bluuebunny/crossref_metadata_2025 huggingface dataset:
True‐amplitude migration of 2D synthetic data1
Authors: Hanitzsch C. , Schleicher J. , Hubral P.
Published: April 2006

True‐amplitude (TA) migration, which is a Kirchhoff‐type modified weighted diffraction stack, recovers (possibly) complex angle‐dependent reflection coefficients which are important for amplitude‐versus‐offset (AVO) inversion. The method can be implemented using existing prestack or post‐stack Kirchhoff migration and fast Green's function computation programs. Here, it is applied to synthetic single‐shot and constant‐offset seismic data that include post‐critical reflections (complex reflection coefficients) and caustics. Comparisons of the amplitudes of the TA migration image with theoretical reflection coefficients show that the (possibly complex) angle‐dependent reflection coefficients are correctly estimated.
Ground penetrating radar sounding of a temperate glacier; modelling of a multilayered medium1
Authors: Nicollin Florence , Kofman Wlodek
Published: April 2006

A numerical simulation of electromagnetic propagation through a multilayered medium is performed in order to explain and interpret the signal received from the radar sounding of a temperate glacier. During the winter of 1990, several radar profiles were obtained on the Mont‐de‐Lans glacier in the French Alps with a ground penetrating radar which uses a phase modulation of the transmitted pulse by coded sequences. The pulse compression is obtained by applying the matched filter to the received signal, which provides a range‐resolution of about 8 m in the ice. The profiles recorded on the temperate glacier do not show a single clear reflection from the ice‐bedrock interface, but they exhibit widely distributed energy decreasing with depth. This may be due to the inhomogeneous inner structure of the temperate glacier and we use a simple model of a layered medium to compute a simulation of the propagation. Thus, partial reflection at each layer and scattering from a rough basal interface may explain the observed signal. A computer‐based technique is used to locate on the data the bottom of the glacier in order to estimate the ice thickness. The results from the different radar profiles are consistent and are a good fit to the thickness which has been determined by other geophysical methods.
Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting1
Authors: Lapenna V. , Macchiato M. , Patella D. , Satriano C. , Serio C. , Tramutoli V.
Published: April 2006

We study the geoelectrical problem of picking out the useful signal from voltage time series, monitored under conditions of a low signal‐to‐noise ratio and non‐stationary noise. Statistical tests performed at different sites show that geoelectrical noise often belongs to the class of non‐stationary phenomena with non‐Gaussian probability distributions. In such cases, the application of conventional methods of geoelectrical useful signal extraction, based on the stationary white‐noise assumption, gives biased estimates. For the on‐line processing of geoelectrical recordings, we recommend the use of the periodogram technique combined with the Kolmogorov–Smirnov test, a suitable algorithm of which is described in detail. The suggested procedure allows data acquisition to stop as soon as the useful signal power is estimated with a relative error smaller than a predetermined value. Finally, we compare the suggested procedure with the autoregressive approach. The previously used and simpler periodogram method, applied to the solution of problems of this kind, appears to give better performances than the autoregressive analysis.
Generalized acoustic diffraction tomogra phy1
Authors: Gelius Leiv‐J.
Published: April 2006

A generalized diffraction tomography algorithm is developed, which in principle can handle irregularly spaced data, curved acquisition lines and non‐uniform background models. By direct comparison with medical diffraction tomography, it is shown that the generalized method involves the same two processing steps: data filtering and back‐propagation. The filter handles the irregular sampling of the model space and the uneven energy coverage, while the back‐propagation operator removes the wave propagation effects. Paraxial ray‐tracing techniques are employed to compute both these quantities.

In medical diffraction tomography, the resolution vector (i.e. the Fourier vector of the model space) is defined by the incident and scattered plane‐wave directions. It is shown here that a similar relationship exists for a non‐uniform background, where the resolution vector at a particular image point is defined by the incident and scattered ray directions. Consequently, the impulse response of the generalized algorithm becomes space variant.

Finally, a general processing procedure for transmission mode seismic data, based on this generalized algorithm, is proposed. The potential of the method is demonstrated using synthetic cross‐hole data.
A new velocity model for clay‐sand mixtures1
Authors: Xu Shiyu , White Roy E.
Published: April 2006

None of the standard porosity‐velocity models (e.g. the time‐average equation, Raymer's equations) is satisfactory for interpreting well‐logging data over a broad depth range. Clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and P‐wave transit time. Because clays are composed of fine sheet‐like particles, they normally form pores with much smaller aspect ratios than those associated with sand grains. This difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations.

