ALBERT

Notes: [Auszug] Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration. But although severe regional heatwaves may become more frequent in a changing climate, their impact on terrestrial carbon cycling is unclear. Here we report measurements of ...

Notes: [Auszug] Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect ...

Notes: The double-exposure electron holographic technique has been put into practical use for the first time. By this method an accurate recording of the distribution of electric and magnetic fields can be directly obtained by the electron microscope without resorting to sophisticated optical manipulation of the holograms. Problems concerning the operative definition of the contour maps are discussed. Experimental results are presented.

Notes: This paper presents CO2 flux data from 18 forest ecosystems, studied in the European Union funded EUROFLUX project. Overall, mean annual gross primary productivity (GPP, the total amount of carbon (C) fixed during photosynthesis) of these forests was 1380 ± 330 gC m−2 y−1 (mean ±SD). On average, 80% of GPP was respired by autotrophs and heterotrophs and released back into the atmosphere (total ecosystem respiration, TER = 1100 ± 260 gC m−2 y−1). Mean annual soil respiration (SR) was 760 ± 340 gC m−2 y−1 (55% of GPP and 69% of TER).Among the investigated forests, large differences were observed in annual SR and TER that were not correlated with mean annual temperature. However, a significant correlation was observed between annual SR and TER and GPP among the relatively undisturbed forests. On the assumption that (i) root respiration is constrained by the allocation of photosynthates to the roots, which is coupled to productivity, and that (ii) the largest fraction of heterotrophic soil respiration originates from decomposition of young organic matter (leaves, fine roots), whose availability also depends on primary productivity, it is hypothesized that differences in SR among forests are likely to depend more on productivity than on temperature.At sites where soil disturbance has occurred (e.g. ploughing, drainage), soil espiration was a larger component of the ecosystem C budget and deviated fromthe relationship between annual SR (and TER) and GPP observed among the less-disturbed forests. At one particular forest, carbon losses from the soil were so large, that in some years the site became a net source of carbon to the atmosphere. Excluding the disturbed sites from the present analysis reduced mean SR to 660 ± 290 gC m−2 y−1, representing 49% of GPP and 63% of TER in the relatively undisturbed forest ecosystems.

Notes: The seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season.The net ecosystem production of the forest equals 472 g C m−2 y−1 while the gross ecosystem production 1016 g C m−2 y−1 and respiration 544 g C m−2 y−1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802 g C m−2 y−1.

Notes: Reliable models are required to assess the impacts of climate change on forest ecosystems. Precise and independent data are essential to assess this accuracy. The flux measurements collected by the EUROFLUX project over a wide range of forest types and climatic regions in Europe allow a critical testing of the process-based models which were developed in the LTEEF project. The ECOCRAFT project complements this with a wealth of independent plant physiological measurements. Thus, it was aimed in this study to test six process-based forest growth models against the flux measurements of six European forest types, taking advantage of a large database with plant physiological parameters.The reliability of both the flux data and parameter values itself was not under discussion in this study. The data provided by the researchers of the EUROFLUX sites, possibly with local corrections, were used with a minor gap-filling procedure to avoid the loss of many days with observations.The model performance is discussed based on their accuracy, generality and realism. Accuracy was evaluated based on the goodness-of-fit with observed values of daily net ecosystem exchange, gross primary production and ecosystem respiration (gC m−2 d−1), and transpiration (kg H2O m−2 d−1). Moreover, accuracy was also evaluated based on systematic and unsystematic errors. Generality was characterized by the applicability of the models to different European forest ecosystems. Reality was evaluated by comparing the modelled and observed responses of gross primary production, ecosystem respiration to radiation and temperature. The results indicated that: Accuracy. All models showed similar high correlation with the measured carbon flux data, and also low systematic and unsystematic prediction errors at one or more sites of flux measurements. The results were similar in the case of several models when the water fluxes were considered. Most models fulfilled the criteria of sufficient accuracy for the ability to predict the carbon and water exchange between forests and the atmosphere. Generality. Three models of six could be applied for both deciduous and coniferous forests. Furthermore, four models were applied both for boreal and temperate conditions. However, no severe water-limited conditions were encountered, and no year-to-year variability could be tested. Realism. Most models fulfil the criterion of realism that the relationships between the modelled phenomena (carbon and water exchange) and environment are described causally. Again several of the models were able to reproduce the responses of measurable variables such as gross primary production (GPP), ecosystem respiration and transpiration to environmental driving factors such as radiation and temperature. Stomatalconductance appears to be the most critical process causing differences in predicted fluxes of carbon and water between those models that accurately describe the annual totals of GPP, ecosystem respiration and transpiration.As a conclusion, several process-based models are available that produce accurate estimates of carbon and water fluxes at several forest sites of Europe. This considerable accuracy fulfils one requirement of models to be able to predict the impacts of climate change on the carbon balance of European forests. However, the generality of the models should be further evaluated by expanding the range of testing over both time and space. In addition, differences in behaviour between models at the process level indicate requirement of further model testing, with special emphasis on modelling stomatal conductance realistically.

Notes: Several electron microscopy techniques were used to investigate in detail the structure of the defects produced by implantation of Si with high current density As+ ion beam under self-annealing conditions (i.e., with simultaneous damage recovery activated by the transient beam heating). The defects, which display a basic octahedral shape, appear if the irradiation is prolonged after the time for which the transient beam heating has produced the complete crystallization of the previously amorphized surface layer. With increasing irradiation time and temperature they grow up to a maximum diameter of about 20 nm, and concentrate in a band located around the position of the maximum of nuclear energy loss of As+ ions, where a segregation peak of electrically inactive As is developed simultaneously. The results of different microstructural and microanalytical techniques used to investigate the defects (conventional and high resolution electron microscopy, electron holography, and energy dispersion x-ray microanalysis) give information which leads us to identify them as large vacancy aggregates, or voids, with As atoms segregated in proximity of the inner surfaces. The role of As atoms is likely to reduce the strain energy which a vacancy aggregate is expected to produce in the surrounding lattice.

Notes: Micrometric, irregularly shaped Fe particles with a nanocrystalline structure have been prepared by mechanical attrition through ball-milling. Electron holography has been employed to visualize the stray field emerging from isolated Fe particles, both at 300 K and at selected temperatures T≤1200 K, from which indirect information on the magnetic domain configuration has been inferred. By complementary x-ray diffraction and transmission electron microscopy investigations a relationship has been established between the changes of the leakage field and of the microstructure upon annealing: it indicates that the structural evolution is accompanied by strong modifications in the interior magnetization pattern. This relationship finds explanation in the framework of the random anisotropy model, including temperature-induced reversible variations in the exchange correlation length and saturation magnetization. Moreover, the role played by the overall geometrical features of the particles in the determination of the actual domain configuration has been investigated. © 2001 American Institute of Physics.

Notes: Electron holography is applied for the study of the leakage field of thin-film ferromagnetic tips used as probes in magnetic force microscopy. We used commercially available pyramidal tips covered on one face with a thin NiCo film, which were then placed in a high external magnetic field directed along the pyramid axis. Good agreement between simulated and experimental electron phase difference maps allows to measure the local flux from the ferromagnetic tips and therefore to evaluate the perturbation induced by the microprobe stray field on the sample area. © 1996 American Institute of Physics.