Measurement of elastic properties of silicates at realistic mantle pressures
- Measurements of seismic wave travel times through the Earth’s interior provide one of the few sources of information on the properties of deep mantle rocks. These travel times can be interpreted in terms of their implications for the mineralogy, chemistry and temperature of the mantle if they can be compared with models determined from laboratory measurements for the elastic properties of mantle minerals at high pressures and temperatures. In this study MHz ultrasonic measurements have been used to determine the velocities of P and S acoustic waves in mantle minerals at mantle conditions. These results are used to constrain the properties of the mantle through comparison with seismic data.
Subduction zones exhibit faster seismic wave velocities compared to the surrounding mantle due to the recycling of relatively cold oceanic lithosphere. In certain subduction zones, however, a 5-10 km thick low velocity layer (LVL) has been inferred to exist along the top surface of the subducting slab at depths of up to 250 km. Shear-wave velocities, in particular, within these layers have been estimated as up to 10% slower than in the surrounding mantle. High-pressure ultrasonic interferometric measurements were performed to gain insight into the elastic properties of lawsonite [CaAl2(Si2O7)(OH)2.H2O], a hydrous mineral phase stabilized under cold subduction zone conditions. It was found that lawsonite has an unusually low shear modulus at high pressure and its formation in subducted oceanic crust can explain some seismic evidence for LVL at depths exceeding 100 km. To approach estimated LVL velocities requires lawsonite to form in the subducting crust as a result of a fluid influx due to the breakdown of other hydrous minerals such as serpentine. The formation of lawsonite additionally lowers seismic velocities because it forms at the expense of garnet, a mineral with relatively fast seismic velocities. LVL observations may therefore be used to place constraints on the amount of H2O subducted into the deep mantle.
Chemical heterogeneities in the transition zone are potentially developed through the subduction of basaltic oceanic crust and lithospheric ultramafic mantle. The mineralogy of these lithologies in the transition zone will mainly differ in terms of the proportion and chemistry of garnet. Garnets formed from originally basaltic and ultramafic compositions will differ in the proportions of Ca, Fe and majoritic component, with the former being richer in all components except majorite. Most models use partial derivatives of elastic properties with respect to composition to determine the elastic properties of complex garnet compositions in the mantle. In this study the effect of chemical variations on seismic wave velocities of garnets up to transition zone conditions have been intensively studied by examining the effects of Ca, Fe and majoritic substitutions on the elasticity of pyrope. In addition, complex multicomponent garnet compositions, expected to form from both subducted basaltic material and typical ultramafic mantle at transition zone conditions, have been studied. The results indicate that elastic properties of multicomponent garnets can be quite reliably interpolated from the end member properties, with an accuracy that is generally within the experimental uncertainties. No evidence was found that large excess properties exist that cause non-linear contributions to multicomponent elastic properties for the mineral garnet. Deviation between the two calculated mantle models, pyrolitic and MORB, is about 3% for both VP and VS, with the modelled slab composition being slower. Within the uncertainties of the seismic data therefore a pyrolite mantle composition is quite consistent with velocities at the base of the transition zone. Relatively low temperatures and olivine-rich nature of the stagnant slab support that there would be few grounds to argue that significant accumulation of MORB composition material may occur at the base of the transition zone.
Ultrasonic interferometry measurements in conjunction with in situ X-ray techniques have been used to measure compressional and shear wave velocities and densities of MgSiO3 perovskite (Mg-Pv) and perovskite ((Mg, Fe)-Pv) in the multianvil at pressures up to 25 GPa and temperatures to 1200 K. Data for Mg-Pv are consistent with previous studies and the (Mg, Fe)-Pv sample has almost identical shear properties to Mg-Pv. The adiabatic bulk modulus, Ks, for (Mg, Fe)-Pv, however, is found to be substantially lower than Mg-Pv, with a refined value of 236 GPa and a pressure derivative of 4.7. It is proposed that this low KS value results from a change in the elasticity of Fe-bearing perovskite at low pressures <30 GPa.
