Step-like Change Research Articles

Spin-singlet dimerization in La2RuO5investigated using magnetic susceptibility and specific heat measurements

The origin of spin-dimerization and concomitant spin-gap opening in the triclinic phase of poly- and single-crystalline La${}_{2}$RuO${}_{5}$ at unusually high temperatures was investigated using magnetic susceptibility and specific-heat measurements. From the low-temperature crystal structure the formation of antiferromagnetically coupled Ru${}^{4+}$ ($S=1$) dimers within the quasi-two-dimensional magnetic system can be deduced, resulting in a nonmagnetic singlet state. It was found that the antiferromagnetic coupling within the dimers is much stronger than the interaction with neighboring dimers. La${}_{2}$RuO${}_{5}$ exhibits a step-like change in the magnetic susceptibility at 161 K, indicating a first-order transition of combined magnetic and structural character. The size of the spin-gap has been estimated from the thermally activated behavior in the low-temperature dimerized phase and was found to be significantly different in the polycrystalline sample when compared to the results obtained from the single crystals. The magnetic entropy obtained from specific-heat measurements amounts to roughly $0.5R\mathrm{ln}(3)$, reflecting solely the contribution of spin degrees of freedom to the entropy change during the phase transition.

Crystal structure, morphotropic phase transition and luminescence in the new cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu

A new series of promising luminescent materials, cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu, has been synthesized via a solid-state reaction. X-ray and neutron powder diffraction studies show that these oxides crystallize in the monoclinic crystal system (S.G. C2/c, Z=4) and have a morphotropic phase transition between Er and Tm compounds followed by a step-like change of the unit cell constants. Isolated [Si3O9] rings located in layers are basic building units and stack with Sr/R layers along the [101¯] direction. The rare earth atoms are distributed among three independent Sr/R sites coordinated by 8, 7 and 6 oxygen atoms, and the Sr-R populations change from mixed to 0.5/0.5 over site (1) and full occupation of sites (2) and (3) by Sr and R, respectively, at the transition. Changes of the conformation and mutual arrangement of [Si3O9] rings, as well as exchange of oxygen atoms from the first and the second coordination sphere of two Sr/R sites also feature the phase transition. Luminescence in Sr3Y2(Si3O9)2:Eu3+ under ultraviolet (UV) excitation has been discussed.

Dephasing by a continuous-time random walk process

Stochastic treatments of magnetic resonance spectroscopy and optical spectroscopy require evaluations of functions such as (exp(i ∫(0)(t) Q(s)ds)), where t is time, Q(s) is the value of a stochastic process at time s, and the angular brackets denote ensemble averaging. This paper gives an exact evaluation of these functions for the case where Q is a continuous-time random walk process. The continuous-time random walk describes an environment that undergoes slow steplike changes in time. It also has a well-defined Gaussian limit and so allows for non-Gaussian and Gaussian stochastic dynamics to be studied within a single framework. We apply the results to extract qubit-lattice interaction parameters from dephasing data of P-doped Si semiconductors (data collected elsewhere) and to calculate the two-dimensional spectrum of a three-level harmonic oscillator undergoing random frequency modulations.

Structural phase transitions in Ti-doped Bi1-xNdxFeO3 ceramics

Recently, it was demonstrated that donor doping with Ti on the B-site significantly reduces the conductivity in Bi0.85Nd0.15FeO3 ceramics [Kalantari et al., Adv. Funct. Mater. 21, 3737 (2011)]. In this contribution, the phase transitions as a function of Nd concentration are investigated in 3% Ti doped Bi1-xNdxFeO3 ceramics. Paraelectric (PE) to ferroelectric (FE) transitions were observed for compositions with x ≤ 0.125 which manifested themselves as peaks in permittivity. In contrast, PE to antiferroelectric (AFE) transitions for 0.15 ≤ x ≤ 0.20 gave rise to a step-like change in the permittivity with x = 0.25 exhibiting no sharp anomalies and remaining PE until room temperature. The large volume change at the PE to FE/AFE transitions, reported by Levin and co-workers [Phys. Rev. B 81, 020103 (2011)] and observed here by dilatometry, coupled with their first-order character constrain the transitions to occur uniformly throughout the material in an avalanche-like manner. Hence, the anomalies in DSC, permittivity and thermal expansion occur over a commensurately narrow temperature interval. However, despite the large volume change and eye-catching anomalies in DSC, the latent heats for the transitions in Ti-doped Bi1-xNdxFeO3 are similar to Pb(Zr,Ti)O3 (1–3 kJ/mol) with each an order of magnitude greater than BaTiO3 (∼0.2 kJ/mol). A broad frequency dependent dielectric anomaly of unknown origin in the temperature range 250–450 °C was also observed in all samples.

