Computational Bounds for Elevator Control Policies by Large Scale Linear Programming
- We computationally assess policies for the elevator control problem by a new column-generation approach for the linear programming method for discounted infinite-horizon Markov decision problems. By analyzing the optimality of given actions in given states, we were able to provably improve the well-known nearest-neighbor policy. Moreover, with the method we could identify an optimal parking policy. This approach can be used to detect and resolve weaknesses in particular policies for Markov decision problems.
Design of Robust Heterogeneous Catalysts for Sustainable Chemistry
- Robust heterogeneous catalysts based on polymer derived non-oxide ceramics (SiC/SiCN) with integrated late transition metal (Ni, Pd) nanoparticles (NPs) were designed, characterized and tested for their catalytic potential. For the decoration of SiCN support with metal NPs, a molecular approach was applied which makes the use of amido metal complexes for the chemical modification of the preceramic polymer (commercially known as HTT 1800, Clariant Advanced Materials GmbH, Sulzbach, Germany) by the transfer of metal to its nitrogen functions. The modified polymer was transformed to an amorphous SiCN material containing metal NPs by controlled pyrolysis under an inert atmosphere. By following aforementioned approach, Cu@SiCN materials (non-porous) have already been developed in our research group.
In present thesis, attempts were made to fabricate porous SiCN/SiC materials with good accessibility of metal (Ni and Pd) NPs. Firstly, the synthesis of palladium containg SiCN materials by the pyrolysis of palladium modified polysilazane was attempted. It was found that during polymer to ceramic transformation, palladium reacts with silicon to form intermetallic palladium silicide particles. The materials were characterized (TGA, FT-IR, NMR, PXRD and TEM) and applied in catalytic hydrogenation reactions. The activity of the catalysts was found low because of the silicidation of metal and non-porous SiCN support.
The issues of the low porosity of SiCN ceramics and the formation of metal silicides were addressed in the synthesis of Ni@SiCN materials. Controlled pyroylsis of Ni modified polysilazane at 600°C provided microporous Ni@SiCN materials, whereas at 1100°C the formation of nickel silicides in non-porous SiCN matrix was observed. Ni@SiCN materials were characterized by solid state NMR, FT-IR, PXRD, TGA, N2-physisorption and TEM. Hyperpolarized 129Xe NMR was used to study the effect of nickel loading and annealing time on pore structure. The microporous materials showed good thermal stability and activity in selective hydrogenation of aryl acetylenes but as polymer to ceramic conversion was not complete at 600°C, the materials possessed low hydrothermal stability at higher temperatures due to the hydrolysis of Si-N bonds.
In order to replace Si-N bonds with more stable Si-C bonds, a commercial polycarbosilane was used to fabricate SiC materials with integrated Ni NPs by the pyrolysis of nanostructured polycarbosilane-block-polyethylene (PCS-b-PE) polymer. PCS-b-PE was synthesized by the reaction of PCS with PE-OH catalyzed by an amido nickel complex followed by its nanostructuring via microphase separation technique. The length of PE block affected the type of pores generated in SiC material and the synthesis of highly porous materials with micro-, meso- and hierarchical porosity was achieved. Ni@SiC materials were characterized by TGA, PXRD, N2-physisorption and TEM. Hierarchically porous Ni@SiC selectively cleaves aromatic C-O bond of aryl ethers in water and was found a reusable catalyst.
Lastly, controlled pyrolysis of palladium modified polysilzane provided nanoporous SiCN materials with very small and highly accessible Pd NPs. This shows the role of SiCN support whose nitrogen atoms have a stabilizing effect on small metal nanoparticles. Moreover, SiCN support ineracts nicely with the NPs leading to the prevention of their leaching. In general, NPs have agglomeration tendencies with increasing metal loading but in the case of Pd@SiCN, size of metal NPs remained small even with a palladium loading of 14 wt%. Pulse titration with hydrogen showed high accessibility of Pd. Pd@SiCN materials showed better activity than commercial Pd/C catalyst in oxidation of alcohols in the absence of air or oxygen with the evolution of hydrogen.
M@SiCN/SiC materials may find applications in sustainable production of chemicals and fuels from renewable sources, for instance, from lignocellulosic biomass.
