bluuebunny/crossref_metadata_2025 huggingface dataset:
Time‐varying time‐shift correction by quasi‐elastic deformation of seismic traces
Authors: Masjukov  , Shlyonkin
Published: March 2003

Residual static correction is based on a model of time shifts (delays) that depend solely on source and receiver locations at the surface. This assumption is valid if all raypaths are vertical in the near‐surface layering. We consider a more general model of the time‐varying time shifts that remain after hyperbolic NMO correction (of non‐hyperbolic trajectories) and static correction. We assume that the wavelet distortion caused by the time‐varying shifts is smooth. We have developed an algorithm for its correction (phase correction), based on minimization of the target functional with a penalty term similar to the quasi‐elastic energy of the time‐axis deformation. The use of a mechanical analogy for optimization is well known (e.g. simulated annealing). We propose here a stable numerical method that deals with a pair of seismic traces. It may be applied to phase correction of CMP (or CDP) gathers and stacked sections. Synthetic and field examples confirm that our method appreciably increases the signal‐to‐noise ratio, and improves the coherence and resolution.
Computing three‐dimensional gravitational fields with equivalent sources
Authors: Furness Peter
Published: March 2003

Existing techniques for computing the gravitational field due to a homogeneous polyhedron all transform the required volume integral, expressing the field due to a volume distribution of mass, into a surface integral, expressing the potential due to a surface mass distribution over the boundary of the source body. An alternative representation is also possible and results in a surface integral expressing the potential due to a variable‐strength double layer located on the polyhedral source boundary. Manipulation of this integral ultimately allows the gravitational field component in an arbitrary direction to be expressed as a weighted sum of the potentials due to two basic source distributions. These are a uniform‐strength double layer located on all faces and a uniform‐strength line source located along all edges. The derivatives of the gravitational field components can also be expressed in a similar form as can the magnetic field components due to a homogeneous magnetic polyhedron. It follows that the present approach can be used to generate a universal program capable of modelling all the commonly used potential field responses due to 3D bodies of arbitrary shape.
Removal of overburden velocity anomaly effects for depth conversion
Authors: Armstrong T. , McAteer J. , Connolly P.
Published: March 2003

A method of compensating for the presence of discrete overburden velocity anomalies during depth conversion of time horizons interpreted on conventional, post‐stack time‐migrated seismic data is presented. Positive and negative time delays are estimated either from the push‐down or pull‐up of reflectors directly beneath the anomalies or from interpreted time thickness of the anomalous body and interval velocities estimated from well data. The critical steps are pre‐stack simulation of seismic acquisition across the velocity anomalies, incorporating the effects of a Fresnel volume which changes its width as a function of depth, and simulation of common‐midpoint (CMP) stacking using a linear regression of time delay, Δt, versus offset‐squared, X2. The time‐correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time. Depth conversion is then performed using a background velocity function. The final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon. The method is implemented by manipulating time grids within an industry‐standard mapping package. The final average velocity map shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies.
Short note: Source geometry identification by simultaneous use of structural index and shape factor
Authors: Roy Lopamudra
Published: March 2003

A cross‐plot of the shape factors and the structural indices, determined from gravity anomalies over various idealized sources, namely horizontal/vertical lines and vertical ribbons with various strike lengths and depth extents, forms a closed loop. Different segments of this loop, termed the source geometry identification loop (SGIL), correspond to different source geometries. Combined use of the structural index and the shape factor determined from an isolated gravity anomaly reduces the ambiguity in characterizing the source geometry. A simulated example and three field examples, namely a Cuban chromite anomaly, an Indian example over manganese ore and a sulphide ore from Quebec, have been analysed by the proposed method in order to identify their respective source geometries.
Three‐dimensional imaging of subsurface structures using resistivity data
Authors: Yi Myeong‐Jong , Kim Jung‐Ho , Song Yoonho , Cho Seong‐Jun , Chung Seung‐Hwan , Suh Jung‐Hee
Published: March 2003

We have developed a three‐dimensional inverse scheme for carrying out DC resistivity surveys, incorporating complicated topography as well as arbitrary electrode arrays. The algorithm is based on the finite‐element approximation to the forward problem, so that the effect of topographic variation on the resistivity data is effectively evaluated and incorporated in the inversion. Furthermore, we have enhanced the resolving power of the inversion using the active constraint balancing method. Numerical verifications show that a correct earth image can be derived even when complicated topographic variation exists. By inverting the real field data acquired at a site for an underground sewage disposal plant, we obtained a reasonable image of the subsurface structures, which correlates well with the surface geology and drill log data.
Effective velocities in fractured media: a numerical study using the rotated staggered finite‐difference grid
Authors: Saenger Erik H. , Shapiro Serge A.
Published: March 2003

