans un second temps, une estimation plus précise de l'allocation du carbone en forêt a été effectuée, en suivant la formation de certains composés structuraux (hémicelluloses et cellulose+lignine). Un possible lien entre la variation de cette allocation selon les conditions climatiques a été étudiée. Les efficiences correspondantes ont également été calculées mensuellement pour avoir une idée plus précise du stockage durable du C dans cet écosystème. Une comparaison avec la prairie a été réalisée en termes d’aBPE afin d'analyser les capacités respectives des 2 écosystèmes à s'adapter aux changements climatiques et à jouer un rôle dans la mitigation des gaz à effet de serre. Dans le cadre de l'Observatoire Pérenne de l'Environnement, deux écosystèmes voisins situés en Meuse (55) ont été instrumentés dans le but de mesurer en continu les échanges de CO2 à l’interface couvert‐atmosphère, ainsi que les conditions micrométéorologiques dans lesquelles ces échanges se produisent. De plus, un suivi régulier du développement des écosystèmes a également été réalisé / The study of the carbon (C) cycle is important in terrestrial ecosystems (forest, grassland) in order to understand their behavior, their role in the C cycle and also the impact of climate change on them. The general study of this thesis which is to monitor the intra-annual allocation of C into different terrestrial ecosystem (forest and grassland), lays within this context. Firstly we calculated aboveground biomass production efficiencies (aBPE, ratio between the quantity of C stored into the aboveground biomass and absorbed) intra-annually, weekly or monthly depending on the ecosystem (grassland and forest respectively). Secondly a more precise assessment of the C allocation in forest was made by analyzing the formation of structural C compounds (hemicelluloses and cellulose + lignins). A possible link between the variability of these allocations and climate was studied. The corresponding efficiencies were then calculated monthly in order to have a better idea of the sustainable C storage in this ecosystem. A comparison with a grassland in term of aBPE was then realized in order to analyze the adaptation capacity of each ecosystem to climate change and to play a role in the mitigation of GHG. Two neighboring ecosystems located in Montiers-sur-Saulx and Osne-le-Val were equipped, as part of the Observatoire Pérenne de l’Environnement (OPE), to measure continuously the CO2 exchange between land cover and atmosphere and the micrometeorological conditions in which these exchanges happen. A regular monitoring of those ecosystems development (biomass, leaves area, senescence) was also made

Fotossíntese é o processo biofísico pelo qual energia luminosa é transformada em energia química armazenada em compostos de carbono. A taxa fotossintética instantânea possui um forte padrão assintótico em resposta ao incremento da intensidade luminosa, porém quando integramos a fotossíntese em escalas espaciais e temporais maiores, observa-se um padrão linear de resposta entre radiação interceptada e produção. Esta abordagem permitiu o surgimento de modelos baseados nas taxas de conversão de energia radiante em biomassa seca, ou eficiência do uso da luz (&epsilon;). Valores publicados para o Eucalyptus estão na faixa de 0,5-2,5 g MJ-1, porém se faz necessário um entendimento mais profundo a respeito da sensibilidade destes valores às flutuações do clima e sua sazonalidade. Para isso, as taxas de crescimento, uso e eficiência do uso da luz foram monitoradas quinzenalmente durante 16 meses em parcelas de 18 clones de Eucalyptus, dos 1,3 aos 2,7 anos de idade. Foram testadas as hipóteses de que a produção de madeira aumentaria em função de incrementos no uso e/ou eficiência de uso da luz, assim como estes valores aumentariam respectivamente com incrementos no índice de área foliar e por uma alocação de carbono para o fuste, respectivamente. Os clones apresentaram uma grande amplitude de produtividade (9,9-22,7 Mg ha-1 ano-1) e arquiteturas de copa, capturando entre 65-95% da radiação incidente. Tais valores resultaram em uma eficiência do uso da luz média de 1,5 g MJ-1, variando entre 0,16-3,14 g MJ-1. Apesar de patamares distintos, os valores de eficiência de uso dos clones oscilaram de maneira similar, de modo que a radiação incidente foi a principal variável afetando a eficiência de uso da luz, estando &epsilon; positivamente relacionada a variáveis que expressam períodos de maior disponibilidade hídrica e negativamente relacionado a períodos de menor disponibilidade. Maiores valores de índice de área foliar efetivo (Le) acarretaram em maior