bluuebunny/crossref_metadata_2025_split huggingface dataset:
P‐wave stacking‐velocity tomography for VTI media
Authors: Grechka Vladimir , Pech Andres , Tsvankin Ilya 
Published: March 2003

ABSTRACTA major complication caused by anisotropy in velocity analysis and imaging is the uncertainty in estimating the vertical velocity and depth scale of the model from surface data. For laterally homogeneous VTI (transversely isotropic with a vertical symmetry axis) media above the target reflector, P‐wave moveout has to be combined with other information (e.g. borehole data or converted waves) to build velocity models for depth imaging. The presence of lateral heterogeneity in the overburden creates the dependence of P‐wave reflection data on all three relevant parameters (the vertical velocity VP0 and the Thomsen coefficients ε and δ) and, therefore, may help to determine the depth scale of the velocity field.Here, we propose a tomographic algorithm designed to invert NMO ellipses (obtained from azimuthally varying stacking velocities) and zero‐offset traveltimes of P‐waves for the parameters of homogeneous VTI layers separated by either plane dipping or curved interfaces. For plane non‐intersecting layer boundaries, the interval parameters cannot be recovered from P‐wave moveout in a unique way. Nonetheless, if the reflectors have sufficiently different azimuths, a priori knowledge of any single interval parameter makes it possible to reconstruct the whole model in depth. For example, the parameter estimation becomes unique if the subsurface layer is known to be isotropic. In the case of 2D inversion on the dip line of co‐orientated reflectors, it is necessary to specify one parameter (e.g. the vertical velocity) per layer.Despite the higher complexity of models with curved interfaces, the increased angle coverage of reflected rays helps to resolve the trade‐offs between the medium parameters. Singular value decomposition (SVD) shows that in the presence of sufficient interface curvature all parameters needed for anisotropic depth processing can be obtained solely from conventional‐spread P‐wave moveout. By performing tests on noise‐contaminated data we demonstrate that the tomographic inversion procedure reconstructs both the interfaces and the VTI parameters with high accuracy. Both SVD analysis and moveout inversion are implemented using an efficient modelling technique based on the theory of NMO‐velocity surfaces generalized for wave propagation through curved interfaces.
Depth imaging in anisotropic media by symmetric non‐stationary phase shift
Authors: Ferguson Robert J. , Margrave Gary F. 
Published: March 2003

ABSTRACTWe present a new depth‐imaging method for seismic data in heterogeneous anisotropic media. This recursive explicit method uses a non‐stationary extrapolation operator to allow lateral velocity variation, and it uses the relationship between phase angle and the spectral coordinates of seismic data to allow velocity variation with phase angle. A qualitative comparison of migration impulse responses suggests that, for an equivalent cost, the symmetric non‐stationary phase‐shift (SNPS) operator is superior to the phase‐shift plus interpolation (PSPI) operator, for very large depth intervals. To demonstrate the potential of the new method, seismic data from a physical model acquired over a transversely isotropic medium are imaged using a shot‐record migration based on the SNPS operator.
3D free‐boundary conditions for coordinate‐transform finite‐difference seismic modelling
Authors: Hestholm Stig , Ruud Bent 
Published: March 2003

New alternative formulations of exact boundary conditions for arbitrary three‐dimensional (3D) free‐surface topographies on seismic media have been derived. They are shown to be equivalent to previously published formulations, thereby verifying the validity of each set of formulations. The top of a curved grid represents the free‐surface topography while the interior of the grid represents the physical medium. We assume the velocity–stress version of the viscoelastic wave equations to be valid in this grid before transforming the equations to a rectangular grid. In order to perform the numerical discretization we apply the latter version of the equations for seismic wave propagation simulation in the medium. The numerical discretization of the free‐surface topography boundary conditions by second‐order finite differences (FDs) is shown, as well as the spatially unconditional stability of the resulting system of equations. The FD order is increased by two for each point away from the free surface up to eight, which is the order used in the interior. We use staggered grids in both space and time and the second‐order leap‐frog and Crank– Nicholson methods for wavefield time propagation. An application using parameters typical of teleseismic earthquakes and explosions is presented using a 200 × 100 km2 area of real topography from southwestern Norway over a homogeneous medium. A dipping plane wave simulates a teleseismic P‐wave incident on the surface topography. Results show conversion from P‐ to Rg‐ (short period fundamental mode Rayleigh) waves in the steepest and/or roughest topography, as well as attenuated waves in valleys and fjords. The codes are parallelized for simulation on fast supercomputers and PC‐clusters to model high frequencies and/or large areas.
Growth, light interception and yield responses of spring wheat (Triticum aestivum L.) grown under elevated CO2 and O3 in open‐top chambers
Authors: Mulholland B. J. , Craigon J. , Black C. R. , Colls J. J. , Atherton J. , Landon G. 
Published: March 2003