A velocity model for Clay–sand mixtures has been developed in terms of the Kuster and Toksöz, effective medium and Gassmann theories. In this model, the total pore space is assumed to consist of two parts: (1) pores associated with sand grains and (2) pores associated with clays (including bound water). The essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays. Because of this, porosity in shales affects elastic compliance differently from porosity in sand‐Stones. The predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales.
A joint inversion algorithm to process geoelectric and sutface wave seismic data. Part I: basic ideas1
Authors: Hering A. , Misiek R. , Gyulai A. , Ormos T. , Dobroka M. , Dresen L.
Published: April 2006

For the exploration of near‐surface structures, seismic and geoelectric methods are often applied. Usually, these two types of method give, independently of each other, a sufficiently exact model of the geological structure. However, sometimes the inversion of the seismic or geoelectric data fails.

These failures can be avoided by combining various methods in one joint inversion which feads to much better parameter estimations of the model than the independent inversions.

A suitable seismic method for exploring near‐surface structures is the use of dispersive surface waves: the dispersive characteristics of Rayleigh and Love surface waves depend strongly on the structural and petrophysical (seismic velocities) features of the near‐surface Underground.

Geoelectric exploration of the structure Underground may be carried out with the well‐known methods of DC resistivity sounding, such as the Schlumberger, the radial‐dipole and the two‐electrode arrays.

The joint inversion algorithm is tested by means of synthetic data. It is demonstrated that the geoelectric joint inversion of Schlumberger, radial‐dipole and two‐electrode sounding data yields more reliable results than the single inversion of a single set of these data. The same holds for the seismic joint inversion of Love and Rayleigh group slowness data. The best inversion result is achieved by performing a joint inversion of both geoelectric and surface‐wave data.

The effect of noise on the accuracy of the solution for both Gaussian and non‐Gaussian (sparsely distributed large) errors is analysed. After a comparison between least‐square (LSQ) and least absolute deviation (LAD) inversion results, the LAD joint inversion is found to be an accurate and robust method.
Direct current electric potential computation in an inhomogeneous and arbitrarily anisotropic layered earthl
Authors: Das Umesh C.
Published: April 2006

We consider the calculation of the electrical field quantities, electric potential and the vertical component of the total volume density of electric current, in a horizontally layered, piecewise homogeneous and arbitrarily anisotropic earth due to a system of direct current point sources. By applying Fourier transformation with respect to the horizontal space coordinates to the static field equations, the field quantities are obtained as the solutions of the system of transform‐domain differential equations in the vertical (depth) coordinates. A recurrence scheme has been given to compute the tranform‐domain field quantities at any depth. The corresponding space‐domain quantities are then obtained by inverse Fast Fourier Transformation (FFT). A complete computer program has been developed for computing the electric potentials at any depth of the layered earth, which is composed of an arbitrary number of anisotropic layers with arbitrary conductivity tensors. By considering the point sources at different depths from the surface, equipotential contours on the surface of arbitrarily anisotropic layered earth models are given.
Imaging beneath an opaque basaltic layer using densely sampled wide‐angle OBS data1
Authors: Samson C. , Barton P.J. , Karwatowski J.
Published: April 2006

A combined reflection/refraction (wide‐angle) seismic survey was conducted on the continental shelf north‐west of Britain, using a conventional streamer with an airgun source, and static ocean‐bottom seismometers (OBS) to record wide‐angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section. The basalts, however, proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons, where an important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wide‐angle OBS data that were coincident with the reflection profile. Eleven OBS instruments were deployed along a 75 km line and recorded signal from a powerful 149 litre (9100 in.3) airgun array fired every 50 m. Data processing was performed using a standard industrial reflection seismic software package prior to first‐arrival picking. Processing steps included geometry definition, trace summation and display of the data using various scaling algorithms. An initial model was constructed from 1D velocity‐time profiles digitized every 4 km along the stacked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modelling revealed the existence of a buried low‐velocity Mesozoic sedimentary basin, of a prominent basement horst and of a normal fault penetrating to the basement.
Cancelling directional EM noise in magnetoteilurics1
Authors: Santarato G. , Spagnolini U.
Published: April 2006

The prospecting of densely urbanized areas by the measurement of magnetic and electric natural fields is severely hampered by electromagnetic (EM) noise. Active man‐made EM noise sources can generally be considered fixed in space, thus affecting the magnetotelluric (MT) signals of a measuring site mainly along their polarization directions.

Taking advantage of the impulsive nature of polarized EM noise, a time‐domain directional noise cancelling (DNC) technique is proposed. The comparison of noisy data with data predicted, using a low noise reference signal or with data interpolated whenever no reference is available, allows the detection of the most likely noise sources with prevailing directional patterns using a Bayes's criterion. The DNC approach is general and can be adapted, depending on the reference signal used (single‐site or remote‐reference).