interceptação de luz, porém as distintas arquiteturas de copa revelaram diferentes estratégias de captura de luz (0,3 < &kappa; < 0,6). Apesar de uma maior interceptação, não houve correlação significativa com a produtividade, no entanto observou-se uma forte correlação entre eficiência do uso da luz e crescimento em madeira, resultado de uma maior alocação para o fuste. Apesar de evidenciar a relação entre alocação e eficiência, existem outros mecanismos associados às alterações observadas em &epsilon; que apenas uma caracterização completa dos fluxos de carbono pode elucidar. / Photosynthesis is the biophysical process by which light energy is converted into chemical energy stored in carbon compounds. The instantaneous photosynthetic rate has a strong asymptotic pattern in response to increases in light intensity, however when we integrate photosynthesis in larger spatial and temporal scales, there is a linear pattern of response between intercepted radiation and production. This approach has allowed the appearance of models based on radiant energy conversion rates into dry biomass, or light use efficiency (&epsilon;). Published values for Eucalyptus range from 0.5 to 2.5 g MJ-1, but a deeper understanding of the sensitivity of these values to climate fluctuations and seasonality is necessary. For this reason, wood growth rates, light use and efficiency were monitored every two weeks for 16 months at 18 Eucalyptus clones plots, from 1.3 to 2.7 years of age. Our hypothesis was that wood production would be positively related to light use and efficiency, as well these values would increase respectively with increases in leaf area index and carbon allocation to the stem. Clones showed a wide range of productivity (9.9 to 22.7 Mg ha-1 yr-1) and canopy architectures, capturing between 65-95% of incident radiation. Such values resulted in an average light use efficiency of 1.8 g MJ-1, ranging from 0.16 to 3.14 g MJ-1. Although different levels, light use efficiency values for the clones fluctuated similarly. Incident radiation was the main variable affecting the efficiency of dry matter conversion, and &epsilon; values were positively related variables expressing periods of greater water availability and negatively related to periods of lower availability. Larger effective leaf area index (Le) values resulted in higher light interception, but the different canopy architectures revealed different light capture strategies (0.3 < &kappa; < 0.6). Despite a higher interception, there was no significant correlation with productivity; however there was a strong correlation between light use efficiency and wood growth, as a result of increased allocation to the stem. While evidencing the relationship between allocation and efficiency, there are other mechanisms associated with changes in &epsilon; observed that only one full characterization of the carbon fluxes can elucidate.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The study aimed to calibrate the model 3-PG for Eucalyptus saligna in the region of Guaíba, RS for the following variables diameter at breast height (DBH), total height, wood volume and biomass of stem (branch, bark and wood). The study took place in the region of Guaíba, RS, and clonal plantations of Eucalyptus saligna in various spacings were used. Six different ages were selected along the crop cycle (2nd to 7th year). A forest inventory was carried out in 60 plants per plot for further collection of biomass (three trees fractionated into wood, bark, branches and leaves). The biomass of roots on the medium tree of each plot was measured to a depth of 1 m. Physical and chemical analysis were performed in the samples of soil in layers from 0 to 20 cm, 20 to 40 cm and 40 to 100 cm. Litter deposition was estimated by allocating 4 square wood collectors of 0.5 m². Leaf area was obtained through digital photographs and thus the specific leaf area was estimated. The physiological parameters were measured using the Li-Cor 6400 in six plots, one at each age. Climatic parameters were provided through the Agronomic Experimental Station of the Federal University of Rio Grande do Sul. Allometric and nonlinear equations adjusted for the fraction branch/bark, wood basic density and specific leaf area did not present a good adjustment because there was variability in the data used. Although the estimated values differed from the values observed, the model performance in terms of adjustment exceeded other parameterizations available in the literature. The parameterization and calibration of the model 3-PG were performed satisfactorily meeting all the predictions proposed by the model. The