187 search hits
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Proceedings of the International Conference of "Atmospheric Transport and chemistry in Forest Ecosystems" Castle of Thurnau, Germany Oct 5 to Oct 8, 2009
(2009)
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Johannes Lüers
Thomas Foken
- no abstract
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Comparison ot the sonic anemometer Young Model 81000 during VOITEX-99
(1999)
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Thomas Foken
- no abstract
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Documentation and Instruction Manual of the Eddy-Covariance Software Package TK3
(2011)
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Matthias Mauder
Thomas Foken
- no abstract
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ExchanGE processes in mountainous Regions (EGER)- Documentation of the Intensive Observation Period (IOP3) June, 13th to July, 26th 2011
(2011)
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Andrei Serafimovich
Fabian Eder
Jörg Hübner
Eva Falge
Linda Voß
Matthias Sörgel
Andreas Held
Qianqian Liu
Rafael Eigenmann
Kilian Huber
Henrique F. Duarte
Peter Werle
Eckhard Gast
Stanislaw Cieslik
Liu Heping
Thomas Foken
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Documentation and Instruction Manual for the Horizontal Mobile Measuring System (HMMS)
(2011)
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Jörg Hübner
Johannes Olesch
Hubert Falke
Franz X. Meixner
Thomas Foken
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The Arctic Turbulence Experiment 2009 - additional laser Scintillometer measurement campaign 2009 at the Bayelva catchment on Svalbard: Technical documentation and visualization of the near surface measurements during the ARCTEX-2009 campaign, August, 10th to August, 20th 2009
(2012)
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Johannes Lüers
Jörg Bareiss
Martin Wagner
- Accurate quantification of turbulent fluxes between the surface and the atmospheric boundary layer in polar environments, characterized by frequent change of weather and exchange conditions (stable to very stable or intermittent; rapid, short term neutral to unstable stratified conditions) is a fundamental problem in soil-snow-ice-vegetation-atmosphere interaction processes. The observed rapid climate warming in the Arctic requires improvements in the permafrost and carbon cycle monitoring. To address these problems, it is essential to improve the databases with high-quality in-situ measurements of turbulent fluxes above tundra landscape surfaces applying the Eddy-Covariance method and the laser scintillometry. Results from the Arctic Turbulence Experiment 2006 on Svalbard helped to better understand physical exchange processes of energy and matter transport and to improve instrumentation standards as well as quality assessment techniques (Lüers and Bareiss 2010, 2011; http://www.arctex.uni-bayreuth.de). Therefore, the primary goal of this additional laser scintillometer measurement campaign is to estimate the flux contributions covering typical tundra surfaces across the Bayelva catchment during a summer season south-west of the Ny-Ålesund village, Kongsfjord, Svalbard. This effort makes it possible to define the spatial context of the fluxes, and to include land use features of the surrounding terrain in the quality assessment of all observations in the Bayelva catchment over the last 10 years performed by the Alfred Wegener Institute for Polar and Marine Research (AWI).
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Klimawanderweg auf der Landesgartenschau in Bamberg 2012
(2012)
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Thomas Foken
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Whole-air relaxed eddy accumulation for the measurement of isotope and trace-gas fluxes
(2012)
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Johannes Ruppert
Michael Riederer
Willi A. Brand
Thomas Foken
- Measuring the isotopic composition of trace gas fluxes can provide additional information on ecosystem gas exchange, when ecosystem processes, like assimilation, discriminate against heavier isotopes. In the case of CO2 exchange, different mass-balances for bulk CO2 and its 13CO2 or CO18O isotopes can be used to separate respiration from photosynthetic assimilation. Up to now, detectors for direct isotope measurements in the field lack the precision needed for fast eddy covariance (EC) flux measurements. The collection of updraft and downdraft whole-air samples using the relaxed eddy accumulation technique (REA) allows simultaneously determining trace gas concentrations and isotope ratios by high precision laboratory analysis. At the same time whole-air REA relaxes several of the technical problems related to REA sampling on traps. In tests using air from a tank the complete whole-air REA sampling system and its foil balloon bag reservoirs showed no signs of contamination after cleaning. The standard deviations of δ13C and δ18O isotope ratios were only slightly higher than the precision specified for the laboratory analysis procedure. First experiment results showed that isotopic differences (up-drafts−downdrafts) were large enough to yield signal to noise ratios greater than five when applying hyperbolic deadbands during REA sampling (HREA). The performance of the instrument and the HREA sampling method are investigated by simulation of the sampling process for bulk CO2, which serves as proxy scalar. Measurements by whole-air HREA in combination with high precision isotope analysis can quantify the isofluxes of 13CO2 and CO18O. Furthermore, additional information is collected on the scalar correlation of bulk CO2 and its stable isotopes, which represents the relatively short timescale of updrafts and downdrafts in the turbulent exchange above an ecosystem. This information is essential to check the scalar similarity assumptions made in the HREA and EC/flask method for the quan-tification of isofluxes.
