Decomposition of organic matter is a critical component in global biogeochemical cycling. While decomposition rates have been robustly predicted for mesic systems, modeling decomposition dynamics in drylands has proven to be problematic, reflecting a need to account for processes that may be unique to dryland systems: low and spatially variable vegetation cover, high rates of soil movement, and high levels of radiant energy exposure at the soil surface. Recent empirical evidence suggests that the discrepancies between measured and predicted decomposition rates in drylands may be due to the greater influence of abiotic drivers, such as soil-litter mixing (SLM) and solar radiation, on plant litter decomposition relative to more mesic systems. UV-driven photodegradation may dominate until SLM reaches a threshold, at which point litter is shielded from radiation and microbial processes become predominant. The overarching goal of this dissertation was to examine the influence of SLM and solar radiation on decomposition of foliar and woody plant litter in a dryland ecosystem undergoing woody plant encroachment. A series of four complimentary experiments sought to quantify the effects of these abiotic drivers on decomposition in relation to variables such as vegetation patch type (e.g., beneath a shrub canopy, in a grass patch, on bare ground), radiant energy regime (e.g., full sun vs. shade), geomorphic surface (e.g., sandy, Holocene-age vs. clay-rich, Pleistocene-age soils), seasonality of litter fall (e.g., summer vs. winter), and litter quality (e.g., grass, shrub leaf, woody). Results indicate that interactions between SLM and photodegradation are complex and mediated by variations in ground cover which influence the local radiant energy environment and the movement of soil across the landscape by wind and water. Decomposition rates were significantly influenced by SLM, UV radiation, radiant energy regime, vegetation structure, and initial litter quality. While these results confirmed the importance of SLM and photodegradation as dryland decomposition drivers, they also reinforced the need for additional research to further clarify the relative importance of these processes under field conditions, particularly the interplay between UV radiation and SLM and their relative influence on biotic and abiotic decomposition processes. Given the changes in climate and vegetation projected for drylands, it is critical to further elucidate the influence of these processes on dryland biogeochemical cycling, as their effects may be magnified or dampened under future conditions. A deeper understanding of the processes driving biogeochemical cycling that may be unique to systems undergoing shifts in plant lifeform composition will allow us to better account for the fate of carbon in these globally important ecosystems.

Made available in DSpace on 2015-03-05T16:19:36Z (GMT). No. of bitstreams: 0 Previous issue date: 31 / Bolsa para curso e programa de Pós Graduação / A Floresta com Araucária apresenta uma grande importância ecológica e econômica para a região sul do Brasil. Durante o último século ela foi intensamente explorada causando a sua conversão para pastagens, agricultura ou monoculturas de espécies arbóreas nativas e exóticas. Essa alteração de paisagem tem efeitos diretos sobre as condições físicas dos habitats, a abundância das populações, a diversidade e composição das comunidades e os processos ecossistêmicos. Dentre as diversas rotas de transferência de nutrientes no ecossistema, a deposição de serrapilheira é uma das principais via, influenciando fortemente o sistema solo-planta. O objetivo deste trabalho foi testar como a substituição da Floresta com Araucária por monoculturas arbóreas afeta os processos de deposição da serrapilheira e suas conseqüências para a ciclagem de nutrientes. Na Floresta Nacional de São Francisco de Paula (RS) foram selecionadas três áreas de um hectare de cada um dos seguintes habitats: Floresta com Araucária (FO), plantação de A / The Araucaria Forest has a great ecologic and economic value for the southern region of Brazil. Along the last century it was intensely explored causing its conversion to grassland, agriculture or monoculture of exotic tree species. This alteration of the landscape had direct effects on physical conditions of the habitats, abundance of populations, diversity and composition of communities, and on ecosystem processes. Among the several routes of nutrient transfer on ecosystems, litter production is one of the most important ones, strongly influencing the soil-plant system. The aim of this paper was to test how the replacement of Araucaria Forest by tree monocultures affects the litter deposition process and its consequences to the nutrient cycle. In the São Francisco de Paula National Forest (RS), three one-hectare areas of each one of the following habitats were selected: Araucaria Forest (FO), Araucaria angustifolia plantation (PA), Pinus plantation (PP) and Eucalyptus plantation (PE). In each area, 12 litte

Dans le cadre de la mission spatiale SMOS (Soil Moisture and Ocean Salinity), nous présentons dans cette thèse une nouvelle approche numérique de modélisation du calcul de l’émissivité et du coefficient bi-statique de systèmes forestiers sol-litière en Bande L. Le système sol-litière est représenté par deux couches diélectriques 3D comportant des interfaces rugueuses, une démarche qui n’apparait pas actuellement dans la littérature. Nous validons notre approche pour une seule couche en comparant les simulations de l'émissivité avec celles produites par la méthode des moments et des données expérimentales. A partir de ce nouveau modèle, nous évaluons la sensibilité de l’émissivité du système sol-litière en fonction de l’humidité et de la rugosité de la litière. Ce nouveau modèle permettra de créer une base de données synthétiques d’émissivités calculées en fonction de nombreux paramètres qui contribuera à améliorer la prise en compte de la litière dans l'algorithme d’inversion des données de la mission spatiale SMOS. / In the context of the SMOS (Soil Moisture and Ocean Salinity) mission, we present a new numerical modelling approach for calculating the emissivity and bistatic scattering coefficient of the soil-litter system found in forests, at L-band. The soil-litter system is modelled as two 3-dimensional dielectric layers, each with a randomly rough surface, which to our knowledge has not previously been achieved. We investigate the validity of the approach for a single layer by comparing emissivity simulations with results of Method of Moments simulations, and experimental data. We then use the approach to evaluate the sensitivity of the soil-litter system as a function of moisture content and the roughness of the litter layer. The numerical modelling approach which has been developed will allow us in the future to create a synthetic database of the emissivity of the soil-litter system as a function of numerous parameters, which will contribute to validating and improving the inversion algorithm used by the SMOS mission to retrieve soil moisture over forests.