In field data, hodograms of the prediction residuals basically confirm the directional noise model assumed in DNC. An example is presented in which the DNC technique has been applied to a single‐site MT survey carried out in northern Italy, where the signal was heavily corrupted by noise with prevailing directional properties due to the densely urbanized area. MT apparent resistivities and phases obtained at the site of the survey before and after DNC are presented and discussed.
Downward continuation of (Vs)L profile in 1P (time‐domain) with linear excitation using point electrodes. A short notet1
Authors: Apparao A. , Sastry R. Sivarama , Sarma V. Subrahmanya
Published: April 2006

It is seen that the apparent chargeability (Ma)L anomaly over a 2D graphite body splits into two distinct (Vs)L anomalies which closely follow the apparent resistivity profile. This suggests that the electric field amplitude is distorted due to a superficial inhomogeneity creating a (Vs)L anomaly, which bears no relation to the polarized body. The target depth obtained by continuation of such a profile is therefore, not acceptable.
3D depth migration by rotated McClellan filters1
Authors: Biondi Biondo , Palacharla Gopal
Published: April 2006

The application of McClellan transformations considerably reduces the computational cost of 3D wavefield depth extrapolation by explicit convolutional methods. The accuracy of migration methods based on McClellan transformation depends on how well the transformation filter (cos !;κ!;) is approximated; errors in this approximation cause anisotropy in the extrapolation operator and frequency dispersion in the migrated results. The anisotropy can be greatly reduced by rotating the approximate filter by 45° and averaging the rotated filter with the original filter. The application of the rotated filter yields a migration method that correctly images very steep dips, with little or no additional computational cost. McClellan migration with the improved circular response enhances the imaging of synthetic and real data.
2.5D full‐wavefield viscoacoustic invercion1
Authors: Liao Oingbo , McMechan George A.
Published: April 2006

Full‐wavefield inversion for distributions of acoustic velocity, density and Q on a vertical slice through a25D model is implemented for common‐source gathers in a cross‐hole geometry. The wavefield extrapolation used is 3D, so all geometrical spreading, scattering, reflection, and transmission effects are correctly and automatically compensated for. In order to keep the number of unknowns tractable, application was limited to 2.5D models of known geometry; the latter assurnes a prior step, such as tomography, to fix the layer geometries. With the model geometry fixed, reliable solutions are obtained using synthetic data from only two independent source locations. Solutions from data with noisy and missing traces are comparable to those from noise‐free data, but with higher residuals. When the source locations are spatially widely separated, conunon‐source gathers may be summed and treated as a single wavefield to yield the same model estimates as when the individual source wavefields are treated separately, at substantially reduced cost. Inversions for full 3D parameter distributions can be handled with the same software, requiring only solution for more unknowns.
Interpretation of slingram conductivity mapping in near‐surface geophysics: using a single parameter fitting with 1D model1
Authors: Guérin R. , Méhéni Y. , Rakotondrasoa G. , Tabbagh A.
Published: April 2006

Electrical conductivity mapping is a prerequisite tool for hydrogeological or environmental studies. Its interpretation still remains qualitative but advantages can be expected from a quantitative approach. However a full 3D interpretation is too laborious a task in comparison with the limited cost and time which are involved in the majority of such field studies. It is then of value to define the situations where lateral variations are sufficiently smooth for a 1D model to describe correctly the underlying features.

For slingram conductivity measurements, criteria allowing an approximate 1D inversion are defined: these mainly consist of a limited rate of variation over three times the intercoil spacing.

In geological contexts where the weathering has generated a conductive intermediate layer between the underlying sound rock and the soil, this processing can be applied to determine the thickness of the conductive layer from the apparent resistivity map when the other geoelectrical parameters are known. The examples presented illustrate this application.
Bayesian inference, Gibbs' sampler and uncertainty estimation in geophysical inversion1
Authors: Sen Mrinal K. , Stoffa Paul L.
Published: April 2006

The posterior probability density function (PPD), σ(m|dobs), of earth model m, where dobs are the measured data, describes the solution of a geophysical inverse problem, when a Bayesian inference model is used to describe the problem. In many applications, the PPD is neither analytically tractable nor easily approximated and simple analytic expressions for the mean and variance of the PPD are not available. Since the complete description of the PPD is impossible in the highly multi‐dimensional model space of many geophysical applications, several measures such as the highest posterior density regions, marginal PPD and several orders of moments are often used to describe the solutions. Calculation of such quantities requires evaluation of multidimensional integrals. A faster alternative to enumeration and blind Monte‐Carlo integration is importance sampling which may be useful in several applications. Thus how to draw samples of m from the PPD becomes an important aspect of geophysical inversion such that importance sampling can be used in the evaluation of these multi‐dimensional integrals. Importance sampling can be carried out most efficiently by a Gibbs' sampler (GS). We also introduce a method which we called parallel Gibbs' sampler (PGS) based on genetic algorithms (GA) and show numerically that the results from the two samplers are nearly identical.