AbstractSpring wheat cv. Minaret was grown to maturity under three carbon dioxide (CO2) and two ozone (O3) concentrations in open‐top chambers (OTC). Green leaf area index (LAI) was increased by elevated CO2 under ambient O3 conditions as a direct result of increases in tillering, rather than individual leaf areas. Yellow LAI was also greater in the 550 and 680 μmol mol–1 CO2 treatments than in the chambered ambient control; individual leaves on the main shoot senesced more rapidly under 550 μmol mol–1 CO2, but senescence was delayed at 680 μmol mol–1 CO2. Fractional light interception (f) during the vegetative period was up to 26% greater under 680 μmol mol–1 CO2 than in the control treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during canopy development, plants grown under elevated CO2 were taller at anthesis and stem and ear biomass were 27 and 16% greater than in control plants. At maturity, yield was 30% greater in the 680 μmol mol–1 CO2 treatment, due to a combination of increases in the number of ears per m–2, grain number per ear and individual grain weight (IGW).Exposure to a seasonal mean (7 h d–1) of 84 nmol mol–1 O3 under ambient CO2 decreased green LAI and increased yellow LAI, thereby reducing both f and accumulated intercepted radiation by ≈ 16%. Individual leaves senesced completely 7–28 days earlier than in control plants. At anthesis, the plants were shorter than controls and exhibited reductions in stem and ear biomass of 15 and 23%. Grain yield at maturity was decreased by 30% due to a combination of reductions in ear number m–2, the numbers of grains per spikelet and per ear and IGW. The presence of elevated CO2 reduced the rate of O3‐induced leaf senescence and resulted in the maintenance of a higher green LAI during vegetative growth under ambient CO2 conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding elevated CO2 treatments in the absence of elevated O3. Thus, although the presence of elevated CO2 reduced the damaging impact of ozone on radiation interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be susceptible to O3‐induced injury during anthesis irrespective of the atmospheric CO2 concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grain filling are discussed.
Impact of elevated atmospheric CO2concentration on soil microbial biomass and activity in a complex, weedy field model ecosystem
Authors: Kampichler Christian , Kandeler Ellen , Bardgett Richard D. , Jones T. Hefin , Thompson Lindsey J. 
Published: March 2003

AbstractAlthough soil organisms play an essential role in the cycling of elements in terrestrial ecosystems, little is known of the impact of increasing atmospheric CO2concentrations on soil microbial processes. We determined microbial biomass and activity in the soil of multitrophic model ecosystems housed in the Ecotron (NERC Centre for Population Biology, Ascot, UK) under two atmospheric CO2concentrations (ambient vs. ambient + 200 ppm). The model communities consist of four annual plant species which naturally co‐occur in weedy fields and disturbed ground throughout southern England, together with their herbivores, parasitoids and soil biota. At the end of two experimental runs lasting 9 and 4.5 months, respectively, root dry weight and quality showed contradictory responses to elevated CO2concentrations, probably as a consequence of the different time‐periods (and hence number of plant generations) in the two experiments. Despite significant root responses no differences in microbial biomass could be detected. Effects of CO2concentration on microbial activity were also negligible. Specific enzymes (protease and xylanase) showed a significant decrease in activity in one of the experimental runs. This could be related to the higher C:N ratio of root tissue. We compare the results with data from the literature and conclude that the response of complex communities cannot be predicted on the basis of oversimplified experimental set‐ups.
Global change and root function
Authors: VAN NOORDWIJK MEINE , MARTIKAINEN PERTTI , BOTTNER PIERRE , CUEVAS ELVIRA , ROULAND CORNINE , DHILLION SHIVCHARN S. 
Published: March 2003