Bottom-up Self-Assembly across Hierarchies: From Triblock Terpolymers to Patchy Particles to Colloidal (Co-)Polymers
- In this work the bottom-up self-assembly of compartmentalized particles on multiple hierarchies was investigated. ABC triblock terpolymers were directed as basic build-ing blocks into nano-scale corona-compartmentalized (patchy) particles via selection of kinetic self-assembly pathways. An extremely efficient and versatile step-wise self-assembly process was developed offering unique nano-engineering capabilities over addressable corona patches. Thereby, carefully chosen solvent sequences were of outmost importance. Depending on the volume ratio of the core forming blocks, VA/VB, two species with different geometrical distribution of the patches were identified: for VA/VB > 1, a Janus-like AB distribution, with patches A and C emanating from opposing sides of the B core and for VA/VB < 1 an ABA distribution, with two A patches on opposing sides of the B core protected by an equatorial C corona. The par-ticles were then used as colloidal building blocks (CBBs) that, upon addition of non-solvent for A, underwent next level hierarchical self-assembly. The AB CBBs self-assemble into spherical multicompartment micelles (MCMs) with precise control over aggregation number (VA/VB). In contrast, ABA CBBs grow into extended linear colloidal polymers of up to several micrometres in length via a step-growth polymerization process. The cluster size (AB)x and the worm length [ABA]m are both conveniently controlled by the solvent quality for the corona block (expansion/contraction). This dynamic tuning of the corona volume is a unique key feature of the bottom-up approach to soft patchy nanoparticles from triblock terpolymers.
In a consecutive work, the AB and ABA CBBs were mixed in specific ratios prior to self-assembly by addition of non-solvent for A. With both CBBs present, aggregation by mutual interaction of A patches into mixed colloidal co-assemblies was accomplished. Colloidal co-assembly is a hierarchical structuring process crossing multiple hierarchies primarily driven by the minimization of interfacial energies. It critically depends on both the dynamic volumes change of the corona and of the aggregating patches with changing solvent polarity. The extraordinary quality of the superstructures is ascribed to the selection of kinetic pathways for co-assembly and similarly, to the dynamic tailoring of patch volume. Particles with a large C corona, but small attractive A patch are stable over broad a range of solvent compositions. On the contrary, particles with a small C corona, but large attractive A patch start to cluster even with at low contents of non-solvent for A. Hence, the mismatch of onset of self-assembly is a set screw to either form the colloidal “substrate” in the first step and decorate subsequently or vice versa. Both approaches lead to well-defined and predictable mixed colloidal co-assemblies comprising colloidal molecules, multiblock co-assemblies, telechelic oligomers, ternary co-assemblies and two-dimensional networks, all of which are exclusively accessible with the presented approach.
The spherical MCM consist of AB CBBs with a B-core with a Janus-like distribution of the A and C blocks as a result of symmetry breaking during cluster formation in non-solvents for A. This phase separation within MCMs represents a novel and versatile route for the template-free synthesis of terpolymer-based, sub 100 nm Janus particles. The synthesis encompasses facile cross-linking of the patches of spherical MCMs to preserve the phase-separated state. This approach yields narrowly dispersed Janus micelles and offers unique options to nano-engineer core diameter, the Janus balance (volume ratio of A and C hemispheres) and the chemistry of the patches. Homogeneous populations of MCMs even at very high concentrations of 100 g/L enable high throughput synthesis of soft Janus micelles making this novel approach technologically relevant. Beyond that, the Janus balance proved decisive for cluster shape and size when the particles were subjected to a selective solvent for either of the hemispheres.
The Janus particles (JPs) with tailored Janus balance were finally applied as disper-sants for multi-walled carbon nanotubes (MWNTs). Thereby, the JPs attach to the tube surface with a suitable hydrophobic patch (polystyrene), while facilitating stabilization in the solvent with the other. Depending on the Janus balance, i.e., the size ratio of adsorbing to stabilizing patch, dense multilayer coatings were obtained or helical arrangements with defined JP-JP interparticle spacing. In both cases, the quantity of attached JPs was substantial and unparalleled. Besides the known applications of JPs in emulsion polymerization and as compatibilizers in polymer blends, JPs proved also effective as non-covalent supracolloidal dispersants for MWNTs and may also find application as general dispersant for other insoluble particulate matter.
