1,721,123 research outputs found
Limits and strengths of tree-ring stable isotopes
No full textThis chapter aims at summarizing strengths and caveats on the suitability of stable carbon and oxygen isotopes in tree rings as recorders for fingerprints of environmental influences. First, environmental constraints limiting tree growth and shaping tree species distribution worldwide are discussed. Second, examples are presented for environmental conditions under which tree-ring isotopes record environmental signals particularly well, but also cases where physiological processes can mask climate signals. Third, the link between leaf-level carbon assimilation and the investment of assimilates in the stem during the annual ring formation are discussed in light of the resulting deviations of the isotopic values between leaves and tree rings. Finally, difficulties and pitfalls in the interpretation of stable isotope signals in tree rings are reviewed. These problems often result from a poor understanding of when and how the tree canopy, stems and roots are physiologically interconnected. Current literature suggests that photosynthesis and radial growth are only loosely coupled, if at all, challenging the interpretation of environmental signals recorded in tree-ring isotopes. Harsh environmental conditions (e.g. low temperatures, drought) often result in a decoupling of carbon assimilation and growth. The chapter closes by providing possible solutions on how to improve the detection of environmental information from stable isotope signals by integrating scales and different methodological approaches
Soil H 2 18 O labelling reveals the effect of drought on C 18 OO fluxes to the atmosphere
No full textAbove- and belowground processes in plants are tightly coupled via carbon and water fluxes through the soil–plant–atmosphere system. The oxygen isotopic composition of atmospheric CO2 and water vapour (H2Ov) provides a valuable tool for investigating the transport and cycling of carbon and water within this system. However, detailed studies on the coupling between ecosystem components and environmental drivers are sparse. Therefore, we conducted a H2 18O-labelling experiment to investigate the effect of drought on the speed of the link between below- and aboveground processes and its subsequent effect on C18OO released by leaves and soils. A custom-made chamber system, separating shoot from soil compartments, allowed separate measurements of shoot- and soil-related processes under controlled conditions. Gas exchange of oxygen stable isotopes in CO2 and H2Ov served as the main tool of investigation and was monitored in real time on Fagus sylvatica saplings using laser spectroscopy. H2 18O-labelling showed that drought caused a slower transport of water molecules from soil to shoot, which was indicated by its direct derivation from independently measured concentrations and 18O/16O ratios of CO2 and H2Ov, respectively. Furthermore, drought reduced the 18O equilibrium between H2O and CO2 at the shoot level, resulting in less-enriched C18OO fluxes from leaf to atmosphere compared with control plants. Compared with the shoot, 18O equilibrium was not instantaneous in the soil and no drought effect was apparent
Impact of traffic on δ15N, δ13C and δ18O of needles and annual tree rings of Norway spruce (Picea abiesL.)
Full text linkIncrease of nitrogen depositions, as oxidized (NOx) and reduced (NHx) compounds, has important implications on ecosystem nitrogen cycle. NOx comes predominantly from fossil fuel combustion in car engines and industrial processes, while agricultural activities (cattle farming, fertilizations) are the main sources for NHx emissions. Such fertilisations could stimulate growth and, therefore, productivity of forests, especially in temperate forests, where nitrogen is a limiting factor. On the other hand an excess of nitrogen can lead to an acidification of the soil and have a negative impact on the microbial fauna and structure of plants. NOx and NHx depositions can be separated with the help of stable isotopes with different 15 N-values reported for NOx originating from combustion sources. Consequently it was observed that the nitrogen isotopic composition of the vegetation reflects the isotopic signature of nitrogen sources. Our preliminary results on needles of Norway spruce trees exposed to NOx emissions along a transect perpendicular to a highway close to Faido and Brittnau show a clear isotopic enrichment in 15 N in trees growing closer to traffic pollution, indicating an uptake of atmospheric nitrogen by stomatal pathway. Carbon and oxygen isotopic composition measured in tree rings reveals the physiological response in plants. Trees more exposed to air pollution from traffic show an increase in δ13 C and δ18 O in tree rings, since mid sixties next to Erstfeld. This could mean a higher photosynthetic activity, enhanced by NOx traffic emissions, under low or not changed stomatal conductance. Our results confirm that stable isotopes of carbon, oxygen and nitrogen measured in needles and tree rings represent an important tool to monitor the impact of NOx load on tree physiology
Stable and radiogenic isotopes as tracers for soil degradation
Full text linkSoil erosion seriously threatens mountain slopes. Direct measurement of soil erosion in the field is difficult and needs long term recording. However, stable and radiogenic isotopes are potential tracers for soil erosion estimation and would enable a quick and easy estimation of soil erosion. Soil degradation of upland soils influences nutrient content and biogeochemistry of wetlands. Our hypothesis was that due to the fact that oxic upland soils with a predominantly assimilatory metabolism differ significantly from anoxic wetland soils and riparian zones with dissimilatory metabolism, stable isotopes of carbon, nitrogen and oxygen can be used as potential tracers for the erosion of soil material from upland to wetland soils.