High temperatures measurements of VP and VS of and perovskite samples lead to the conclusion that the substitution of Mg by Fe seems to have a consistent effect on Ks, strongly lowering it in both Al and Al-free perovskites while it does not have any noticeable effect on the shear elastic properties. By substituting Al into the Si octahedral site, the rigidity of this framework is being reduced which clearly has an influence on the shear modulus G. G of the Al-bearing perovskites is obviously controlled by the Al content in the structure. Seismic observations indicate low visibility of underside PP reflections from the 660 km discontinuity. Seismologists have used this in the past to argue that the P velocity contrast at the discontinuity is much lower than would be expected from seismic reference models. The data collected on Fe-Al perovskite in this study provide excellent support for this argument as model calculations based on these data show very little contrast in Vp at 660 km depth. Instead, the model derived in this study indicates that much of the change in P velocity between the transition zone and lower mantle occurs over a much broader depth, 200km, interval and is mainly attributed to the transformation of garnet to perovskite. A major conclusion therefore is that a sharp 660 km discontinuity reflection should be visible in S but not P wave seismic data. The model calculation for VS is in a good agreement with the seismic reference models up to depths as great as 1071 km, while VP remains 3 to 4% lower at lower mantle conditions. The extrapolations of pressure and temperature dependences significantly out of the ranges over that they were measured may well explain this discrepancy however.
Domänenspezifische Evaluation und Optimierung von Datenstandards und Infrastrukturen
- In den letzten Jahren und Jahrzehnten ist die Menge der Daten, die in wissenschaftlichen Projekten erhoben wird, exorbitant gestiegen. Mit diesem gestiegenen Datenaufkommen geht eine Zunahme der Anforderungen an die Datenspeicherung und den Datenaustausch einher. Dazu wurden in den letzten Jahrzehnten Infrastrukturen entwickelt, welche diese Aufgaben für eine bestimmte Domäne übernehmen. Insbesondere in der Domäne der Lebenswissenschaften sind sogenannte Megascience-Plattformen entstanden, welche für den globalen Datenaustausch verantwortlich sind. Ein wesentliches Merkmal dieser Infrastrukturen sind die unterstützten Datenstandards. Um die Eignung eines Datenstandards oder einer Infrastruktur in einer Domäne zu bestimmen, werden generische Evaluationssysteme benötigt. Auf Basis der Evaluation mit diesen Systemen ist es möglich, Schwachstellen in Infrastrukturen und Datenmodellen zu erkennen und diese Mängel zu beseitigen. Grundlage hierfür ist die Strukturierung der Anwendungsdomäne in Prozessen.
In dieser Arbeit werden die Themenbereiche der Datenstandards und Infrastrukturen in der Anwendungsdomäne der Biodiversitätsinformatik getrennt untersucht. Im ersten Teil der Arbeit werden Datenstandards betrachtet. Ausgangspunkt sind die Datenstandards, da diese den Datenaustausch in einer Infrastruktur limitieren. Dazu wird ein generisches Evaluationssystem für Datenstandards entwickelt, domänenspezifisch angepasst und auf Standards der Biodiversitätsinformatik angewendet. Kern des Frameworks ist die Vollständigkeitsanalyse mit der 'Process Oriented Schema Evaluation' (POSE), welche die Basis des Frameworks bildet. Dabei konnte keiner der untersuchten Datenstandards die Anforderungen der Biodiversitätsinformatik vollständig erfüllen. Auf Basis dieser Erkenntnisse wurde mit der 'Process Oriented Data Schema Language' (PODSL) mit PODSL-Biodiv ein flexibles Datenmodell für die Datenspeicherung in der Biodiversität entwickelt.
Der zweite Teil der Arbeit behandelt Infrastrukturen. Dazu wird zunächst mit dem 'Infrastructure Evaluation Framework' (IEF) ein Evaluationssystem für Infrastrukturen auf der Grundlage von Prozessen entwickelt und auf die wichtigsten Infrastrukturen in der Biodiversitätsinformatik angewendet. Dabei ist für die Qualität einer Infrastruktur neben der technischen Umsetzung auch deren Organisation dieser maßgeblich. Im Rahmen der Evaluation konnte ermittelt werden, dass die Infrastruktur der 'Global Biodiversity Information Facility' (GBIF) als wichtigste Infrastruktur der Biodiversitätsinformatik die Kriterien an die Funktionalität einer Infrastruktur nicht vollständig erfüllen kann. Mit BDEI wird ein Konzept zur Weiterentwicklung von GBIF vorgestellt, um diese Mängel zu beseitigen.
Wage Policy in a Global World
- This thesis contains an extended literature review and three essays on the interaction of globalization and wage policy, employment, income distribution and welfare. A specific focus lies thereby on the role of trade unions as one major labor market institution. A brief introductory chapter motivates the general topic before an extended literature review highlights main findings from previous studies.