We first investigate the performance of enumeration and several sampling based techniques such as a GS, PGS and several multiple maximum a posteriori (MAP) algorithms for a simple geophysical problem of inversion of resistivity sounding data. Several non‐linear optimization methods based on simulated annealing (SA), GA and some of their variants can be devised which can be made to reach very close to the maximum of the PPD. Such MAP estimation algorithms also sample different points in the model space. By repeating these MAP inversions several times, it is possible to sample adequately the most significant portion(s) of the PPD and all these models can be used to construct the marginal PPD, mean) covariance, etc. We observe that the GS and PGS results are identical and indistinguishable from the enumeration scheme. Multiple MAP algorithms slightly underestimate the posterior variances although the correlation values obtained by all the methods agree very well. Multiple MAP estimation required 0.3% of the computational effort of enumeration and 40% of the effort of a GS or PGS for this problem. Next, we apply GS to the inversion of a marine seismic data set to quantify uncertainties in the derived model, given the prior distribution determined from several common midpoint gathers.
Reflection from a deep interface in a strongly heterogeneous layered medium1
Authors: Lewicki Pawel , Burridge Robert
Published: April 2006

We consider the problem of acoustic pulse propagation through a layered medium with a reflector at one end. The fluctuations in the medium properties are assumed to be strong, i.e. of finite amplitude, rapid in comparison to the typical wavelength and to have statistical structure. The depth of the reflector is assumed to be large in comparison to the wavelength. In this regime, simple formulae for the reflected pulse and its arrival time at the surface are obtained. The amplitude of the pulse is broadened and attenuated as a result of multiple scattering: the fine‐layered structure of the medium can be characterized by a single constant which appears in the formula for the limiting waveform and which measures the size of the fluctuations in the medium. Within the theory, the commonly observed discrepancy between the integrated sonic traveltime and the seismic traveltime can be studied and understood. The theory is a natural extension of the long‐wavelength effective medium theory of Backus. The analysis is rigorous and based on the invariant embedding technique.
Seismic migration and absorbing boundaries with a one‐way wave system for heterogeneous media1
Authors: Dai Nanxun , Vafidis Antonios , Kanasewich Ernest R.
Published: April 2006

A first‐order one‐way wave system has been created based on characteristic analysis of the acoustic wave system and optimization of the dispersion relation. We demonstrate that this system is equivalent to a third‐order scalar partial‐differential equation which, for a homogeneous medium, reduces to a form similar to the 45° paraxial wave equation. This system describes accurately waves propagating in a 2D heterogeneous medium at angles up to 75°.

The one‐way wave system representing downgoing waves is used for a modified reverse time migration method. As a wavefield extrapolator in migration, the downgoing wave system propagates the reflection events backwards to their reflectors without scattering at the discontinuities in the velocity model. Hence, images with amplitudes proportional to reflectivity can be obtained from this migration technique. We present examples of the application of the new migration method to synthetic seismic data where P‐P reflections P‐SV converted waves are present.

Absorbing boundaries, useful in the generation of synthetic seismograms, have been constructed by using the one‐way wave system. These boundaries absorb effectively waves impinging over a wide range of angles of incidence.
Simulated climate change: are passive greenhouses a valid microcosm for testing the biological effects of environmental perturbations?
Authors: KENNEDY ANDREW D.
Published: April 2006

This paper considers the use of passive greenhouse apparatus in field experiments investigating the biological consequences of climate change. The literature contains many accounts of such experiments claiming relevance of greenhouse treatment effects to global change scenarios. However, inadequacies in microclimate monitoring, together with incomplete understanding of greenhouse modes of action, cast doubt upon such claims. Here, treatment effects upon temperature (magnitude, range, variation, rates of change), moisture (humidity, precipitation, soil water content), light (intensity, spectral distribution), gas composition, snow cover, and wind speed are reviewed in the context of Intergovernmental Panel on Climate Change (IPCC) predictions. It is revealed that greenhouses modify each of these potentially limiting factors in a complex and interactive manner, but that the relationship between this modification and forecast conditions of climate change is poor. Interpretation of biological responses, and their extrapolation to predictive models, is thus unreliable. In order that future greenhouse experiments may overcome criticisms of artefact and lack of rigour, two amendments to methodology are proposed: (1) objective‐orientated design of greenhouse apparatus (2) multiple controls addressing individual environmental factors. The importance of a priori testing of microclimate treatment effects is stressed.
Growth response of subalpine fir (Abies lasiocarpa) to climate in the Olympic Mountains, Washington, USA
Authors: ETTL GREGORY J. , PETERSON DAVID L.
Published: April 2006

