Le lien qui existe entre la diversité et la productivité des écosystèmes constitue un sujet central en écologie. De nombreuses études ont montré une relation positive entre la diversité et la productivité des forêts, ainsi que leur résistance à différents stress comme à la sécheresse. En revanche, peu de travaux ont permis de comprendre les mécanismes à l'origine des avantages observés dans les forêts à plusieurs espèces comparés aux forêts monospécifiques. En particulier, en raison de la difficulté d'accès aux racines, le rôle du compartiment souterrain dans les interactions entre espèces est particulièrement méconnu. L'objectif de ma thèse a été d'étudier l’influence des interactions entre espèces d’arbres sur le système racinaire du hêtre (Fagus sylvatica) sous différents niveaux de contrainte hydrique. Ces travaux ont montré que, au stade jeune plant, la compétition souterraine entre hêtre, chêne et pin était forte et que mélanger les espèces à ce stade pouvait influencer la croissance du hêtre. La disponibilité en eau a un fort impact sur la croissance des plantules de hêtre mais la présence d’autres espèces à proximité des jeunes plants de hêtre n’a pas amélioré leur réponse à la sécheresse. Au stade adulte, dans une plantation forestière, nous avons montré que la présence de l’érable n’influençait que légèrement la profondeur d’extraction de l’eau du hêtre ou la distribution verticale de ses racines. Nous concluons (i) à une absence de séparation des niches souterraines entre ces deux espèces, tant au niveau spatial que fonctionnel (vis à vis de l'acquisition de l’eau), et (ii) que les mécanismes d’interaction souterraine ne semblent pas expliquer l’origine de la productivité plus forte dans la zone de mélange que dans les zones de monocultures. Enfin, j'ai montré que les peuplements mélangés hébergent une communauté fongique plus riche par rapport aux peuplements purs. Cette plus forte richesse de la communauté fongique dans le mélange pourrait contribuer à une meilleure acquisition des ressources hydriques et minérales dans le mélange. Mon travail suggère que les interactions souterraines entre le hêtre et d’autres espèces forestières ne sont probablement pas le mécanisme principal expliquant les effets positifs des mélanges sur la productivité / The link between species diversity and ecosystem productivity is a central issue in ecology. Numerous studies have shown a positive relationship between forest diversity and productivity, as well as a greater resistance to various stresses such as droughts. On the other hand, few studies demonstrated the mechanisms behind the benefits observed in multi-species forests compared to monospecific ones. In particular, the role of belowground interactions among species in explaining the origin of positive effects of species diversity on ecosystem functioning is unknown. The aim of my PhD work was to study the influence of tree species interactions on the root development of beech (Fagus sylvatica) under different levels of water conditions. We showed that at the seedling stage, underground competition between beech, oak and pine was strong and that mixing these species could influence the growth of the beech. Water availability had a strong impact on the growth of beech seedlings, but the presence of seedlings of other species competing with beech did not particularly improve its response to drought. At the adult stage, in a forest plantation, we showed that maple trees competing with beech did not strongly modify the mean depth of soil water extraction of beech trees or the vertical distribution of beech roots. We conclude that (i) there was no separation of the underground ecological niches of these two species, both spatially and functionally (with respect to water acquisition), and (ii) that the mechanisms of belowground interaction between these two species do not seem to explain the origin of the higher productivity in the mixture than in the monoculture zone. Finally, the richest fungal communities were found in the mixed species zone: we hypothesize that greater fungal community richness in the mixture may improve water and nutrient acquisition and then contribute to higher productivity in the mixed species zone. My work suggests that underground interactions between beech and other forest species are probably not the main mechanism explaining beneficial effects of mixtures on productivity

A Floresta Ombrófila Mista também chamada de Floresta de Araucária representa um dos mais ricos remanescentes de florestas pluviais subtropicais brasileiras, tendo como principal representante a Araucaria angustifolia, espécie considerada ameaçada de extinção. A diversidade microbiana possui um importante papel no funcionamento e manutenção do equilíbrio dos ecossistemas florestais, mas é desconhecida em solos com araucária. O presente estudo teve como objetivo avaliar a diversidade, funcionalidade e estrutura das comunidades microbianas em florestas de Araucaria angustifolia natural, introduzida e impactada pela queima acidental. O estudo foi realizado no Parque Estadual de Campos de Jordão, SP, tendo como áreas de estudo: floresta nativa com predomínio de araucária (FN), reflorestamento de araucária (RF), e reflorestamento de araucária com queima acidental em julho de 2001 (RQ). Em cada área, foram escolhidas ao