L’objectif de ce travail était de caractériser la variabilité saisonnière des échanges gazeux des arbres de la canopée en forêt tropicale humide guyanaise confrontés à des saisons sèches.L’étude s’est orientée sur la mesure des flux de carbone et d’eau au cours des saisons sèches dans deux habitats contrastés du point de vue hydrique, afin de préciser leur sensibilité et réponse à la sècheresse. Nos résultats montrent qu’une proportion non négligeable d’arbres ne subit pas de sécheresse marquée en saison sèche, malgré la forte diminution de la teneur en eau du sol en surface. Nous faisons l’hypothèse que ces arbres possèdent alors des racines en profondeur qui leur permettent d’accéder aux horizons qui restent humides malgré la faible pluviométrie. Durant les longues saisons sèches, nous observons une diminution de l’assimilation nette de CO2 foliaire, de l’efflux de CO2 des troncs ou de la densité de flux de sève, et dans une moindre mesure de la respiration foliaire. Cette diminution concerne un nombre plus important d’arbres, quand l’intensité de la sécheresse est plus forte (2008 par rapport à 2007). Néanmoins, de grandes différences de réponse à la sècheresse du sol ont été observées, suggérant des sensibilités et des mécanismes de réponse différents entre ces arbres. En habitat de bas-fond, nous montrons une plus faible diminution des flux carbonés et hydriques en saison sèche, suggérant un accès pour ces arbres aux horizons humides, proches de la nappe phréatique. Par ailleurs, nos résultats suggèrent pour certains arbres un effet négatif des conditions d’anoxie sur ces flux en saison des pluies.Si l’accentuation de l’intensité des saisons sèches en forêt tropicale humide Amazonienne se confirme, comme suggérée par les modèles climatiques, ce travail suggère des modifications rapides de la composition spécifique des forêts, au profit des espèces plus résistantes aux contraintes hydriques / The aim of this work was to characterize the seasonal variations in leaf and trunk gas exchange of tropical rainforest canopy trees exposed to dry seasons. This study focused on measuring gas exchange (CO2 and H2O) at the leaf and trunk level during dry seasons in a Terra firme forest and in a seasonally flooded forest in order to specify their sensitivity and response to drought.Our results showed that part of the studied trees do not encounter a large reduction in soil water content during dry seasons, suggesting that they display deep rooting systems.During long dry periods, we recorded a decrease in leaf photosynthesis, trunk CO2 efflux or sap flow density, and foliar respiration to a lesser extent, for a majority of the trees. Nevertheless, large differences among trees in their response to these conditions were observed, suggesting distinct sensitivities and response to soil drought among trees. Furthermore, these variations were greatest when soil drought was highest (2008 as compared to 2007).In seasonally flooded forest, the decrease in gas exchange was less, suggesting that these trees do have access to wet layers during the dry season, close to the groundwater. However, during wet seasons, we observed a negative effect of anoxia for some trees, whereas others did not display any response, suggesting large differences in tolerance to anoxia among trees abundant in these seasonally flooded forests.We conclude that would seasonal soil droughts increase over the next decades, large vegetation changes might occur because of the large differences among trees in their functional response to soil drought conditions

Orientador: Célio Fernando Baptista Haddad / Resumo: Nesta tese exploramos a diversificação de espécies de anuros da Tribo Lophyohylini distribuídas na Mata Atlântica (MA) e Caatinga, biomas do Brasil. Para tanto, utilizamos dados genômicos e métodos de filogeografia estatística para analisá-los, uma abordagem robusta e até então ainda escassa na filogeografia dos organismos sul-americanos. Através do sequenciamento Sanger amostramos trechos de DNA mitocondrial (mtDNA) e através do sequenciamento de alto rendimento obtivemos polimorfismos de nucleotídeo único (SNPs) do DNA nuclear (nDNA). No primeiro capítulo, reunimos 82 amostras da espécie Aparasphenodon brunoi que apresenta distribuição nas terras baixas e costeiras do bioma MA, sendo considerada ideal para investigar o papel da instabilidade climática do Pleistoceno em seu processo de diversificação. Ainda no primeiro capítulo, para avaliar a influência do comportamento filopátrico, associado às diferenças no modo reprodutivo, na dispersão e consequentemente, no fluxo gênico entre populações, reunimos 72 amostras de A. arapapa e A. aff. brunoi. Estas tratam-se de espécies simpátricas, de distribuição mais restrita e com modos reprodutivos contrastantes o que as tornam ideais para esse tipo de comparação. De acordo com a inferência baseada em modelos demográficos, por volta do último máximo glacial (UMG) a população norte de A. brunoi se manteve estável enquanto que a do sul experimentou um gargalo seguido de expansão recente. Além disso, a população norte de A. brunoi apres... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this dissertation we explored the diversification of anuran species of the Lophyohylini Tribe distributed in the Atlantic Forest (AF) and Caatinga biomes of Brazil. To do so, we used genomic data and methods of statistical phylogeography to analyze them, a robust and until then still scarce approach in the phylogeography of South American organisms. Through the Sanger sequencing we sampled mitochondrial DNA (mtDNA) and through high-throughput sequencing we obtained single nucleotide polymorphisms (SNPs) from nuclear DNA (nDNA). In the first chapter, we collected 82 samples of the species Aparasphenodon brunoi that is distributed in the low and coastal lands of the AF biome, being considered ideal to investigate the role of Pleistocene climatic instability in its diversification process. In the first chapter, to evaluate the influence of philopatric behavior, associated with differences in reproductive mode, dispersion and consequent gene flow among populations, we also collected 72 samples of A. arapapa and A. aff. brunoi species. These are sympatric species with a narrower distribution and with contrasting reproductive modes, making them ideal for this type of comparison. According to the inference based on demographic models, around the last glacial maximum (LGM) the northern population of A. brunoi remained stable while the southern one experienced a bottleneck followed by recent expansion. In addition, the northern population of A. brunoi exhibits greater nuclear diver... (Complete abstract click electronic access below) / Doutor

No description available.