Apparent Dispersion Research Articles

Residual CO2 saturation estimate using noble gas tracers in a single-well field test: The CO2CRC Otway project

This paper presents the results of a partitioning tracer test using noble gases to measure residual CO2 saturation in the field as one part of the CO2CRC residual saturation and dissolution test sequence. Noble gas tracers were used in a sequential partitioning tracer test. The first tracer test was performed prior to CO2 injection to characterise the aquifer under single-phase conditions. The second tracer test was performed after CO2 had been driven to residual saturation. The noble gas tracers can be regarded as non-partitioning aqueous-phase tracers in the undisturbed aquifer and subsequently as water/CO2 partitioning tracers in the presence of the residual CO2 saturation.One-dimensional radial simulations are used to estimate residual saturation from the tracer tests by fitting only two independent parameters: the apparent dispersivity of each tracer in the aqueous phase and the residual CO2 saturation. This analysis method is validated by successful application to krypton (Kr) and xenon (Xe) production curves from the field data.Both tracers give the same estimate of residual saturation to within the accuracy of the method. Furthermore the estimated residual saturation from the noble gas tracer tests is consistent with independent measurements. However the analysis of the two tracer production curves indicates that Xe has higher dispersivity than Kr, despite having been injected and produced simultaneously and measured from the same samples. Finally the application of this sequential tracer test method to other field settings is discussed.

B42 ファイバーレーザによるSUS304薄板の精密溶接と継手特性に関する研究(OS9 レーザ応用加工(2))

There is a great demand for the development of welding techniques to join thin stainless sheets with the reduction of size and weight of the electronic components. In this study, we searched for butt welding conditions of SUS304 0.2mm thickness using a fiber laser and then found the welding conditions which have no burn through and complete penetration. As a result of investigating the joint properties, tensile tests indicated good joint properties of the welded sheets, as evidenced from the strength of approximately 90% when compared with the base metal and ductile fracture as same as the base metal. We found that the shield gas is appropriate way to supply and prevent from oxidation of welded sheets. However, we could not obtain the reproducibility of welding as evidenced from apparent dispersion in strength under the same conditions.

Migration behavior of plutonium affected by ferrous ion in compacted bentonite by using electrochemical technique

ABSTRACTThe migration behavior of plutonium is expected to be affected by the corrosion products of carbon steel in compacted bentonite at high-level waste repositories. Electrochemical experiments were carried out to simulate the reducing environment created by ferrous iron ions in equilibrium with anoxic corrosion products of iron. The concentration profiles of plutonium could be described by the convection -dispersion equation to obtain two migration parameters: apparent migration velocity Va and apparent dispersion coefficient Da. The apparent migration velocity was evaluated within 1 nm/s and was found to be independent of the experiment duration and the dry density of bentonite in the interval 0.8-1.4 Mg/m3. The apparent dispersion coefficient increased with the experiment duration at a dry density of 1.4 Mg/m3. The results for other dry densities also showed the same trend. These findings indicate that plutonium migration likely starts after ferrous ions reach the plutonium, in other words, the reducing environment due to ferrous ions could change the chemical form of plutonium and/or the characteristics of compacted bentonite. The apparent diffusion coefficient was estimated to be around 0.5 to 2.2 µm2/s and increased with decreasing the dry density of bentonite.

Effect of temperature on the equilibrium and kinetics of galactose, glucose, and lactose adsorption on a cation exchanger

AbstractGalacto-oligosaccharides are typically produced by an enzymatic reaction when the post-reaction mixture contains considerable amounts of lactose and glucose and a smaller amount of galactose. In order to develop a process of chromatographic removal of saccharide impurities, adsorption equilibria and kinetics of these di- and monosaccharides were investigated for Diaion UBK 530, an industrialgrade strong cation-exchanger in the Na+ form. Frontal chromatographic experiments were carried out in the temperature range of 30–70°C and a broad interval of saccharide concentrations up to 350 g L−1. Breakthrough curves were described using the equilibrium-dispersive model with the linear adsorption isotherm. Both the distribution and the axial dispersion coefficient values depended on the saccharide molecule type and size. No significant effect of temperature or concentration on the distribution coefficient was observed. The apparent dispersion coefficients of all saccharides exhibited some decrease with the temperature, which was caused by the decrease of the intraparticle mass transfer resistance. An analysis showed that both the intraparticle mass transfer and the axial dispersion had a significant influence on the front dispersion.