model validation was not statistically accepted, but their estimated data can be used as a source of estimates for the variables studied, demonstrating the potential of its use, however, the need for additional studies to better understanding of this species in this region. The use of the parameters raised by other authors in other regions cannot be used for the region studied. / O trabalho teve como objetivo principal calibrar o modelo 3-PG para Eucalyptus saligna na região de Guaíba, RS para as varáveis diâmetro à altura do peito (DAP), altura total, volume de madeira e biomassa de stem (galho, casca e lenho). O estudo ocorreu na região de Guaíba, RS, sendo utilizados plantios clonais de Eucalyptus saligna, em diversos espaçamentos. Foram selecionadas seis idades ao longo do ciclo de cultivo (2º ao 7º ano). Realizou-se o inventário florestal em 60 plantas por parcela, para posterior coleta da biomassa (três árvores, sendo fracionadas em lenho, casca, galhos e folhas). Na árvore média de cada parcela, mensurou-se a biomassa de raízes até a profundidade de 1 m. Análises físicas e químicas em amostras de solo foram realizadas nas camadas de 0 a 20 cm, 20 a 40 cm e 40 a 100 cm. A deposição de serapilheira foi estimada através da alocação de quatro coletores quadrados de madeira de 0,5 m². A área foliar obteve-se através de fotografias digitais e, a partir destas, estimou-se a área foliar específica. Os parâmetros fisiológicos foram mensurados através do aparelho Li-Cor 6400, em seis parcelas, uma em cada idade. Os parâmetros climáticos foram fornecidos através da Estação Experimental Agronômica da Universidade Federal do Rio Grande do Sul. As equações alométricas e as não-lineares ajustadas para fração galho/casca, densidade básica da madeira e área foliar específica não tiveram bom ajuste, pois há variabilidade nos dados utilizados. Apesar dos valores estimados serem diferentes dos valores observados, o desempenho do modelo, em termos de ajuste, superou outras parametrizações disponíveis na literatura. A parametrização e a calibração do modelo 3-PG foram feitas de modo satisfatório, atendendo todas as predições propostas pelo modelo. A validação do modelo não foi estatisticamente aceita, mas os seus dados estimados podem ser utilizados como fonte de estimativa para as variáveis estudadas, evidenciando o potencial do seu uso, porém, com a necessidade de estudos adicionais para melhor compreensão desta espécie para esta região. O emprego dos parâmetros levantados por outros autores em outras regiões não pode ser utilizada para a região de estudo.

Abstract Mycorrhizal fungi are important contributors to the functioning of boreal forests, since they act in the bilateral carbon and nutrient transport between above- and belowground parts of the ecosystem. In ectomycorrhizal (ECM) symbiosis of woody host plants, both fungal and plant partners depend on resources provided by the other. A single tree may simultaneously host several ECM fungal partners, which greatly enhance the host's nutrient uptake. At the same time nearly 20% of host primary production is allocated to mycorrhizal fungi. Although fungi depend on host-derived carbon, it is poorly understood how reduced carbon availability, e.g., due to herbivory, affects the ECM fungal symbionts. In this thesis I studied the impact of simulated insect defoliation or mammal browsing on mycorrhizal fungi of boreal woody hosts. Quantitative and qualitative changes in biomass partitioning in different fungal compartments were detected. None of the experiments showed that defoliation or shoot clipping treatments reduced the intensity of ECM colonisation, while treatments often shifted fungal composition towards less biomass producing ECM morphotypes. Above- and belowground diversity in ECM symbionts tended to decrease due to shoot or foliar damage. In addition, in some cases defoliation also reduced fungal biomass in fine roots and decreased ECM sexual reproduction by reducing the number of sporocarps produced. Defoliation induced a similar response pattern in the host and in ECM fungi with a stronger response to increasing severity of treatment (e.g. degree of removed foliage or repeated years of defoliation). This was also confirmed when relating the effects of host and ECM fungal symbionts to defoliation using present and previously published data. The present results suggest that belowground adaptation of boreal trees to the changing environment is mediated by changes in fungal community or biomass partitioning. The lack of response in the intensity of ECM colonisation further emphasises the importance of the symbiosis to boreal trees.