A QUASI-LINEAR VISCOELASTIC MODEL FOR THE PASSIVE PROPERTIES OF THE HUMAN HIP JOINT

Properties of passive elastic structures constituting the human hip joint can be exploited to increase efficiency of human locomotion. As studies estimating the passive contributions to the net joint moment often disregard damping properties of the joint such contributions overestimate the energy gained during leg retraction within swing and stance phase. We built an experimental apparatus to measure moment-angle-relations during motor guided cyclic movements over a wide range of angular velocities and step-like changes in hip angle. On the basis of the experimentally gained data set the objective of this study was to model the elastic as well as the damping characteristics of the joint in the sagittal plane utilizing the Quasi-Linear Viscoelastic theory (QLV). A double exponential function was conveniently employed to describe the elastic response. The dependency of the hip joint stiffness on biarticular muscles was incorporated by repeating the measurement protocol for different knee angles. Due to the fact that the stiffness characteristics of the elastic response were merely shifted over knee angles we introduced an equilibrium angle at the hip joint as exponential function of the knee angle eventually yielding an elastic response as a function of hip and knee angle. In order to cover the damping characteristics the reduced relaxation function comprising a continuous spectrum of relaxation was utilized. We exemplify the applicability of the QLV model on published kinematic data on human walking and estimated that approximately 27% of the energy passively stored at the hip dissipates during the gait cycle.

Cholesterol-Induced Variations in the Domain Structure Fluctuations and Microdynamics of Lipid Membranes

Broadband ultrasonic attenuation spectra (100 kHz to 2 GHz) of aqueous solutions of vesicles from 1,2-dimyristoyl-l-3-phosphatidylcholine, with different amounts of cholesterol admixed, have been measured at temperatures between 20 and 28 °C. The spectra have been evaluated in terms of suitable relaxation functions. They are discussed in view of the effect of cholesterol on the membrane behavior around the gel-fluid phase transition temperature T(m). In addition to a frequency-independent asymptotic high-frequency term, all spectra reveal a critical term and a Debye-type relaxation term with relaxation time around 0.5 ns. The former is evaluated in the light of the Bhattacharjee-Ferrell dynamic scaling theory. It is assigned to the critical domain structure fluctuations of the membranes. Critical slowing of fluctuations is demonstrated. Also shown are relations of the critical amplitudes to thermodynamic parameters. The Debye term reflects the rotational isomerization of the phospholipid alkyl chains. The relaxation time of isomerization reveals a significant steplike change at T(m). At moderate cholesterol content an additional Debye relaxation term exists. It is assigned to the axial diffusion of the membrane molecules. Because it likewise shows effects of slowing near T(m), the diffusion appears to be coupled to the domain structure fluctuations. A further relaxation term at small cholesterol concentration is assumed to be due to small-range shape fluctuations of vesicles near the phase transition temperature.

Ambulance call-outs and ‘disruptive technology’

Disruptive technology is a term coined by Harvard Business School professor Clayton M. Christensen to describe a new technology that unexpectedly displaces an established one. His 1997 best-selling book, The Innovator’s Dilemma separates new technology into two categories: sustaining and disruptive. Sustaining technology relies on incremental improvements to an established technology. Disruptive technology lacks refinement, often has performance problems because it is new, appeals to a limited audience, and may not yet have a proven practical application. For example, Alexander Graham Bell’s ‘electrical speech machine’, now the telephone. Of relevance to healthcare is the fact that large corporations (i.e. Department of Health) are designed to work with sustaining technologies. They excel at knowing their market, staying close to their customers, and having a mechanism in place to develop existing technology. Conversely, they have trouble capitalising on the potential efficiencies, cost-savings, or new marketing opportunities created by low-margin disruptive technologies. Christensen demonstrates how it is not unusual for a big corporation to dismiss the value of a disruptive technology because it does not reinforce current company goals, only to be blindsided as the technology matures, gains a larger audience and market share and threatens the status quo (http://tinyurl.com/38lury5). Over the last three years, a series of articles in BJHCM have introduced the conceptual equivalent to ‘disruptive technology’ into our understanding of how healthcare demand behaves through time (see Jones, 2009; 2010; 2011a–b; 2012 for further references). The existing incremental technology (demographic-based growth) has been demonstrated to fail in the real world and unheard of ‘disruptive’ concepts such as ‘cycles’, ‘step-like changes’ and ‘wavelets’ have been proposed to offer better insight into how costs behave over time. The combined and interactive effects of the environment (weather, air quality, infectious outbreaks) have been proposed as a major contributing factor in these disruptive changes and explain why it is almost impossible to run a balanced healthcare budget. Hence, changing commissioners from primary care trusts (PCTs) to clinical commissionig groups (CCGs) is mere window dressing based on the old incremental ‘technology’ and its hidden assumptions. For those of you who are bewildered, Figure 1 gives an illustration of the issues at stake. In this figure, weekly ambulance call outs across England have been compared pair-wise for the same week of the year, i.e. first week Figure 1. Category A+B ambulance call-outs for England