The first essay sets up a multi-sector general oligopolistic equilibrium trade model in which all firms face wage claims of firm-level unions. By accounting for productivity differences across industries, the model features income inequality along multiple lines, including inequality between firm owners and workers as well as within these two groups of agents, and involuntary unemployment. This setting is used to study the impact of trade liberalization on key macroeconomic performance measures. In particular, the study shows that a movement from autarky to free trade with a fully symmetric partner country lowers union wage claims and therefore stimulates employment and raises welfare. Whether firms can extract a larger share of rents in the open economy depends on the competitive environment in the product market. Furthermore, the distribution of profit income becomes more equal when a country opens up to trade with a fully symmetric trading partner. It is also shown how country size differences and technological dissimilarity of trading partners affect the results from the analysis.
The second essay also builds upon the framework of general oligopolistic equilibrium with two countries that, however, differ in the centralization of union wage setting. Being interested in the consequences of openness, this study shows that, in the short-run, trade increases welfare and employment in both locations, and it raises income of capital owners as well as workers. In the long run, capital outflows from the country with the more centralized wage setting generate winners and losers and make the two countries more dissimilar in terms of unemployment and welfare. Decentralization of wage setting can successfully prevent capital outflow and the export of jobs.
The third and final essay is of an empirical nature and investigates the role of wages as a potential driving force for German export activity. In the past 15 years Germany has been characterized by a strong export activity while at the same time initiating structural reforms on the labor market. It is often argued that German firms and plants are particularly successful in exporting since they are very competitive internationally. By computing unit labor costs as a measure of international competitiveness based on OECD STAN data and the IAB establishment panel this study investigates the role of unit labor costs for the decision to export. The results show that (i) German plants’ export intensity is positively correlated with competitiveness and (ii) that the relationship is spuriously driven by a non-industry specific common time trend. The study furthermore applies a corner solution model that allows to disentangle the total effect into its effect at the extensive and intensive margin of trade. Results indicate a positive and significant effect of competitiveness at both margins but the effect turns out insignificant before the introduction of the Euro.
Iron spin crossovers at high pressures and temperatures and their effects on materials relevant tot he Earth’s lower mantle and core
- Iron is the most abundant element by mass in the Earth. The iron content and its spin or oxidation state have a major influence on the physical properties of the main phases in the Earth’s interior. Therefore it is of vast importance to understand the behavior of iron in mineral phases at the temperature and pressure conditions of the Earth’s interior. This cumulative thesis investigates Fe spin crossovers in iron-containing magnesium aluminum silicates, iron-bearing silicate glasses, the iron carbide Fe3C and the effect of Fe spin crossovers on the Fe/Mg partitioning between perovskite and ferropericlase in pyrolitic model system of the Earth’s lower mantle. The goal is first to understand the nature of the Fe spin crossover in respect to its oxidation state and second to estimate the consequences of their occurrence to processes and the structure in the Earth. Central tools in these studies are laser heated diamond anvil cells, to reach the pressure and temperature conditions of the Earth’s interior, Mössbauer spectroscopy, which is a sensitive probe for detecting structural and spin changes in Fe-bearing materials, and analytical transmission electron microscopy, as a probe of chemistry and oxidation state on the nm-scale. In this cumulative thesis I present the results of five research articles. For the analysis of conventional and recently developed synchrotron energy domain Mössbauer spectra the computer program MossA is introduced, which builds the basis for the analysis and interpretation of the results for the other studies. Based on synchrotron Mössbauer spectroscopy and electrical conductivity measurements of Fe-bearing silicate aluminum perovskite it is shown that Fe3+ occupies the dodecahedral A-site of the perovskite structure and remains in the high-spin state throughout the pressure and temperature conditions of the Earth’s lower mantle. Furthermore, a study on the electronic behavior of Fe in a Fe2+-rich aluminous silicate glass and a Fe3+-rich sodium silicate glass infers that no sharp high spin to low spin crossover occurs in silicate melts in the Earth’s lower mantle. This result excludes the possibility of negatively buoyant melts in the lower mantle in an early magma ocean solely due to strong preferential partitioning of iron into the melt phase, which would be induced by a Fe low-spin bearing melt. New insights into to decoupled partitioning behavior of Fe2+ and Fe3+ between the two dominant phases of the Earth’s lower mantle, perovskite and ferropericlase, are presented. The intermediate spin to low spin crossover of Fe2+ in perovskite at about 110 GPa seems to have a strong effect on partitioning and oxidation state of Fe. It leads to a change of the partitioning behavior of Fe between perovskite and ferropericlase and induces a reduction of Fe3+ to Fe2+ in perovskite. Finally, a Mössbauer spectroscopic and single-crystal x-ray diffraction study of Fe3C reveals a two-stage loss of magnetism in Fe3C at high pressures at room temperature: a ferro- to paramagnetic transition around 8-10 GPa and a para- to nonmagnetic transition at about 22 GPa.
