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Institute
- Institut für Biochemie und Biologie (61)
- Institut für Chemie (24)
- Institut für Physik und Astronomie (21)
- Wirtschaftswissenschaften (19)
- Institut für Ernährungswissenschaft (18)
- Institut für Geowissenschaften (15)
- Bürgerliches Recht (10)
- Institut für Umweltwissenschaften und Geographie (10)
- Öffentliches Recht (10)
- Department Erziehungswissenschaft (7)
Motivations and research objectives: During the passage of rain water through a forest canopy two main processes take place. First, water is redistributed; and second, its chemical properties change substantially. The rain water redistribution and the brief contact with plant surfaces results in a large variability of both throughfall and its chemical composition. Since throughfall and its chemistry influence a range of physical, chemical and biological processes at or below the forest floor the understanding of throughfall variability and the prediction of throughfall patterns potentially improves the understanding of near-surface processes in forest ecosystems. This thesis comprises three main research objectives. The first objective is to determine the variability of throughfall and its chemistry, and to investigate some of the controlling factors. Second, I explored throughfall spatial patterns. Finally, I attempted to assess the temporal persistence of throughfall and its chemical composition. Research sites and methods: The thesis is based on investigations in a tropical montane rain forest in Ecuador, and lowland rain forest ecosystems in Brazil and Panama. The first two studies investigate both throughfall and throughfall chemistry following a deterministic approach. The third study investigates throughfall patterns with geostatistical methods, and hence, relies on a stochastic approach. Results and Conclusions: Throughfall is highly variable. The variability of throughfall in tropical forests seems to exceed that of many temperate forests. These differences, however, do not solely reflect ecosystem-inherent characteristics, more likely they also mirror management practices. Apart from biotic factors that influence throughfall variability, rainfall magnitude is an important control. Throughfall solute concentrations and solute deposition are even more variable than throughfall. In contrast to throughfall volumes, the variability of solute deposition shows no clear differences between tropical and temperate forests, hence, biodiversity is not a strong predictor of solute deposition heterogeneity. Many other factors control solute deposition patterns, for instance, solute concentration in rainfall and antecedent dry period. The temporal variability of the latter factors partly accounts for the low temporal persistence of solute deposition. In contrast, measurements of throughfall volume are quite stable over time. Results from the Panamanian research site indicate that wet and dry areas outlast consecutive wet seasons. At this research site, throughfall exhibited only weak or pure nugget autocorrelation structures over the studies lag distances. A close look at the geostatistical tools at hand provided evidence that throughfall datasets, in particular those of large events, require robust variogram estimation if one wants to avoid outlier removal. This finding is important because all geostatistical throughfall studies that have been published so far analyzed their data using the classical, non-robust variogram estimator.
The recent discovery of an intricate and nontrivial interaction topology among the elements of a wide range of natural systems has altered the manner we understand complexity. For example, the axonal fibres transmitting electrical information between cortical regions form a network which is neither regular nor completely random. Their structure seems to follow functional principles to balance between segregation (functional specialisation) and integration. Cortical regions are clustered into modules specialised in processing different kinds of information, e.g. visual or auditory. However, in order to generate a global perception of the real world, the brain needs to integrate the distinct types of information. Where this integration happens, nobody knows. We have performed an extensive and detailed graph theoretical analysis of the cortico-cortical organisation in the brain of cats, trying to relate the individual and collective topological properties of the cortical areas to their function. We conclude that the cortex possesses a very rich communication structure, composed of a mixture of parallel and serial processing paths capable of accommodating dynamical processes with a wide variety of time scales. The communication paths between the sensory systems are not random, but largely mediated by a small set of areas. Far from acting as mere transmitters of information, these central areas are densely connected to each other, strongly indicating their functional role as integrators of the multisensory information. In the quest of uncovering the structure-function relationship of cortical networks, the peculiarities of this network have led us to continuously reconsider the stablished graph measures. For example, a normalised formalism to identify the “functional roles” of vertices in networks with community structure is proposed. The tools developed for this purpose open the door to novel community detection techniques which may also characterise the overlap between modules. The concept of integration has been revisited and adapted to the necessities of the network under study. Additionally, analytical and numerical methods have been introduced to facilitate understanding of the complicated statistical interrelations between the distinct network measures. These methods are helpful to construct new significance tests which may help to discriminate the relevant properties of real networks from side-effects of the evolutionary-growth processes.