Growth response of subalpine fir (Abies lasiocarpa) to climate was studied across its local geographical and elevation range in the Olympic Mountains, Washington. A dendroecological analysis of subalpine fir across a range of elevations (1350‐1850 m) and annual precipitation (125‐350 cm y−1), was used to compare environmental factors affecting growth. Climate‐growth relationships were explored using Pearson product‐moment correlation coefficients; partial correlation analysis was used to assess relationships among site chronologies and climatic variables. Radial growth is negatively correlated with winter precipitation at high elevation and wet sites, but not at low and middle elevation dry sites. Growth is positively correlated with current growing season temperature at all sites; however, growth is negatively correlated with previous year August temperature, indicating that climate affects growth in subsequent years. Positive correlations between growth and summer precipitation during the growing season at low and middle elevation dry sites suggest that soil moisture is partially limiting to growth on these sites. If the climate of the Pacific Northwest becomes warmer and drier, then subalpine fir growth may increase at high elevation and wet sites, but may decrease at lower elevation dry sites in the Olympic Mountains. However, the growth response of subalpine fir to potentially rapid climate change will not be uniform because subalpine fir grows over a wide range of topographic features, habitats, and local climates at different geographical scales. A comparison of growth response to current growing season temperature suggests that the temperature‐related growth response of subalpine fir is not adequately described by the parabolic curve used in JABOWA‐based models.
A new technique for estimating rates of carboxylation and electron transport in leaves of C3 plants for use in dynamic global vegetation models
Authors: BEERLING D.J. , QUICK W.P.
Published: April 2006

The possible responses of the terrestrial biosphere to future CO2 increases and associated climatic change are being investigated using dynamic global vegetation models (DG VMs) which include the Farquhar et al. (1980) biochemical model of leaf assimilation as the primary means of carbon capture. This model requires representative values of the maximum rates of Rubisco activity, Vmax, and electron transport, Jmax, for different vegetation types when applied at the global scale. Here, we describe an approach for calculating these values based on measurements of the maximum rate of leaf photosynthesis (Amax) 13C discrimination. The approach is tested and validated by comparison with measurements of Rubisco activity assayed directly on wild‐type and transgenic Nicotiana tabacum (tobacco) plants with altered Rubisco activity grown under ambient and elevated CO2 mole fractions with high and low N‐supply. Vmax and Jmax values are reported for 18 different vegetation types with global coverage. Both variables were linearly related reinforcing the idea of optimal allocation of resources to photosynthesis (light harvesting vs. Rubisco) at the global scale. The reported figures should be of value to the further development of vegetation and ecosystem models employing mechanistic DGVMs.
The influence of elevated CO2 on community structure, biomass and carbon balance of mediterranean old‐fleld microcosms
Authors: NAVAS MARIE‐LAURE , GUILLERM JEAN‐LOUIS , FABREGUETTES JACQUES , ROY JACQUES
Published: April 2006

We studied the effects of a doubling of atmospheric CO2 concentration on intact monoliths of Mediterranean grassland in growth chambers where climatic field conditions were simulated. During the six month growing season, changes in community structure were monitored by quantifying species richness and cover. The CO2 exchange of microcosms was measured continuously and the resulting quantity and quality of biomass were evaluated. Species richness and cover did not respond to elevated CO2. After one month of treatment, CO2 exchange measured during the day did not differ between CO2 levels but the night respiration was two‐fold higher under elevated CO2. Stimulations of both day and night CO2 flux by short‐term CO2 enrichment were recorded several times during the growing season. These results suggest that despite some downward adjustment of photosynthesis, net canopy photosynthesis was stimulated by elevated CO2, but this stimulation was compensated for by an increased respiration. The 20% stimulation of final phytomass under elevated CO2 was not significant: it resulted from unchanged live plant matter but a significant, 100% increase in litter accumulation. These results suggest that in low‐productivity Mediterranean herbaceous systems, the greatest effect of CO2 is not on the storage of carbon in biomass but on the turnover of the carbon in the plants.
Collembolan water relations and environmental change in the maritime Antarctic
Authors: BLOCK WILLIAM , HARRISSON PAUL M.
Published: April 2006

Abstrach

The water status of the collembolan Cryptopygtus antarcticus (Willem) was investigated from April 1984 to December 1987 at Signy Island, maritime Antarctic, by monthly field sampling to determine body water content. Water content, expressed either as the weight of water per unit dry weight or as a proportion of fresh weight, exhibited both a seasonal cycle and an upward trend over the 44‐month study, both of which were highly significant. On an annual basis, body water content was at a minimum (1.21 g g−1) in July and maximal (1.98 g g−1) in September, whilst over the entire study water contents increased from 1.3 to 2.0 g g−1 (or 57‐66% of fresh weight) calculated from the fitted linear regression line. Field water contents were below those found for this species in culture (2.9‐5.9 g g−1). Individual C. antarcticus survived experimental loss of 20% of their body water with a resultant significant rise in haemolymph osmolarity from 285 to 397 mOsm L−1 and there was no evidence of osmoregulation under the experimental conditions of 20 °C and 35% relative humidity. The cuticular permeability (mean conductance) of individual Collembola in dry air increased exponentially with temperature over the range D‐45 °C (Q10= 2.0) showing no control of water loss.