High-Resolution Modelling of Surface-Atmosphere Interactions and Convection Development at Nam Co Lake, Tibetan Plateau
- The Tibetan Plateau has recently become an area of increased interest for the atmospheric and environmental sciences. Surface-atmosphere interactions and specifically the exchange of momentum, turbulent energy and water vapour as well as the development of convection are not only important for the surface energy balance and local water resources, but also have influence on the evolution of the monsoon system and climate. High-resolution, numerical atmospheric models with a fully coupled surface model are a valuable tool for the systematic investigation of surface-atmosphere interactions. Nam Co Lake, located at the northern extent of the monsoon's influence, was selected as a complex system in order to study the interaction of the land and the lake in the generation of mesoscale circulations and the development from boundary-layer clouds to moist convection.
Turbulent fluxes estimated by eddy-covariance and atmospheric profiles measured by radiosondes are used in this work in conjunction with the ATHAM (Active Tracer High-resolution Atmospheric Model) and Hybrid models. Substantial model development is undertaken for both ATHAM and Hybrid. This means a more consistent formulation of tracer and heat transport in ATHAM and improved model stability. Hybrid has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and surface model base temperature is assumed in each layer and extended to the surface. Compared to eddy-covariance measurements and a Surface-Vegetation-Atmosphere Transport (SVAT) Model there is an overall reasonable model performance, when tested on four days for two sites with variable environmental conditions during the 2009 summer monsoon season. At the same time, errors are reduced by 40-60% compared to the unmodified Hybrid.
Subsequently, the coupled modelling system is used for 2-dimensional cross-sections through the Nam Co Lake basin with horizontal resolutions of
200 m and at least 150 vertical layers between the surface and the model top located in the lower stratosphere. The 2-dimensional modelling approach has a
tendency to overestimate convective strength due to the underestimation of dry air entrainment and cannot reproduce fully realistic flow fields. Nevertheless,
it provides a valuable tool for systematic investigations of environmental factors, where 3D simulations are prohibitively expensive.
In simulations with several background wind speeds it is found that the model adequately simulates the mesoscale circulation system between the lake and the surrounding mountain chains. Dependent on the geostrophic wind direction there are two different mechanisms for the triggering of convection: Convective triggering, when overflowing topography, and triggering due to convergence between the lake-breeze front and the background wind. It is concluded that coupled modelling setup is capable of reproducing the system's most important dynamics, such as realistic turbulent surface fluxes, mesoscale circulations and cloud evolution.
Thereafter, the influence of the atmospheric profiles of temperature and relative humidity and the uncertainty that arises from them is discussed.
Simulations are initialised with profiles based on direct measurements (radiosondes), NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on two
days during the summer of 2012. The simulated convection from radiosondes compares reasonably well with weather observations for the first day, but
less well for the second day, when large-scale synoptic effects, which are not included in the model, gain importance. The choice of vertical profile information leads to strongly differing convection development, causing modifications of the surface energy balance and thus of the energy and water
cycle for Nam Co Lake.
With respect to precipitation it is found that a large fraction of the precipitation that is generated in the simulations is deposited within the basin
and on the slopes of the surrounding mountain chains and thus locally recycled. This also means that a weather station in the centre of the basin is not representative of the system. Furthermore, Nam Co Lake may be of importance as a water supply for the region. Additionally, the choice of profile and the initial water vapour contents determine the amount of precipitation so that there are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles.
The findings from the thesis provide an example of the impacts of surface-atmosphere interactions, mesoscale circulations and convective evolution on
the Tibetan Plateau. Scaled to the entire plateau these processes are highly relevant to ecosystems, climate and the water cycle.
