458 search hits
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Experimental deformation of forsterite, wadsleyite and ringwoodite: Implications for seismic anisotropy of the Earth’s mantle
(2004)
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Hélène Couvy
- The rheological properties of the major minerals of the Earth’s mantle are still not well constrained. However, these properties are crucial for the understanding of a wide range of processes in the Earth’s interior such as mantle convection. The purpose of this work is to address the issue of the rheology of the lowermost upper mantle and of the transition zone through the mechanical properties at high pressure of olivine (with forsterite composition Mg2SiO4) and of its high-pressure polymorphs wadsleyite and ringwoodite. Indeed, the properties of the Earth’s mantle can be inferred as a first approximation from the mechanical properties of those polymorphs which volumetrically dominate the mineralogy of the region of concern. Deformation experiments have been performed on hot-pressed forsterite samples and on pre-synthesized wadsleyite and ringwoodite samples under pressure conditions of the Earth’s mantle and at 1300-1400°C. The possible influence of the phase transformation from forsterite to wadsleyite on rheology has been also investigated. Deformation has been achieved by shear using the Kawai-type multianvil apparatus. Complementary experiments on forsterite have been performed in the newly developed Deformation-DIA. Some of them have been carried out on a synchrotron beam line to perform in-situ stress and strain measurements. In order to gain a maximum of information on the deformation mechanisms and on the Crystallographic Preferred Orientation (CPO), a special attention has been devoted to the microstructural characterisation of the samples. Electron BackScattering Diffraction (ESBD) and Transmission Electron Microscope (TEM) have been mainly used. An important pressure-induced change in deformation mechanism is shown in forsterite. The deformation of forsterite at high pressure and temperature is dominated by the [001]{hk0} slip system rather than the [100](010) glide which is extensively observed at low pressure and high temperature.. Concerning the high-pressure polymorphs, their plastic behaviour has been studied with a strong emphasis on the formation of CPO. ViscoPlastic Self Consistent (VPSC) modelling is used to link the CPO with known elementary deformation mechanisms of these phases. The main features of the CPO of wadsleyite are characterized by the alignment of the [100] axes parallel to the shear direction and the alignment of the [001] axes toward the normal to the shear plane. Too many uncertainties remain on the ringwoodite CPO for them being used to interpret seismic anisotropy. Finally, we suggest that strain-induced CPO might be responsible for the seismic anisotropy observed in the lowermost upper mantle and in the upper part of the transition zone. The low seismic anisotropy of the lowermost upper mantle can be explained from the slip system change in forsterite and the CPO of wadsleyite point toward a dominant tangential flow in the upper part of the transition zone.
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The Aggregation Behavior of Mixture of Alkylmethylaminoxides with Their Protonated Analogues in Aqueous Solution
(2005)
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Yuji Yamashita
- The C12C8MAO aqueous solution at 100 mM consisted of two phases which were optically isotropic and low viscous. Addition of chloric acid induced a phase transition, and the following lamellar (L-alpha) phase was formed in the range of low protonation degree, X = 0.007 – 0.35. When the surfactant was protonated further (X > 0.35), the single L-alpha phase separated again into two isotropic phases. The abnormal phase sequence could be interpreted by the result that mixtures of protonated and non-protonated C12C8MAO were more surface active than each component. The surface and interfacial measurements showed synergism of mixing two components. This synergistic effect arised from the peculiar interaction of hydrogen bonding between protonated and non-protonated head groups. This short-range interaction would cause the C12C8MAO molecules to be more lipophilic with protonation, resulting in the phase separation at high protonation degree. The SAXS measurements in the L-alpha phase also showed the synergistic effect between the head groups. The rheological measurements and microscope observations demonstrated that the morphologies of L-alpha phase could be controlled by preparation routes. It was found that the vesicles were transformed into the classical lamellar phase by the simple process of heating and cooling through the phase transition (L-alpha <-> L-alpha;/L1) temperature. Furthermore, the classical, planar lamellar morphology could be prepared by means of kinetic protonation of the C12C8MAO molecules using hydrolysis reaction. Any classical L phase was modified to the vesicle form under shearing, and its transformation was irreversible in terms of shear force. Various acids were treated as protonation agent in 100mM C14DMAO aqueous solution, and their contributions to the viscosity of solution were examined ranging X = 0 to 1. It was elucidated that the aggregate structure remarkably depended on the ion-pair (counte-ion) thermodynamics. For the Cl-, HCOO-, and H2PO4- ions, no remarkable structural change took place with increasing protonation. For the Br-, NO3-, Oxalate, Tartarate, Tartronate, and SO42- ions, the small micelles of C14DMAO grew up with protonation. For the ClO4-, SCN-, and Salicylate ions, the L-alpha phase was formed with protonation. The interfacial curvature thus followed mostly the sequence of Hofmeister series, while the sulfuric ion was completely excluded from the series because of its divalency. The viscosity change was interpreted quantitatively by the hydration free energy (Delta-Ghyd) of counter-ion. However the divalent counter-ions did not arrange in the same order as the monovalent ones, that could be considered to arise from (1) depressing pKa of C14DMAO with increasing amount of acid and (2) electrostatic force within the diffuse