Supramolecular Nanofibers - Preparation, Structure-Property Relations, and Applications
- Conventional polymer nanofibers have gained tremendous interest in the last years in the fields of catalysis as templates, in medicine as tissue engineering, in functional textiles as protective suits, and especially in air filtration as filter media. Generally, nanofibers are prepared by top-down approaches. However, these processes feature several disadvantages. As consequence cost-effective alternative strategies are required. One strategy to this problem is the bottom-up approach – the self-assembly of small molecules. Therefore, this thesis covers different topics with respect to the preparation, structure-property relations, and application of supramolecular nanofibers:
To investigate the impact of the molecular structure on the stacking behavior in the self-assembly process, a set of pyrene-containing model compounds was synthesized. Here, the focus was set on the influences of sterical demanding side groups as well as hydrogen bonding motifs on the stacking of the pyrene units. These influences were, besides others, detected by excimer formation in dilute solution, in the aggregated state and in solid films. It was demonstrated that stacking of the pyrene units is the driving force of the self-assembly process in solution in this system. However, hydrogen bonds are required to obtain well-defined supramolecular nanofibers. The influence of the hydrogen bonding motif and the sterical hindrance on the pyrene stacking becomes more and more significant the closer the molecules are forced together. Hence, the columnar stacking is increasingly disturbed in solid films compared to solution.
The class of 1,3,5-benzenetrisamides is one of the simplest and most-versatile motifs in supramolecular chemistry. Within this thesis, two different self-assembly processing pathways of benzenetrisamides from solution; in particular self-assembly upon cooling at constant concentration and self-assembly during solvent evaporation at constant temperature were explored. One factor that determines the actual processing pathway is the solubility of the benzenetrisamide molecule. Exclusive self-assembly upon cooling takes place when the benzenetrisamide is almost completely insoluble in the used solvent at room temperature. The prerequisite for self-assembly during solvent evaporation is certain solubility of the BTAs at room temperature. In addition, these self-assembly pathways were compared with respect to control the supramolecular nanofiber morphology in view of homogeneity, fiber diameter, and fiber diameter distribution. Thereby, influences of external parameters such as temperature, solvent, and concentration were investigated in detail.
Especially in air filtration industry nanofibers are an important tool because of their beneficial effects due to their high surface-to-volume ratio. In industry, electrospinning is the standard technique to post-modify nonwoven filters with conventional polymer nanofibers on the filter surface. However, this process is limited to the surface of the scaffold. In this thesis, the principle utilization of supramolecular nanofibers in air filtration is demonstrated for the first time. Here, a solution-based immersion process was developed, which allows a successful in-situ formation of supramolecular nanofibers in nonwoven scaffolds. This results in a stable microfiber-nanofiber composite. The main advantage of this process is the effective incorporation of nanofibers in the volume of the nonwoven fabrics. For supramolecular systems, it was claimed that they are too fragile to be competitive with conventional polymers. But the herein prepared supramolecular nanofibers possess enough stability even upon applied airstreams of 3.0 meter per second. This stability is by far superior than it is required at standard vacuum cleaners which possess flow velocities of 0.25-0.40 meter per second at the filter element. First filtration tests revealed promising filtration efficiencies.
Building on these promising results a comprehensive study on structure-property relations at the preparation of microfiber-nanofiber composites in view of optimized filtration efficiencies was investigated. Depending on the selected benzenetrisamide, solvent, and concentration of the immersion solution, the filtration efficiency of the filters can be adjusted. By varying the thickness of the filters by means of double- and triple-layer filters, for supramolecular modified filters, excellent filtration efficiencies over 90 percent were obtained for aerosol particles with the size of 0.2 micrometer.
Challenging notions of development and change from everyday life in Africa
Mohamed A.G. Bakhit
Girum Getachew Alemu
- The fourth volume of BIGSASworks! seeks to present the interdependence of paradigms and practices in global, national, and local spheres from different disciplinary perspectives and foci - Social Anthropology, Geography, Media Studies, Political Sciences and Sociology. The contributing papers present various ways in which the daily livelihood activities of community people in different parts of Africa represent this interdependence. Together and in interlinked ways, the authors address the question of how global development paradigms affect people’s lives, what meanings are there in the everyday things people do to live that may synchronise or be at variance with these global paradigms. The contributing papers challenge us all to take another look at our approach to development in Africa and its entanglements with broader forces.