Polyelectrolyte Complexes and their Therapeutic Potential
- This thesis describes the preparation of polyelectrolyte nanostructures, the characterization of interpolyelectrolyte complexes (IPECs) made from these structures and their use in a therapeutic context. The therapeutic use of such IPECs connects the two major topics of this work: First, the delivery of genes into eukaryotic cells in vitro by means of new star-shaped polycations was explored. Second, the structure of ionic multicompartment micelles (MCMs) when complexed with polyions was studied and the performance of these nanostructures as delivery agents of anti-cancer drugs both in vitro and in vivo was tested.
To better understand structure-property relationships of polycations relevant for gene delivery, a library of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) homopolymers was synthesized. Star-shaped polymers with a different number of arms and molecular weights were created by using sugar-based or inorganic nanoparticles as core molecules with varying number of initiation sites. The cytotoxicity as well as transfection performance of polyplexes from these polymers and plasmid DNA was determined for different PDMAEMA-nitrogen/DNA phosphate ratios in CHO-K1 cells. A decrease of the cytotoxicity of polymers with a given molecular weight was observed with increasing degree of branching, i.e., with increased arm-number. Star-shaped PDMAEMA with roughly 20 arms (Si-PDMAEMA) showed exceptionally high transfection efficiency. The superior transfection behavior of this specific polymer was demonstrated in non-dividing or differentiated (C2C12 and human T lymphocytes) cell lines. Additionally, polymeric micelles were produced from a polybutadiene-block-PDMAEMA diblock copolymer in aqueous solution and subsequently used for gene transfection. Their transfection efficiency was in the same range as that of Si-PDMAEMA, hinting towards a general design principle for highly effective gene vectors. This consists of a star-shaped architecture of PDMAEMA chains emanating from a common center.
The second major part of this thesis deals with the structure of ionic MCMs complexed with diverse polycations as well as the drug delivery capabilities of some of these complex micellar structures. MCMs from polybutadiene-block-poly(1-methyl-2-vinyl pyridinium)-block-poly(methacrylic acid) (BVqMAA) triblock terpolymers were used as the basis for further structural modifications. These micelles exhibit a core-shell-corona morphology, where PB forms the core of the micelles, a discontinuous (patchy) shell consisting of an IPEC between P2VPq and PMAA is present and finally a corona of excess PMAA stabilizes the micelles in aqueous solution. At sufficiently high pH a portion of the corona carries negative charges, which were then used to form further IPECs with either cationic homopolymers or double-hydrophilic block copolymers featuring one positively charged block. If polycations other than poly(2-vinyl pyridine) such as quaternized PDMAEMA were used for the complexation, a new and distinguishable IPEC compartment was formed on top of the already existing P2VPq/PMAA IPEC. In the case of MCMs with a short to moderate block length of the corona, i.e., the degree of polymerization (DP) of PMAA was between 345 to 550 units, a layered arrangement of the newly formed IPEC compartment was found.
For BVqMAA micelles with a long PMAA corona (DP of PMAA = 1350) complexed with different quaternized homopolymers, a patchy arrangement of the newly formed IPEC compartment instead of a layered one was found. The formation of this new structure is due to an interfacial energy minimization between the new IPEC and the compartmentalized core of the micelles that became possible due to the exceptionally long PMAA corona of the precursor micelles.
Finally, MCMs from BVqMAA were tested for their capacity to deliver a hydrophobic anti-cancer drug for photodynamic therapy to lung cancer cells in vitro and in vivo. The influence of the corona composition was studied by forming complexes with a double-hydrophilic diblock copolymer, poly(L-lysine)-block-poly(ethylene glycol) (PLL-b-PEG). A new cylinder-on-sphere morphology was observed in electron microscopy for BVqMAA/PLL-b-PEG complex micelles at high complexation ratios between lysine and MAA units. Highest cytotoxicity and uptake were found for pure BVqMAA micelles, both decreasing with increasing amount of PLL-b-PEG attached to the micelles. In mice, a prolonged blood circulation time in the range of several hours was exclusively observed when fully PEGylated micelles were injected. Those micelles were the only ones showing an enhanced accumulation in a subcutaneous tumor model 24 h after intravenous injection. The amount of drug delivered to the tumor tissue by the micelles suppressed tumor growth for up to 21 days after a single dose injection and photoirradiation step. The potential of complex micelles on the basis of BVqMAA MCMs could thus be proven.