acaso dez árvores de araucária e ao redor de cada árvore coletou-se uma amostra composta de três subamostras. Foram avaliados atributos químicos, microbiológicos, estrutura das comunidades de Bacteria e Archaea por PCR-DGGE e seqüenciamento parcial do gene rRNA 16S de Bacteria, análise da capacidade de utilização de substratos de carbono (Biolog) e perfis de ácidos graxos ligados a ésteres de fosfolipídios (PLFAs). As áreas são caracterizadas por solos ácidos, com elevado conteúdo de matéria orgânica (MO) e baixa disponibilidade de cátions metálicos básicos. A área FN apresentou maiores teores de carbono da biomassa microbiana (CBM), atividade respiratória basal (C-CO2) e relação carbono da biomassa microbiana: carbono orgânico total (CBM:COT), comparada ao RF e RQ. Os maiores valores de quociente metabólico (qCO2) foram encontrados no RQ quando comparado a FN e RF. A análise canônica discriminante identificou o atributo microbiano qCO2 e químicos, Mg e pH, como sendo responsáveis pela discriminação das áreas, seguidos do teor de P. A análise de PCRDGGE revelou que as estruturas das comunidades bacterianas de FN e RQ foram mais similares entre si do que em RF. A análise de escala multidimensional (NMDS) com ANOSIM baseada nos perfis de amplicons de Bacteria mostrou que as três áreas são diferentes entre si, enquanto para Archaea não houve diferença entre as áreas estudadas. A afiliação taxonômica das seqüências de clones do gene rRNA 16S mostrou que o solo de FN apresenta uma maior diversidade de táxons. Os filos Proteobacteria e Actinobacteria foram os mais freqüentes nas três áreas. A maior diversidade estimada pelo índice de Shannon foi encontrada em RQ, em comparação a FN e RF. A análise por Biolog mostrou que área FN apresenta a maior taxa de utilização de substratos, em relação RF e RQ, as quais não diferiram entre si. Os perfis de PLFAs não apresentaram diferenças entre as áreas estudadas. Observou-se uma maior biomassa bacteriana, principalmente de Gram-positivas, quando comparada a biomassa fúngica de PLFAs das áreas estudadas. As análises multivariadas apresentaram-se como importantes ferramentas no estudo de diversidade microbiana, sendo os atributos químicos e microbiológicos do solo úteis para a interpretação dos resultados obtidos. / The Ombrophilic Mixed Forest, also called Araucaria Forest, represents one of the richest remainders of subtropical pluvial forests in Brazil. Its main representative species is the endangered Araucaria angustifolia. The microbial diversity plays an important role in functioning of forest ecosystems. However, the microbial diversity in soils with araucaria forests is mostly unknown. The aim of this work was to evaluate the diversity, structure of microbial communities in their possible functions in a natural preserved araucaria forest (FN), a planted araucaria forest (RF) and a planted araucaria forest impacted by accidental fire (RQ). The study was carried out at the State Park of Campos of Jordão (SP). For each area, ten araucaria trees were randomly selected and a sample composed of three sub-samples was collected at approximately one meter from the trunk of each tree. Chemical and microbiological attributes, as well as structures of bacterial and archaeal communities were evaluated using PCR-DGGE and the partial sequencing of the 16S rRNA gene from Bacteria, community level physiological profiles using Biolog, the phospholipid fatty acids profiles (PLFAs). The studied areas were characterized by acidic soils, with high content of organic matter (OM) and low availability of basic metallic cations. The area FN presented the highest contents of carbon in the microbial biomass (CBM), higher basal respiration activity (C-CO2) and higher microbial biomass carbon: total organic carbon ratio (CBM:TOC), compared to RF and RQ. The highest values of metabolic quotient (qCO2) were observed in RQ, when compared to FN and RF. Using canonical discriminant analysis (CDA), qCO2, Mg concentration and pH were identified as the main attributes responsible for the discrimination of the areas, followed by the P concentration. The PCR-DGGE analysis revealed that the bacterial community structures in FN and RQ share higher levels of similarity, as compared to RF. Non-metric multidimensional scale analysis (NMDS) and ANOSIM based on the profiles of bacterial 16S rRNA gene amplicons showed that the all three areas had different bacterial communities, whereas archaeal communities were similar, based on 16S rRNA genes amplicon profiles. The phylogenetic affiliation of 16S rRNA gene clone sequences showed that soil from FN presents higher taxa diversity, as compared to RF and RQ. The phyla Proteobacteria and Actinobacteria were the most frequent in the three areas studied. Higher Shannon index was observed in RQ soil than FN and RF soils. Biolog analysis showed that FN has the highest substrate utilization rates, when compared to RF and RQ, which did not show significant differences. In general, PLFAs profiles did not show differences for the areas studied. Estimated bacterial biomass was higher than fugal biomass, with predominance of Gram-positive bacteria. Integration of chemical and microbial attributes through multivariate analyses is essential for identifying the factors determining microbial community structure in forest soils.