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The Nature of Fluids in Hydrothermal Copper and Molybdenum Ore Deposits - An experimental and analytical study
(2012)
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Linda Lerchbaumer
- The evolution of magmatic-hydrothermal fluids in porphyry Cu and porphyry Mo deposits was studied using synthetic and natural fluid inclusions by optical microscopy, microthermometry, Raman spectroscopy, and LA-ICP-MS.
The partitioning of Cu between vapor and brine in aqueous NaCl-S ± KCl ± FeCl2-rich fluids was investigated by means of hydrothermal experiments in rapid quench autoclaves at 600-800°C, 70-130 MPa and at both oxidizing and reducing fO2, covering all geologically relevant conditions. Resulting partition coefficients (DCuvap/brine) are between 0.2 and 0.4 for the range of studied S-concentrations, fluid pH, fO2, and P-T conditions. These values indicate that Cu does not partition into the vapor phase at any plausible condition in contrast to data from natural quartz-hosted vapor and brine inclusions which appear to indicate Cu enrichment in the vapor.
The formation of such Cu-rich vapor-type fluid inclusions was investigated in hydrothermal re-equilibration experiments. For this purpose, coexisting vapor and brine inclusions of known composition were re-equilibrated in a fluid of slightly different composition and lower pH than the trapped one at 800°C, 70-130 MPa. This procedure led to a dramatic increase in Cu concentrations in the vapor phase from 0.3 ± 0.03 to 5.7 ± 3.3 wt% after re-equilibration and the change of DCuvap/brine from a true value of 0.4 ± 0.05 to an apparent value of 8.3 ± 4.9. This post-entrapment modification can be traced back to the difference in fluid pH between the trapped and the surrounding fluid, inducing diffusion of H+ out of the inclusion and the diffusion of Cu+ (and Na+, Ag+) into the inclusion in order to maintain charge balance. Moreover, the presence of larger amounts of S within vapor inclusions as compared to brine inclusions can bind larger amounts of Cu. The re-equilibration of trails of vapor and brine inclusions in a natural quartz sample in a fluid similar to the trapped one, yet more acidic, showed that this modification process can be reversed, resulting in the loss of major amounts of Cu from natural vapor inclusions.
The composition of metal-bearing melts and magmatic fluids of Mo-mineralized granites resembling porphyry Mo deposits was investigated using natural melt-, fluid-, and solid-inclusions in quartz crystals found in miarolitic cavities from minor Mo-occurrences in Colorado and Norway. Melt inclusions from the Treasure Mountain Dome are highly enriched in Mo (4-43 ppm), also melt inclusions from the Drammen and Glitrevann granites (5-32 ppm, and 12 ppm, respectively), resembling highly fractionated melts. Copper concentrations are low (<1-30 ppm) in the melts, but high in the fluids: intermediate density, supercritical fluid inclusions of these three locations host 6-1900 ppm, 8-3500 ppm, and 5-180 ppm Cu, respectively. The comparison of these results with data from economic porphyry Mo and porphyry Cu (Mo, Au) deposits shows no difference in Mo concentrations in the particular melts and fluids. Hence, other factors probably control the evolution of a granitic intrusion into large, economic Mo deposits, minor Mo occurrences, or just into barren plutons. These may be mainly the size, position, and geometry of the parental magma chamber, the multiplicity of intrusions maintaining a constant flux of metalliferous, S-rich, hot melts, and the extent of fluid focusing within small rock volumes forming high-grade ore shells.