The modelling of elastic waves in fractured media with an explicit finite‐difference scheme causes instability problems on a staggered grid when the medium possesses high‐contrast discontinuities (strong heterogeneities). For the present study we apply the rotated staggered grid. Using this modified grid it is possible to simulate the propagation of elastic waves in a 2D or 3D medium containing cracks, pores or free surfaces without hard‐coded boundary conditions. Therefore it allows an efficient and precise numerical study of effective velocities in fractured structures. We model the propagation of plane waves through a set of different, randomly cracked media. In these numerical experiments we vary the wavelength of the plane waves, the crack porosity and the crack density. The synthetic results are compared with several static theories that predict the effective P‐ and S‐wave velocities in fractured materials in the long wavelength limit. For randomly distributed and randomly orientated, rectilinear, non‐intersecting, thin, dry cracks, the numerical simulations of velocities of P‐, SV‐ and SH‐waves are in excellent agreement with the results of the modified (or differential) self‐consistent theory. On the other hand for intersecting cracks, the critical crack‐density (porosity) concept must be taken into account. To describe the wave velocities in media with intersecting cracks, we propose introducing the critical crack‐density concept into the modified self‐consistent theory. Numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case.
Improved NMO correction with a specific application to shallow‐seismic data
Authors: Brouwer Jan H.
Published: March 2003

The application of traditional NMO‐correction techniques in the processing of seismic data may result in severe distortion. This distortion is observed as a decrease in frequency content (NMO stretch) or even a total loss of data for steep velocity gradients. It is shown that the specific nature of the observed distortion is introduced by the particular implementation of the traditional NMO‐correction technique. It is also shown that other techniques that have been proposed in the literature suffer from similar artefacts. An alternative approach is suggested, based on the correction of tapered blocks of seismic data, followed by a coherence filter to compensate for the specific artefacts thus introduced. Correction of seismic amplitudes (e.g. geometrical spreading and attenuation) is implemented as an integral part of the NMO‐correction method thus allowing for both dynamic and kinematic reconstruction of interfering events. From the application of this method to synthetic and field data, it is concluded that the proposed technique may be particularly useful in the processing of shallow seismic (shear‐wave) data. Although not illustrated in this paper, it is emphasized that application of the proposed method is by no means restricted to shear waves; application to P‐wave data will prove equally useful, although the stretch effects involved may not be as severe as in the examples given in this paper.
Seismic buffer recognition using wavelet‐based features and neural classification
Authors: Hoffman Alwyn , Hoogenboezem Riaan , Van Der Merwe Theo , Tollig Tonie
Published: March 2003

One of the first operations in a seismic signal processing system applied to earthquake data is to distinguish between valid and invalid records. Since valid signals are characterized by a combination of their time and frequency properties, wavelets are natural candidates for describing seismic features in a compact way. This paper develops a seismic buffer pattern recognition technique, comprising wavelet‐based feature extraction, feature selection based on the mutual information criterion, and neural classification based on feedforward networks. The ability of the wavelet transform to capture discriminating information from seismic data in a small number of features is compared with alternative feature reduction techniques, including statistical moments. Three different variations of the wavelet transform are used to extract features: the discrete wavelet transform, the single wavelet transform and the continuous wavelet transform. The mutual information criterion is employed to select a relatively small set of wavelets from the time–frequency grid. Firstly, it is determined whether wavelets can capture more informative data in an equal number of features compared with other features derived from raw data. Secondly, wavelet‐based features are compared with features selected based on prior knowledge of class differences. Thirdly, a technique is developed to optimize wavelet features as part of the neural network training process, by using the wavelet neural network architecture. The automated classification techniques developed in this paper are shown to perform similarly to human operators trained for this function. Wavelet‐based techniques are found to be useful, both for preprocessing of the raw data and for extracting features from the data. It is demonstrated that the definition of wavelet features can be optimized using the classification wavelet network architecture.
Velocity anisotropy and attenuation of shale in under‐ and overpressured conditions
Authors: Domnesteanu Patricia , McCann Clive , Sothcott Jeremy
Published: March 2003

The seismic velocity and attenuation of fully saturated shales were measured for the first time under overpressured conditions, using the ultrasonic reflection technique. Shale cores from naturally overpressured horizons in the North Sea were tested in the laboratory, at confining and pore pressures relevant to in situ conditions.