Environmental Instability as a Motor for Dispersal: A Case Study from a Growing Population of Glossy Ibis

Dispersal is a life-history trait directly affecting population dynamics and species range shifts and thus playing a prominent role in the response to climate change. Nonetheless, the relationship between extreme climatic events and dispersal has received little attention in birds. Here we focused on climatic, demographic and individual factors affecting the dispersal propensity of a major glossy ibis population. We performed a capture-resighting analysis on individuals born and observed at Doñana (South-West Spain) over fourteen years. We applied a multiple analytical approach to show that single-site capture-resighting estimates were a reliable index of dispersal propensity from the area. We focused on the emigration of Doñana-born individuals sporadically (transients) and regularly (residents) frequenting their natal area. Droughts during two out of 14 study years caused higher apparent dispersal rates, explaining most of the annual variation in these rates. The age structure of Doñana-born individuals resighted simultaneously locally and in Morocco in one week over the 2010 autumn confirmed that the 2005 drought boosted permanent emigration. As numbers increased steadily during non-drought years since the formation of the colony in 1996 to several thousand pairs, philopatry increased gradually, while transients probability appeared to be related to average breeding success. Age, sex, density, quality of foraging habitat and breeding success in the previous season were not found to directly affect apparent dispersal. Nonetheless, autumn sex ratio gradually switched from male (≈0.68) to female-skewed (≈0.44) by the end of the study period, suggesting that males and females respond differently to high densities reached in recent years. This study demonstrates the importance of extreme climatic events as a powerful motor for spread of species in expansion. Also, it suggests different factors drive emigration of individuals according to their amount of experience in the area (e.g. transients vs residents).

Effects of past copper contamination and soil structure on copper leaching from soil.

Copper contamination affects biological, chemical, and physical soil properties and associated ecological functions. Changes in soil pore organization as a result of Cu contamination can dramatically affect flow and contaminant transport in polluted soils. This study assessed the influence of soil structure on the movement of water and Cu in a long-term polluted soil. Undisturbed soil cores collected along a Cu gradient (from about 20 to about 3800 mg Cu kg soil) were scanned using X-ray computed tomography (CT). Leaching experiments were performed to analyze tracer transport, colloid leaching, and dissolved organic carbon (DOC) and Cu losses. The 5% arrival time () and apparent dispersivity (λ) for tracer breakthrough were calculated by fitting the experimental data to a nonparametric, double-lognormal probability density function. Soil bulk density, which did not follow the Cu gradient, was the main driver of preferential flow, while macroporosity determined by X-ray CT (for pores >180 μm) proved the best predictor of solute transport. Higher preferential flow due to the presence of well-aligned pores and small cracks controlled water movement in compacted soil. Transport of Cu was rapid during the first flush (≈1 pore volume) in association with the movement of colloid particles, followed by slower transport in association with the movement of DOC in the soil solution. The relative amount of Cu released was strongly correlated with macroporosity as determined by X-ray CT, indicating the promising potential of this visualization technique for predicting contaminant transport through soil.

Effect of Cu content on the surface and catalytic properties of Cu/ZrO2 catalyst for ethanol dehydrogenation

ZrO2-supported Cu catalysts with different Cu content (5–30wt.%) were prepared by impregnation method. The effect of Cu content on the structure, surface and catalytic properties of Cu/ZrO2 catalysts in the reaction of ethanol conversion was studied. The physicochemical characterization of the calcined and reduced samples was carried out by: N2 adsorption–desorption isotherms, N2O titration, XRD, XPS, TPR and DRFTS of CO adsorption. It was observed that the increase of Cu content leads to decrease of the apparent copper metal dispersion caused by the strong agglomeration of the metal particles. The selectivity to different reaction products was connected with the electronic properties of the catalysts defined by the copper particle size and the interface at metal-oxide support. The highest selectivity to ethyl acetate over samples with Cu content ≥10wt.% was assigned to the high density of basic sites of O2− ions and more heterogeneous distribution of copper species (Cu0/Cu+) defined by DRIFTS of CO adsorption and XPS.