Effect of Dopants on the Local Atomic Structure and Sintering Behavior of Bismuth Sodium Titanate
- The most commonly used piezoceramic is lead zirconate titanate Pb(ZrxTi(1-x))O (PZT). It possesses outstanding piezoelectric properties which can be modified for numerous applications by the addition of dopants. However, because of environmental and health concerns regarding lead, lead-free alternatives are demanded by politics.
One of the two most promising lead-free replacement materials is the ferroelectric bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT). Like PZT, it crystallizes in the perovskite structure.
Since the dielectric and piezoelectric properties of pure BNT ceramics are insufficient for application, BNT is often modified by the addition of dopants. These influence a great variety of material properties to different degrees, e.g. the sintering behavior, the dielectric and piezoelectric properties and their respective temperature stabilities. Doping of BNT aims to decrease the sintering temperature in order to avoid Bi vaporization, to increase the depolarization temperature and to enhance the piezoelectric coefficient.
The effects of numerous dopants on the resulting performance of BNT were studied extensively in the literature. However, so far little attention has been payed to the way in which dopants interact with the piezomaterial. Nevertheless, it is the understanding of these relationships that would make targeted modifications and improvements of BNT possible.
The primary goal of this study was to investigate and explain the effects of a model dopant -cobalt- on the phase formation, sintering behavior and microstructure of BNT as well as on the resulting dielectric and piezoelectric properties. In this regard, a core issue was to determine the preferred lattice site of Co in BNT.
BNT was synthesized from oxide powders using the classic solid-state route and sintered at temperatures ranging from 1000 °C to 1150 °C. Cobalt was added in concentrations between 0.1 mol% and 2.6 mol% Co prior to the calcination as Co3O4.
About one third of the total cobalt amount was incorporated into the BNT lattice on the perovskite B-site, that is, it substituted for Ti. The cobalt in BNT appeared to be in equilibrium with the secondary phase Co2TiO4, which invariably formed at cobalt concentrations greater than 0.1 mol% Co. For charge balancing reasons, oxygen vacancies were created in the lattice of cobalt-doped BNT. These markedly enhanced the diffusivity. As a result, the sintering temperature of doped BNT decreased with increasing cobalt concentrations, and high final densities were achieved. However, in highly doped BNT sample swelling occurred at elevated temperatures of the sintering cycle. This phenomenon was attributed to evaporating oxygen caused by the valence transition of Co3+ to Co2+.
Up to 950 °C, BNT was found to densify via solid state sintering mechanisms. Above this temperature, a small amount of liquid phase was present, which probably formed from decomposing BNT because of a slight Ti-deficiency due to doping. Increased Bi vaporization from the melt above 1000 °C appeared to have stabilized sodium cobalt titanate, an additional secondary phase.
The rotation of the iso-lines in the kinetic field diagram of doped BNT was interpreted such that the activation energy for grain growth was higher than the activation energy for densification. Possible reasons are the solute-drag effect and the pinning of domain walls by secondary phase particles.
Both the depolarization temperature and the piezoelectric coefficient d33 decreased with increasing cobalt concentrations. The dielectric properties deteriorated as well. This was attributed to the high electrical conductivity of the doped samples, which prevented full poling.
Plasticity, Intraspecific Variability and Local Adaptation to Climatic Extreme Events of Ecotypes/Provenances of Key Plant Species
- Climate change, and especially an increase of magnitude and frequency of climatic extreme events such as drought periods or heatwaves, will alter growing conditions for plants in the future. Persistent ecosystems, with long-living organisms, such as forest or permanent grassland will be particularly impacted by this development. The velocity of these changes is likely to occur at a pace, which species may not be able to keep track with by natural dispersal or genetic adaptation. Agriculture, forestry and ecosystem management must develop counteracting practices to secure the persistence and functioning of these ecosystems and thus their provision of goods and services. Therefore it is important to develop a better understanding how species and ecosystems may respond to future climatic stressors. Impact assessments, e.g. via climatic envelope modelling are prone to misinterpretations of the adaptive capacity of species, as they do not incorporate the intraspecific genetic and phenotypic differences that exist within the populations accross the distribution range of a species.
Yet, intraspecific variation may exhibit potential tools for the development of climate change adaptation strategies. Here, I focus on key ecosystems in Central Europe. In particular the selective use of plant provenances or ecotypes may help to make ecosystems climate-resilient without a potentially more problematic introduction of exotic species. Especially provenances from warmer, drought-prone regions, with a current climate similar to the projected one for Central Europe recently came into focus as potential substitutes for local provenances, as they might have developed local adaptations to climate conditions at their location of origin. Insights about the response of these provenances to changing averages and extreme event regimes are crucial for a reasonable use of within-species diversity in climate change adaptation.