<p>Earth’s ecosystems emerge from interconnected biosphere, geosphere, and atmosphere processes. Changes to any one process ripple through the Earth system, affecting other processes. As global climate change continues, nitrogen deposition is anticipated to increase and precipitation is expected to have varied changes across the globe. These changes to the atmosphere and geosphere will have implications for the biosphere. Namely, vegetation will be impacted by changes to nutrient and precipitation regimes. Vegetation comprises the aggregate strategies of individual plants, which are also influenced by changes in nutrient and water availability. The responses of individual plants to nitrogen, water, and uncertainty are the main focus of this dissertation, as understanding those will be critical to scaling up to the aggregate.</p> <p> First, I describe a mathematical model that predicts grassland root and shoot biomass across carbon, nitrogen, and water gradients. The model simulates competition among plants by dynamically allocating carbon to either root or shoot growth depending on the growth strategy employed by the other plant. I show that the model accurately predicts root net primary productivity (NPP), but performs poorly for shoot and total NPP. At the biome scale, modeled NPP does not vary with water alone but rather water and nitrogen interact to influence NPP. Second, I conduct a greenhouse experiment using <em>Eragrostis capillaris</em> (L.) Nees to examine the predictions of the model mentioned above to answer the question: how do water and nitrogen affect fitness and biomass allocation in a drought-tolerant C4 grass? And ask: what is the nature of the relationship between water and nitrogen as resources? I show that water was important for increasing shoot and total biomass, but that root biomass and root:shoot ratio was influenced interactively by water and nitrogen as predicted by the model. I conclude that the nature of the relationship between water and nitrogen was that of either interacting or hemi-essential resources. That is, additional water was able to partially substitute for limited nitrogen to maintain biomass. Third, I explore how information theory can apply to plants that face uncertainty in resource availability and briefly review the types and sources of information and the mechanisms that plants use to perceive and respond to their environment. Overall, my framework posits that plants interpret information from their surroundings as an emergent property of distributed information processed by a network of cells. I end with a prospectus of directions for future research, including decoding signal from noise, storage of information, strategies to cope with information entropy, additional means of information transmission, and two-way information signaling with biotic partners. Finally, I use the information theory framework discussed above to answer the questions: can plants sense and respond to information entropy? I explore this question using data from an experiment which altered the temporal supply of nutrients and found no support that <em>P. sativum</em> can sense and respond to entropy. Understanding the relationships of water, nitrogen, and uncertainty is critical to predicting plant growth, especially as climate change continues to influence the global system.</p>

Les hydrocarbures aromatiques polycycliques (HAP) sont des polluants organiques persistants potentiellement mutagènes et cancérigènes. Leur transfert de l’atmosphère vers les écosystèmes, notamment vers les plantes, conditionne leur entrée dans les chaines alimentaires mais les modalités de ce transfert restent encore mal connues. L’objectif de ce travail était donc de caractériser le transfert et d’identifier les effets biologiques des HAP atmosphériques sur un système sol-plante-microorganismes symbiotiques.Un dispositif expérimental a été conçu afin de recréer en laboratoire une pollution atmosphérique par les HAP avec comme HAP modèle le phénanthrène (PHE). Le dispositif a fait l’objet d’une validation et d’une calibration élaborée de manière originale par une double approche mêlant l’expérimental à la simulation mathématique. Les niveaux d’exposition en polluant (150 µg m-3), contrôlés par des échantillonneurs passifs, se sont avérés pertinents par rapport aux conditions in situ. Ce dispositif a donc été utilisé pour exposer durant un mois des