The influence of mesoscopic flow on the P-wave attenuation and dispersion in a porous media permeated by aligned fractures

In sedimentary rocks attenuation/dispersion is dominated by fluid-rock interactions. Wave-induced fluid flow in the pores causes energy loss through several mechanisms, and as a result attenuation is strongly frequency dependent. However, the fluid motion process governing the frequency dependent attenuation and velocity remains unclear. We propose a new approach to obtain the analytical expressions of pore pressure, relative fluxes distribution and frame displacement within the double-layer porous media based on quasi-static poroelastic theory. The dispersion equation for a P-wave propagating in a porous medium permeated by aligned fractures is given by considering fractures as thin and highly compliant layers. The influence of mesoscopic fluid flow on phase velocity dispersion and attenuation is discussed under the condition of varying fracture weakness. In this model conversion of the compression wave energy into Biot slow wave diffusion at the facture surface can result in apparent attenuation and dispersion within the usual seismic frequency band. The magnitude of velocity dispersion and attenuation of P-wave increases with increasing fracture weakness, and the relaxation peak and maximum attenuation shift towards lower frequency. Because of its periodic structure, the fractured porous media can be considered as a phononic crystal with several pass and stop bands in the high frequency band. Therefore, the velocity and attenuation of the P-wave show an oscillatory behavior with increasing frequency when resonance occurs. The evolutions of the pore pressure and the relative fluxes as a function of frequency are presented, giving more physical insight into the behavior of P-wave velocity dispersion and the attenuation of fractured porous medium due to the wave-induced mesoscopic flow. We show that the specific behavior of attenuation as function of frequency is mainly controlled by the energy dissipated per wave cycle in the background layer.

Contrasting patterns of survival and dispersal in multiple habitats reveal an ecological trap in a food-caching bird

A comprehensive understanding of how natural and anthropogenic variation in habitat influences populations requires long-term information on how such variation affects survival and dispersal throughout the annual cycle. Gray jays Perisoreus canadensis are widespread boreal resident passerines that use cached food to survive over the winter and to begin breeding during the late winter. Using multistate capture-recapture analysis, we examined apparent survival and dispersal in relation to habitat quality in a gray jay population over 34 years (1977-2010). Prior evidence suggests that natural variation in habitat quality is driven by the proportion of conifers on territories because of their superior ability to preserve cached food. Although neither adults (>1 year) nor juveniles (<1 year) had higher survival rates on high-conifer territories, both age classes were less likely to leave high-conifer territories and, when they did move, were more likely to disperse to high-conifer territories. In contrast, survival rates were lower on territories that were adjacent to a major highway compared to territories that did not border the highway but there was no evidence for directional dispersal towards or away from highway territories. Our results support the notion that natural variation in habitat quality is driven by the proportion of coniferous trees on territories and provide the first evidence that high-mortality highway habitats can act as an equal-preference ecological trap for birds. Reproductive success, as shown in a previous study, but not survival, is sensitive to natural variation in habitat quality, suggesting that gray jays, despite living in harsh winter conditions, likely favor the allocation of limited resources towards self-maintenance over reproduction.

Measuring Artificial Recharge with Fiber Optic Distributed Temperature Sensing

Heat was used as a tracer to measure infiltration rates from a recharge basin. The propagation of diurnal oscillation of surface water temperature into the basin bed was monitored along a transect using Fiber Optic Distributed Temperature Sensing (FODTS). The propagation rate was related to downward specific discharge using standard theory of heat advection and dispersion in saturated porous media. An estimate of the temporal variation of heat propagation was achieved using a wavelet transform to find the phase lag between the surface temperature diurnal oscillation and the correlated oscillation at 0.33 and 0.98 m below the bed surface. The wavelet results compared well to a constant velocity model of thermal advection and dispersion during periods of relatively constant discharge rates. The apparent dispersion of heat was found to be due primarily to hydrodynamic mechanisms rather than thermal diffusion. Specific discharge estimates using the FODTS technique also compared well to water balance estimates over a four month period, although there were occasional deviations that have yet to be adequately explained. The FODTS technique is superior to water balance in that it produces estimates of infiltration rate every meter along the cable transect, every half hour. These high resolution measurements highlighted areas of low infiltration and demonstrated the degradation of basin efficiency due to source waters of high suspended solids. FODTS monitoring promises to be a useful tool for diagnosing basin performance in an era of increasing groundwater demand.