The physiological response of C. antarcticus suggests that it experiences water stress in its maritime Antarctic habitats with significant seasonal variations of body water content, which correlate with annual cycles of water availability. It is concluded that the significant rise in its mean body water content over the 44‐month field study was associated with increased glacial ablation due to higher levels of irradiation and windspeed making available more liquid water. Analyses of climate records for Signy Island from 1947 to 1990 showed that mean monthly air temperature rose by 0.93 °C over this period and by 2.29 °C during the 1980s, both statistically significant increases. Mean monthly windspeeds also increased significantly during 1970–90, and it is suggested that this parameter is the primary climatic driving force behind the increase in glacial ablation during the last two decades. The field water status of species such as C. antarcticus may reflect changes in the patterns of atmospheric circulation, associated with the circumpolar vortex, through increased ozone depletion due to increased tropospheric concentrations of halocarbons.
Whole plant respiration and photosynthesis of wheat under increased CO2 concentration and temperature: long‐term vs. short‐term distinctions for modelling
Authors: GIFFORD ROGER M.
Published: April 2006

Short‐ and long‐term effects of elevated CO2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO2 concentrations. Whole plant CO2 exchange was measured on a 24 h basis and measurement conditions varied both to observe short‐term effects and to determine the growth respiration coefficient (rg), dry weight maintenance coefficient (rm), basal (i.e. dark acclimated) respiration coefficient (rg), and 24 h respiration:photosynthesis ratio (R:P). There was no response of rg to short‐term variation in CO2 concentration. For plants with adequate N supply, rg was unaffected by the growth‐CO2 despite a 10% reduction in the plant's N concentration (%N). However, rm was decreased 13%, and rb was decreased 20% by growth in elevated CO2 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO2. Whole plant respiration responded to short‐term variation of ± 5 °C around the growth temperature with low sensitivity (Q10= 1.8 at 15 °C, 1.3 at 30 °C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S‐shape declining between 25 and 30 °C. While rm, increased, rg decreased when growth temperature increased between 15 and 20 °C. Above 20 °C rm became temperature insensitive while rg increased with growth temperature. Despite these complex component responses, R:P increased only from 0.40 to 0.43 between 15° and 30 °C growth temperatures. Giving the plants a step increase in temperature caused a transient increase in R:P which recovered to the pre‐transient value in 3 days. It is concluded that use of a constant R:P with respect to average temperature and CO2 concentration may be a more simple and accurate way to model the responses of wheat crop respiration to ‘climate change’ than the more complex and mechanistically dubious functional analysis into growth and maintenance components.
Significance of ocean carbonate budgets for the global carbon cycle
Authors: HOLLIGAN PATRICK M. , ROBERTSON JANE E.
Published: April 2006

Changes in the trace gas composition of the atmosphere over glacial–interglacial cycles are linked to changes in the oceanic carbon cycle. This paper examines the role of biologically driven fluxes of organic and inorganic carbon in modifying the carbon dioxide chemistry of the oceans, and the corresponding implications for the partitioning of CO2 between the atmosphere and ocean. Relevant details of the marine carbon system are presented together with an assessment of the significance of remineralization and dissolution processes. Recent estimates of the marine carbonate fluxes show significant uncertainties and inconsistencies which must be resolved in order to assess fully the role of the oceans' biota in the marine carbon system. Various types of ocean carbon cycle models have been developed in order to interpret the changes in past atmospheric carbon dioxide. Some take account of the role of the oceans' biota, focussing in the main on the cycling of organic matter. Relatively few have considered the role of the carbonate pump and the subtle interactions between organic and inorganic carbon cycling. The significance of carbonate formation and dissolution, and of the effects of global change on the marine carbonate system, for air–sea fluxes of CO2 are discussed. Finally some recommendations for future research are made in order to improve our understanding of how spatial and temporal variation in marine carbonate fluxes, in conjunction with processes determining the oxidation and burial of organic matter in the oceans, affect levels of CO2 in the atmosphere.
Effects of growth under elevated UV‐B on photosynthesis and isoprene emission in Quercus gambelii and Mucuna pruriens
Authors: HARLEY PETER , DEEM GREG , FLINT STEPHAN , CALDWELL MARTYN
Published: April 2006

Increasing surface levels of UV‐B resulting from stratospheric ozone reduction directly affect tropospheric photochemistry. There may also be indirect tropospheric effects due to changes in emission of organic compounds from vegetation. We treated woody and herbaceous isoprene‐emitting species in the field with supplemental UV‐B simulating 30% ozone depletion. For Quercus gambelii, photosynthesis and isoprene emission were significantly greater in elevated UV‐B treatments when expressed on a leaf area basis, but not on a leaf mass basis. Leaves of Mucuna pruriens, however, showed no significant differences in photosynthesis or isoprene emission between treatments, nor when exposed for 45 min to acute high levels of UV‐B. Elevated UV‐B during growth did not elicit significant isoprene emission from Acer platanoides, a non‐emitting species. Other potential UV‐B effects, such as changes in leaf area or species composition, which may influence regional isoprene emissions, should be examined.
Growth at elevated carbon dioxide concentration reduces hydraulic conductance in alfalfa and soybean
Authors: BUNCE JAMES A.
Published: April 2006