Our main objective was to test different stable (δ13C, δ15N and δ18O) and radiogenic (Cs-137) isotopes on their suitability for soil erosion detection and especially for detection of early stage soil erosion in alpine environments. Transects in stable isotope dynamics were determined from upland soils (oxic environment and erosion source) to wetlands (anoxic environments and sink of erosion material) as they often occur in alpine environments. Cs-137 which is a common tracer for soil erosion in lowlands was adapted for application in high alpine environments. Cs-137 provides information about the spatial distribution and the extent of soil erosion in the investigation area and could be used as a control for stable isotope measurements. A NaI in-situ spectrometer was calibrated for Cs-137 determination at steep mountain slopes. Small scale heterogeneity in Cs-137 distribution in the field is averaged out by in-situ measurements which integrate over a distinct area. Calibration was done by comparing Cs-137 activities measured with GeLi detector in the laboratory and with NaI in-situ spectrometry at the same site. A close correlation between the two methods proved the validity of the in-situ measurements of the NaI detector system. No dependency of Cs-137 on pH, clay content and carbon content was found but in-situ data must be corrected on soil moisture. In-situ measurements with a NaI detector provide, thus, an easy to handle and quick method for soil erosion determination in remote mountain areas.
For the use of stable isotopes to detect soil erosion in a transect from uplands to wetlands the prerequisite of differing isotopic signatures for upland and wetland soils must be met. This is given for all tested elements. Carbon isotopic signature (δ13C) and SOC content have shown to be a promising tool to reflect short and long term soil erosion processes. A close correlation of δ13C and carbon content in uplands indicates that depth profiles of δ13C mainly reflect decomposition of SOC. Long term disturbance of an upland soil is indicated by decreasing correlation of δ13C and SOC. Mixed signatures between typical upland and wetland δ13C values were measured for wetlands influenced by soil erosion indicating that soil input of upland soil material has happened recently. Similar mixed signatures were measured for δ15N at the site Seebodenalp indicating that δ15N can be used as a tracer for soil erosion. However, influence of farmyard manure on the stable isotope signature of wetland soils was identified as a factor changing δ15N signatures dramatically, so that erosion detection with δ15N was not possible in the Urseren Valley.
As oxygen is the most abundant element in soil, δ18O was tested analogue to δ13C as a tool for soil erosion detection. Soil δ18O measurements have not yet been done in the past, so that a pyrolysis temperature for the conversion of the sample in the TC/EA had to be determined first. A temperature of 1325 °C has shown to be suitable because at this temperature standard materials and soil samples yield a constant δ18O signature but quartz signature is still negligible. Soil δ18O signatures of uplands and wetlands were significantly different reflecting the proportion of organic and mineral constituents. First measurements at wetlands influenced by soil erosion indicate that δ18O might be used as erosion tracer parallel to δ13C. Cs-137 measurements in the Urseren Valley fully support δ13C erosion measurements and mostly δ18O results. This indicates that stable isotopes are indeed a suitable tool for tracking down soil erosion. Thus, both, qualitative and quantitative soil erosion information is available for single hillslopes. The combination of stable and radiogenic isotope data helps to better understand soil erosion processes and might enables developing an early warning system for soil erosion
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stable isotope dendroclimatology in the Swiss Alps: a 1200-year record from European larch: a 1200-year record from European larch
Full text linkTemperature versus species-specific influences on the stable oxygen isotope ratio of tree rings
Full text linkAssessing the carbon and water vapor fluxes in a temperate grassland using 13CO2 and H218O as system tracers
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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