double layer. For different counter-ions, the characteristic scaling laws of viscosity evolution against X were observed. The scaling laws also obeyed Delta-Ghyd. Sulfate, however, could not be manipulated by Delta-Ghyd: regardless of its strong hydrophilicity (high Delta-Ghyd), the excess amount of SO42- caused the micelles to grow up. The micelle growth therefore would be attributed by counter-ion condensation onto the micelles rather than hydrophilicity itself. Comprehensively it was proved that Delta-Ghyd and the ionic valency (electrostatic force) of counter-ion played strikingly significant roles in the structural properties. Trifluoro acetic acid CF3COOH behaved as hydrophobic acid, however, the viscosity trend could stand in neither the sequence of Delta-Ghyd nor the Hofmeister series. The interfacial tension measurments suggested that the CF3COO- ion was incorporated into the micelle, behaving like co-surfactant. Synergism on mixing was also observed in the CF3COOH system, by which the minimal CMC was obtained. The interaction parameter beta indicated that CF3COOH caused the stronger synergistic effect than HNO3. However, some experiments showed the successively hydrophobic C14DMAO with protonation. This would be due to the orientation of hydrophobic trifluoro group in the palisade layer of micelle. The increasing volume fraction of hydrophobic moiety in the surfactant molecule (OleylDMAO) built up the bilayer structure even using the strongly hydrated acids. The L-alpha formation was referred to synergism because the isotropic micellar phase was present on both the sides of high and low protonation degrees. The L-alpha phase melted on elevating temperature, and the subsequent L1 phase was highly viscoelastic. The L-alpha - L1 transition temperatures for different acids were almost correlated by the enthalpy of hydration (DeltaHhyd). And the viscoelastic properties of the L-alpha phases were dependent on the Hofmeister series.
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Factors regulating signal composition and task allocation in colonies of the ant Myrmicaria eumenoides
(2004)
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Lengyel Florian
- The integrity of social insect colonies is facilitated by nestmate recognition. A defining characteristic of social insect colonies is their elaborate division of labour system. Cuticular hydrocarbons (CHCs) have long been implicated in nestmate recognition. In the African ant Myrmicaria eumenoides they also appear to vary with task. The factors regulating the task allocation (i.e. division of labour), as well as the CHC profiles (i.e. signalling) are still not well understood. Both external and internal (endocrine) factors have both been postulated in the past. Most of the evidence to-date suggests that juvenile hormones (JHs) and ecdysteroids are the prime endocrine control candidates. The aim of this project is to investigate how signalling and task allocation are dynamically governed in M. eumenoides colonies. Firstly, the CHC profiles of individual workers in relation to their task (behavioural development phase) over their life span are assessed. Secondly, the influences of the two endocrinological factors (JHs, ecdysteroids) hypothesised to play a role in task allocation, as well as signalling, are investigated. 1) The behavioural transition from callow to brood-tender, and on to forager was shown to be continuous and to change in a synchronous manner in all individual M. eumenoides focus workers. In addition, all workers were shown to go through the “reserve” phase (characterised by a high degree of inactivity) before changing to foraging activities. 2) The CHC profiles of all individual workers were shown to change parallel to the behavioural development transition noted above. Overall, the CHC profile change is only quantitative, and characterised by a decrease in short-chain, and a simultaneous increase in long-chain components, as well as an increase in saturated, and a decrease in unsaturated CHCs. 3) Direct behavioural evidence, and the strong correlation of signalling with task suggests that in M. eumenoides the CHC profiles inform task decisions. Only five components were shown to contribute more then 90% to the variance between brood-tender-type and forager-type CHC profiles. This suggests that perhaps “task signalling” requires only a few CHCs. These may be of a saturated and/or unsaturated nature, but must be of a high or medium high relative concentration. 4) The levels of ecdysone and 20-hydroxyecdysone (found to be the main free ecdysteroids in workers), as well as the sum of the total free ecdysteroids in workers, are only high after eclosion, and then fall to a low, steady level within two weeks. Thus no correlation was found between the above and signalling, or task. 5) Interestingly, JH I was found (only) in the haemolymph of female M. eumenoides alates: this is the first time this JH homologue has been identified in the Hymenoptera. It is hypothesised that JH I plays a role in either flight or migratory behaviour, and is thus absent from worker haemolymph. 6) Foragers were demonstrated to have significantly higher JH III titres than brood-tenders. Exogenous JH III and JH analogue application however did not result in an observable change in, or acceleration of, the behavioural development of workers. This may however be due to the much reduced longevity of the applied workers, and the hence decreased observational period. 7) The changes from a brood-tender-type to a forager-type CHC profile are accelerated in a concentration-dependent manner by the application of JH III, and JH analogue. This appears to take place independent of task changes, resulting in e.g. brood-tending workers that display clear forager-type CHC profiles. Thus these results demonstrate for the first time that signalling in Hymenoptera is strongly influenced by JH. Several possible pathways for the action of JH III are discussed. 8) Finally, it is suggested that workers assess the colony needs using CHCs as simple, localised cues. The CHCs would thus play a key role in efficiently regulating a colony’s behaviour. Several promising approaches were also detailed with respect to further work on this topic.
