A l’heure actuelle, seuls des facteurs locaux, stationnels, sont considérés pour le calcul des risques liés au vent alors que le vent qui aborde un peuplement forestier est affecté par les surfaces sur lesquelles il vient de passer ; les lisières en particulier jouent un rôle important sur l’écoulement atmosphérique, en contribuant à générer de la turbulence. Dans un paysage fragmenté, constitué d’une mosaïque de surfaces de différentes hauteurs et rugosités, la multiplicité des lisières est ainsi susceptible d’avoir des effets cumulatifs perceptibles à l’échelle régionale, qui pourraient contribuer de manière significative à la fragilité des massifs face à des tempêtes. Certains niveaux de fragmentation semblent susceptibles de conduire à un accroissement des risques en cas de vent violent. Bien que la région de lisière a été étudiée de manière approfondie dans le passé en raison de leur importance pour la détermination des vitesses de vent, des niveaux de turbulence et des échanges entre l’atmosphère et la canopée, il n’y a aucune étude de l’impact de lisières multiples ou de la fragmentation des forêts sur les caractéristiques de la couche limite à l’échelle du paysage. Quelques rares études laissent penser que la fragmentation du paysage pourrait moduler de manière significative la structure turbulente de la couche limite atmosphérique mais ces études concernent des réseaux de brisevents plutôt qu’un ensemble de parcelles forestières. On cherche par conséquent à caractériser les champs de vent et de turbulence pour ces différentes configurations. Pour ce faire, une expérimentation en soufflerie à été réalisée, visant à caractériser l’écoulement sur des maquettes de paysage présentant cinq degrés de fragmentation (L = ~ 5, ~ 10, ~15, ~20, ~30h, où L est la distance entre deux patchs de forêts régulièrement espacés et h est la hauteur de la canopée). Un cas homogène a également été simulé et sert ici de référence. Pour le modèle de canopée choisi, ces expérimentations montrent que l’énergie cinétique turbulente présente dans la basse atmosphère ne passe pas par un maximum pour une valeur de l’espacement intermédiaire comme il était supposé à l’origine. Le cas homogène est la configuration la plus rugueuse. Pour de grands espacements l’influence d’une parcelle ne se fait guère sentir sur la suivante et lorsqu’ils sont faibles l’écoulement ne "ressent" guère les clairières et présente des caractéristiques semblables au cas homogène. Nous avons également évalué un modèle atmosphérique de type "simulation des grandes échelles" à l’aide des données présentées précédemment. Le modèle est capable de reproduire les grandes caractéristiques de la turbulence telles que les vitesses de vent horizontales et verticales, l’énergie cinétique turbulente, les contraintes de Reynolds et les coefficients d’asymétrie horizontale et verticale en tous points du domaine. Cela nous a permis de confirmer la validité des calculs numériques et de simuler l’écoulement sur une plus large gamme de paysages fragmentés. Les résultats démontrent l’importance de l’indice foliaire pour le calcul de la rugosité effective sur une succession de patchs de forêt. / At present only the characteristics of a forest stand and its immediate environment are taken into account in calculating forest wind risk. However, it is known that the wind is strongly affected by the surfaces over which it has previously flowed. Forest edges in particular play an important role in determining the characteristics of the atmospheric flow by generating increased turbulence, triggering the formation of coherent tree scale structures. In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may have cumulative effects at the regional scale leading to increased forest damage during storms. Flow changes in the atmospheric boundary-layer across surface roughness changes have received extensive study in the past because of their importance in determining velocities, turbulence levels and exchange between the atmosphere and biosphere or ground. There have also been a number of studies across single forest edges both in the field, wind-tunnels and computer models. However, there have been no studies of flow across multiple forest edges or the impact of forest fragmentation on the characteristics of the boundary-layer. The only studies on multiple surface changes have been wind-tunnel examination of the flow though and across multiple wind-breaks. In this thesis we show results from a series of wind tunnel experiments on a range of levels of forest fragmentation. Five gap spacings (L = ~ 5, ~ 10, ~15, ~20, ~30h, where L is the length of the gap and h is the canopy height) were investigated using 3D laser doppler velocimetry in order to assess the effects of fragmented landscapes on mean and turbulent wind characteristics. The fragmentation was two-dimensional with the transition between forest and gaps only being along the wind direction and the forest and gaps were continuous perpendicular to the wind direction. The wind speeds and turbulence characteristics are compared against measurements from a single forest edge in the wind tunnel, which acts as a reference. No enhancement of turbulence formation at a particular level of fragmentation was observed but there was a consistent pattern of wind speed and turbulence back from the first edge of each simulation with the horizontal velocity at tree top increasing and the turbulent kinetic energy decreasing as gap size increased. We also compare mean wind speeds (U and W) and turbulence characteristics (variance in u, v, and w; skewness in U, V, and W; Reynold’s stress, and TKE) at all points in the experimental measurement domain of the wind tunnel with Large Eddy Simulation (LES) results, which allows us to confirm the validity of the LES calculations and to conduct a wider range of experiments than was possible in the wind-tunnel. The results demonstrate the importance of the frontal area index or roughness density of elements (in this case trees) in determining the nature of the flow and the effective roughness of the overall surface. They also show that as the gaps between forest blocks increases the flow transitions (at a gap size between 10 to 15 tree heights) from flow comparable to that over a continuous forest to flow across a set of isolated forest blocks.