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Beyond productivity- Effects of extreme weather events on ecosystem processes and biotic interactions
(2012)
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Julia Walter
- Under global climate change, extreme weather events, such as heat waves, drought or heavy rain spells, are projected to increase in magnitude and frequency. As these may affect vegetation and ecosystems more than gradual shifts in mean climatic parameters, investigating the consequences of extreme weather events recently became an important issue in climate change research. The main focus of most experiments investigating effects of extreme weather events on vegetation is on primary productivity. In our experiment in artificially planted communities, even an extreme drought of 1000-year recurrence did not have effects on above- or below-ground biomass production from 2005-2010.
Thus, the main objectives of this thesis were (1) to investigate if extreme weather events have an effect on ecosystem functions beyond productivity, (2) to test if such a high resistance or resilience in response to drought regarding productivity also exists in more naturally grown plant communities and (3) to further elucidate possible mechanisms of the surprisingly large stability of the plant communities.
To investigate these objectives, several experimental studies were conducted in artificially planted, as well as in naturally grown grassland communities and consequences of extreme weather events for ecosystem processes, such as decomposition and herbivory were investigated. In a pot experiment, it was studied, if grass plants react improved towards repeated drought when compared to a first drought and thus reveal a kind of drought memory. Such a memory might be one possible, but up until now widely neglected mechanism of resilience.
Even though biomass production remained stable in our experiment in artificially planted communities, biomass quality was severely affected by extreme drought, thereby strongly affecting the development of a herbivore caterpillar feeding on drought-exposed leaves. Further, plant compounds of the host plant depended on the composition of the plant community it was grown in. This in turn resulted in strong effects on the larval mortality of herbivores feeding on such plants.
In contrast to the study in artificially planted communities, aboveground net primary productivity (ANPP) was reduced in naturally composed grassland in response to extreme rainfall variability, including an extreme drought followed by heavy rainfall. Forage quality was altered by drought. Furthermore, mowing frequency strongly altered forage quality and biomass production, but did not interact with rainfall variability and thus did neither buffer, nor amplify effects of extreme rainfall variability. Despite effects of rainfall variability on ANPP, grassland showed high resilience after drought followed by heavy rain, as effects were large shortly after the extreme event, but did not persist until a second harvest later in the year.
In natural grassland, rainfall variability and drought also affected ecosystem processes, here litter decomposition, beyond productivity. Drought followed by heavy rain pulses decreased decomposition rates. Decomposition in more frequently mown meadows was more vulnerable towards drought exposure. Winter warming and additional winter rain had no long-term effect on decomposition. To conclude, projected increases in drought frequency under climate change may inhibit decomposition and alter nutrient and carbon cycling along with soil quality in temperate grassland, whereas a reduction of snow cover leading to more variable soil surface temperatures may counteract increased decomposition under winter warming.
In this thesis, an ecological stress memory as one possible mechanism of resilience is defined as any response of a single plant after a stress experience that improves the reaction of the plant towards future stress experience and which is assessed on a whole plant level. This thesis further provides evidence of a drought memory in grass plants: Plants repeatedly subjected to drought showed improved photo-protection and a higher rate of living biomass when compared to plants faced with their first drought. Similarly, tree seedlings exposed to drought in summer revealed higher frost resistance during winter, providing evidence of a long-lasting “cross-stress-memory” .
To sum up, the thesis shows that extreme weather events, even though neither severely affecting biomass production in artificially composed, nor in naturally growing communities in the long-term, exert strong influence on physiological or biogeochemical parameters, such as plant compounds or soil biotic activity. These changes in turn modify ecosystem functions beyond productivity, for example herbivory or decomposition, possibly altering biotic interactions and nutrient cycling. Furthermore, the findings imply that plants exhibit a stress memory after stress exposure, which may be one mechanisms leading to a high stability and resilience upon frequent stress.