Multispecies hydrodynamic dispersion under high concentration gradients

Recent research suggests that when high concentration gradients (HCG) are present, resulting sharp density differences can cause the dispersive flux relationship to deviate from its classical Fickian form. This paper presents stable, upward, miscible displacement experiments conducted in two different types of porous media for a wide range of concentration differences between resident and displacing fluids. The considered groundwater velocities ranged from advection-dominated transport to velocities where the contribution of molecular diffusion is important, with the corresponding Peclet numbers ranging from 0.2 to 320. In addition to single component displacing fluids, mixtures consisting of multiple solutes were considered. The results of this study provide further evidence that classical Fick's law over-estimates the dispersion coefficient under HCG conditions. The decrease in the apparent dispersion coefficient is shown to be a nonlinear function of both concentration difference and groundwater velocity. This observation is attributed to gravitational effects at the sub-continuum scale which are not directly accounted for in classical variable density advection/dispersion models. Mixture experiments showed that the dispersive behaviors of individual components in a groundwater contaminant mixture are coupled.

Meso-scale modelling of shock wave propagation in a SiC/Al nanocomposite reinforced with WS2-inorganic fullerene nanoparticles

It has been postulated that nanocomposites incorporating IF-WS2 nanoparticles within a strong matrix might form the next generation of highly shock-resistant materials. The present work describes initial analyses into the shock response of such materials via a sequential multi-scale dynamic analysis. Density functional theory is used to calculate the elastic properties of the multilayered WS2 nanoparticles. These properties are then used within an explicit finite element (FE) analysis of wave propagation through an embedded statistical volume element (SVE) of a two-phase nanocomposite consisting of a matrix with IF-WS2 nanoparticles. Some wave front dispersion was noted, particularly where the modulus of the matrix is significantly different from that of the particles. A three-phase nanocomposite consisting of an aluminium matrix with IF-WS2 and SiC nanoparticles was also considered, and showed more apparent wave front dispersion than for the two-phase nanocomposite. Hugoniot shock propagation data have been derived from the simulation outputs. It is concluded that sequential multiscale modelling of these systems is appropriate and can provide useful information about shock wave propagation in the elastic region. The work also provides a foundation for more realistic simulations at higher rate loading, where it will be necessary to incorporate material failure in the models.

Mammalian herbivores as potential seed dispersal vectors in ancient woodland fragments

Europe's remaining ancient woodland is highly fragmented and many specialist woodland plants persist as isolated relictual populations. Their apparent dispersal limitation and failure to colonise more recently established secondary woodlands may reflect a loss of vectors and mechanisms for dispersal. This is in contrast to long‐distance dispersal events evidenced both by paleoecology and some contemporary observations. Although increasing populations of deer are recognised as important dispersers of plant seeds, particularly for species with no apparent dispersal mechanism, the potential for deer to disperse ancient woodland plants has not been studied previously in the UK, where remaining ancient woodland fragments are frequently surrounded by arable landscapes. Viable seed content of 616 faecal samples collected from a guild of mammalian herbivores; red deer Cervus elaphus, fallow deer Dama dama, roe deer Capreolus capreolus, reeves' muntjac Muntiacus reevesi and brown hare Lepus europaeus, over five months and from four coppiced ancient semi‐natural woodlands in eastern England, was assessed. Following cold stratification and sowing in a controlled greenhouse, 502 seedlings of 41 species germinated. Three species, constituting just 1.8% of the total individual seedlings recorded, were characteristic of woodland habitats, including only one ancient woodland indicator species, wood speedwell Veronica montana. Germinated plant species were instead characteristic of non‐woodland habitats including grassland, arable and ruderal communities. The four most abundant species were the widespread grass common bent Agrostis capillaris, the ruderal greater plantain Plantago major and the two crop species, wheat Triticum spp. and rape Brassica napus. Mammalian herbivory in these ancient woodland fragments provided few dispersal benefits for the woodland plant community. Instead, larger free‐ranging herbivores transported large volumes of propagules of ruderal agrarian and open‐habitat species from surrounding habitats into ancient woodland fragments.

