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Berlin Brandenburg

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  • 1
    Language: English
    In: Nature, 28 September 2011, Vol.477(7366), pp.547-8
    Description: On the global scale, the net flux is only a few per cent of the gross fluxes. Because small changes in photosynthesis and respiration can have big consequences for the net carbon uptake of terrestrial ecosystems, the interplay between photosynthesis and respiration must be well described in carbon-cycle models if they are to reliably project into the future.
    Keywords: El Nino-Southern Oscillation ; Atmosphere -- Chemistry ; Carbon Dioxide -- Analysis ; Oxygen Isotopes -- Analysis
    ISSN: 00280836
    E-ISSN: 1476-4687
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  • 2
    Language: English
    In: Nature, Sept 29, 2011, Vol.477(7366), p.547(2)
    Keywords: Carbon Cycle -- Environmental Aspects ; Carbon Cycle -- Research ; Atmospheric Carbon Dioxide -- Environmental Aspects ; Atmospheric Carbon Dioxide -- Research
    ISSN: 0028-0836
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  • 3
    In: Nature, 2011, Vol.477(7366), p.547
    ISSN: 0028-0836
    Source: Nature Publishing Group
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  • 4
    Language: English
    In: Journal of Hydrology, 2010, Vol.388(3), pp.438-455
    Description: We present a new isotopically enabled hydrologic scheme, “Soil–Litter–Iso”, suitable for use as part of an isotopically enabled land surface model. Soil–Litter–Iso is a one-dimensional model for coupled transport of heat, water and stable isotopes (HDO and H O) in soil and litter. It is sufficiently efficient for use at regional scale, yet includes the complexity of coupled heat and water transport enabling decomposition of the total moisture flux into liquid and vapour components. The numerical implementation is based on Ross’ fast solution to the Richards equation ( ). This, combined with the explicit solution of the energy and moisture equations at the soil/air interface, permit the isotopic calculations to be performed with thick soil layers and large times steps, resulting in significantly improved computational efficiency compared with existing isotopically-enabled soil models of similar complexity. We demonstrate the model’s numerical accuracy by conducting a series of established test-cases and comparing predictions of steady-state isotopic concentration profiles with corresponding analytical solutions. We also demonstrate the model’s operation within a land surface model by performing simulations for the forested flux site at Tumbarumba in south-eastern Australia. These simulations show that the total evapotranspiration (ET) flux, its components and their isotopic signatures are very sensitive to the inclusion of litter, and that the model is a useful tool for assessing when the isotopic signatures of the ET components are sufficiently distinct to be useful for flux partitioning.
    Keywords: Coupled Heat and Water Transport ; Water Isotopes ; Litter ; Evapotranspiration ; Soil Moisture ; Land Surface Model ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 5
    In: New Phytologist, September 2014, Vol.203(4), pp.1242-1252
    Description: The oxygen isotope signature of water is a powerful tracer of water movement from plants to the global scale. However, little is known about the short‐term variability of oxygen isotopes leaving the ecosystem via transpiration, as high‐frequency measurements are lacking. A laser spectrometer was coupled to a gas‐exchange chamber directly estimating branch‐level fluxes in order to evaluate the short‐term variability of the isotopic composition of transpiration (δE) and to investigate the role of isotopic non‐steady‐state transpiration under natural conditions in cork‐oak trees (Quercus suber) during distinct Mediterranean seasons. The measured δ18O of transpiration (δE) deviated from isotopic steady state throughout most of the day even when leaf water at the evaporating sites was near isotopic steady state. High agreement was found between estimated and modeled δE values assuming non‐steady‐state enrichment of leaf water. Isoforcing, that is, the influence of the transpirational δ18O flux on atmospheric values, deviated from steady‐state calculations but daily means were similar between steady state and non‐steady state. However, strong daytime isoforcing on the atmosphere implies that short‐term variations in δE are likely to have consequences for large‐scale applications, for example, partitioning of ecosystem fluxes or satellite‐based applications.