Hydraulic conductances of alfalfa and soybean plants grown in controlled environment chambers at the current ambient carbon dioxide concentration and at twice the current ambient concentration were determined from measurements of transpiration rate and leaf and stem water potentials in the growth conditions. Growth at elevated carbon dioxide concentration reduced both transpiration rate and hydraulic conductance from the soil to the leaf in both species. Hydraulic conductance from the soil to the base of the stem was also lower at elevated carbon dioxide in soybean, but not alfalfa. These measurements identified the stem to leaf hydraulic pathway as a major target of the carbon dioxide effect in both species. The conductance of excised stem segments was much less in plants grown at elevated carbon dioxide in soybeans.
Effects of ultraviolet radiation on corals and other coral reef organisms
Authors: SHICK J. MALCOLM , LESSER MICHAEL P. , JOKIEL PAUL L.
Published: April 2006

The discovery of the importance of solar ultraviolet radiation (UVR) as a factor affecting the biology of coral reefs dates only to about 1980. Interest has heightened during the past five years owing to the demonstration of loss of stratospheric ozone through human activities. We have only begun to document gross, qualitative effects of UVR on coral reef organisms, usually in experiments comparing the biological response to the presence or absence of UVR through the use of UV‐cutoff filters, or to varying levels of UVR in transplantation studies. Most such studies have not distinguished between the effects of UVA (320–400 nm) and those of UVB (290–320 nm), although in the context of global change involving stratospheric ozone loss, it is the latter wavelengths that are relevant. To date we have been addressing physiological and ecological questions, not yet attempting to evaluate quantitatively the impact of forecast increases in solar UVB penetration. Interacting and synergistic effects of UVR with increased temperature, pollutants, sedimentation, visible light, etc. have scarcely been studied but will be essential to understanding and predicting the fate of coral reefs under conditions of global change.

Here we comprehensively review the effects of UVR on corals and other reef macroorganisms, mindful that although much is known of proximal effects, little of this knowledge is directly useful in making long‐term predictions regarding the health of coral reefs. We conclude that even small anthropogenic increases in UVB levels will have sublethal physiological manifestations in corals and other reef organisms, but that this will have relatively small impact on the distribution of reef corals and coral reefs, perhaps affecting their minimum depths of occurrence.
Seasonal net carbon dioxide exchange of a beech forest with the atmosphere
Authors: VALENTINI R. , De ANGELIS P. , MATTEUCCI G. , MONACO R. , DORE S. , MUCNOZZA G. E. SCARASCIA
Published: April 2006

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.
A feasible Global Carbon Cycle Observing System: a plan to decipher today's carbon cycle based on observations
Authors: TANS PIETER P. , BAKWIN PETER S. , GUENTHER DOUGLAS W.
Published: April 2006

A design is presented to continuously monitor the transfers of carbon between the atmosphere and the terrestrial biosphere and oceans on large spatial scales. A consideration of the expected signal‐to‐noise suggests that a very cost effective method is to perform repeated measurements of the mixing ratios of trace gases in the atmospheric column at an increased number of sites compared to the present geographical coverage. This can be accomplished with existing technology, provided that more automation is applied to the sampling process as well as to the analytical procedures. The resulting data will be crucial to test extrapolations based on direct flux measurements to regional and global scales.
Effects of enhanced UV‐B radiation on pea (Pisum sativum L.) grown under field conditions in the UK
Authors: MEPSTED R. , PAUL N.D. , STEPHEN J. , CORLETT J. E. , NOGUÉS S. , BAKER N. R. , JONES H. G. , AYRES P. G.
Published: April 2006

A new modulated lamp system is described. This system has successfully provided an ultraviolet‐B (UV‐B) supplement in proportion to ambient UV‐B. The modulated system was used to simulate the UV‐B environment resulting from an annual mean reduction of 15% in the stratospheric ozone under UK field conditions, but taking account of seasonal variation in depletion. The effects of this enhanced level of UV‐B on the growth, physiology and yield of four cultivars of pea were assessed.