Quantifying water use by temperate deciduous forests in South Korea: roles of species diversity, canopy structure, and complex terrain
- About seventy percent of South Korea is covered with forests, most of which are found in the mountain regions since mountains receive more rainfall and are difficult terrains not suitable for agriculture. Because mountains are important water sources for cities and human population downstream, performing water balance for forest catchments has become a research priority. The ongoing shift from coniferous to species-rich deciduous forests due to a changing government policy and the anticipated changes in future climate, associated with increasing amount of rainfall and temperature will also impact forest water use, calling for an urgent need to understand how forests, in their current status, use water. The knowledge is vital for predicting water requirements for the future forest. The warm-deciduous temperate forests found in South Korea, however, have a high diversity of tree species, have multi-layered canopies and are mostly located on rugged mountainous terrains, which make it difficult to quantify forest water use, a basic requirement for catchment water budgeting. The main objectives of this study were to: (1) identify the roles of species diversity in tree and forest water use, (2) examine the impact of canopy structure on forest transpiration, and (3) evaluate the influence of terrain on forest water use.
Site-specific studies were carried out in three different natural deciduous forests, namely, Gyebang (GB), Gwangneung (GN) and Haean (HA) forest sites, representing the general structure of S. Korean forests. GB site is known for its high species diversity, GN site is an old forest growth at climax, with clearly defined understory and overstory canopy layers while the HA site was located with in a catchment, with strong elevation changes within short horizontal distances, rising from 400 to 1,000 m a.s.l., and in different aspects. Four locations with varying elevations and aspects were chosen in the HA site. Tree water use (TWU) and canopy transpiration (EC) were estimated from sap flux density measured with thermal dissipation probes. Understory transpiration (EU) was measured using stem heat balance while ecosystem evapotranspiration (Eeco) was determined using eddy covariance technique. Air temperatures (Ta), precipitation, solar radiation, vapor pressure deficit (VPD), wind speed were measured from weather stations and soil water content was measured from frequency domain reflectometry (FDR) sensors at the respective study sites. Vegetation surveys, including diameter at breast height (DBH), tree density, species composition, sapwood area (AS), and leaf area index were performed in all the sites. Canopy conductance (GC) and stomatal sensitivity to VPD were assessed based on transpiration and microclimate measured at each site.
A functional allometric relationship was established between AS and DBH, and also between TWU and DBH for all the study sites; first for single species and then combining all the species either in a single site or in all the sites. Irrespective of tree species, AS and maximum TWU were significantly correlated with DBH in a power function for AS (R2 = 0.77, P <0.0001) and both in power (R2 = 0.63, P <0.0001) and sigmoid functions (R2 = 0.66, P <0.0001) for TWU, for the co-occurring species as well as across the sites, suggesting that DBH can be a good predictor of stand AS and maximum TWU, based on the established allometric functions.
Early bud break and development of the understory compared to the overstory canopy resulted in an earlier onset of forest transpiration, with EU contributing 22% and 14% between April and May to the total forest transpiration. This high contribution was favored by high radiation and VPD in the understory, since the overstory was still undeveloped and open. Despite diminishing VPD and light conditions in the understory between June and August, the understory continued to transpire a substantial amount of water, contributing 10% of the total transpiration. The seasonal patterns of both EO and EU were synchronized to canopy development, while VPD and radiation determined daily trends. EO and EU accounted for 80% of Eeco in spring but only 60% during the monsoon period due to lowered radiation input, VPD, and plant area index (PAI). Thus, Eeco is largely influenced by transpiration rate and its seasonal variation and also canopy structure.
Early saturation of EC at relatively low VPD and also a rapid decrease in GC with increasing VPD were observed in the forest stand located at the highest elevation studied (950 m) in the HA site, compared to the GN and the other forest stands in HA. These differences in transpiration rates and stomatal response can be explained by greater stomatal sensitivity to VPD of 0.83 found at the 950 m site compared to 0.63–0.66 in the other study sites. However, the main controlling factor of the change in stomatal sensitivity at the 950 m stand is uncertain. Although maximum daily EC were correlated with AS of the forest stands at different sites (R2 = 0.78, P <0.01), annual EC declined with increasing elevation, i.e., 176 >175 >110 >90 mm year−1 at 340 >450 >650 >950 m, respectively. Decline in total EC was due to the decline in annual Ta, daytime VPD, and length of growing season at higher elevations. The GB site, which was located at 960 m elevation, however, did not display a same response pattern as those observed at the 950 m site. It is likely because these sites were under different environmental conditions, i.e., GB site is exposed to higher Ta and higher humidity, and is sheltered (lower wind speeds). These observations emphasize the complexity associated with estimation of transpiration in rugged terrains, since general principles do not always apply and the spatial patterns of forest transpiration are complex.