A single‐frequency tone‐burst pulse wave was used to determine the seismic wave velocities and quality factors of the shale samples, with errors less than 0.3% and 0.1 dB/cm, respectively, at a frequency of 0.75 MHz. Sample length changes with varying confining and pore pressure were measured and the pore pressure equilibration time was monitored for each sample.

The anisotropy of the seismic attributes (Vp, Vs, Qp and Qs) was determined over a range of differential pressures from 5 to 60 MPa, with respect to the predominant foliation. The ultrasonic velocity data followed a transversely isotropic pattern depending on the direction of wave propagation with respect to the laminations. The Poisson's ratio was found to rise by 5% as the shale material progressed from a normally pressured to an overpressured state. The quality factor (Q) characteristics were interpreted in terms of pore geometry and connectivity as well as the directional permeability of the transversely isotropic shale material. The results were converted to bulk and shear loss modulus defects, and a positive bulk loss was observed for waves propagating perpendicular to the lamination plane even above differential pressures of 20 MPa. This indicates different levels of Biot‐flow and squirt‐flow attenuation mechanisms acting within the shale structure, depending on the wave propagation and vibration directions.
Towards high‐resolution 3D marine seismic surveying using Boomer sources
Authors: Müller Christof , Milkereit Bernd , Bohlen Thomas , Theilen Friedrich
Published: March 2003

The use of Boomer sources for 3D seismic imaging of shallow marine structures was investigated in a feasibility study. Boomers show sufficient stability to be used in multichannel seismic surveys. The acquisition of a high‐frequency, densely sampled seismic data volume was successfully performed in the Baltic Sea. A Pleistocene fluvial channel system and shallow gas accumulations were revealed beneath the unconsolidated sediments which constitute the sea‐floor in the southern Kiel Bay.
Antialiasing conditions in the delay‐time Radon transform
Authors: Wang Yanghua
Published: March 2003

The delay‐time Radon transform parametrizes coherent events in a seismic gather by the far‐offset trace delay time, instead of the conventional parabolic curvature or ray parameter. The reformulation may give a different physical insight into the aliasing effect in the Radon transformation and may also lead to a different algorithm. The delay‐time parametrization enables modelling of a seismic gather as the sum of coherent events with any form of moveout curve. For example, a parabolic curve can be used for traces within a moderate offset range and a linear moveout for far‐offset traces. When using this delay‐time Radon transform, it is the number of traces, rather than the spatial sampling, of the input gather that directly controls aliasing in the Radon transform image. A preconditioning operator that implicitly increases the number of input traces by spatial reconstruction (without physically performing the spatial resampling) may minimize aliasing noise in the Radon transform image.
Velocity–depth and time–depth relationships for a decompacted uplifted unit
Authors: Al‐Chalabi M. , Rosenkranz Peter L.
Published: March 2003

A linear instantaneous velocity model is used to describe the velocity variations in an uplifted unit that has been partly decompacted as a result of the reduction in overburden that often accompanies uplift. The model results in a series of equations for deriving values for the function parameters in the velocity–depth and the time– depth domains and for carrying out time‐to‐depth conversions. The formulation uses the base of the unit as a reference level to generate the reference datum from a combination of the depth of the base of the unit and a parameter that represents the decompaction factor.
A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle
Authors: Kleidon AxeL. , Heimann Martin
Published: March 2003

We outline a method of inferring rooting depth from a Terrestrial Biosphere Model by maximizing the benefit of the vegetation within the model. This corresponds to the evolutionary principle that vegetation has adapted to make best use of its local environment. We demonstrate this method with a simple coupled biosphere/soil hydrology model and find that deep rooted vegetation is predicted in most parts of the tropics. Even with a simple model like the one we use, it is possible to reproduce biome averages of observations fairly well. By using the optimized rooting depths global Annual Net Primary Production (and transpiration) increases substantially compared to a standard rooting depth of one meter, especially in tropical regions that have a dry season. The decreased river discharge due to the enhanced evaporation complies better with observations. We also found that the optimization process is primarily driven by the water deficit/surplus during the dry/wet season for humid and arid regions, respectively. Climate variability further enhances rooting depth estimates. In a sensitivity analysis where we simulate changes in the water use efficiency of the vegetation we find that vegetation with an optimized rooting depth is less vulnerable to variations in the forcing. We see the main application of this method in the modelling communities of land surface schemes of General Circulation Models and of global Terrestrial Biosphere Models. We conclude that in these models, the increased soil water storage is likely to have a significant impact on the simulated climate and the carbon budget, respectively. Also, effects of land use change like tropical deforestation are likely to be larger than previously thought.
Regional application of an ecosystem production model for studies of biogeochemistry in Brazilian Amazonia
Authors: Potter Christopher S. , Davidson EriC. A. , Klooster Steve A. , Nepstad Daniel C. , De Negreiros Gustavo H. , Brooks Vanessa
Published: March 2003