First, the concept of assisted colonization or migration of species or ecotypes and the role it can play as an adaptation strategy in agriculture, forestry or nature conservation is introduced (Manuscript1). It is suggested that a focus should be laid on keystone species that ensure ecosystem persistence and functioning as they govern the habitat structure and microclimate of a site. The assisted colonization of pre-adapted ecotypes of keystone species from climates similar to future projections for the target site is proposed.
Furthermore, provenances of selected grassland and forest key-species were exposed to drought and warming in two experiments in Bayreuth and Landau, and their ecological responses were analysed. Results suggest that local adaptations to climatic stressors exist. However, the magnitude and direction of responses strongly depend on species and climatic variables. For grassland species, e.g. differences in drought sensitivity could be demonstrated in some cases (Manuscript 4). Fagus sylvatica exhibited differences between the provenances in response to drought conditions, as well (Manuscript 3). It seems that marginal provenances, from the dry margins of the distribution range, show less increment reduction due to the drought treatment. Yet, under more favourable conditions of water supply these provenances did not yield the same high increment rates than more central provenances, indicating a trade-off between stability under stress and yield under non-stress conditions. A pine species that is generally considered to be rather drought-resistant, Pinus nigra, which is a potential substitute for climate-threatened conifers on dry sites in Central Europe, did not show any differences in response to drought and warming (Manuscript 2), maybe due to a weak selective pressure as a result of high drought-resistance across the whole distribution range. The impacts of drought on increment became not visible before the second year after the treatment, stressing the need for more long-time experiments in climate impact research.
Even in a generally warmer environment, cold extremes in winter or spring are expected still to prevail in the future. Therefore, the provenances of the selected species were tested for their cold-hardiness and late frost resistance (Manuscripts 5-7). Growth of the grassland species and F. sylvatica were negatively impacted by a late frost event and differences in late-frost sensitivity between provenances or ecotypes were identified. The (sub-) mediterranean species P. nigra showed differences between provenances in their winter cold hardiness. Correlations between performance under cold stress and winter conditions or late frost proneness of the places of origin could be established for almost all species. However, preceding climate experience, such as the warming or drought treatment of the plants altered their reaction to cold extremes compared to the control treatment, indicating the complexity of the interactive impacts of climate factors on ecosystem and plant performance.
The uncertainty of climate projections and the multitude of changing climatic stressors, though, make the prospect of an easy and rapid success in the search for single “best-adapted” provenances very questionable. In economics the portfolio effect shows that a diversification of investments decreases the risk of a total loss of profits. Hence, in a modelling procedure based on the increment data from the above mentioned experiment it was tested if a “portfolio investment” in several provenances in one stand decreases the risk of yield losses (Manuscript 8). Results indicate that the higher the number of provenances the higher the chance for a “best-performer” to be included in the set. So the likelihood of higher yields, under different climatic conditions increases, yet the risk of low yields stays stable.
Generally, it seems that the selective use of plant species and ecotypes in climate change adaptation can be a feasible tool to maintain ecosystem functionality and productivity. However, the uncertain projections, the multitude of climatic stressors and their interplay with other environmental factors and the potential impacts of assisted colonization of ecotypes on the genetic diversity within species and populations require further research.
Monsoonal affected dynamics of nitrate and dissolved organic carbon in a mountainous catchment under intensive land-use
- In recent decades, complex mountainous landscapes have been increasingly under pressure by deforestation and intensified highland agriculture. This trend not only compromises the quality of water in the highlands, but also impacts the availability of water resources downstream. Hence, an effective and sustainable management of these mountainous regions is essential and needed to mitigate this risk. Developing sustainable management principles to ensure the freshwater supply, however, requires a profound understanding of decisive factors and processes that control the water quality in mountainous landscapes. Amongst other substances, nitrate and dissolved organic carbon (DOC) play a critical role in the ecosystem health of water bodies. The major focus of this dissertation is on determining the nitrate and DOC mobilization processes and dynamics in the Haean Catchment, a mountainous watershed located in South Korea, which is agriculturally productive and strongly influenced by the prevailing East-Asian monsoon. The primary objective was to identify decisive factors that control the nitrate and DOC mobilization processes and dynamics in such catchments.