Modern anthropogenic forcing of atmospheric chemistry poses the question of how the Earth System will respond as thousands of gigatons of greenhouse gas are rapidly added to the atmosphere. A similar, albeit nonanthropogenic, situation occurred during the early Paleogene, when catastrophic release of carbon to the atmosphere triggered abrupt increase in global temperatures. The best documented of these events is the Paleocene-Eocene Thermal Maximum (PETM, ~55 Ma) when the magnitude of carbon addition to the oceans and atmosphere was similar to those expected for the future. This event initiated global warming, changes in hydrological cycles, biotic extinction and migrations. A recently proposed hypothesis concerning changes in marine ecosystems suggests that this global warming strongly influenced the shallow-water biosphere, triggering extinctions and turnover in the Larger Foraminifera (LF) community and the demise of corals. The successions from the Adriatic Carbonate Platform (SW Slovenia) represent an ideal location to test the hypothesis of a possible causal link between the PETM and evolution of shallow-water organisms because they record continuous sedimentation from the Late Paleocene to the Early Eocene and are characterized by a rich biota, especially LF, fundamental for detailed biostratigraphic studies. In order to reconstruct paleoenvironmental conditions during deposition, I focused on sedimentological analysis and paleoecological study of benthic assemblages. During the Late Paleocene-earliest Eocene, sedimentation occurred on a shallow-water carbonate ramp system characterized by enhanced nutrient levels. LF represent the common constituent of the benthic assemblages that thrived in this setting throughout the Late Paleocene to the Early Eocene. With detailed biostratigraphic and chemostratigraphic analyses documenting the most complete record to date available for the PETM event in a shallow-water marine environment, I correlated chemostratigraphically for the first time the evolution of LF with the δ¹³C curves. This correlation demonstrated that no major turnover in the LF communities occurred synchronous with the PETM; thus the evolution of LF was mainly controlled by endogenous biotic forces. The study of Late Thanetian metric-sized microbialite-coral mounds which developed in the middle part of the ramp, documented the first Cenozoic occurrence of microbially-cemented mounds. The development of these mounds, with temporary dominance of microbial communities over corals, suggest environmentally-triggered “phase shifts” related to frequent fluctuations of nutrient/turbidity levels during recurrent wet phases which preceding the extreme greenhouse conditions of the PETM. The paleoecological study of the coral community in the microbialites-coral mounds, the study of corals from Early Eocene platform from SW France, and a critical, extensive literature research of Late Paleocene – Early Eocene coral occurrences from the Tethys, the Atlantic, the Caribbean realms suggested that these corals types, even if not forming extensive reefs, are common in the biofacies as small isolated colonies, piles of rubble or small patch-reefs. These corals might have developed ‘alternative’ life strategies to cope with harsh conditions (high/fluctuating nutrients/turbidity, extreme temperatures, perturbation of aragonite saturation state) during the greenhouse times of the early Paleogene, representing a good fossil analogue to modern corals thriving close to their thresholds for survival. These results demonstrate the complexity of the biological responses to extreme conditions, not only in terms of temperature but also nutrient supply, physical disturbance and their temporal variability and oscillating character.
The aim of this thesis is to achieve a deep understanding of the working mechanism of polymer based solar cells and to improve the device performance. Two types of the polymer based solar cells are studied here: all-polymer solar cells comprising macromolecular donors and acceptors based on poly(p-phenylene vinylene) and hybrid cells comprising a PPV copolymer in combination with a novel small molecule electron acceptor. To understand the interplay between morphology and photovoltaic properties in all-polymer devices, I compared the photocurrent characteristics and excited state properties of bilayer and blend devices with different nano-morphology, which was fine tuned by using solvents with different boiling points. The main conclusion from these complementary measurements was that the performance-limiting step is the field-dependent generation of free charge carriers, while bimolecular recombination and charge extraction do not compromise device performance. These findings imply that the proper design of the donor-acceptor heterojunction is of major importance towards the goal of high photovoltaic efficiencies. Regarding polymer-small molecular hybrid solar cells I combined the hole-transporting polymer M3EH-PPV with a novel Vinazene-based electron acceptor. This molecule can be either deposited from solution or by thermal evaporation, allowing for a large variety of layer architectures to be realized. I then demonstrated that the layer architecture has a large influence on the photovoltaic properties. Solar cells with very high fill factors of up to 57 % and an open circuit voltage of 1V could be achieved by realizing a sharp and well-defined donor-acceptor heterojunction. In the past, fill factors exceeding 50 % have only been observed for polymers in combination with soluble fullerene-derivatives or nanocrystalline inorganic semiconductors as the electron-accepting component. The finding that proper processing of polymer-vinazene devices leads to similar high values is a major step towards the design of efficient polymer-based solar cells.