AbstractGlobal change includes land‐use change, elevated CO2concentrations, increased temperature and increased rainfall variability. All four aspects by themselves and in combination will influence the role of roots in linking below‐ and above‐ground ecosystem function via organic and inorganic resource flows. Root‐mediated ecosystem functions which may be modified by global change include below‐ground resource (water, nutrients) capture, creation and exploitation of spatial heterogeneity, buffering of temporal variations in above‐ground factors, supply and storage of C and nutrients to the below‐ground ecosystem, mobilization of nutrients and C from stored soil reserves, and gas exchange between soil and atmosphere including the emission from soil of greenhouse gases.The theory of a functional equilibrium between root and shoot allocation is used to explore predicted responses to elevated CO2in relation to water or nutrient supply as limiting root function. The theory predicts no change in root:shoot allocation where water uptake is the limiting root function, but substantial shifts where nutrient uptake is (or becomes) the limiting function. Root turnover will not likely be influenced by elevated CO2, but by changes in regularity of water supply. A number of possible mechanisms for root‐mediated N mineralization is discussed in the light of climate change factors. Rhizovory (root consumption) may increase under global change as the balance between plant chemical defense and adapted root consuming organisms may be modified during biome shifts in response to climate change. Root‐mediated gas exchange allows oxygen to penetrate into soils and methane (CH4) to escape from wetland soils of tundra ecosystems as well as tropical rice production systems. The effect on net greenhouse gas emissions of biome shifts (fens replacing bogs) as well as of agricultural land management will depend partly on aerenchyma in roots.
Global change, soil biodiversity, and nitrogen cycling in terrestrial ecosystems: three case studies
Authors: SWIFT M. J. , ANDRÉN O. , BRUSSAARD L. , BRIONES M. , COUTEAUX M. ‐M. , EKSCHMITT K. , KJOLLER A. , LOISEAU P. , SMITH P. 
Published: March 2003

AbstractThe relative contribution of different soil organism groups to nutrient cycling has been quantified for a number of ecosystems. Some functions, particularly within the N‐cycle, are carried out by very specific organisms. Others, including those of decomposition and nutrient release from organic inputs are, however, mediated by a diverse group of bacteria, protozoa, fungi and invertebrate animals. Many authors have hypothesized that there is a high degree of equivalence and flexibility in function within this decomposer community and thence a substantial extent of redundancy in species richness and resilience in functional capacity. Three case studies are presented to examine the relationship between soil biodiversity and nitrogen cycling under global change in ecosystem types from three latitudes, i.e. tundra, temperate grassland and tropical rainforest.In all three ecosystems evidence exists for the potential impact of global change factors (temperature change, CO2enrichment, land‐use‐change) on the composition and diversity of the soil community as well as on various aspects of the nitrogen and other cycles. There is, however, very little unequivocal evidence of direct causal linkage between species richness and nutrient cycling efficiency. Most of the changes detected are shifts in the influence of major functional groups of the soil biota (e.g. between microflora and fauna in decomposition). There seem to be few data, however, from which to judge the significance of changes in diversity within functional groups. Nonetheless the soil biota are hypothesized to be a sensitive link between plant detritus and the availability of nutrients to plant uptake. Any factors affecting the quantity or quality of plant detritus is likely to change this link. Rigorous experimentation on the relationships between soil species richness and the regulation or resilience of nutrient cycles under global change thus remains a high priority.
The effect of experimental ecosystem warming on CO2 fluxes in a montane meadow
Authors: Saleska Scott R. , Harte JohN. , Torn Margaret S. 
Published: March 2003