Submitted by (ana.araujo@ufrpe.br) on 2016-07-05T14:12:49Z No. of bitstreams: 1 Elias Dias Coelho Neto.pdf: 2570519 bytes, checksum: 45c9ec25f4446657c86b65924c054a2d (MD5) / Made available in DSpace on 2016-07-05T14:12:49Z (GMT). No. of bitstreams: 1 Elias Dias Coelho Neto.pdf: 2570519 bytes, checksum: 45c9ec25f4446657c86b65924c054a2d (MD5) Previous issue date: 2012-12-20 / The rate at which species accumulate with increased sample size - the species-area relationship - is one of the most basic and fundamental problem in biogeography. This relationship has profound significance in understanding the generation and maintenance of biodiversity in an ecosystem. In this sense, this thesis introduces two models of community dynamics within two different frameworks: The first investigate the spatial patterns of species distribution in fragmented landscapes within the framework of the neutral theory. In the second approach the spatial patterns of species distribution on the genetic variability is studied under the assumption that natural selection has a prominent role in driving the evolution of such populations. Additionally, the model assumes that the environment is heterogeneous, such that the strength of natural selection depends on the localization of the species in the lattice. Our results for the neutral community model show that fragmentation has an important influence in shaping the specie-area relationship. In particular, the level of fragmentation than changes the size of the area interval when the species-area relation is well described by a power-law, S ∼ Az. We also investigate the biodiversity on the percolating cluster. In the non-neutral model our simulation results demonstrate that the level of heterogeneity of the environment affects the shape of the genetic-area relationships. But it is possible to recover the triphasic scenario for low and intermediate level of heterogeneity. / A taxa com que as espécies acumulam com o crescimento da área de amostragem - a relação espécie-área - é um dos problemas mais básicos e fundamentais em biogeografia. Esta relação tem significado profundo na compreensão da geração e manutenção da biodiversidade no ambiente. Nesse sentido, nesta tese introduzimos dois modelos para populações de organismos que interagem em ambiente finito e saturado: O primeiro para populações que sofrem variação em suas abundâncias de forma nula - modelo neutro; o segundo, as populações estão sob seleção natural e variabilidade genética - modelo não neutro. Em ambas as abordagens, para caracterizarmos a relação espécie-área, realizamos simulações computacionais para gerar diversidade de espécies em comunidades em equilíbrio. Na abordagem neutra utilizamos o método da coalescência na versão estendida a habitats fragmentados. Enquanto que na abordagem com seleção natural utilizamos o modelo NK para o ambiente com níveis de heterogeneidade entre habitats controlados pelo parâmetro λ . Nossos resultados para comunidade neutra mostram que o aumento da fragmentação do habitat influência o padrão da curva espécie-área, principalmente em áreas pequenas e intermediárias, aonde ocorre o encurtamento do comprimento do intervalo de áreas em que o regime de lei de potências é verificado. Nós notamos também que um pequeno valor da taxa de especiação ν , o expoente z da relação espécie-área se eleva com o crescimento da fragmentação. Por outro lado, quando pressão de seleção é considerada, o parâmetro de correlação λ também exerce uma importante influência sobre a formação do tamanho do regime intermediário da relação espécie-área, que decresce com o aumento do nível de correlação entre habitats. Quanto maior for a epistasia, mais pronunciado é esse efeito.