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New block copolymers of Isobutylene by combination of cationic and anionic polymerizations
(2004)
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Nemesio Martínez-Castro
- The studies presented in this thesis deal with the new block copolymers of isobutylene by combination of cationic and anionic polymerizations, which are new materials with numerous promising potential applications. A new method for the synthesis of tailored polyisobutylene(PIB)-based block copolymer by combination of controlled / living cationic and anionic polymerizations has been developed. In addition and parallel to these subjects, new synthetic routes for preparation of telechelic PIBs and conductive polymers have been investigated. The PIB precursors used for subsequent anionic polymerization and other processes were prepared by controlled / living cationic polymerization of isobutylene followed by quenching with thiophene under selected conditions. Quantitative functionalization of living PIB with thiophene (T) has been achieved. The process is complicated by coupling between living PIB and PIB-T formed by in situ deprotonation. By lithiation of PIB-T a new, convenient method has been demonstrated for the synthesis of PIB-based block copolymers, involving anionic initiation of tert-butyl methacrylate. A major improvement is that for an industrial process, lithiation by n-BuLi is much more convenient than metalation by Na/K alloy, which had to be used in former processes. Block copolymers with narrow and unimodal molecular weight distribution (MWD) were synthesized under well-controlled conditions. The lithiated thiophene-capped PIB was also used to prepare four-armed star PIB via chlorosilane coupling. Using the same method, new amphiphilic block copolymers, namely PIB-b-poly(N,N-dimethylacrylamide) and PIB-b-poly(ethylene oxide) were synthesized. The anionic polymerization of N,N-dimethylacrylamide (DMAAm) was carried out with a binary initiator system prepared from thienyllithium and the Lewis acids triethylaluminium (Et3Al), diethylzinc (Et2Zn) or triethylborane (Et3B) in THF. Polymerizations proceeded in a homogeneous manner and gave polymers having controlled molecular weights. However, in presence of alkoxides the polymerization results in polymers insoluble in THF, even using Et3Al. Finally, lithiated PIB-T, in conjunction with Lewis acids, was used to initiate the living anionic polymerization of DMAAm, resulting in the new amphiphilic block copolymer PIB-b-PDMAAm. The anionic polymerization of ethylene oxide (EO) was carried out with a binary initiator system prepared from thienyllithium and the polyiminophosphazene base tBu-P4 in THF. Lithilated PIB-T- in conjunction with tBu-P4 was used to initiate the living anionic polymerization of ethylene oxide. In a second method, PIB-b-PEO was synthesized using hydroxyl end-capped PIB as macroinitiator in conjunction with tBu-P4. The aggregation behavior in dilute aqueous solutions of a PIB-b-PEO copolymer, produced from the combination of cationic and anionic polymerizations, was investigated by dynamic light scattering in aqueous solution. The monoaddition of ethylene oxide to lithiated PIB-T is demonstrated as a new pathway for the synthesis of hydroxy-functional PIB (PIB-OH). This is a useful alternative to the tedious procedures used in the literature so far. PIB-OH was successfully used as a macroinitiator for the ring-opening polymerizations of L-lactide and epsilon-caprolactone catalyzed by stannous octoate. Combination of chromatographic methods and MALDI-TOF ass spectrometry gave information about the side reactions during polymerization. Quantitative functionalization of living PIB with 2-bromothiophene has been achieved. Using various coupling methods, this polymer was convertet to a macromonomer with a pendant thiophene group, which is accessible in the 1- and 5-position. Oxidative copolymerization with thiophene led to graft copolymers consisting of polythiophene-graft-PIB.