Effect of Root Water and Solute Uptake on Apparent Soil Dispersivity: A Simulation Study

Plants take up water from the root zone and thus affect the three-dimensional water flow field and solute transport processes in the soil. In this study, the impacts of root architecture, plant solute uptake mechanisms (passive, active, and solute exclusion), and plant transpiration rate on the water flow field in the soil and on solute spreading were simulated. Therefore, a fully mechanistic model was used to simulate water flow along water potential gradients in the root–soil continuum by coupling a three-dimensional Richards equation in the soil with a flow equation in the root xylem vessels. Solute transport was simulated using a three-dimensional random walk particle tracking algorithm. To quantify the effect of root water and nutrient uptake on solute transport, an equivalent one-dimensional flow and transport model was fitted to horizontally averaged simulation results, and the fitted apparent parameters were compared with the parameters of the three-dimensional model. Our simulation results showed that the apparent dispersivity length is affected by the heterogeneous flow field, caused by root water uptake, and changed in a range of 50%, depending on solute redistribution in the root zone that depends on solute uptake type and soil dispersivity length. In addition, simulation results indicate that local concentration gradients within the root zone have an impact on apparent solute uptake rate parameters used in one-dimensional models to calculate uptake rates from spatially averaged concentrations. This shows the importance of small scale three-dimensional water and solute fluxes induced by root water and nutrient uptake.

Development of Organoclays for Use in Oil-Based Drilling Fluids

The organoclays are derived from bentonite clays treated in aqueous solution with surfactants, which are incorporated into the interlayer surface of clays, covering them with an organic layer; so, the naturally hydrophilic clay becomes hydrophobic. This paper proposes a study to find the dispersant media (diesel, paraffin or ester) which is more suitable for use in fluids, using the bentonite clay Cloisite Na +, treated with ionic surfactants Praepagen HY and Praepagen WB. The organoclay obtained was characterized by X-ray diffraction, and we measured the apparent viscosity and plastic dispersions. We observed the importance of the type of surfactant and dispersant amid to obtain organoclays for non-aqueous drilling fluids with the dispersants that meet existing specifications for drilling oil wells.

Meta-analysis of the effects of soil properties, site factors and experimental conditions on solute transport

Abstract. Preferential flow is a widespread phenomenon that is known to strongly affect solute transport in soil, but our understanding and knowledge is still poor of the site factors and soil properties that promote it. To investigate these relationships, we assembled a database from the peer-reviewed literature containing information on 733 breakthrough curve experiments under steady-state flow conditions. Most of the collected experiments (585 of the 733 datasets) had been conducted on undisturbed soil columns, although some experiments on repacked soil, clean sands, and glass beads were also included. In addition to the apparent dispersivity, we focused our attention on three indicators of preferential solute transport: namely the 5%-arrival time, the holdback factor, and the ratio of piston-flow and average transport velocities. Our results suggest that, in contrast to the 5%-arrival time and the holdback factor, the piston-flow to transport velocity ratio is not related to preferential macropore transport but rather to the exclusion or retardation of the applied tracer. Confirming that the apparent longitudinal dispersivity is positively correlated with the travel distance of the tracer, our results also illustrate that this relationship is refined if the normalized 5%-tracer arrival time is also taken into account. In particular, we found that the degree of preferential solute transport increases with apparent dispersivity and decreases with travel distance. A similar but weaker relationship was observed between apparent dispersivity, 5%-tracer arrival time, and lateral observation scale, such that the degree of preferential transport increases with lateral observation scale. However, we also found that the travel distance and the lateral observation scale in the investigated dataset are correlated, which makes it difficult to distinguish their influence on these transport characteristics. We also found that the strength of preferential transport increased at larger flow rates and water saturations, which suggests that macropore flow was a more important flow mechanism than heterogeneous flow in the soil matrix. Nevertheless, our data show that heterogeneous flow in the soil matrix also occasionally leads to strong preferential transport. Furthermore, we show that preferential solute transport under steady-state flow depends on soil texture in a threshold-like manner: moderate to strong preferential transport was found to occur only for undisturbed soils that contain more than 8% clay. Preferential flow characteristics were also absent for columns filled with glass beads, clean sands, or sieved soil. No clear effect of land use on the pattern of solute transport could be discerned, probably because the available dataset was too small and too strongly affected by cross-correlations with experimental conditions. Our results suggest that, in developing pedotransfer functions for solute transport properties of soils, it is critically important to account for travel distance, lateral observation scale, and water flow rate and saturation.