The degree to which primary production, soil carbon, and trace gas fluxes in tropical forests of the Amazon are limited by moisture availability and other environmental factors was examined using an ecosystem modelling application for the country of Brazil. A regional geographical information system (GIS) serves as the data source of climate drivers, satellite images, land cover, and soil properties for input to the NASA Ames‐CASA (Carnegie‐Ames‐Stanford Approach) model over a 8‐km grid resolution. Simulation results lead us to hypothesize that net primary production (NPP) is limited by cloud interception of solar radiation over the humid north‐western portion of the region. Peak annual rates for NPP of nearly 1.4 kg C m–2 year–1 are localized in the seasonally dry eastern Amazon in areas that we assume are primarily deep‐rooted evergreen forest cover. Regional effects of forest conversion on NPP and soil carbon content are indicated in the model results, especially in seasonally dry areas. Comparison of model flux predictions along selected eco‐climatic transects reveal moisture, soil, and land use controls on gradients of ecosystem production and soil trace gas emissions (CO2, N2O, and NO). These results are used to formulate a series of research hypotheses for testing in the next phase of regional modelling, which includes recalibration of the light‐use efficiency term in NASA‐CASA using field measurements of NPP, and refinements of vegetation index and soil property (texture and potential rooting depth) maps for the region.
Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations
Authors: WANG Y. ‐P. , REY A. , JARVIS P. G.
Published: March 2003

We constructed a carbon budget for young birch trees grown in ambient and elevated CO2 concentrations over their fourth year of growth. The annual total of net leaf photosynthesis was 110% more in elevated CO2 than in ambient CO2. However, the trees in elevated CO2 grew only 59% more biomass than the trees in ambient CO2 over the year. Modelling studies showed that larger loss of carbon from fine‐root production and growth of the root‐associated mycorrhiza by the trees in elevated CO2 probably accounted for all the remaining difference in net photosynthesis between the two treatments. Our modelling also showed that the fraction of net photosynthate consumed by respiration of nonleaf tissue was similar in the two CO2 treatments, and was 26% and 24% for trees in ambient and elevated CO2, respectively.

Trees in elevated CO2 had 43% more leaves, and produced 110% more net photosynthate than trees in ambient CO2, even though the maximum rate of carboxylation per unit leaf nitrogen decreased by 21%. Sensitivity studies showed that down‐regulation reduced the annual net photosynthetic production of the trees in elevated CO2 by only 6%. Direct effects of higher CO2 on photosynthesis and greater leaf area of the trees in elevated CO2 increased the net photosynthesis of the trees by 68% and 60%, respectively; and together accounted for most of the difference in net photosynthesis between the two treatments.
A field study of the effects of elevated CO2 and plant species diversity on ecosystem‐level gas exchange in a planted calcareous grassland
Authors: Stocker R. , Körner CH. , Schmid B. , Niklaus P. A. , Leadley P. W.
Published: March 2003

The relationship between plant species diversity and ecosystem CO2 and water vapour fluxes was investigated for planted calcareous grassland communities composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination with a CO2 enrichment experiment in order to investigate the degree to which ecosystem responses to elevated CO2 are altered by a loss of plant diversity. Ecosystem CO2 and H2O fluxes were measured over several 24‐h periods during the 1994 and 1995 growing seasons. Ecosystem CO2 assimilation on a ground area basis decreased with decreasing plant diversity in the first year and this was related to a decline in above‐ground plant biomass. In the second year, however, CO2 assimilation was not affected by diversity, and this corresponded to the disappearance of a diversity effect on above‐ground biomass. Irrespective of diversity treatment, CO2 assimilation on a ground area basis was linearly related to peak above‐ground biomass in both years. Elevated CO2 significantly increased ecosystem CO2 assimilation in both years with no interaction between diversity and CO2 treatment, and no corresponding increase in above‐ground biomass. There were no significant effects of diversity on water vapour flux, which was measured only in the second year. There were indications of a small CO2 effect on water vapour flux (3–9% lower at elevated CO2 depending on the light level). Our findings suggest that decreasing plant species diversity may substantially decrease ecosystem CO2 assimilation during the establishment of such planted calcareous grassland communities, but also suggest that this effect may not persist. In addition, we find no evidence that plant species diversity alters the response of ecosystem CO2 assimilation to elevated CO2.
Short‐term effects of changing water table on N2O fluxes from peat monoliths from natural and drained boreal peatlands
Authors: Regina Kristiina , Silvola Jouko , Martikainen Pertti J.
Published: March 2003

