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  • 1
    Electronic Resource
    Electronic Resource
    Climatic and ecological determinants of leaf lifespan in polar forests of the high CO2 Cretaceous ‘greenhouse’ world (2005)
    Oxford, UK : Blackwell Science Ltd
    Global change biology 11 (2005), S. 0 
    Details
    ISSN: 1365-2486
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Notes: Polar forests once extended across the high-latitude landmasses during ice-free ‘greenhouse’ intervals in Earth history. In the Cretaceous ‘greenhouse’ world, Arctic conifer forests were considered predominantly deciduous, while those on Antarctica contained a significantly greater proportion of evergreens. To investigate the causes of this distinctive biogeographical pattern, we developed a coupled model of conifer growth, soil biogeochemistry and forest dynamics. Our approach emphasized general relationships between leaf lifespan (LL) and function, and incorporated the feedback of LL on soil nutrient status. The model was forced with a mid-Cretaceous ‘greenhouse’ climate simulated by the Hadley Centre GCM. Simulated polar forests contained mixtures of dominant LLs, which reproduced observed biogeographical patterns of deciduous, mixed and evergreen biomes. It emerged that disturbance by fire was a critical factor. Frequent fires in simulated Arctic ecosystems promoted the dominance of trees with short LLs that were characterized by the rapid growth and colonization rates typical of today's boreal pioneer species. In Antarctica, however, infrequent fires allowed trees with longer LLs to dominate because they attained greater height, despite slower growth rates. A direct test of the approach was successfully achieved by comparing modelled LLs with quantitative estimates using Cretaceous fossil woods from Svalbard in the European Arctic and Alexander Island, Antarctica. Observations and the model both revealed mixed Arctic and evergreen Antarctic communities with peak dominance of trees with the same LLs. Our study represents a significant departure from the long-held belief that leaf habit was an adaptation to warm, dark winter climates, and highlights a previously unrecognized role for disturbance (in whatever guise) in polar forest ecology.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2486.2005.001068.x
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  • 2
    Electronic Resource
    Electronic Resource
    Seperable models for the EEG and ERP
    Amsterdam : Elsevier
    International Journal of Psychophysiology 11 (1991), S. 83 
    Details
    ISSN: 0167-8760
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Medicine , Psychology
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0167-8760(91)90347-Z
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  • 3
    Electronic Resource
    Electronic Resource
    ERP component estimation
    Amsterdam : Elsevier
    International Journal of Psychophysiology 11 (1991), S. 69 
    Details
    ISSN: 0167-8760
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Medicine , Psychology
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0167-8760(91)90290-E
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  • 4
    Electronic Resource
    Electronic Resource
    Weather regimes in past climate atmospheric general circulation model simulations (1999)
    Springer
    Climate dynamics 15 (1999), S. 773-793 
    Details
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract  We investigate the climates of the present-day, Inception of the Last Glaciation (115 000 y ago) and Last Glacial Maximum (21 000 y ago) in the extra-tropical north Atlantic and Europe, as simulated by the Laboratoire de Météorologie Dynamique Atmospheric General Circulation Model. We use these simulations to investigate the low-frequency variability of the model in different climates. The aim is to evaluate whether changes in the intraseasonal variability, which we characterize using weather regimes, can help describe the impact of different boundary conditions on climate and give a better understanding of climate change processes. Weather regimes are defined as the most recurrent patterns in the 500 hPa geopotential height, using a clustering algorithm method. The regimes found in the climate simulations of the present-day and inception of the last glaciation are similar in their number and their structure. It is the regimes’ populations which are found to be different for these climates, with an increase of the model’s blocked regime and a decrease in the zonal regime at the inception of the last glaciation. This description reinforces the conclusions from a study of the differences between the climatological averages of the different runs and confirms the northeastward shift to the tail of the Atlantic storm-track, which would favour more precipitation over the site of growth of the Fennoscandian ice-sheet. On the other hand, the Last Glacial Maximum results over this sector are not found to be classifiable, showing that the change in boundary conditions can be responsible for severe changes in the weather regime and low-frequency dynamics. The LGM Atlantic low-frequency variability appears to be dominated by a large-scale retrogressing wave with a period 40 to 50 days. In addition weather regimes are found in a sector located further eastward over the east Atlantic and European continent and are proved to be linked to this low-frequency oscillation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003820050315