microsystèmes sol-plante-microorganismes au PHE par voie atmosphérique.Les différentes études ont mis en évidence un transfert du PHE depuis l’atmosphère vers tous les compartiments du microsystème, avec une accumulation majeure vers les feuilles de trèfle ou de ray-grass (respectivement 170 et 70 µg g־ ¹MS) et un transfert phloémien vers les racines est suggéré. Chez le trèfle, la mycorhization n’a pas été affectée, tandis que le nombre de nodules actifs a diminué de manière significative. Contrairement aux racines, la biomasse aérienne du trèfle a été significativement affectée (environ – 25%) par l’exposition au PHE atmosphérique, suggérant un impact sur le métabolisme carboné de la plante. Une expérience de marquage des trèfles au ¹³C-CO2 a effectivement montré un impact négatif du PHE atmosphérique sur la croissance, l’allocation de biomasse et l’allocation carbonée. Pour conclure, ces études ont permis non seulement de caractériser les effets biologiques et physiologiques des HAP atmosphériques sur les végétaux mais également de proposer l’utilisation du potentiel mycorhizien comme indicateur de pollution atmosphérique par les HAP. / Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants potentially mutagenic and carcinogenic. Transfer from the atmosphere to ecosystems, especially to plants, conditioning their entry into food chains, but the terms of this transfer are still poorly understood. The aim of this study was to characterize the transfer and identify the biological effects of atmospheric PAHs on soil-plant-symbiotic microorganisms.An experimental device was designed to recreate in the laboratory air pollution with phenanthrene (PHE) as a model PAH. The device was been validated and a calibration developed in an original way by a dual approach combining the experimental mathematical simulation. The levels of exposure to pollutant (150 mg m־³), controlled by passive samplers, were relevant with field conditions. This device has been used to expose a month of the soil-plant micro-organisms in the PHE through the air.Various studies have demonstrated a transfer of PHE from the atmosphere to all compartments of the microsystem, with a major accumulation to leaves in clover or ryegrass (respectively 170 and 70 µg g ־¹ dry weight) and a phloemic transfer to the roots is suggested. In clover, mycorrhization was not affected, while the number of active nodules decreased significantly. Unlike roots, aboveground biomass of clover was significantly affected (approximately – 25%) by exposure to air PHE, suggesting an impact on the carbon metabolism of the plant. A labelling experiment with ¹³C- CO2 in clover has actually shown a negative impact of PHE air on growth, biomass and carbon allocation.In conclusion, these studies have not only characterized the biological and physiological effects of atmospheric PAHs on plants but also proposed the use of mycorrhizal potential as an indicator of air pollution by PAHs.

Le mutualisme entre les plantes et les champignons arbusculaires mycorhiziens est extrêmement répandu (~ 80% des plantes sont colonisées par ces organismes) et ancien (il ya plus de 450 millions d'années). Cette relation symbiotique est une composante essentielle du fonctionnement des écosystèmes et de leur productivité, et est fortement impliqué dans le cycle de deux éléments clés: le phosphore et le carbone. Le maintien de ce mutualisme est devenu particulièrement important dans le contexte actuel de perte de biodiversité. Un des objectifs de cette thèse était de comprendre la stabilité de ce mutualisme. L'accent a tout d'abord été mis sur les échanges de nutriments impliqués dans cette symbiose, en testant si la plante hôte et les symbiotes fongiques sont capables de discriminer leurs différents partenaires, et d'allouer davantage de ressources aux partenaires fournissant plus de nutriments. J'ai ensuite étudié la possibilité de l'implication de la plante hôte dans la protection des symbiotes mycorhiziens via un transfert de métabolites secondaires dans les hyphes. Nous avons alors pu emettre une nouvelle hypothèse suggérant que la protection en métabolites secondaires venant de la plante serait positivement corrélée avec le niveau de coopération (à savoir le transfert des nutriments) du champignon symbiotique. L'echelle d'étude est ensuite passée de l'individu à la communauté en étudiant les effets de la diminution de la diversité végétale