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AFRICAN TRADITIONAL PLANT KNOWLEDGE TODAY: An ethnobotanical study of the Digo at the Kenya Coast
(2005)
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Mohamed PAKIA
- The Digo are farmers and fishermen living in the coastal belt stretching from Mombasa in Kenya to Tanga in Tanzania. They settled there in the 16th Century or earlier, and their original settlements were forest villages, kaya. During the kaya life, wild plants were important to them for most basic needs, hence they accumulated a traditional plant knowledge. In the 19th Century they moved out of the kaya, but the forests were protected as sacred sites, and this preserved the traditional plant knowledge. On the other hand, modern scientific botany was introduced to the Digo through teaching in schools and as modern agriculture to farmers, creating two types of plant knowledge with little overlap. Although Digo history indicates intimacy with their plant world, in this technologically advanced era traditional knowledge systems are threatened of being lost. It was on this premise that the study was established, aimed at ‘documenting the traditional Digo plant knowledge, and examining the global influence on it’. Information was collected with formal and informal interviews with both the traditional Digo plant users (herbalists, farmers, and carpenters), and ‘modernists’ (pupils, students, and teachers). In summary, Digo plant knowledge has a considerable verbal component, minimal description and no conception of internal plant processes. The knowledge is voluntarily incomplete, with a conscious ignorance of some areas of plant life. Also the knowledge is mainly value oriented, and confined to the known, with the objective of addressing realities of social life. Globalisation will affect the plant lexicon and description, but less so the application of traditional materials. The subject areas of the thesis: Digo plant lexicon and description Digo plant knowledge has a considerable verbal component, with limited description and no conception of internal plant processes. And there are non-verbalised areas and under-labelled elements of plants and plant life. Clearly there is intentional selectivity of what to label and describe, meaning the Digo are not striving for completeness of plant knowledge. The incentive for labelling and description is predominantly value oriented, as commonly used plants and plant parts are labelled in detail. Some Digo plant names have a prefix borrowed from ‘female’ human names, meaning the species are perceived as ‘female’. ‘Unmarked’ Digo plant names refer to them in ‘female’ status, as in reference to ‘male’ plants the name is always marked with the term mlume [male]. Semantic analysis of the interpretable plant names indicates that Digo plant naming is guided by several unwritten principles. Digo plant identification methods Digo plant identification is characterised by familiarity and little verbal descriptions. Experienced plant collectors identify plants using ‘fixed images’ in their memory, without rigorous procedures. ‘Procedural’ identifications are used by less experienced collectors, for doubted identifications or in new environments. In procedural identification different plant user groups focus on the part ‘useful’ to them, creating a variety of approaches in identification of the same species. Digo folk taxonomy Digo folk taxonomy is shallow. It does not correspond to comprehensive folk taxonomies reported by most other ethnographers. In Digo there is no term for ‘unique beginner’, thus ‘plant kingdom’ is not a recognised rank. Recognised ranks are life-forms and folk species, with occasional presence of folk generics and folk varietals. Plant life-forms are recognised on the basis of discontinuity of kinds, which is consistent with rationalism theory, but lower taxonomic ranks are constructed on basis of utility value, which agrees with utilitarian view. The Digo folk taxonomy, therefore, is an intermediate kind of knowledge. Digo perception of internal plant processes The perception of plants as being ‘female’ is twined with an understanding that male plants are irrelevant in plant reproduction and propagation, hence associated processes (pollination and fertilization) are not perceived; neither are other scientific concepts e.g. photosynthesis and transpiration. At a community level, it is not obligatory for the Digo to understand details in plant processes, which contrasts labelling and descriptions of plants. Digo farming practices Digo farmers have a knowledge built by experience, hence a stabilized understanding of correlations between different components such as demand of crop plants, performance of soils, diseases and pest menace. A Digo farmer recognizes and classifies soils in relation to crop production. Although some old farming practices have been dropped due to various socio-political changes, the Digo farmers maintain to a considerable degree traditional practices, magico-religious practices and preference of local crop varieties. The future of Digo plant knowledge The Digo plant knowledge reacts to global influence in a multitude of ways, depending on the risk of material loss. In plant lexicon and description, where there is no risk of material loss, change is acceptable. Thus school-mediated plant knowledge complements Digo lexicon and description, sometimes only temporarily. On the other hand, in plant description the role of primary Digo colour terms is fading off. National and global support has changed the Digo healing only in the tools i.e., in hygiene, processing and dosage of medicinal plants, but not in content, as traditional cures are still the main phytotherapical substances. Digo farmers however, resist modern agriculture, partly due to the economic implications. The continued lack of material support to the farmers strengthens the future of traditional farming practices.