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  • 5
    Electronic Resource
    Electronic Resource
    Nonlinear EEG analysis based on a neural mass model (1999)
    Springer
    Biological cybernetics 81 (1999), S. 415-424 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract. The well-known neural mass model described by Lopes da Silva et al. (1976) and Zetterberg et al. (1978) is fitted to actual EEG data. This is achieved by reformulating the original set of integral equations as a continuous-discrete state space model. The local linearization approach is then used to discretize the state equation and to construct a nonlinear Kalman filter. On this basis, a maximum likelihood procedure is used for estimating the model parameters for several EEG recordings. The analysis of the noise-free differential equations of the estimated models suggests that there are two different types of alpha rhythms: those with a point attractor and others with a limit cycle attractor. These attractors are also found by means of a nonlinear time series analysis of the EEG recordings. We conclude that the Hopf bifurcation described by Zetterberg et al. (1978) is present in actual brain dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004220050572
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  • 6
    Electronic Resource
    Electronic Resource
    Simulations of the Last Glacial Maximum climates using a general circulation model: prescribed versus computed sea surface temperatures (1998)
    Springer
    Climate dynamics 14 (1998), S. 571-591 
    Details
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract  The climate during the Last Glacial Maximum (LGM) has been simulated using the UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (GCM) with both prescribed sea surface temperatures (SSTs) based on the CLIMAP reconstruction and computed SSTs with a simple thermodynamic slab ocean. Consistent with the Paleoclimate Modelling Intercomparison Project (PMIP), the other boundary conditions include the large changes in ice-sheet topography and geography, a lower sea level, a lower concentration of CO2 in the atmosphere, and a slightly different insolation pattern at the top of the atmosphere. The results are analysed in terms of changes in atmospheric circulation. Emphasis is given to the changes in surface temperatures, planetary waves, storm tracks and the associated changes in distribution of precipitation. The model responds in a similar manner to the changes in boundary conditions to previous studies in global mean statistics, but differs in its treatment of regional climates. Results also suggest that both the land ice sheets and sea ice introduce significant changes in planetary waves and transient eddy activity, which in turn affect regional climates. The computed SST simulations predict less sea ice and cooler tropical temperatures than those based on CLIMAP SSTs. It is unclear as to whether this is a model and/or a data problem, but the resulting changes in land temperatures and precipitation can be large. Snow mass budget analysis suggests that there is net ice loss along the southern edges of the Laurentide and Fennoscandian ice sheets and net ice gain over other parts of the two ice sheets. The net accumulation is mainly due to the decrease in ablation in the cold climate rather than to the changes in snowfall. The characteristics of the Greenland ice-sheet mass balance in the LGM simulations is also quite different from those in the present-day (PD) simulations. The ablation in the LGM simulations is negligible while it is a very important process in the ice mass budget in the PD simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003820050242
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  • 7
    Electronic Resource
    Electronic Resource
    Synoptic-scale perturbations in AGCM simulations of the present and Last Glacial Maximum climates (2000)
    Springer
    Climate dynamics 16 (2000), S. 517-533 
    Details
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract  The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes at the Last Glacial Maximum (LGM, 21 000 years ago) are very different from the present ones: this period is characterised by a general cooling of the extra-tropics, with massive ice sheets over the Northern Hemisphere continents and sea-ice extending very far south over the North Atlantic. The present work uses regression analysis to study the characteristics of the synoptic eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (AGCM) of the UK Universities' Global Atmospheric Programme (UGAMP). In the LGM experiment, the structure of the Pacific eddies is similar to the present-day (PD) situation, but they are weaker. On the other hand, the Atlantic eddies show an increased zonal wavelength and a much shallower structure in the temperature and vertical wind perturbations. To understand the changes of these characteristics from present-day to LGM, we compare them to those computed for the most unstable modes of the corresponding mean flows, determined using a dry primitive equation model. A normal-mode stability analysis is carried both on zonally symmetric and asymmetric flows for each of the Northern Hemisphere storm-tracks. The changes in the most unstable normal modes found by both these analyses give a good account of changes in the structure of the perturbations as retrieved from the AGCM, suggesting that changes in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of the present-day Atlantic storm-track, the growth rate of these modes is found to be very low compared to the other cases. A complementary analysis evaluates the importance of non-modal growth, in the form of downstream development of perturbations, for each of the storm-tracks. This type of growth is found to be especially important in the case of the present-day Atlantic storm-track.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00007923