sur la diversité des symbiotes racinaires. Pour ce faire, des analyses moléculaires et des outils novateurs ont été utilisés, tels que le séquençage à haut débit. Pour faciliter encore l'étude des séquences obtenues et d'autres séquences fongiques, j'ai collaboré avec des collègues afin de créer une base de données 'Phymyco-DB' rendue publique en 2012. Enfin, je discute de l'implication du mutualisme mycorhizien dans le contexte des systèmes agricoles actuels et propose de nouvelles trajectoires pour gérer ces systèmes. Ce projet de thèse apporte un nouvel éclairage sur la façon dont fonctionnent ces interactions entre les plantes et champignons MA et sur la manière dont ils façonnent les processus écologiques et les trajectoires évolutives dans les écosystèmes naturels et agricoles. Ces points sont d'une importance majeure pour développer une agriculture plus écologiquement intensive et durable. Le projet a fourni de nouvelles connaissances et perspectives sur la perte de la diversité végétale, et ses conséquences pour la stabilité de la symbiose AM. Comme les champignons mycorhiziens sont essentiels dans les processus des écosystèmes et l'entretien de la fertilité des sols, ce travail devrait avoir un large impact dans (i) la politique de protection des sols, (ii) la recherche sur l'amélioration des plantes et (iii) la conception de systèmes agricoles durables. / The mutualism between plants and arbuscular mycorrhizal fungi is extremely widespread (~ 80% of plants are colonized by these organisms) and ancient (over 450 million years ago). This symbiotic relationship is an essential component of healthy ecosystem functioning and productivity, and is strongly involved in the cycle of two key elements: phosphorus and carbon. Maintaining this mutualism has become especially important in the current context of a biodiversity loss. One goal of this thesis was to understand the stability of the mutualism. I first focused on nutrient exchange, testing whether plant host and fungal symbionts are able to discriminate among partners, and allocate more resources to those individuals providing more nutrients. I then explored the possibility of the host-plant involvement in the protection of mycorrhizal symbionts via a transfer of secondary metabolites into fungal hyphae. We introduced a new hypothesis suggesting that chemcial protection from the plant is positively correlated with the level of cooperation (i.e. nutrient transfer) of the fungal symbiont. I then moved from the individual to the community by studying the effects of decreasing plant diversity on the diversity of root symbionts. To this aim, I utilized molecular analyzes and innovative tools, such as high throughput sequencing. To further facilitate the study of the obtained sequences and other fungal sequences, I worked with colleagues to create a database ‘Phymyco-DB’ which was released to the public in 2012. Finally, I discuss the implication of the mycorrhizal mutualism in the context of current agricultural systems and propose new trajectories to manage these systems. This PhD project provides new insights on how plant and AM fungi interactions work and how they shape ecological processes and evolutionary trajectories in natural and agricultural ecosystems. These points are of major importance to develop a more ecologically intensive agriculture. The project has provided new knowledge and perspectives on the loss of plant diversity, and its consequences for AM symbiosis stability. As arbuscular mycorrhizal fungi are essential in ecosystem processes and soil fertility maintenance, this work should have a broad impact in (i) the soil protection policy, (ii) the research on plant breeding and (iii) the design of sustainable agricultural systems.

Wetland plant species rely largely on vegetative reproduction. Although all types of clonal growth organs are found in wetlands, special adaptations, e.g., turions, fragmentation and budding, are more frequent in true aquatic communities. However, root-sprouting is underrepresented, although it can be beneficial under disturbed conditions. This thesis focuses on ecological constrains potentially hindering root-sprouting in wetlands. This ability was studied in a wetland herb Rorippa palustris in relation to life history, injury timing and carbon economy of a plant. In addition, plant regeneration following submergence and severe disturbance was assessed.