The effect of the water table on nitrous oxide (N2O) fluxes from peat profiles representing boreal peatlands of differing nutrient status was studied in the laboratory. Lowering of the water table in peat monoliths taken from two natural waterlogged peatlands for 14 weeks in a greenhouse at 20 °C increased the fluxes of N2O, an effect that was enhanced further by incubation in the dark. Raising of the water table in monoliths from two drained and forested peatlands caused cessation of the N2O fluxes from the drained peats, which had previously been sources of N2O. It is known that N2O fluxes have increased in peatlands drained several decades ago. The results suggest that it is not necessary for the water table to be lowered for several years to change a boreal peatland from a N2O sink to a source of the gas. In addition to the draining of peatlands, climate change can be expected to lower ground water levels during the summertime in the boreal zone, and this could cause marked changes in N2O fluxes from boreal peatlands by enhancing the microbial processes involved in nitrogen transformations.
Litter quality and decomposition in Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth
Authors: Lutze Jason L. , Gifford Roger M. , Adams Helen N.
Published: March 2003

Litter quality parameters of Danthonia richardsonii grown under CO2 concentrations of ≈ 359 & ≈ 719 μL L− 1 at three mineral N supply rates (2.2, 6.7 & 19.8 g m− 2 y− 1) were determined. C:N ratio was increased in senesced leaf (enhancement ratios, Re/c, of 1.25–1.67), surface litter (1.34–1.64) and root (1.13–1.30) by CO2 enrichment. After 3 years of growth, nonstructural carbohydrate concentrations were reduced in senesced leaf lamina (avg. Re/c=  0.84) but not in root in response to CO2 enrichment. Cellulose concentrations increased slightly in senesced leaf (avg. Re/c=  1.07) but not in root in response to CO2 enrichment. Lignin and polyphenolic concentrations in senesced leaf and root were not changed by CO2 enrichment. Decomposition, measured as cumulative respiration in standard conditions in vitro, was reduced in leaf litter grown under CO2 enrichment. Root decomposition in vitro was lower in the material produced under CO2 enrichment at the two higher rates of mineral N supply. Significant correlations between decomposition of leaf litter and initial %N, C:N ratio and lignin:N ratio were found. Decomposition in vivo, measured as carbon disappearance from the surface litter was not affected by CO2 concentration. Arbuscular mycorrhizal infection was not changed by CO2 enrichment. Microbial carbon was higher under CO2 enrichment at the two higher rates of mineral N supply. Possible reasons for the lack of effect of changes in litter quality on in‐sward decomposition rates are discussed.
Direct effects of atmospheric carbon dioxide concentration on whole canopy dark respiration of rice
Authors: Baker J.E.ffrey T. , Jr L. H.A.rtwell Allen , Boote K.E.nneth J. , Pickering Nigel B.
Published: March 2003