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  • 8
    Electronic Resource
    Electronic Resource
    Local Linearization method for the numerical solution of stochastic differential equations (1996)
    Springer
    Annals of the Institute of Statistical Mathematics 48 (1996), S. 631-644 
    Details
    ISSN: 1572-9052
    Keywords: Stochastic differential equations ; numerical solution ; local linearization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The Local Linearization (LL) approach for the numerical solution of stochastic differential equations (SDEs) is extended to general scalar SDEs, as well as to non-autonomous multidimensional SDEs with additive noise. In case of autonomous SDEs, the derivation of the method introduced gives theoretical support to one of the previously proposed variants of the LL approach. Some numerical examples are given to demonstrate the practical performance of the method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00052324
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  • 9
    Electronic Resource
    Electronic Resource
    Biochemical and physiological parameters and estimated work output in draught horses pulling loads for long periods (1992)
    Springer
    Veterinary research communications 16 (1992), S. 231-246 
    Details
    ISSN: 1573-7446
    Keywords: biochemistry ; draught animals ; exercise ; horses ; physiology ; respiration ; serum ; work
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract A study was undertaken in five draught horses of 648±33 kg body weight to find the effects of continuously pulling loads on their cardiovascular, respiratory and metabolic responses. A cart equipped with an odometer, for measuring distance, and a hydraulic dynamometer, for measuring draught force, was used. Heart and respiration rates and rectal temperatures were recorded. Blood samples for measuring arterial and venous pH and blood gases, haemoglobin, glucose and lactic acid concentrations and the serum activity of the enzymes creatine phosphokinase (CK), lactate dehydrogenase, aspartate aminotransferase and alkaline phosphatase were taken before exercise and immediately after each journey (morning and afternoon) of the daily work. Draught exercise, with loads which generated forces of between 0.57 and 0.59 kN, at speeds of 1.60 to 2.11 m/s, for 8 h daily for five consecutive days, with resting intervals of 10 min each hour, was well tolerated. Exercise tolerance was evaluated from the recovery from the changes observed in the biochemical and physiological parameters induced by the work. The analysis of these showed that, when the horses were subjected to prolonged periods of resting, their loss of fitness for work was shown by significant increases in the serum activity of muscle-derived enzymes and in blood lactate concentrations during the first day of work. However, over the following days the horses adapted to the work, so that the decreases in serum enzyme activities and blood lactate concentrations were reduced. Since similar observations have been described for racehorses, the determination of blood lactate concentrations and the serum activities of muscle-derived enzymes, specifically CK, seem to be good indicators of fitness in draught horses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01839160
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  • 10
    Electronic Resource
    Electronic Resource
    High resolution quantitative EEG analysis (1994)
    Springer
    Brain topography 6 (1994), S. 211-219 
    Details
    ISSN: 1573-6792
    Keywords: EEG spectrum ; Normality ; Statistics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary High resolution spectral methods are explored as an alternative to broad band spectral parameters (BBSP) in quantitative EEG analysis. In a previous paper (Valdes et al. 1990b) regression equations (“Developmental surfaces”) were introduced to characterize the age-frequency distribution of the mean and standard deviation of the log spectral EEG power in a normative sample. These normative surfaces allow the calculation of z transformed spectra for all derivations of the 10/20 system and z maps for each frequency. Clinical material is presented that illustrates how these procedures may pinpoint frequencies of abnormal brain activity and their topographic distribution, avoiding the frequency and spatial “smearing” that may occur using BBSP. The increased diagnostic accuracy of high resolution spectral methods is demonstrated by means of receiver operator characteristic (ROC) curve analysis. Procedures are introduced to avoid type I error inflation due to the use of more variables in this type of procedure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01187711
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