<p>This thesis is a collection of three research articles to quantify carbon fluxes and isotopic signature changes across global terrestrial ecosystems. Chapter 2, the first article of this thesis, focuses on the importance of an under-estimated methane soil sink for contemporary and future methane budgets in the pan-Arctic region. Methane emissions from organic-rich soils in the Arctic have been extensively studied due to their potential to increase the atmospheric methane burden as permafrost thaws. However, this methane source might have been overestimated without considering high affinity methanotrophs (HAM, methane oxidizing bacteria) recently identified in Arctic mineral soils. From this study, we find that HAM dynamics double the upland methane sink (~5.5 TgCH₄yr^-1) north of 50°N in simulations from 2000 to 2016 by integrating the dynamics of HAM and methanogens into a biogeochemistry model that includes permafrost soil organic carbon (SOC) dynamics. The increase is equivalent to at least half of the difference in net methane emissions estimated between process-based models and observation-based inversions, and the revised estimates better match site-level and regional observations. The new model projects double wetland methane emissions between 2017-2100 due to more accessible permafrost carbon. However, most of the increase in wetland emissions is offset by a concordant increase in the upland sink, leading to only an 18% increase in net methane emission (from 29 to 35 TgCH₄yr^-1). The projected net methane emissions may decrease further due to different physiological responses between HAM and methanogens in response to increasing temperature. This article was published in <i>Nature Climate Change</i> in March 2020.</p> <p>In Chapter 3, the second article of this thesis, I develop and validate the first biogeochemistry model to simulate carbon isotopic signatures (δ¹³C) of methane emitted from global wetlands, and examined the importance of the wetland carbon isotope map for studying the global methane cycle. I incorporated a carbon isotope-enabled module into an extant biogeochemistry model to mechanistically simulate the spatial and temporal variability of global wetland δ¹³C-CH₄. The new model explicitly considers isotopic fractionation during methane production, oxidation, and transport processes. I estimate a mean global wetland δ¹³C-CH₄ of -60.78‰ with its seasonal and inter-annual variability. I find that the new model matches field chamber observations 35% better in terms of root mean square estimates compared to an empirical static wetland δ¹³C-CH₄ map. The model also reasonably reproduces the regional heterogeneity of wetland δ¹³C-CH₄ in Alaska, consistent with vertical profiles of δ¹³C-CH₄ from NOAA aircraft measurements. Furthermore, I show that the latitudinal gradient of atmospheric δ¹³C-CH₄ simulated by a chemical transport model using the new wetland δ¹³C-CH₄ map reproduces the observed latitudinal gradient based on NOAA/INSTAAR global flask-air measurements. I believe this study is the first process-based biogeochemistry model to map the global distribution of wetland δ¹³C-CH₄, which will significantly help atmospheric chemistry transport models partition global methane emissions. This article is in preparation for submission to <i>Nature Geoscience</i>.</p> <p>Chapter 4 of this thesis, the third article, investigates the importance of leaf carbon allocation for seasonal leaf carbon isotopic signature changes and water use efficiency in temperate forests. Temperate deciduous trees remobilize stored carbon early in the growing season to produce new leaves and xylem vessels. The use of remobilized carbon for building leaf tissue dampens the link between environmental stomatal response and inferred intrinsic water use efficiency (iWUE) using leaf carbon isotopic signatures (δ¹³C). So far, few studies consider carbon allocation processes in interpreting leaf δ¹³C signals. To understand effects of carbon allocation on δ¹³C and iWUE estimates, we analyzed and modeled the seasonal leaf δ¹³C of four temperate deciduous species (<i>Acer saccharum, Liriodendron tulipifera, Sassafras albidum, </i>and <i>Quercus alba</i>) and compared the iWUE estimates from different methods, species, and drought conditions. At the start of the growing season, leaf δ¹³C values were more enriched, due to remobilized carbon during leaf-out. The bias towards enriched leaf δ¹³C values explains the higher iWUE from leaf isotopic methods compared with iWUE from leaf gas exchange measurements. I further showed that the discrepancy of iWUE estimates between methods may be species-specific and drought sensitive. The use of δ¹³C of plant tissues as a proxy for stomatal response to environmental processes, through iWUE, is complicated due to carbon allocation and care must be taken when interpreting estimates to avoid proxy bias. This article is in review for publication in <i>New Phytologist</i>.</p> <p> </p>

No description available.