Complexity arising from multiple tree species composition when estimating forest water use can be reduced by applying functional allometric relationship linking tree size and water use. Forest canopy structure and physical location should be taken into account since they influence the way forests use water resources by altering microclimate and plant physiology. Based on our findings, estimation of forest water use on rugged terrains require repeated measurements at relatively small spatial scales since the driving factors change rapidly over very narrow vertical distances.
Structural and electronic properties of transition metal nanoalloys and magnetic compounds
- In transition metal clusters, potentially profitable technological applications and fascinating fundamental questions are closely connected. Bimetallic nanoalloys, e.g., have become increasingly popular as their performance in catalysis is often superior to their pure counterparts. Exemplary for this are gold-platinum (Au-Pt) nanoalloys that have been used as highly potent catalysts in electrocatalysis and in a variety of oxidation reactions. However, the mere existence of Au-Pt nanoalloys is astonishing, as Au and Pt cannot be mixed in bulk over a wide range of compositions. Furthermore, how a combination of Au and Pt in nanoalloys results in their special properties has not yet been determined conclusively.
It has been shown in empirical simulations and first-principles density functional theory (DFT) calculations that Au-Pt nanoalloys preferably arrange in a core-shell mixing pattern with Au forming a shell around a Pt core. This is in contradiction to many experimental studies that report the formation of solid solutions of Au and Pt. In the present work, this seeming discrepancy is addressed by simulating x-ray diffraction patterns that are experimentally used to characterize nanoalloys. It is shown that the interpretation of the diffraction patterns relies on questionable assumptions and therefore does not suffice as a definite characterization tool for Au-Pt nanoalloys.
To shed light on the special catalytic properties of Au-Pt nanoalloys under rather different experimental conditions, a thorough investigation of their electronic and structural properties has been carried out. It is found that features favorable for catalysis in Au-Pt nanoalloys emerge as a consequence of combining two fundamental properties: Pt contributes a high density of states close to the Fermi level, which promotes chemical activity. Au increases the structural flexibility of the Au-Pt system, which might be beneficial for the formation of active and element-specific binding sites as well as regeneration of the catalyst after the reaction.
Although DFT offers an attractive compromise between computational effort and accuracy for a theoretical description of Au-Pt nanoalloys, other transition metal compounds severely challenge existing DFT approximations. Manganese (Mn) doped silicon (Si) clusters represent an ideal model system to study the interaction of a single magnetic impurity with a semiconducting host both experimentally and theoretically. The transition from exohedral (lowly coordinated) to endohedral (highly coordinated) doping that occurs for Si clusters with more than ten atoms, is accompanied by complete quenching of the magnetic moment of Mn. We show that MnSi11+, the smallest endohedral cluster found in experiment, suffers strongly from a well-known general problem of most DFT approximations: the self-interaction error. Finally, a universal correlation between magnetic moment and the coordination of the Mn dopant is established that can be generalized to extended systems and suggests a route to stabilize the magnetic moment of bulk Mn-Si compounds.
Synthesis and Self-Assembly of Novel ABC Miktoarm Star Terpolymers
- A novel synthesis for ABC miktoarm star terpolymers and their self-assembly into complex superstructures in aqueous solution are described within this thesis. To this aim a modular route for such materials was developed, combining anionic polymerization and copper-catalyzed azide-alkyne cycloaddition. At the example of ABC miktoarm star terpolymers and an ABA’ miktoarm star copolymer containing a poly(N-methyl-2-vinypyridinium iodide) (P2VPq) segment, the counterion-mediated superstructure-formation of complex shaped aggregates was thoroughly investigated.
The key compound of the combinatorial synthesis is the newly synthesized 4-alkyne-substitued diphenylethylene derivative 1-[(4-(tert-butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene (“click-DPE”). This was applied in sequential anionic polymerization to prepare well-defined alkyne mid-functional diblock copolymers composed of polybutadiene (PB) as first and poly(tert-butyl methacrylate) (PtBMA), poly(2-vinylpyridine) (P2VP), or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as second block. The alkyne-midfunctional diblock copolymers were afterwards conjugated with azido-functional polystyrenes (PS), poly(ethylene oxide) (PEO), PtBMA and PDMAEMA to successfully obtain different novel ABC miktoarm star terpolymers with narrow molecular weight distributions.