    Keywords: Isotopic Non‐Steady‐State Transpiration ; Isotopic Steady‐State Transpiration ; Laser Spectrometer ; Oxygen Isotopes ; Plant Transpiration ; Quercus Suber
    ISSN: 0028-646X
    E-ISSN: 1469-8137
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  • 6
    Language: English
    In: Forest Ecology and Management, 15 March 2015, Vol.340, pp.103-113
    Description: Leaf area index is a key vegetation parameter that can be used in soil–vegetation–atmosphere exchange modeling. To represent the structure of ecosystems in vertically distributed modeling, vertically resolved distributions as well as vertical and angular gap probability distributions are needed, but they are rarely available. On the experimental side, studies often neglect woody plant components when using indirect methods for or observations. This can lead to significantly biased results, particularly in semi-arid savannah-type ecosystems with low values. The objective of this study is to compare three non-destructive leaf area index measurement techniques in a sparse savannah-type cork oak canopy in central Portugal in order to derive vertically resolved as well as vertically and angularly resolved . We used the established LAI-2000 device as well as fast digital cover photography (DCP), which was vertically and angularly distributed. We applied object-based image analysis to DCP to exclude woody plant components. We compared the results with vertically distributed LAI-2000 measurements and with vertical estimates based on easily measurable crown parameters. Height and angularly distributed DCP was successfully applied here for the first time. It delivers gap probability and effective leaf area index measurements that are comparable to the established LAI-2000. The height and angularly dependent leaf clumping index could be determined with DCP, which led to a 30% higher total leaf area index for DCP compared to LAI-2000. The exclusion of woody tissue from DCP yields on average a 6.9% lower leaf area index . Including and excluding woody tissue, the of DCP matched precisely with direct measurements using litter traps. However, the set-up and site-specific adjustment of the image analysis algorithm remains challenging. We propose a special filter for LAI-2000 to enhance data quality when used in open canopies. Finally, if height-dependent observations are not feasible, ground-based observations of crown parameters can be used to derive very reasonable height distributions from a single, ground-based observation.
    Keywords: Leaf Area Index ; Gap Fraction ; Clumping Index ; Woody Tissue ; Lai-2000 ; Digital Cover Photography ; Forestry ; Biology
    ISSN: 0378-1127
    E-ISSN: 1872-7042
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  • 7
    Language: English
    In: Forest Ecology and Management, March 15, Vol.340, p.103(11)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.foreco.2014.12.026 Byline: Arndt Piayda, Maren Dubbert, Christiane Werner, Alexandre Vaz Correia, Joao Santos Pereira, Matthias Cuntz Abstract: * Height and angular dependent gap probability and leaf area index L. * Compare three methods: LAI-2000, plain photographs, photos with crown parameters. * Considering clumping increases L by 30% and excluding wood decreases L by 6.9%. * Photos with simple crown parameters yields very reasonable L height distributions. Article History: Received 15 October 2014; Revised 19 December 2014; Accepted 22 December 2014
    ISSN: 0378-1127
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: Journal of Hydrology, 24 July 2013, Vol.496, pp.142-153
    Description: Stable oxygen isotopes of water provide a valuable tracer for water movements within ecosystems and are used to estimate the contribution of transpiration to total ecosystem evapotranspiration ( ). We tested the Craig and Gordon equation against continuous field measurements of isotopic composition of evaporation and assessed the impact for partitioning evapotranspiration. Therefore, evaporation ( ) and its isotopic signature ( ) on bare soil plots, as well as evapotranspiration ( ) and its corresponding isotopic composition of ( ) of an herbaceous layer was measured with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We quantified the variation in arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature ( ) and the temperature at the evaporating site ( ), and the kinetic fractionation factor ( ). We could hence quantify based on measured , modeled from observed soil water isotopic composition at the evaporating site ( ), and modeled δ O of transpiration ( ) from observed total soil water isotopic composition. Our results demonstrate that predicting using the Craig and Gordon equation leads to good agreement with measured given that the temperature and O isotope profiles of the soil are thoroughly characterized. However, modeled is highly sensitive to changes in and as well as . This markedly affected the partition results of transpiration and evaporation from the total flux: The fraction of transpiration ( ) varied strongly using different formulations for and assuming steady or non-steady state transpiration. These findings provide a first comparison of laser-based and modeled isotopic compositions of evaporation based on the Craig and Gordon equation under field conditions. This is of special interest for studies using stable isotopes to separate soil evaporation and plant transpiration fluxes and highlights the need for a thorough characterization of the micrometeorological and isotopic constitution of the upper soil layer to locate the evaporating front with a resolution of a few cm soil depths. We also call on a better characterization of the kinetic fractionation factor of soil evaporation.