CO2 sequestration for enhanced gas recovery: New measurements of supercritical CO2–CH4 dispersion in porous media and a review of recent research

The enhanced recovery of natural gas by the injection and sequestration of CO2 is an attractive scenario for certain prospective field developments if the risks of gas contamination or early CO2 breakthrough can be assessed reliably. Simulations of enhanced gas recovery (EGR) scenarios require accurate dispersion parameters at reservoir conditions to quantify the size of the miscible CO2–CH4 displacement front; several experimental studies using core-flooding equipment aimed at measuring such parameters have been reported over the last decade. However, such measurements are particularly challenging and the data produced are generally afflicted in their repeatability by limited experimental control and in their accuracy by systematic errors such as gravitational and core-entrance/exit effects. We review here the existing experimental data pertaining to EGR by CO2 sequestration and also report new measurements of longitudinal CO2–CH4 dispersion coefficients at temperatures of 40–80°C, pressures of 8–12MPa and interstitial velocities of 0.05–1.13mms−1 [14.2–320ftday−1] in 5–10cm long sandstone cores with permeabilities of 12 and 460mD. The core-floods were conducted in both a horizontal and vertical orientation, with significant gravitational effects observed for low velocity floods in horizontal cores with high permeabilities. We also analyzed the effects of tubing and core entrance/exit effects on the measurements and found that the latter resulted in apparent dispersion coefficients up to 63% larger than would be due to the core alone. Our results indicate that dispersivities for CO2–CH4 at these supercritical conditions are less than 0.001m, which indicates that excessive mixing will not occur in EGR scenarios in the absence of conformance effects such as heterogeneity coupled with injection well pattern. Inclusion of such conformance effects is essential for detailed reservoir simulation.

Commentary: Tree refugia and slow forest development in response to post-LGM warming in North‐Eastern European Russia

news and update ISSN 1948‐6596 commentary Tree refugia and slow forest development in response to post‐ LGM warming in North‐Eastern European Russia Though the Postglacial period counts its thousands of years, it was not indefinitely long, and few plants that merely scatter their seed could advance more than a yard in a year […]. The oak, to gain its present most northerly position in North Britain after being driven out by the cold, probably had to travel fully six hundred miles, and this without external aid would take some‐ thing like a million years. (Clement Reid 1899, p. 25) Driven by an urge to understand how the compo‐ sition of the British flora arose, Clement Reid was puzzled by the discrepancy between the apparent low dispersal capacity of big seeded trees and the high postglacial migration rates deduced from fos‐ sil evidence. Possible explanations resolving Reid’s paradox, as it was later termed, were fast migra‐ tion by long‐distance dispersal events or glacial survival of small scattered populations at shel‐ tered sites further north than evidenced by the fossil record (Reid 1899, Skellam 1951). More than 100 years have passed since the early writings of Reid, but two questions leading from his thoughts are still debated: How far north did tree species survive the Last Glacial Maximum (LGM, ca. 21 kya)? How fast did tree species spread and thus current forest structure develop during post‐LGM warming? A new paleoecological study by Valiranta et al. (2011, Journal of Biogeography) provides, in addition to several other recent studies (e.g., Bin‐ ney et al. 2009, Heikkila et al. 2009), insight into these unresolved questions. Valiranta et al. (2011) summarized macrofossil and stomata records from five sites at the late‐glacial–early Holocene transition at the European Russian arctic tree line; an area east of the margin of the Scandinavian ice sheet and west of the Ural Mountains (Fig. 1). Their data evidenced the presence of Norway spruce (Picea abies) and tree birch (Betula pen‐ dula/pubescens) during the early Holocene (11,500–10,000 cal. yr BP) at all five sites. At one site, two radiocarbon dates even suggested the presence of these taxa up to 1000 yrs prior to the beginning of the Holocene. Historical biogeogra‐ phy, however, depends on reliable chronologies of fossil records. Ideally, each site should have a well established chronology based on an adequate number of radiocarbon dated fragile and precisely identified terrestrial plant remains that show no signs of redeposition. In this respect, the chro‐ nologies of Valiranta et al. (2011) were not opti‐ mal and the late‐glacial presence of P. abies and tree Betula can be questioned, but their early Holocene records were well documented. Hence, the smoking gun, i.e., direct palaeoecological proof of LGM‐populations, might be evasive, but scattered glacial and widespread early Holocene fossil records indeed reflect consistent patterns of early expansions in North‐Eastern European Rus‐ sia. Furthermore, independent climate reconstruc‐ tions are crucial when evaluating the possible LGM survival of boreal tree populations so far north. Glacier‐based climate models do indeed indicate relatively modest LGM temperature de‐ pressions in the arctic Ural Mountains (Allen et al. 2008). This might have facilitated survival of trees. Besides, the ability to produce krummholz forms (Oberg & Kullman 2011) or other strategies for non‐reproductive persistence through millennia (Bhagwat & Willis 2008) may have been decisive. Nevertheless, as concluded by Valiranta et al. (2011), it certainly seems likely that postglacial development of high‐latitude forests occurred through expansion of local tree populations rather than through re‐colonisation from distant south‐ ern refugia. Using molecular markers, recent phy‐ logeographic studies for both P. abies and Betula support glacial survival in north‐eastern Europe (Maliouchenko et al. 2007, Tollefsrud et al. 2008). For example, P. abies shows distinct gene pools of similar within‐population genetic diversity for northern and central‐southern Europe (Tollefsrud et al. 2008): a pattern that supports west expan‐ sion from such north‐eastern refugia and not re‐ colonisation of northern Europe from the south. For decades, pollen data have suggested early frontiers of biogeography 2.4, 2011 — © 2011 the authors; journal compilation © 2011 The International Biogeography Society