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Biogeochemical Consequences of Hydrologic Conditions in a Tropical Montane Rain Forest in Ecuador
(2004)
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Rainer Goller
- Tropical montane forests regulate the hydrological cycles of high-elevation areas being an important service for the local population. The objectives of my work were (1) to determine the major hydrological flow paths, (2) to quantify concentrations of dissolved organic C and organic and inorganic forms of N, P, and S, and (3) to identify the major controls of the dissolved C, N, P, and S dynamics in a tropical montane forest in south Ecuador. Three 8-13 ha large microcatchments (MCs) under montane forest at an altitude of 1900-2200 m above sea level were selected. Scientific equipment was installed on five transects, about 20 m long with an altitude range of 10 m. Three unforested sites near the microcatchments were used for rainfall gauging. Within the three monitored years between May 1999 and April 2002, on average 2448 mm of precipitation fell on the study area. The delta 18O of rainfall shows large variations (-12.6 to +2.1 per mil) related to different air-masses. The delta 18O values of throughfall and lateral flow are similar to those in rainfall. Variations in delta 18O values of the soil solution and the stream water are smaller (-9.1 to -3.0 and -5.8 to -8.7 per mil) than those of rainfall, throughfall, and lateral flow. The delta 18O values in stream water increased immediately after an intense rainstorm event to isotope values similar to those of rainfall and lateral flow. This indicated that during elevated rainfall the water flows rapidly in the organic layers to the stream channel paralleling the surface. During this time the water content of the organic layer was higher than of the upper mineral soil. Results from an isotope two-component hydrograph separation for the three microcatchments showed that new rain water ('event water') contributed 44-81 % to the total stormflow runoff during one selected peak discharge. The canopy was a small and the organic layer the major source of DON, DOP, and DOS, which were almost completely retained in the mineral soil. The organically bound forms contributed, on average in all solutions, 54 %, 78 %, and 59 % to the total concentrations of N, P, and S, respectively. The contribution of organically bound forms to total nutrient concentrations was element-specific and differed among the ecosystem fluxes. The portions of DON (23-81 %) were similar and those of DOP (5-100 %) highly variable in all solutions. There were high DOS portions in the aboveground fluxes (51-100 %) and lower portions in the mineral soil (8-47 %). The pH was positively correlated with the DOC and organic N, P, and S concentrations at the scale of pedons. Increasing DOC concentrations in stream water following rainstorms indicated that rainstorms were an important control of the dissolved organic matter dynamics on a regional scale. During the passage of the water through the forest, dissolved inorganic N (DIN) and DIP concentrations increased between the canopy and the mineral soil and strongly decreased in the mineral soil. In contrast, DIS concentrations were highest in the mineral soil and stream water. Thus, the organic layer was the major source for DIN, the canopy for DIP and the mineral soil for DIS. The mineral soil was a sink for DIN and particularly for DIP. Soil drying and rewetting promoted the release of inorganic N. High discharge levels following heavy rainstorm events were associated with pulses of NO3-N and partly also NH4-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively related to runoff conditions. The DIP and DIS concentrations in throughfall and stemflow were negatively correlated to the respective water fluxes, whereas DIS concentrations in the mineral soil solutions of both studied soil depths were positively related to the rainfall volume. DIN and DIP concentrations and fluxes tended to be positively related to the pH of the organic layer. My results demonstrate that nutrient dynamics were mainly controlled by hydrological conditions in the studied steep forested catchments. Furthermore, small ecosystem inputs and outputs indicated tight cycles of dissolved N, P and S in the study forest, except for DIS.