Conformational Effects in Non-Stoichiometric Complexes of Two Hyperbranched Molecules with a Linear Polyelectrolyte

We report results from Brownian dynamics computer simulations of systems comprised by two terminally charged hyperbranched molecules preferentially branched in the periphery, with an oppositely charged linear chain of varying length. Comparison of the findings from the present study to stoichiometric counterparts and to analogous dendrimer-based complexes, reveal that the presence of the second hyperbranched molecule incurs significant changes in the conformational characteristics of both components of the complex. Instead of step-like changes in the average size and shape of the hyperbranched component that were noted in the previously studied stoichiometric systems, a rather smooth change is observed upon increase of the length of the linear component. In addition, a markedly different behavior is also noticed in the conformational characteristics of the linear chain when compared to that in similar dendrimer-based systems. The above findings are consistent with the higher degree of deformability of the peripherally branched molecules which allow appropriate rearrangements in shape in order to accommodate the favorable Coulombic interactions between the two components of the complex. This behavior offers new insight towards the design of more efficient hyperbranched-based systems which can take advantage of the multifunctionality and the structural properties of the highly branched polymer components.

Region‐wide changes in marine ecosystem dynamics: state‐space models to distinguish trends from step changes

AbstractRegime shifts are sudden changes in ecosystem structure that can be detected across several ecosystem components. The concept that regime shifts are common in marine ecosystems has gained popularity in recent years. Many studies have searched for the step‐like changes in ecosystem state expected under a simple interpretation of this idea. However, other kinds of change, such as pervasive trends, have often been ignored. We assembled over 300 ecological time series from seven UK marine regions, covering two to three decades. We developed state‐space models for the first principal component of the time series in each region, a common measure of ecosystem state. Our models allowed both trends and step changes, possibly in combination. We found trends in three of seven regions and step changes in two of seven regions. Gradual and sudden changes are therefore important trajectories to consider in marine ecosystems.

Strain compatibility and fault linkage in relay zones on normal faults

Relay zones on normal faults are unlikely to have tabular geometries as depicted in idealised models. Rotation of a relay ramp between non-parallel and non-planar relay-bounding faults will inevitably lead to strain compatibility problems causing open gaps or overlaps within the relay zone. Linkage of relay-bounding faults does not evolve from a single branch point. Rather, linkage occurs at multiple points along the fault tip lines giving rise to initially discontinuous branch lines. Where linkage occurs along a discontinuous slip-aligned branch line, displacement at different levels within the relay zone is partitioned between variable amounts of ramp rotation and slip across the branch line. The linking fault propagates when strain compatibility can no longer be maintained by continuous deformation processes, such as thickening or thinning of incompetent layers within the relay ramp. Step-like changes in vertical displacement vs. distance (d − x) profiles on horizons containing apparently intact relay ramps are probably indicative of incipient breaching and can be used predict the presence of a slip-aligned branch line in the sub-surface. Despite the complexity of the strain distribution within relay zones, the total vertical displacement across the relay remains geometrically coherent at all levels.