    Keywords: Craig and Gordon ; Laser Spectroscopy ; Evaporation ; Evapotranspiration ; Stable Oxygen Isotopes ; Kinetic Fractionation ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 9
    In: Journal of Geophysical Research: Atmospheres, 16 September 2003, Vol.108(D17), pp.n/a-n/a
    Description: We have built the first comprehensive global three‐dimensional model of δO in atmospheric CO. The constructed model goes beyond all other approaches made until now, by simulating the diurnal variations and transport of CO, δO of water, and δO of CO. The COO fluxes are thereby dependent on the atmospheric COO composition. We have validated the model surface processes, showing that it compares well to other estimates and measurements of NPP, NEE, and stomata‐internal CO mixing ratio (), except for high northern latitudes. Here, the model is considerably lower in NPP and higher in than other model estimates. However, estimates derived indirectly from observations tend to support our model findings. The water isotopes of rain are reproduced very well at all latitudes. The soil bucket model used in the model integrates incoming rain in one single value. The bucket approach overattenuates the isotopic variations of rain, and hence our isotopic source signature of respiration shows almost no seasonal cycle and is thus isotopically too depleted during summer.
    Keywords: Isotope Model ; O In Co ; O In Ho ; Ecosystem Model ; Biosphere‐Atmosphere Exchange ; Isotope Discrimination
    ISSN: 0148-0227
    E-ISSN: 2156-2202
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  • 10
    In: Journal of Geophysical Research: Atmospheres, 16 September 2003, Vol.108(D17), pp.n/a-n/a
    Description: We have modeled the distribution of δO in atmospheric CO with a new comprehensive global three‐dimensional model. We have focused in this study on the seasonal cycle and the meridional gradient in the atmosphere. The model has been compared with a data set of δO‐CO, which merges measurements made by different laboratories, with allowance for recently elucidated calibration biases. The model compares well with the seasonal cycle of CO, but advances the measured δO‐CO seasonal cycle by two months. The calculated seasonal amplitude is typically 2/3 of the measured value, but the sensitivity to uncertainties in the input parameter set is such that a range of amplitudes over a factor of 3 is accommodated. Unlike the case for the amplitude, the sensitivity analyses demonstrate that the modeled phase of the seasonal cycle and the north‐south gradient are practically unaffected by uncertainty in the parameter set. The north‐south gradient comes, on the one hand, from the disequilibrium of the δO‐CO isofluxes at every grid point and, on the other hand, from rectification gradients, a covariance of the varying δO‐CO source with the atmospheric transport. The model exhibits a very strong rectification gradient that can lead to a misinterpretation of the measurements compared to the model. We therefore restrict comparison to the latitudinal means of only ocean grid cells with measurements from stations sampling the marine boundary layer. Assimilation and respiration are the determining factors of the seasonal cycle and the north‐south gradient of δO‐CO. In a number of sensitivity studies we have explored the range of possible processes affecting the simulated seasonal cycle and hemispheric gradient. None of these processes contributed significantly to improve the model‐observation mismatch. The contribution of assimilation and respiration to the total signal does change significantly in the sensitivity studies, but, because of feedback processes, they change in such a way that the overall response of the model is only marginally altered. In particular, prescribing δO‐HO soil values to monthly means of rain does not significantly change the modeled signal, either in the seasonal cycle or in the meridional gradient. This highlights the need to accurately model assimilation and respiration in order to understand δO in atmospheric CO.
    Keywords: Isotope Model ; O In Co ; Isotope Feedback Mechanism ; Rectifier Effect ; Global Model
    ISSN: 0148-0227
    E-ISSN: 2156-2202
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