For an ABC miktoarm star terpolymer consisting of arms of PB, PtBMA and P2VP it was demonstrated that after quaternization with methyl iodide (yielding BVqT) and dialysis to water the nature of the counterion allows for manipulation of the obtained structures. The miktoarm star architecture together with iodide as counterion is essential for this directed self-assembly. Transformation of iodide to triiodide, via the addition of iodine before dialysis to water, decreases the hydrophilicity of the P2VPq corona and therefore induces the directed self-assembly of spherical micelles with a PB/PtBMA core, into cylinders, superstructures thereof and finally barrel-shaped aggregates of up to 1 µm with an internal lamellar fine structure. Based on their appearance in transmission electron micrographs these were termed “woodlouse” aggregates. The compact particles consist of alternating lamellae of a partially demixed PB/PtBMA phase and a swollen P2VPq phase.
The general applicability of this counterion-mediated hierarchical self-assembly was furthermore demonstrated by using two other miktoarm star systems. For three ABC miktoarm star terpolymers of different composition, consisting of PB, PS and P2VPq segments (BVqS), a dependence of the morphology on the fraction of the hydrophilic block was determined, in analogy to diblock copolymers. For long P2VPq blocks stacked lamellar/disk-like structures evolve from micellar building units. In contrast, a short P2VPq segment yields multilamellar vesicles via fusion of vesicular primary building blocks. The vesicle walls are supposed to consist of a lamellar structure with the PB phase in the centre, shielded from the P2VPq corona by thin PS layers. At the example of one BVqS miktoarm star terpolymer the successful formation of nanohybrids containing gold nanoparticles within the P2VPq phase is demonstrated.
In the second system the low-Tg PB segment was replaced by a second PS block of different length (SVqS’). Even though vesicles serve as initial building units, the triiodide-induced superstructure formation leads to anisotropic aggregation of deformed vesicles, rather than to the fusion into multilamellar vesicles. This is attributed to the two glassy PS core blocks which minimize the dynamics during self-assembly and allow only minor rearrangement of the aggregated structures. Similar to the “woodlouse” aggregates from BVqT, lamellar structured particles of elongated shape were obtained from SVqS’, despite vesicles serving as primary building units. Consequently, the presented triiodide-directed self-assembly into complex superstructures is not restricted to miktoarm star polymers containing a low-Tg segment, as the rearrangement processes take place during the dialysis process, where the organic co-solvent enables sufficient mobility of the core-forming blocks.
Besides the introduction of a novel synthetic approach for the construction of miktoarm star terpolymers and the synthetic advance of the alkyne-functionalized DPE, the presented triiodide-mediated superstructure formation represents an interesting concept for directed self-assembly processes.
The Effect of Spatial and Environmental Drivers on Patterns in Species Richness and Composition
- This thesis includes eight manuscripts with methodological, empirical and theoretical contributions that aim to enhance the understanding of species richness and composition patterns and their underlying drivers. Islands and isolated systems are in the focus of this work.
Islands provide optimal conditions to study biogeographic patterns. Theoretical advances in ecology have been initiated by island biogeography. Theory on island biogeography has particularly been improved by a better representation of time related components including speciation and environmental change. Oceanic islands are not stable systems but follow a characteristic ontogeny. After the volcanic emergence over the sea surface, erosion processes, shaping the island first more heterogeneous and then flatter, transform islands. This thesis shows how particular characteristics of the classic theory of island biogeography can be included in the currently most advanced theoretical framework. While MacArthur & Wilson (1963) particulary focussed on processes (colonisation and extinction) for generating species richness patterns, current theory assumes a defined upper limit for species richness (“carrying capacity”). By reinforcing the importance of processes in the current theory, as suggested in this thesis, it is much simpler to formulate hypothesis that can be tested by empirical data. Carrying capacity is linked to “habitat heterogeneity”, both, in the meaning of topographic variability as well as the number of vegetation units that are present in a given area. This thesis demonstrates that a clear terminology is a prerequisite for a profound understanding of the effects of “heterogeneity” on species diversity patterns in general and the underlying biogeographic processes in particular. The heterogeneity of surfaces influences species diversity not only on scales larger than kilometres but also is important on very fine scales of meters and smaller. Novel methods to measure different aspects of surface variability are introduced and discussed and their effect on species richness and composition of plant species groups in different ecological systems is presented.