High Temperature Mechanical Spectrum and Interstitial Oxygen in YBaCuFeO<sub>5+δ</sub>

Double perovskite oxide YBaCuFeO5+δ samples (labeled as “as-prepared”) were sintered by a solid-state reaction technique. Mechanical spectra of as-prepared sample were measured under vacuum condition from room temperature up to 720K by the reed vibration method. In the first heating run, two internal friction peaks were observed around 380 and 620K (labeled as P1 and P2, respectively) in YBaCuFeO5+δ. In the subsequent cooling run, both of them disappeared and a small step-like decrease of the internal friction around 500K was found. This step-like change was reproduced in the following thermal cycles. Thermogravimetric analysis (TGA) was also performed in order to monitor the oxygen content of the sample. It showed that the interstitial oxygen concentration decreased in the first heating run. The interstitial oxygen relaxation is suggested to be the origin of P1 peak. In dielectric measurements, a dielectric loss peak appeared around the temperature of the P2 internal friction peak and the peak temperature was independent on the measuring frequency. It is expected that P2 is associated with some kind of transitions of the interstitial oxygen. Further mechanical loss measurement of the annealed sample and scanning electron microscopy characterizing the as-prepared and annealed samples, provided more information on the interstitial oxygen in YBaCuFeO5+δ samples.

On the possibility to determine the electrical conductivity of 67P/CG from ROSETTA magnetic field observations

The ROSETTA mission to comet 67P/Churyumov-Gerasimenko presents us for the first time the opportunity to study the interior of a cometary nucleus. Information about the electrical conductivity and its distribution inside the nucleus can be derived by measuring the magnetic fields induced in the nucleus by variations of the interplanetary magnetic field. Due to the relatively small size of the nucleus, this task is not trivial and requires very precise magnetic field measurements.Here we analyze the response of a conductive nucleus to transient step-like magnetic field changes as well as the response in frequency domain to electromagnetic waves. For the nucleus we use a spherical model with either uniform conductivity or consisting of a core and an outer layer with different conductivities. The space outside of the nucleus is modeled either by highly conductive solar wind plasma or by zero conductivity void.Various combinations of conductivities, radii of the nucleus and of the core, and properties of the space outside the nucleus result in different inductive responses. Several such combinations which could be encountered by ROSETTA are studied in detail in order to estimate which conductivity ranges and core radii can be determined using the ROSETTA instruments and which mission operations would facilitate such measurements.

Role of variability in determining the vertical wind speeds and structure

[1] The University of Michigan's Global Ionosphere Thermosphere Model (GITM) is used to study the temporal and spatial variability of high-latitude vertical winds during the December solstice. Underlying mechanisms controlling the high-latitude distribution and the magnitude of vertical winds in the thermosphere are investigated in a suite of systematic model simulations. First, in a series of six model simulations GITM longitude × latitude resolution is gradually increased from 5° × 5° to 2.5° × 0.3125°, imposing constant moderate solar and low magnetospheric activity in all simulations. Analysis of the high-latitude mean parameters shows that polar distributions of vertical winds and Joule heating demonstrate localized enhancements at high spatial resolution that are not well captured at coarse grid resolution. Second, in simulations with fixed spatial resolution of 2.5° × 2.5°, the impact of temporally variable magnetospheric conditions on the morphology of the high-latitude vertical winds has been investigated. For this, hemispheric power and the cross polar cap potential values are modified in a series of systematic simulations. The magnitude and temporal variations of ion flows significantly impact the high-latitude Joule heating, which in turn dramatically effects the vertical wind structure. It is demonstrated that variability in the vertical winds are driven by variability in the ion flows and that sudden step-like changes in ion flows can cause the largest enhancements in the vertical winds, in particular, via nonhydrostatic effects. The vertical wind variability is seen to be the largest in the summer Southern Hemisphere with much larger vertical wind magnitudes.

Magnetic and quadrupole effects at the crossover in a strong magnetic field in rare-earth tetragonal compounds

Anomalies of the magnetic and magnetoelastic properties of rare-earth (RE) tetragonal paramagnetic materials with zircon and scheelite structures resulting from the interaction of the energy levels (crossover) of an RE ion in a strong magnetic field up to 400 kOe are discussed. Using the example of HoPO4, the specific features of magnetic properties are compared for a crossover with and without a gap in the magnetic field along the [100] and [110] axes. Anomalies of linear magnetostriction at a crossover in a strong magnetic field with DyPO4 are discussed. Calculations within the formalism of the crystal field are shown to give an adequate description of magnetic and magnetoelastic anomalies and critical parameters of the crossover of the investigated crystals when using known interaction parameters, under the assumption of an adiabatic magnetization process in pulsed fields. Taking HoVO4 as example, the anomalous magnetocaloric effect near the crossover is discussed when a crystal is cooled to temperatures of approximately 0.1 K as the field increases, with the starting temperature being Tst = 4.2 K. The step-like change of the contribution of the quadrupole interactions in a number of the crystals caused by the change of the quadrupole moments at the crossover is noted as an analogue of the induced quadrupole transition.