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Biogeochemical Investigations in Old Growth and Disturbed Forest Sites at Mount Kilimanjaro
(2004)
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Marion Schrumpf
- Areas covered by mountain rainforests in East Africa have important functions as catchment areas for the streams in the savannah. Still, little is known about the biogeochemical processes in these often threatened ecosystems. The current study aims to present basic information on soils and different parameters of the water and nutrient cycle in intact as well as disturbed tropical mountain rainforest sites at Mt. Kilimanjaro in order to assess the long term effects of forest dissection on biogeochemical parameters. For this purpose, the chemical and physical soil properties, soil water tension, rainfall, throughfall, litter percolate and soil solution were determined on twelve study sites in mature forest, secondary forest and elder clearings between 2100 and 2300 m a.s.l. on the southwestern slopes of the mountain. The soil water tension was monitored from May 2000 to August 2002. The simultaneously collected water samples were combined to form fortnight samples and subsequently analysed for their content of K, Mg, Ca, Na, NO3-N, NH4-N, TOC, TON, TOS and TOP. The soils, which comprised of several buried horizons, were Andisols with high C and N stocks, some hydromorphic properties and comparatively low pH values. The latter led to very small ECEC values and low base saturation. Consequently, the retention of cationic nutrients in the soil was considered to be poor. Rainfall amounts were slightly less at the higher sampling sites where the mature forest was located and differed between the study years (2600 mm and 2480 mm at 2100 m, 2210 mm and 1960 mm at 2250 m). Rainfall interception was close to zero in the clearings, but reached up to 30% of incident rainfall in both studied forest types. The analyses of the soil water tension revealed the driest conditions to be under secondary forests. The reason for this was probably a combination of the topographic position of the secondary forest sites and the forest fragmentation. Since the secondary forest sites and the clearings had higher sand contents, the water holding capacity of these sites was lower than in the mature forest. With the exception of NO3-N in the forests, all nutrients in rainfall were increased upon passage through the forest canopy, but nutrient fluxes in rainfall as well as throughfall were still low compared to other mountain rainforests, especially for Mg, Ca and K. Considering the lower biomass of the clearings compared to the forest, the amount of nutrients leached from the vegetation of the clearings was relatively high. Since NO3-N was not assimilated by the vegetation of the clearings, NO3-N throughfall fluxes were higher in the clearings as were the K fluxes, which was probably attributable to different leaf properties of the vegetation at the different sites. In the mature forest, the highest concentrations of nutrients were found in the litter percolate, followed by a pronounced decline in the soil solution. This was especially the case for K. NO3-N was an exception since it exhibited the highest concentrations only in the topsoil solution. Nutrient concentrations in litter percolate and the topsoil solution were usually higher in the clearings than in both forest types. This probably resulted in higher belowground nutrient fluxes since greater water amounts reached the ground surface in the clearings. The high belowground nutrient concentrations were likely the result of the higher mineralisation rates in the clearings induced by higher temperatures and the greater nutrient contents of the litter. With increasing soil depths, nutrient concentrations in seepage water below the clearings declined so that the differences among sites were not significant at deeper soil layers. The contribution of organically bound nutrients to the total concentrations of respective nutrients was highest in throughfall water and lowest in the soil solution, in which OM concentrations were overall lower than in other mountain forests. This was most likely attributable to the adsorption of organically bound nutrients to the mineral phase of the Andisols studied. Again, the highest concentrations in seepage water were measured in the clearings. The results show that mature forests at Mt. Kilimanjaro exhibit a more closed nutrient cycle, especially for basic cations, while the nutrient cycle in the clearings is more open. This probably reflects the different nutrient usage and conservation strategies of the pioneer and the late successional vegetation. Therefore, the forest disturbance on Mt. Kilimanjaro leads to long term changes in biogeochemical cycles. The opening of the forest at lower elevations, which resulted in the formation of large clearings with impeded regeneration and highly fragmented secondary forest patches, led to a higher spatial and seasonal variability of soil moisture and nutrient contents in seepage water. The large scale effects of these forest conversions on water yields and nutrient outputs need to be tested in the future.
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Supercurrents in Restricted Geometries and Driven by Time-Dependent Electric Fields
(2004)
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Martin Endres
- NS Contact We studied the linear response of a normal metal superconducting metal contact to a small electric field. In a preparatory section the order-parameter profile and the density of states were calculated in equilibrium. We showed that the density of states in the normal metal is unaltered if the impurity self-energies are not taken into account while the coherence in the superconductor is always affected by the presence of the normal metal. Self-consistent calculations result in an impurity-induced proximity effect in the normal metal. This proximity effect causes a spatially constant gap in the density of states of the normal metal if the normal metal is sandwiched between two superconductors. The dynamics of the NS contact is strongly dominated by the conservation law for charge and local charge neutrality which together fully determine the current in one-dimensional systems. For answering the question how this constant current is established in the non-homogeneous NS contact, the quasiclassical equations were solved including the self-consistencies for the order parameter, the impurity self-energies, and the electrochemical potential. The latter was used to deduce an internal electric field as response to the external perturbation. The internal field is of same order as the perturbation and is caused by charges which are either bound to the interface or spread over several coherence lengths. The surface charges are not due to the step in the order parameter at the interface but solely to abrupt changes of the impurity scattering. The order parameter itself can only produce continuous charge densities. The charges are indirectly calculated using Maxwell's equations. They are of higher order in the expansion parameters of Fermi-liquid theory and are hence beyond this theory. Nevertheless, their effect has to be considered to be consistent in leading order. Weak links in He3 We investigated several methods of calculating the current-phase relation of weak links in He3. In the limit of small holes the hole itself and the current through it does not affect the order parameter in the superfluid and the current can hence be calculated using the pinhole model. This leads to a periodic current-phase relation. It was shown that the pair-breaking effect of the separating wall has no significant influence on the functional dependence of the current on the phase difference. The wall mainly reduces the amplitude of the current. For orifices with radii comparable with the coherence length, self-consistent order-parameter fields were calculated. The two fixed phases of the pinhole model are then replaced by a field which allows the phase to wind up continuously. This not only breaks the periodicity, but also leads to multivalued current-phase relations. Over a wide range the current through the orifice is linear in the phase difference between the reservoirs. Although this is expected in the hydrodynamic limit, the hydrodynamic equations are not applicable as they always fail at the edges of the circular apertures. However, calculating the current quasiclassically with the phase determined via the Laplace equation gives a fairly good approximation to the fully self-consistent solution. This approximation becomes weak for larger phase differences when pair-breaking due to the current itself has to be taken into account. Remarkably, the maximal current through the aperture is sandwiched between the pinhole current and the depairing current for a homogeneous superfluid which differ only by a factor of about two at low temperature in spite of the drastic difference of the models. A quasiclassical free-energy functional was introduced and it was stressed that this choice is not unique and that a whole zoo of different functionals exists. The functional was used to investigate the change in free energy due to the wall, the orifice, and the phase difference.