AbstractClimatic change is predicted to alter rates of soil respiration and assimilation of carbon by plants. Net loss of carbon from ecosystems would form a positive feedback enhancing anthropogenic global warming. We tested the effect of increased heat input, one of the most certain impacts of global warming, on net ecosystem carbon exchange in a Rocky Mountain montane meadow. Overhead heaters were used to increase the radiative heat flux into plots spanning a moisture and vegetation gradient. We measured net whole‐ecosystem CO2 fluxes using a closed‐path chamber system, relatively nondisturbing bases, and a simple model to compensate for both slow chamber leaks and the CO2 concentration‐dependence of photosynthetic uptake, in 1993 and 1994. In 1994, we also measured soil respiration separately. The heating treatment altered the timing and magnitude of net carbon fluxes into the dry zone of the plots in 1993 (reducing uptake by ≈100 g carbon m–2), but had an undetectable effect on carbon fluxes into the moist zone. During a strong drought year (1994), heating altered the timing, but did not significantly alter the cumulative magnitude, of net carbon uptake in the dry zone. Soil respiration measurements showed that when differences were detected in dry zone carbon fluxes, they were caused by changes in carbon input from photosynthesis, not by temperature‐driven changes in carbon output from soil respiration. When differences were detected in dry‐zone carbon fluxes, they were caused by changes in carbon input from photosynthesis, not by a temperature‐driven changes in carbon output from soil respiration. Regression analysis suggested that the reduction in carbon inputs from plants was due to a combination of two soil moisture effects: a direct physiological response to decreased soil moisture, and a shift in plant community composition from high‐productivity species to low‐productivity species that are more drought tolerant. These results partially support predictions that warming may cause net carbon losses from some terrestrial ecosystems. They also suggest, however, that changes in soil moisture caused by global warming may be as important in driving ecosystem response as the direct effects of increased soil temperature.
Seasonal trends of gross N mineralization in a natural calcareous grassland
Authors: Jamieson Nicola , Monaghan RosS. , Barraclough Declan 
Published: March 2003

AbstractThe effect of climate change on biologically mediated soil processes is likely to be complex and difficult to predict. The direct effect of changes in soil moisture and temperature may be obscured or even reversed by changes in the return of organic substrates to the soil. This paper is the first in a series reporting the results from a medium‐term investigation into the effect of simulated climate change on soil N mineralization in a seminatural calcareous grassland in southern England. Gross rates of N mineralization were determined by isotope dilution in plots subjected to winter warming (3 °C above ambient), enhanced or decreased summer rainfall and combinations of these treatments. The results from the control treatment, reported here, show a strong seasonality of gross N mineralization with rates highest in the Spring and Autumn and lowest in Summer. They indicate that water availability is the main restraint on microbial processes and plant growth and that, in the short term, enhanced summer rainfall is likely to be the main factor influencing nutrient turnover. Further papers will report the results from the imposed treatments.
Carry‐over of enhanced ultraviolet‐B exposure effects to successive generations of a desert annual: interaction with atmospheric CO2 and nutrient supply
Authors: Musil C. F. , Midgley G. F. , Wand S. J. E. 
Published: March 2003

AbstractThe performance of fifth generation offspring of a desert annual (Dimorphotheca sinuataDC.) were compared in the absence of UV‐B, under variable atmospheric CO2and nutrient supply, after four consecutive generations of concurrent exposure of their progenitors to UV‐B at ambient (seasonal range: 2.55–8.85 kJ m–2d–1) and enhanced (seasonal range: 4.70–11.41 kJ m–2d–1) levels. Offspring of progenitors grown under elevated UV‐B exhibited a diminished photosynthetic rate, a consequence of a reduced leaf density, and diminished foliar levels of carotenoids, polyphenolics and anthocyanins. Conversely, nonstructural carbohydrate and chlorophyllblevels were increased. Altered physiology was accompanied by reduced apical dominance and earlier flowering, features generally considered under photomorphogenic control, increased branching and inflorescence production and greater partitioning of biomass to reproductive structures, but diminished seed production. Many of these changes were magnified under nutrient limitation and intensified under atmospheric CO2enriched conditions. The latter disagrees with current opinion that elevated CO2may reduce detrimental UV‐B effects, at least over the long‐term. Observed correlations between seed production and polyphenolic, especially anthocyanin, levels in offspring, and indications of diminished lignification (thinner leaves, less robust stems and fewer lignified seeds set) all pointed to the involvement of the phenylpropanoid pathway in seed formation and plant structural development and its disruption during long‐term UV‐B exposure. Comparisons with earlier generations revealed trends with cumulative generations of enhanced UV‐B exposure of increasing chlorophyll b and nonstructural carbohydrates, decreasing polyphenolics and biomass allocation to vegetative structures, and diminishing seed production despite increasing biomass allocation to reproductive structures. Notwithstanding some physiological compensation (increased chlorophyll b), the accumulation and persistence of these ostensibly inherited changes in physiological and reproductive performance suggest a greater impact of elevated UV‐B on vegetation, primary production and regeneration over the long‐term than presently envisaged.
Acclimation of the respiration/photosynthesis ratio to temperature: insights from a model
Authors: Dewar Roderick C. , Medlyn Belinda E. , Mcmurtrie Ross. E. 
Published: March 2003