Synthesis and applicable condition of vector wave envelopes in layered random elastic media with anisotropic autocorrelation function based on the Markov approximation

SUMMARY The Markov approximation is a powerful stochastic method for the direct synthesis of wave envelopes in random velocity fluctuated media when the wavelength is shorter than their correlation distance. To apply the Markov approximation to realistic cases, we consider horizontal layered random media characterized by an anisotropic autocorrelation function (ACF), where different layers have different randomnesses and different background velocities having step-like changes. Solving the parabolic master equation for the two frequency mutual coherence function (TFMCF) in random elastic media for the vertical incidence of a plane wavelet from the bottom, we calculate the angular spectrum just before the first velocity boundary. Multiplying transmission or PS conversion coefficients of the boundary by the angular spectrum, we calculate the angular spectrum and TFMCF on the other side of the boundary. Then we solve the master equation for the forward propagating wavelet in the second layer. Taking the same procedure for each layer boundary, we finally obtain the mean square (MS) vector envelopes on the free surface on the top layer. For the practical simulation, we use 2-D random media characterized by a Gaussian ACF. We numerically confirm the validity of the envelope synthesis for a specific case of layered random media with anisotropic ACF by comparing with finite difference (FD) simulations of elastic waves. Considering the Earth structure, the horizontal correlation distance is larger than the vertical one and the velocity fractional fluctuation becomes weak as depth increases, the Markov approximation is good for modelling the primary wavelet and also the converted wavelet for the vertical incident wavelet. We derive an applicable range of the Markov approximation for random media with anisotropic ACF by comparing with FD simulations. The results show that the Markov approximation is accurate when the wavelength is comparable or shorter than the both of vertical and horizontal correlation distances and the MS fractional fluctuation is much smaller than the ratio of squared horizontal correlation distance to the product of the vertical correlation distance and propagation distance.

Representation of Dynamical Stimuli in Populations of Threshold Neurons

Many sensory or cognitive events are associated with dynamic current modulations in cortical neurons. This raises an urgent demand for tractable model approaches addressing the merits and limits of potential encoding strategies. Yet, current theoretical approaches addressing the response to mean- and variance-encoded stimuli rarely provide complete response functions for both modes of encoding in the presence of correlated noise. Here, we investigate the neuronal population response to dynamical modifications of the mean or variance of the synaptic bombardment using an alternative threshold model framework. In the variance and mean channel, we provide explicit expressions for the linear and non-linear frequency response functions in the presence of correlated noise and use them to derive population rate response to step-like stimuli. For mean-encoded signals, we find that the complete response function depends only on the temporal width of the input correlation function, but not on other functional specifics. Furthermore, we show that both mean- and variance-encoded signals can relay high-frequency inputs, and in both schemes step-like changes can be detected instantaneously. Finally, we obtain the pairwise spike correlation function and the spike triggered average from the linear mean-evoked response function. These results provide a maximally tractable limiting case that complements and extends previous results obtained in the integrate and fire framework.

The Na conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers

Na (and Li) currents and fluorescence transients were recorded simultaneously under voltage-clamp conditions from mouse flexor digitorum brevis fibers stained with the potentiometric dye di-8-ANEPPS to investigate the distribution of Na channels between the surface and transverse tubular system (TTS) membranes. In fibers rendered electrically passive, voltage pulses resulted in step-like fluorescence changes that were used to calibrate the dye response. The effects of Na channel activation on the TTS voltage were investigated using Li, instead of Na, because di-8-ANEPPS transients show anomalies in the presence of the latter. Na and Li inward currents (INa, ILi; using half of the physiological ion concentration) showed very steep voltage dependences, with no reversal for depolarizations beyond the calculated equilibrium potential, suggesting that most of the current originates from a noncontrolled membrane compartment. Maximum peak ILi was ∼30% smaller than for INa, suggesting a Li-blocking effect. ILi activation resulted in the appearance of overshoots in otherwise step-like di-8-ANEPPS transients. Overshoots had comparable durations and voltage dependence as those of ILi. Simultaneously measured maximal overshoot and peak ILi were 54 ± 5% and 773 ± 53 µA/cm2, respectively. Radial cable model simulations predicted the properties of ILi and di-8-ANEPPS transients when TTS access resistances of 10–20 Ωcm2, and TTS-to-surface Na permeability density ratios in the range of 40:60 to 70:30, were used. Formamide-based osmotic shock resulted in incomplete detubulation. However, results from a subpopulation of treated fibers (low capacitance) provide confirmatory evidence that a significant proportion of ILi, and the overshoot in the optical signals, arises from the TTS in normal fibers. The quantitative evaluation of the distribution of Na channels between the sarcolemma and the TTS membranes, as provided here, is crucial for the understanding of the radial and longitudinal propagation of the action potential, which ultimately govern the mechanical activation of muscle in normal and diseased conditions.