La biodiversité favorise un grand nombre de fonctions et services écosystémiques des écosystèmes forestiers tels que la production de bois ou la résistance aux attaques d’insectes et aux maladies. Cependant l’impact de la diversité sur l’acquisition et l’utilisation de l’eau et du carbone reste largement méconnu dans ces écosystèmes. De plus, dans le contexte actuel de changement climatique, l’influence de la diversité sur la réponse des écosystèmes forestiers à des événements climatiques extrêmes tels que la sécheresse reste à étudier. L’objectif de ce travail est donc de déterminer l’impact de la diversité en espèces d’arbre sur d’importantes fonctions du cycle de l’eau et du carbone telles que la transpiration, la composition isotopique du carbone ou la profondeur d’extraction de l’eau à l’échelle de l’arbre et de l’écosystème sous des conditions contrastées dedisponibilité en eau du sol. Ce travail a été réalisé dans le cadre du projet FunDivEUROPE sur un réseau de parcelles forestières ainsi que dans des plantations expérimentales le long d’un gradient Nord-Sud en Europe afin de couvrir une importante gamme de conditions climatiques. Nos travaux ont montré une importante variabilité de la réponse à la diversité à l’échelle de l’arbre et de l’écosystème en termes de relations hydriques et carbonées à travers l’Europe. La diversité en espèces ne semble pas influencer les relations hydriques et carbonées des espèces et des écosystèmes forestiers dans des conditions non limitantes de disponibilité en eau. Cependant, un fort effet de la diversité a été observé en conditions de sécheresse pour certains types forestiers. A partir de ces résultats, je discute des mécanismes d’interaction entre espèces qui peuvent expliquer les effets observés. Nos données ont montré que l’influence de la diversité en espèces est fortement dépendante du contexte et peut êtremodifiée par les conditions environnementales locales et les conditions climatiques. En terme de gestion forestière, je suggère que pour certaines régions en Europe, promouvoir la diversité en espèces ainsi que contrôler la densité des parcelles doit être recommandé afin d’adapter les écosystèmes forestiers aux futures conditions climatiques / Biodiversity is known to support and boost a wide range of forest ecosystem functions and services like productivity and resistance against insect pests and diseases. However, whether tree species diversity also promotes water and carbon acquisition and use in forest ecosystems is still unclear. Furthermore, in the current context of global warming, information on how tree species diversity can influence the response of forest ecosystems to extreme climatic events such as drought are urgently needed. In this framework, the objective of my PhD thesis was to determine how tree species diversity influences important functions of the water and carbon cycle including transpiration, carbon isotope composition and water extraction depth at the tree- and ecosystem-Scale under contrasting soil water conditions. My work was conducted within the FunDivEUROPE project in a network of permanent forest stands and tree plantations across a North-South gradient in Europecovering a wide range of climatic conditions. I found considerable variability among species or forest types in the response of transpiration and carbon isotope composition at the tree- and ecosystem-Scale across Europe. Species diversity did not affect the water and carbon relations of tree species and forest ecosystems under non-Limiting soil water conditions. However, a strong effect of species diversity was observed under drought conditions in some forest types. Based on these data, I discuss the potential mechanisms of species interactions that may explain the observed patterns. I also point out that the influence of species diversity is highly context-Dependent, and changes with local environmental and climatic conditions. In terms of forest management applications, I suggest that, at least in some regions, controlling for tree species diversity along with stand density and total basal area could be recommended to help forests adapt to drier conditions

The fungal domain has one of the highest biodiversities among eukaryotes and species within the domain fill important ecological roles, such as mutualistic mycorrhiza, decomposers, parasites and pathogens. The development of forest ecosystems and their related processes has not only been linked to fungal diversity but the composition and abundance of fungi benefits the abundance and diversity of other species as well. In this study three questions concerning the differences in the fungal communities between mixed forests and coniferous forests were considered. Inventories were done in coniferous forest and mixed forest areas classified as nature reserves and later statistically analyzed. The study could show that while the probability of finding ectomycorrhiza was significantly higher in mixed forests than coniferous forests, the probability of finding saprotrophs was significantly higher in coniferous forests than in mixed forests. Results in contrast to other studies were also found. The study revealed that more is needed to be done on the topic of forest fungi in mixed forest and coniferous forest areas. / Svampdomänen har en av de högsta biologiska mångfalderna bland eukaryoter och domänens arter innehar viktiga ekologiska roller, såsom mutualistisk mykorrhiza, nedbrytare, parasiter och patogener. Utvecklingen av skogsekosystem och deras relaterade processer har inte bara kopplats till svampdiversitet utan svamparnas sammansättning och förekomst gynnar också förekomst av och mångfald hos andra arter. I denna studie behandlades tre frågor om skillnaderna i svampsamhällen mellan blandskogar och barrskogar. Inventeringar gjordes i barrskogs- och blandskogsområden klassificerade som naturreservat och analyserades senare statistiskt. Studien kunde visa att även om sannolikheten för att hitta ektomykorrhiza var signifikant högre i blandskogar än barrskogar, var sannolikheten för att hitta saprotrofer signifikant högre i barrskogar än i blandskogar. Resultat i motsats till andra studier hittades också. Studien visade att det behövs mer forskning om skogssvampar i blandskog och barrskogsområden.

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