The purpose of this study was to test for direct inhibition of rice canopy apparent respiration by elevated atmospheric carbon dioxide concentration ([CO2]) across a range of short‐term air temperature treatments. Rice (cv. IR‐72) was grown in eight naturally sunlit, semiclosed, plant growth chambers at daytime [CO2] treatments of 350 and 700 μmol mol−1. Short‐term night‐time air temperature treatments ranged from 21 to 40 °C. Whole canopy respiration, expressed on a ground area basis (Rd), was measured at night by periodically venting the chambers with ambient air. This night‐time chamber venting and resealing procedure produced a range of increasing chamber [CO2] which we used to test for potential inhibitory effects of rising [CO2] on Rd. A nitrous oxide leak detection system was used to correct Rd measurements for chamber leakage rate (L) and also to determine if apparent reductions in night‐time Rd with rising [CO2] could be completely accounted for by L. The L was affected by both CO2 concentration gradient between the chamber and ambient air and the inherent leakiness of each individual chamber. Nevertheless, after correcting Rd for L, we detected a rapid and reversible, direct inhibition of Rd with rising chamber [CO2] for air temperatures above 21 °C. This effect was larger for the 350 compared with the 700 μmol mol−1 daytime [CO2] treatment and was also increased with increasing short‐term air temperature treatments. However, little difference in Rd was found between the two daytime [CO2] treatments when night‐time [CO2] was at the respective daytime [CO2]. These results suggest that naturally occurring diurnal changes in both ambient [CO2] and air temperature can affect Rd. Because naturally occurring diurnal changes in both [CO2] and air temperature can be expected in a future higher CO2 world, short‐term direct effects of these environmental variables on rice Rd can also be expected.
Predicting N2O emissions from agricultural land through related soil parameters
Authors: Conen F. , Dobbie K. E. , Smith K. A.
Published: March 2003

An empirical model of nitrous oxide emission from agricultural soils has been developed. It is based on the relationship between N2O and three soil parameters – soil mineral N (ammonium plus nitrate) content in the topsoil, soil water‐filled pore space and soil temperature – determined in a study on a fertilized grassland in 1992 and 1993. The model gave a satisfactory prediction of seasonal fluxes in other seasons when fluxes were much higher, and also from other grassland sites and from cereal and oilseed rape crops, over a wide flux range (< 1 to > 20 kg N2O‐N ha−1 y−1). However, the model underestimated emissions from potato and broccoli crops; possible reasons for this are discussed. This modelling approach, based as it is on well‐established and widely used soil measurements, has the potential to provide flux estimates from a much wider range of agricultural sites than would be possible by direct measurement of N2O emissions.
Temperature‐ and moisture‐induced changes in the structure of the nematode fauna of a semiarid grassland –‐ patterns and mechanisms
Authors: Bakonyi Gábor , Nagy Péter
Published: March 2003

A field experiment was established to examine the effects of temperature and moisture modifications on the nematode fauna of a semiarid grassland. Several combinations of drying, wetting, warming and cooling were applied to plots and compared with untreated control plots. The experiment was performed from July to October 1996. A significant shift was observed in the structure of the nematode fauna between late summer and early autumn. This shift was manifested in the disappearance of four rare genera; Ecumenicus, Eucephalobus, Paraphelenchus and Pungentus. A significant decrease was found in the density of Acrobeles, Aphelenchoides, Ditylenchus and Prismatolaimus; in addition there was a significant increase in the density of Cephalobus, Helicotylenchus, Paratylenchus and Tylenchorhynchus. Community structural change was represented by an initial decrease in nematode generic richness of 30–50%, and in a statistically significant decrease of nematode diversity in the control and all treated plots. Thereafter, emergence of a new community was demonstrated. Data show that temperature manipulation was the main factor to influence nematode diversity, Maturity Index, and Plant Parasite Index. However, nematode population density was influenced predominantly by the soil moisture content. Coenological analysis of soil nematode fauna appears to be a useful tool for the biological monitoring of the effects of global change on semiarid grasslands.
Biotic validation of small open‐top chambers in a tundra ecosystem
Authors: Hollister Robert D. , Webber Patrick J.
Published: March 2003

Small open‐top chambers (OTC) are used widely in ecosystem warming experiments. The efficacy of the open‐top chamber as an analogue of climatic warming is examined. Twenty‐four small OTCs were used to passively warm canopy temperatures in wet meadow tundra at Barrow, Alaska, during two consecutive summers with contrasting surface air‐temperatures. Fortuitously, the seasonal average temperature regime within chambers in the colder year (1995) was similar to the controls of the warmer year (1996); this allowed a comparison of natural vs. chamber warming. All measured plant responses behaved similarly to both year and treatment 68% of the time. A comparison of the populations of the warmer summer's control with the cooler summer's OTC found no statistical difference in 80% of the response variables measured. A meta‐analysis also found no significant difference between the responses of the two populations. These results give empirical biotic validation for the use of the OTC as an analogue of regional climate warming.
The impact of elevated CO2 on yield loss from a C3 and C4 weed in field‐grown soybean
Authors: Ziska Lewis H.
Published: March 2003