Enhanced UV‐B resulted in small reductions in the number of stems and total stem length per plant (respectively 4.7 and 8.7%). There were also significant decreases in the dry weight of peas (10.1%), pods (10.3%) and stems (7.8%) per plant. UV‐B treatment had no effect on the number of peas per pod or average pea weight, but did significantly reduce (12.1%) the number of pods per plant. This decrease in pod number was partly due to enhanced abscission of pods during the final month of plant growth. UV‐B treatment had no significant effect on chlorophyll fluorescence characteristics or CO2assimilation rate per unit leaf area. These results are consistent with previous controlled environment experiments, and suggest that reduction in yield may be due to direct effects of UV‐B on plant growth rather than a decrease in photosynthetic capacity per unit leaf area.
Effects of elevated CO2 on model calcareous grasslands: community, species, and genotype level responses
Authors: LEADLEY P. W. , STÖCKLIN J.
Published: April 2006

We investigated the responses of model calcareous grassland communities to three CO2concentrations: 330, 500, and 660 μL L‐1, The communities were composed of six species, Bromus erectus Hudson, Festuca ovina L., Prunella vulgaris L., Prunella grandiflora (L.) Scholler, Hieracium pilosella L., and Trifolium repens L., that are native to the calcareous grasslands of Europe. Genotypic variation in CO2 response was studied in Bromus erectus and Festuca ovina. Plants were harvested after c. 126 days of growth. We found that:

At the community level, there were marginally significant (0.1≥P > 0.05) increases in leaf and litter dry weight with increasing CO2 concentration.

There were significant differences between species in CO2 response, including both negative and positive responses. Prunella vulgaris had a significant negative response; Hieraciunt pilosella and Festuca ovina had significant positive responses; Prunella grandiflora had a marginally significant positive response; and Bromus erectus and Trifotium repens did not have significant responses.

There was significant variation among genotypes in the response to elevated CO2 in Bromus erectus, but not in Festuca ovina.

Based on the observed species‐ and genotype‐level variation in CO2 response of calcareous grassland plants in this and other studies, we speculate that increasing atmospheric CO2 concentrations will alter community structure in calcareous grasslands.
Possible impacts of environmental change on pelagic fish recruitment: modelling anchovy transport by advective processes in the southern Benguela
Authors: SHANNON LYNNE J. , NELSON GREVILLE , CRAWFORD ROBERT J. M. , BOYD ALAN J.
Published: April 2006

Climate change may influence pelagic fish by altering advective processes or by changing where fish choose to spawn. Using a simulation model, the effects of altered advection and spatial distribution of spawning by anchovy on recruitment off South Africa were explored. Cape anchovy Engraulis capensis spawn on the region of the Agulhas Bank, south of South Africa. Currents transport eggs and larvae to nursery areas. Transport of eggs and larvae was modelled using a flow field based on averaged Acoustic Doppler Current Profile data, Feasible Scenarious of altered advection were modelled. For modelling purposes, the ocean surrounding South Africa was divided into blocks with dimensions of a quarter‐degree latitude by a quarter‐degree longitude. Acoustically measured distributions of spawner biomass for the years 1986–92 were used to calculate egg production per block. In the model, batches were released from each of these blocks each day of the spawning season. The modelling study indicates that passive transport of young anchovy may account for a substantial proportion of year‐class variability. Model results show that distribution of spawners influences the distribution of young of the year, as well as the number and the location of advective losses across offshore boundaries.
Climate implications of biomass burning since the 19th century in eastern North America
Authors: CLARK JAMES S. , STOCKS BRIAN J. , RICHARD PIERRE J.H.
Published: April 2006

Recent predictions that tropospheric aerosols have counterbalanced greenhouse warming assume aerosol emissions were low beforead1850 and then increased dramatically with industrialization of the Northern Hemisphere and biomass burning in the Tropics. We assembled the lake sediment record of emissions across northeastern North America, where temperatures are predicted to have been substantially affected by industrial aerosols. Sediment evidence suggests a systematic shift in source and an overall decline in emissions since the 19th century. The geographical shift results from high presettlement emissions from wildfires in the Midwest that collapsed with tillage and fire suppression. Meanwhile, emissions were increasing in the North‐east with European settlement. These regional changes produced a shift from the continental interior to the North‐east. An overall decline results because decreases in the Midwest more than compensate for increases in the North‐east. Results suggest the Central Plains as an important source of emissions in the recent past, consistent with pioneer accounts of dense smoke clouds emanating from prairie in the 19th century. Contrary to recent models that suggest increased 20th century combustion emissions could have offset warming effects of rising greenhouse gases, our data suggest that aerosols could have actually decreased over this interval. Although we cannot directly quantify aerosols from our methods, the emissions of large particles suggest assumptions of 20th century aerosol declines should be reconsidered.
NETHERLANDS
Authors: Roseveare G. M.
Published: April 2006

This contribution from the Commonwealth Bureau of Pastures and Field Crops gives brief notes on questions of current concern in countries where conditions and types of grassland resemble those in Great Britain. Items in this issue are provided by G. M. Roseveare (Netherlands); R. Peter Jones (Sweden); G. Melvyn Howe (South Africa), and P. Mackintosh (Canada).
HILL DRAINAGE
Authors: Thompson C. K.
Published: April 2006

He could not deny that, looking round upon the dreary region, and seeing nothing but bleak fells and naked trees, hills obscured by fog and flats covered with undulations, he did for sometime suffer melancholy to prevail upon him, and wished himself safe at home.(1)