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Dynamics of vortices in the two-dimensional anisotropic Heisenberg model with magnetic fields.
(2003)
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Juan Pablo Zagorodny
- The subject of this work is the dynamics of a vortex in a classical 2-dimensional spin system with anisotropic exchange interaction under the combined action of magnetic fields and damping. Static as well as dynamic magnetic fields were employed (as dynamical field we used a homogeneous field which is rotating in the XY-plane). The most important goal of this work was to demonstrate that there is a coupling between the inner and translational freedom degrees of the vortex, coupling which is responsible for at least 2 phenomena that we study in detail in this Thesis: 1. the switching or flipping of the vortex polarization (for negative field frequency), and 2. the formation of stable orbits of the vortex center around the center of the system driven by the rotating field (for positive frequency). It was known to us that the polarization can change abruptly its sign under the action of a field rotating in the XY-plane, for p omega < 0 and appropriate field amplitudes. In the Chapter 4 we have investigated the possible underlying mechanisms for this phenomenon. Our main results can be summarized as follows: a) The flipping times do not depend essentially on the size of the system, provided that the lattice is large enough (radius L >~ 36 lattice constants). In other words, the switching of the vortex polarization is not much affected by the presence of boundaries. b) In our numerical simulations we observed a clear correlation between the core magnetization dynamics (the oscillations of the core spins in the out-of-plane direction) and the velocity of the vortex center in the plane of the lattice. c) A diagram of flipping events as a function of the field parameters, from extensive numerical simulations with an OP vortex in a rotating magnetic field, was presented. We found out that in the (omega, h) parameters space there is no well-defined curve which separates the regime where the flips do not occur from the regime where they do. We found intervals ("windows'') of intermittent flip and non-flip events. d) The switching of the vortex polarization can be achieved also by applying a static magnetic field with both in-plane (IP) and out-of-plane (OP) components. The IP component of the field sets the vortex into translational movement in the XY-plane, while the OP component breaks the vertical symmetry favoring one of the two possible orientations. e) The switching dynamics may be described in terms of a core model which takes into account a coupling between the vortex polarization dynamics and the motion of the vortex center. We showed that a reduced core model, which is valid near the threshold of the IP-OP vortex instability (lambda ~ lambda c), can be mapped to a generalized Thiele equation with an inertial term. f) It is plausible that the phenomenon of switching we described will not be essentially affected by the inclusion of a dipole-dipole interaction. The experimental works on nanodisks mentioned in the Introduction of this Thesis reported the observation of vortices in either of two polarization states, and the switching between them was forced by means of static fields perpendicular to the plane of the disks. Rotating magnetic fields might be used as well static fields with both IP and OP components to make this switching more favorable. In the Chapter 5 we turned to the study of the movement of the vortex in the XY plane, in the presence of the IP rotating field. Attention was directed to the existence of stable orbits, where the vortex stays inside the system in a stationary movement, forming circular limit cycles. We discussed then the failure of the conventional Thiele approach to describe this phenomenon, and this motivated us to formulate an extended collective coordinate Theory, which leads to a qualitative agreement with the results of the simulations. A diagram of the different types of trajectories, as a function of the field parameters, showed the presence of non-monotonous effects and "windows'', like in the case of the switching diagram. We are led to conclude that for some regions of the field parameters space, the system exhibits chaos -which is typical for many-body systems-, though no particular tool of the chaos theory was used to study our discrete and collective coordinate models, from this viewpoint. Our theoretical work qualitatively suggests that it would be interesting to apply in the experiments weak rotating fields like those used here, to control both the mean position of a vortex in larger magnetic dots (where the vortex center could show dynamics) and at the same time the sign of the out-of-plane core magnetization. Future directions of this work may include the use of inhomogeneous fields, particularly with a gaussian localization in a small region of the lattice or "spot'', as a model of the field of a laser beam.