Soybean (Glycine max) was grown at ambient and enhanced carbon dioxide (CO2, + 250 μL L−1 above ambient) with and without the presence of a C3 weed (lambsquarters, Chenopodium album L.) and a C4 weed (redroot pigweed, Amaranthus retroflexus L.), in order to evaluate the impact of rising atmospheric carbon dioxide concentration [CO2] on crop production losses due to weeds. Weeds of a given species were sown at a density of two per metre of row. A significant reduction in soybean seed yield was observed with either weed species relative to the weed‐free control at either [CO2]. However, for lambsquarters the reduction in soybean seed yield relative to the weed‐free condition increased from 28 to 39% as CO2 increased, with a 65% increase in the average dry weight of lambsquarters at enhanced [CO2]. Conversely, for pigweed, soybean seed yield losses diminished with increasing [CO2] from 45 to 30%, with no change in the average dry weight of pigweed. In a weed‐free environment, elevated [CO2] resulted in a significant increase in vegetative dry weight and seed yield at maturity for soybean (33 and 24%, respectively) compared to the ambient CO2 condition. Interestingly, the presence of either weed negated the ability of soybean to respond either vegetatively or reproductively to enhanced [CO2]. Results from this experiment suggest: (i) that rising [CO2] could alter current yield losses associated with competition from weeds; and (ii) that weed control will be crucial in realizing any potential increase in economic yield of agronomic crops such as soybean as atmospheric [CO2] increases.
Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems
Authors: Nijs Ivan , Roy Jacques , Salager Jean‐Louis , Fabreguettes Jacques
Published: March 2003

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO2 for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO2 maintained at ambient (370 μmol mol−1) or elevated (700 μmol mol−1) concentration.

During the third year of study, CO2 enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m−2 y−1 (one assemblage became a net CO2 sink, another just reached equilibrium, and the remaining two remained as a CO2 source). At least 50% of the shift under elevated CO2 originated from a decrease in belowground respiration. This indicates that, during this year, CO2 enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization.

Although elevated‐CO2‐grown systems acted as a net CO2 sink during a longer period of the year (4–7 months) compared with ambient‐CO2‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between −5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO2 coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO2 on productivity. Water regime appears to strongly govern the influence of CO2 on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.
Geographic patterns and dynamics of Alaskan climate interpolated from a sparse station record
Authors: Fleming Michael D. , Chapin F. Stuart , Cramer Wolfgang , Hufford Gary L. , Serreze Mark C.
Published: March 2003

Data from a sparse network of climate stations in Alaska were interpolated to provide 1‐km resolution maps of mean monthly temperature and precipitation–‐variables that are required at high spatial resolution for input into regional models of ecological processes and resource management. The interpolation model is based on thin‐plate smoothing splines, which uses the spatial data along with a digital elevation model to incorporate local topography. The model provides maps that are consistent with regional climatology and with patterns recognized by experienced weather forecasters. The broad patterns of Alaskan climate are well represented and include latitudinal and altitudinal trends in temperature and precipitation and gradients in continentality. Variations within these broad patterns reflect both the weakening and reduction in frequency of low‐pressure centres in their eastward movement across southern Alaska during the summer, and the shift of the storm tracks into central and northern Alaska in late summer. Not surprisingly, apparent artifacts of the interpolated climate occur primarily in regions with few or no stations. The interpolation model did not accurately represent low‐level winter temperature inversions that occur within large valleys and basins. Along with well‐recognized climate patterns, the model captures local topographic effects that would not be depicted using standard interpolation techniques. This suggests that similar procedures could be used to generate high‐ resolution maps for other high‐latitude regions with a sparse density of data.
Hierarchical subdivision of Arctic tundra based on vegetation response to climate, parent material and topography
Authors: Walker D. A.
Published: March 2003

Current land‐cover classifications used for global modelling portray Arctic tundra as one or two classes. This is insufficient for analysis of climate–vegetation interactions. This paper presents a simple three‐level vegetation‐map legend system useful for modelling at global, regional, and landscape scales. At the highest level (global scale: 107−108 km2) the Tundra Zone is divided into four subzones based on vegetation response to temperature along the latitudinal temperature gradient from north to south: (1) Cushion‐forb, (2) Prostrate Dwarf‐shrub, (3) Erect Dwarf‐shrub, and (4) Low Shrub subzones. The boundaries follow a modification of Yurtsev's phytogeographic subzones. Parent material and topography are also major considerations at global, regional, and landscape scales. Soil pH is a key variable for many ecosystem responses, and a division into acidic (pH 5.5 or less) and nonacidic soils is used. A conceptual mesotopographic gradient is used to characterize the influence of soil‐moisture and snow regimes. The example legend framework focuses on the Northern Alaska floristic subprovince, and could be expanded to other floristic provinces using local expert knowledge and available literature. Dominant plant functional types within each habitat type within the four subzones are also presented. Modellers could include or ignore different levels of resolution depending on the purpose of the model. The approach resolves conflicts in terminology that have previously been encountered between the Russian, North American, and Fennoscandian approaches to Arctic zonation.
Biotic disturbance agents in the boreal forest: considerations for vegetation change models
Authors: Malmström Carolyn M. , Raffa Kenneth F.
Published: March 2003