SummaryBased on short‐term experiments, many plant growth models – including those used in global change research – assume that an increase in temperature stimulates plant respiration (R) more than photosynthesis (P), leading to an increase in the R/P ratio. Longer‐term experiments, however, have demonstrated that R/P is relatively insensitive to growth temperature. We show that both types of temperature response may be reconciled within a simple substrate‐based model of plant acclimation to temperature, in which respiration is effectively limited by the supply of carbohydrates fixed through photosynthesis. The short‐term, positive temperature response of R/P reflects the transient dynamics of the nonstructural carbohydrate and protein pools; the insensitivity of R/P to temperature on longer time‐scales reflects the steady‐state behaviour of these pools. Thus the substrate approach may provide a basis for predicting plant respiration responses to temperature that is more robust than the current modelling paradigm based on the extrapolation of results from short‐term experiments. The present model predicts that the acclimated R/P depends mainly on the internal allocation of carbohydrates to protein synthesis, a better understanding of which is therefore required to underpin the wider use of a constant R/P as an alternative modelling paradigm in global change research.
Influence of elevated ozone and limited nitrogen availability on conifer seedlings in an open‐air fumigation system: effects on growth, nutrient content, mycorrhiza, needle ultrastructure, starch and secondary compounds
Authors: Kainulainen Pirjo , Utriainen Jarkko , Holopainen Jarmo K. , Oksanen J.A.R.I. , Holopainen Toini 
Published: March 2003

SummaryScots pine (Pinus sylvestris L.) and Norway spruce (Picea abies Karst.) seedlings were exposed to realistically elevated O3 levels in open‐air experiments over three growing seasons. The total O3 exposure doses were 1.2 × (1991), 1.5 × (1992) and 1.7 × (1993) ambient levels. During the 1992 and 1993 growing seasons pine and spruce seedlings received two different levels of nitrogen supply. Effects on growth, mycorrhiza formation, needle ultrastructure, primary and secondary compounds were studied. Ozone exposure had only slight effects on biomass production, growth height and nutrient content of studied conifers. Higher nitrogen availability improved growth of the seedlings and resulted in higher concentration of nitrogen in needles. In Scots pine O3 exposure did not have effects on quantity of total mycorrhizas and short roots, while higher nitrogen availability decreased quantity of mycorrhizas and short roots. In both tree species O3 exposure induced O3‐related ultrastructural symptoms, e.g. granulation and dark staining of the chloroplast stroma in the needle mesophyll cells, at both nitrogen availability levels. Ozone exposure and nitrogen availability did not have significant effects on starch concentrations in either tree species. Concentrations of some individual terpenes were higher in O3‐exposed needles, while concentrations of individual and total resin acids, total phenolics and catechins were not affected by O3 exposure. Nitrogen availability did not have substantial effects on concentrations of monoterpenes. By contrast, concentrations of some individual and total resin acids were lower in pine needles and higher in spruce needles with higher nitrogen availability, while phenolic concentration in spruce needles decreased at higher nitrogen availability. The results suggest that realistically elevated levels of O3 in the field can have some negative effects on the mesophyll ultrastructure of conifer needles, but carbon allocation to root and shoot growth and secondary metabolites are not affected substantially.