To these ends, we conducted stream water quality and discharge measurements during a range of conditions, from monsoonal rainfall events to dry, baseflow conditions. Assessments were completed along the topographic elevation gradient of the catchment, from an upland deciduous forest, over areas intensively used for agriculture, down to the catchment outlet. In order to gain a better understanding of nutrient fate within the Haean Catchment, we investigated river-aquifer exchange fluxes. In addition to hydraulic gradient monitoring, we used heat as tracer. To resolve the river-aquifer exchange fluxes, we set up a two-dimensional flow and heat transport model using the numerical code HydroGeoSphere (HGS). Potential effects of river-aquifer exchange dynamics on local water quality were investigated by collecting both, river and groundwater samples. Furthermore, we determined the impact of the ridge and furrow cultivation that is commonly applied in South Korean dryland agriculture, on nitrate leaching and evaluated fertilizer best management practices (FBMPs) using a three-dimensional flow and solute transport model (HGS).
Our results show that DOC and nitrate sources as well as their mobilization differ between the forest and agricultural river sites. In the forest, elevated in-stream DOC concentrations during rainfalls were due to hydrologic flushing of soluble organic matter in upper soil horizons with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the stream occurred predominantly along interflow transport pathways.
At the agricultural sites, in-stream DOC concentrations were considerably higher and supplied from adjacent rice paddies. The highest in-stream nitrate concentrations occurred in the lower agricultural part of the catchment. Through hydraulic gradient monitoring, we identified in this part of the catchment a distinct connection between the river and aquifer, and nitrate-rich groundwater inputs to the river elevated the in-stream nitrate concentration. In the mid-elevation portion of the catchment, however, a limited connectivity between the river and aquifer was identified and river water quality was consequently unaffected in these areas.
The results of our HGS modeling study show a high temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals during the monsoon season. Our results also suggest that these flow reversals may enhance the natural attenuation of nitrate in the shallow groundwater below the stream. The simulation results on evaluating FBMPs demonstrate that applying a combination of several FBMPs such as an adapted placement and timing is recommended to minimize the risk of groundwater nitrate contamination.
Overall, this dissertation demonstrates that the hydrologic pathways resulting from the monsoon climate drive the in-stream DOC dynamics in the forested catchment, whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the specific land-use type. In particular, the widely used rice paddy “plot-to-plot” irrigation system in Korean highlands was shown to control in-stream DOC concentrations. Nitrate dynamics in streams and aquifers in the agricultural areas of the catchment reflect the combined effects of land-use type and monsoonal hydrology. Particularly, the highly variable river-aquifer exchange fluxes with frequent flow reversals, which may enhance the nitrate attenuation, are driven by monsoonal extreme precipitation events. Since it has been forecasted that global warming will increase the frequency and the intensity of extreme precipitation events also in other regions of the world, our results will become increasingly important in future water quality assessments and research.
An Investigation of Organizational Level Continuance of Cloud-Based Enterprise Systems
- Cloud-based enterprise systems are a growing trend in today’s business software market. With a steadily expanding number of implementations, cloud service providers are now turning their attention from adoption issues towards retaining their existing customer base. The difficulties even established cloud players, like e.g. Salesforce.com, face in retaining their customers have been emphasized by tech bloggers, where the subscriptions of cloud-based enterprise systems are cancelled even at an early stage of adoption. This discontinuance of enterprise systems at an early stage is a rather new phenomenon, which is related to the subscription-based payment model of cloud services, which (theoretically) allows service cancellation without the customers having to fear financial penalties. In contrast, traditional on-premise systems (e.g. SAP ERP) are on a long term license base, where customers are contractually bound. Therefore the research question of the thesis is as follows: What factors influence the organizational level continuance intention of cloud-based enterprise systems? In an effort to answer this research question, the thesis presents five inter-related papers. The first paper develops a conceptual model to study the continuance of cloud-based enterprise systems. Building on this, paper two develops a formative measurement instrument to assess the success of operational cloud-based enterprise systems. The third paper quantitatively explores the influence of the variables identified in the conceptual model. Building on these findings, paper four conducts a stakeholder analysis to solve the problem of broad samples. Finally, the fifth paper uses the formative measurement instrument to test the final research model, which is a revision of the conceptual model developed in the first paper. The results show that continuance intention is influenced both, by information systems success variables, as well as continuance inertia. In addition, behavioral variables, such as attitude towards usage also explained a decent amount of variance in the dependent variable.