Furthermore, this thesis highlights the isolating effect of elevation (elevation-driven ecological isolation hypothesis). Environmental filtering along an elevational gradient differentiates ecosystems. Isolation increases with elevation, as comparable ecosystems are much farther apart at high elevations than is the case for lowland ecosystems. In addition, ecosystems on neighbouring islands or on the continent that serve as source regions for colonising species are smaller in area in high elevations in comparison to low elevation ecosystems. Consequently, an above average speciation rate reflected in a high percentage of endemic species can be expected for higher elevations on islands and high mountains. The elevation driven ecological isolation hypothesis is tested for a number of islands and a new hypothesis indicating a complex interaction with isolation is developed. The difference in isolation between low and high elevation ecosystem diminishes as the overall isolation of the island increases. Thus the relation between the percentage of endemic species and elevation should reverse with an increase in isolation. On very isolated islands, low and high elevation ecosystems are alike isolated but low elevation ecosystems should have an above average speciation rate as they provide more area and higher temperatures relative to the ecosystems above (e.g. metabolic theory of ecology).
The scale dependence of diversity patterns are attributed to ecological processes that operate differently over varying extents and grain sizes. This thesis demonstrates that scale dependencies in distance-decay analyses cannot be traced back to processes that are specific for the ecological scale, but can largely be attributed to sampling design and are highly sensitive to grain size and study extent. Distance-decay analyses are an adequate method to assess spatial turnover in species composition. However, this thesis shows that frequent practise of making comparisons among studies is not possible within the current methodological framework.
Finally, this thesis provides an overview on patterns in species richness and composition and elaborates interconnections between associated theories and underling drivers. Promising novel research questions and directions are identified in the field of island biogeography and in an adequate formalisation of a “heterogeneity” concept.
Impact of time and spatial averages on the energy balance closure
- Secondary circulations are large and relatively stationary eddies, which are caused by the surface heterogeneity and normally reside away from the ground. They are believed to be the cause of the energy balance closure problem at the earth's surface, because their contribution to the turbulent fluxes is missed by a fixed eddy-covariance tower measurement that has a typical averaging time of 30 minutes. In this thesis, data from the LITFASS-2003 experiment was used to investigate the impact of time and spatial averages on the energy balance closure. This data consisted of many observations over a large heterogeneous landscape that could generate secondary circulations; some of which might be still near the earth's surface.
For the time average analysis, the averaging time was extended to increase the possibility that secondary circulations were picked up by the sensor. Two approaches, which were the modified ogive analysis and the block ensemble average, were applied to analyze the data from ground-based measurements. The modified ogive analysis requiring a steady state condition, could extend the averaging time up to a few hours and suggested that an averaging time of 30 minutes was still overall sufficient for the eddy-covariance measurement over low vegetation. The block ensemble average, on the contrary, did not require a steady state condition, but could extend the averaging time to several days. However, this approach could only improve the energy balance closure for some sites during specific periods, when secondary circulations existed in the vicinity of the sensor. Based on this approach, it was found that the near-surface secondary circulations mainly transported sensible heat, which led to an alternative energy balance correction by the buoyancy flux ratio approach, in which the attribution of the residual depended on the relative contribution of the sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction was larger than in the energy balance correction that preserved the Bowen ratio.
In the spatial average analysis, two energy balance correction approaches, the buoyancy flux ratio and the Bowen ratio approaches, were applied to the area-averaged fluxes (composite fluxes) in order to include contribution from secondary circulations. These composite fluxes were aggregated from multiple ground-based measurements. The energy balance corrected fluxes were validated against the spatial average fluxes, which were measured by an aircraft and a large aperture scintillometer (LAS). In this validation, the backward Lagrangian footprint model was used to estimate the source area of the measurement. It was found that both energy balance correction approaches did improve the agreement between time and spatial averages fluxes. This suggested that the contribution from secondary circulations could be properly accounted by the energy balance correction.
All findings in this thesis, therefore, depict that secondary circulations significantly transport energy in the atmospheric surface layer. The energy balance correction, accomplished by using either the Bowen ratio approach or the buoyancy flux ratio approach, is necessary to estimate the actual vertical transport of energy at the earth's surface.