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Coordinated Tree Responses to Drought -Vulnerability and Sustainable Production: Hypotheses on Arid Ecosystem Adjustments to Limitations in Water Resources
(2004)
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Dennis Otieno
- Field and controlled greenhouse experiments were carried out to investigate tree responses to declining soil water content. Field experiments were conducted on naturally growing trees of Acacia tortilis and A. xanthophloea in the savanna region of Kenya and Quercus suber in the Mediterranean region of Portugal. The selected field sites were regions that experience regular drought periods during the year. Greenhouse experiments constituted two watering regimes. Seedlings of A. tortilis and A. xanthophloea grown from seeds initially obtained from the Kenya field site were raised and arranged on a greenhouse bench into two groups per species. The first set of plants were watered every other day (controls) while the second set were watered every seven days (water stress treatments). Field measurements included weather parameters, soil and plant water status, growth, sap flux density, leaf transpiration and stomatal conductance, tissue water relations and isotope labeling. Similar measurements were conducted on plants growing in the greenhouse. Also examined in the greenhouse were root biomass, root structure as well as whole plant biomass accumulation. A second set of experiments was carried out in the greenhouse by subjecting plants initially stressed and non-stressed to severe water stress by withholding water until plants were wilted overnight. The wilted plants were then re-watered regularly and their recovery after stress alleviation was monitored. Declining soil water content significantly affected plant water status in all the trees studied. Lowest psi pd recorded during the study period occurred in the month of June and were –2.0 and –1.1 MPa for A. xanthophloea and A. tortilis respectively. The same species subjected to repeated water stress in the greenhouse attained mean minimum psi pd of –2.4 and –1.2 MPa for A. xanthophloea and A. tortilis respectively at the end of the drying cycle. Mean minimum psi pd recorded for Q. suber during summer was –1.8 MPa and occurred in September. There were however, significant differences among trees. Decline in psi associated with increasing soil drought led to decline in leaf initiation and leaf expansion and both processes ceased at higher water stress levels. For the Acacia species, even leaf shedding occurred at higher stress levels. There was also a decline in stomatal conductance (gs) during water stress, leading to decrease in transpiration rates (E). Maximum stomatal conductance of 340 mmol m-2 s-1 were observed during rainy seasons for the Acacia trees while mean maximum values of 300 mmol m-2 s-1 were recorded for Q. suber when soil moisture conditions were favorable. Stomatal conductance declined by 31%, 67% and 67% in A. tortilis and A. xanthophloea in the savanna and Q. suber in the Mediterranean regions respectively. Daily tree water use (Qtree) as well as leaf transpiration reflected changes in psi and gs. Root to leaf hydraulic conductance equally declined with increasing soil drought. Q. suber trees adjusted osmotically by a magnitude of 0.7 MPa, while bulk modulus of elasticity (epsilon) increased by a magnitude of 17 MPa. Osmotic adjustment of 0.48 MPa was observed in greenhouse plants of A. tortilis while epsilon declined by a magnitude of 7 MPa in A. xanthophloea. A. tortilis plants in the greenhouse showed increased absolute root growth, root depth and root:shoot (r:s) ratio. The dimorphic rooting pattern in Q. suber resulted into hydraulic lift and this could as well occur in A. tortilis because of similarity in their rooting patterns. Most plant responses were reactionary and were aimed at enhancing soil water uptake and reducing transpiration water loss when soil water content was declining. Similar responses were observed for both greenhouse and naturally growing field plants of the same species. Decline in leaf initiation and leaf expansion as well as leaf senescence reduced tree crown size hence potential tree transpiration. This however, had negative impact on plant productivity. Increased root growth as well as osmotic adjustment increased tree water uptake from the soil. The balance between root water uptake and leaf transpiration through growth and stomatal regulation was aimed at protecting xylem integrity. The overall results showed that soil characteristics, root activities and root distribution patterns are the main factors determining tree functioning and productivity in drylands, while the coordinated interaction between the aboveground shoot and belowground root activities ensures survival during drought. Maintained production and survival will ensure distribution and success in the arid environments. Repeated water stress imparted water stress resistance qualities on seedlings enabling them to survive longer during severe stress. The study emphasizes the role of soil resource base as well as species interactions in the functioning and balance of dryland ecosystems.