Species formation and geographical range evolution in a genus of Central American cloud forest salamanders (Dendrotriton)

AbstractAim Montane Central America offers an ideal system for testing geographical hypotheses of species diversification. We examined how the complex geological history of Nuclear Central America has shaped the diversification of a genus of cloud‐forest‐inhabiting salamanders (Dendrotriton). We applied parametric models of geographical range evolution to determine the predominant mode of species formation within the genus and to test existing hypotheses of geographical species formation in the region.Location Montane cloud forests of Nuclear Central America.Methods We estimated a species tree for Dendrotriton using a multi‐locus DNA sequence data set and several coalescent methods, and performed molecular dating for divergence events within the genus. We then applied the species‐tree estimate to a likelihood‐based time‐stratified model of geographical range evolution, based on current species distributions and available geological information for Central America.Results Species trees from all methods contain two groups, one corresponding to species from the Sierra de los Cuchumatanes and the other containing all remaining species. In most cases, species formation within the genus involved an even division of the geographical range of the ancestral species between descendant species. The ancestor of extant Dendrotriton species was estimated to have occurred in either the Sierra de los Cuchumatanes or the Sierra Madre de Chiapas, and both of these areas appear to have been important for diversification within the genus. The single species found in the Quaternary‐age Guatemalan volcanic cordillera dispersed to the volcanoes from an older highland area.Main conclusions Models of geographical range evolution, when combined with robust species‐tree estimates, provide insight into the historical biogeography of taxa not available from phylogenies or distributional data alone. Vicariant species formation, rather than peripatric or gradient speciation, appears to have been the dominant process of diversification, with most divergence events occurring within or between ancient highland areas. The apparent dispersal of Dendrotriton to the Quaternary‐age volcanoes raises the possibility that the rich salamander community there is composed of species that dispersed from geologically older areas. The Motagua Valley appears not to have been as important in vicariant species formation within Dendrotriton as it is within other groups.

P-wave attenuation and dispersion in a porous medium permeated by aligned fractures—a new poroelastic approach

An alternative method is advocated to obtain the analytical expressions of pore pressure, relative flux distribution and frame displacement in a periodic layered porous medium (White model) based on quasi-static poroelastic theory, from which the expression of the effective medium model for a P-wave propagating vertical to the layer can be obtained. Then a dispersion equation for a P-wave propagating in the porous medium permeated by aligned fractures is also presented by considering fractures as thin and highly compliant layers, with which the influence of mesoscopic fluid flow on phase velocity dispersion and attenuation is discussed under the condition of varying fracture weakness and the length scale ratio of the background layer to the fracture layer. For both cases, fluid flow induced by the Biot slow wave can result in apparent attenuation and dispersion in the usual seismic frequency band (5–500 Hz). The magnitude of velocity dispersion and attenuation of the P-wave will increase with increasing fracture weakness or with decreasing length scale ratio, and also relaxation peak and maximum attenuation move towards low frequencies. In particular, the evolution of pore pressure and relative fluxes as a function of frequency in the medium for those two cases is present, which gives us a more physical understanding of the behaviours of P-wave velocity dispersion and attenuation due to wave-induced mesoscopic flow.