Sea level variability influencing coastal flooding in the Swan River region, Western Australia

Coastal flooding refers to the incidence of high water levels produced by water level fluctuations of marine origin, rather than riverine floods. An understanding of the amplitude and frequency of high water level events is essential to foreshore management and the design of many coastal and estuarine facilities. Coastal flooding events generally determine public perception of sea level phenomena, as they are commonly associated with erosion events. This investigation has explored the nature of coastal flooding events affecting the Swan River Region, Western Australia, considering water level records at four sites in the estuary and lower river, extending from the mouth of the Swan River to 40 km upstream. The analysis examined the significance of tides, storms and mean sea level fluctuations over both seasonal and inter-annual time scales. The relative timing of these processes is significant for the enhanced or reduced frequency of coastal flooding. These variations overlie net sea level rise previously reported from the coastal Fremantle record, which is further supported by changes to the distribution of high water level events at an estuarine tidal station. Seasonally, coastal flooding events observed in the Swan River region are largely restricted to the period from May to July due to the relative phases of the annual mean sea fluctuation and biannual tidal cycle. Although significant storm surge events occur outside this period, their impact is normally reduced, as they are superimposed on lower tidal and mean sea level conditions. Over inter-annual time scales tide, storminess and mean sea level produce cycles of enhanced and depressed frequency of coastal flooding. For the Swan River region, the inter-annual tidal variation is regular, dominated by the 18.6 year lunar nodal cycle. Storminess and mean sea level variations are independent and irregular, with cycles from 3 to 10 year duration. Since 1960, these fluctuations have not occurred in phase, suggesting that recent historic records may not provide a real indication of inundation risk, exclusive of factors linked to climate change. The burst-like nature of coastal flooding incidents, with respect to frequency, has implications for both public perception and coastal management effort. The result, when combined with sea level rise, produces step-like change, with short periods of frequent coastal flooding, followed by extended, slowly varying quiescent periods. This presents challenges for coastal managers to incorporate variability into projections of future management needs, and to ensure that public and political recognition of coastal flooding hazard is not downplayed during quiet periods.

Surface acoustic wave response to ambient humidity in graphite oxide structures

The effect of ambient air humidity on surface acoustic waves (SAW) in graphite oxide-lithium niobate structures was investigated. The acoustic wave amplitude and phase strongly depended on the humidity level in structures with graphite oxide films deposited on the free substrate surface. These dependencies were significantly reduced in structures with graphite oxide films deposited on a metalized substrate surface. Fast (sub-second range) response in the acoustic amplitude and phase to step-like humidity changes was observed. Humidity-induced variations in the SAW amplitude and velocity are attributed to acoustoelectric interaction and mass loading due to moisture adsorption by graphite oxide layers.

The pace of Holocene vegetation change – testing for synchronous developments

Mid to high latitude forest ecosystems have undergone several major compositional changes during the Holocene. The temporal and spatial patterns of these vegetation changes hold potential information to their causes and triggers. Here we test the hypothesis that the timing of vegetation change was synchronous on a sub-continental scale, which implies a common trigger or a step-like change in climate parameters. Pollen diagrams from selected European regions were statistically divided into assemblage zones and the temporal pattern of the zone boundaries analysed. The results show that the temporal pattern of vegetation change was significantly different from random. Times of change cluster around 8.2, 4.8, 3.7, and 1.2 ka, while times of higher than average stability were found around 2.1 and 5.1 ka. Compositional changes linked to the expansion of Corylus avellana and Alnus glutinosa centre around 10.6 and 9.5 ka, respectively. A climatic trigger initiating these changes may have occurred 0.5 to 1 ka earlier, respectively. The synchronous expansion of C. avellana and A. glutinosa exemplify that dispersal is not necessarily followed by population expansion. The partly synchronous, partly random expansion of A. glutinosa in adjacent European regions exemplifies that sudden synchronous population expansions are not species specific traits but vary regionally.