Disturbance regimes strongly determine vegetation patterns and succession in the boreal landscape. One of the current challenges for boreal vegetation modellers is to represent disturbance agents as dynamic factors that can respond to climate change. Outbreak species of insects and plant pathogens can cause marked changes in vegetation patterns and should be incorporated into vegetation change models. This introduction to the ecology of boreal biotic disturbance agents is designed as a brief overview for global change researchers and modellers. We discuss the importance of biotic disturbance agents in the boreal forest, offer an overview of their ecology, and review modelling approaches. We illustrate these issues with examples from different systems, drawing largely from our experience with bark beetles.
An empirical model of carbon fluxes in Russian tundra
Authors: Zamolodchikov Dmitri G. , Karelin Dmitri V.
Published: March 2003

This study presents an empirical model based on a GIS approach, which was constructed to estimate the large‐scale carbon fluxes over the entire Russian tundra zone. The model has four main blocks: (i) the computer map of tundra landscapes; (ii) data base of long‐term weather records; (iii) the submodel of phytomass seasonal dynamics; and (iv) the submodel of carbon fluxes. The model uses exclusively original in situ diurnal CO2 flux chamber measurements (423 sample plots) conducted during six field seasons (1993–98). The research sites represent the main tundra biome landscapes (arctic, typical, south shrub and mountain tundras) in the latitudinal diapason of 65–74°N and longitudinal profile of 63°E−172°W. The greatest possible diversity of major ecosystem types within the different landscapes was investigated. The majority of the phytomass data used was obtained from the same sample plots. The submodel of carbon fluxes has two dependent [GPP, Gross Respiration (GR)] and several input variables (air temperature, PAR, aboveground phytomass components). The model demonstrates a good correspondence with other independent regional and biome estimates and carbon flux seasonal patterns. The annual GPP of Russian tundra zone for the area of 235 × 106 ha was estimated as −485.8 ± 34.6 × 106 tC, GR as +474.2 ± 35.0 × 106 tC, and NF as −11.6 ± 40.8 × 106 tC, which possibly corresponds to an equilibrium state of carbon balance during the climatic period studied (the first half of the 20th century). The results advocate that simple regression‐based models are useful for extrapolating carbon fluxes from small to large spatial scales.
Interactive effects of elevated CO2 and temperature on the leaf‐miner Dialectica scalariella Zeller (Lepidoptera: Gracillariidae) in Paterson's Curse, Echium plantagineum (Boraginaceae)
Authors: Johns Caitlin V. , Hughes Lesley
Published: March 2003

Doubling of the current atmospheric CO2 concentration, and an increase in global mean annual temperatures of 1.5–6 °C, have been predicted to occur by the end of this century. Whilst the separate effects of CO2 and temperature on plant–insect interactions have been examined in a number of studies, few have investigated their combined impact. We carried out a factorial experiment to explore the effect of a doubling of CO2 concentration and a 3 °C temperature increase on the development of a complete generation of the leaf‐miner, Dialectica scalariella, in the host plant Paterson's Curse, Echium plantagineum.

Elevated CO2 increased biomass, reduced leaf N and increased C:N ratios in the host plants. Leaf thickness also increased under elevated CO2, but only in the high‐temperature treatment. Female D. scalariella did not discriminate between plants grown at the different CO2 levels when ovipositing, despite the reduction in foliage quality under elevated CO2. Overall, the negative response of D. scalariella to elevated CO2 was greater than for many species of free‐living insects, presumably because of the limited mobility imposed by the leaf‐mining habit. Development was accelerated at the high temperature and slowed under elevated CO2. The net result was a reduction in development time of ∼14 days in the elevated CO2/high temperature treatment, compared to the ambient CO2/low temperature treatment. Larval survivorship and adult moth weight were both affected by a significant interaction between CO2 and temperature. At the low temperature, CO2 had little effect on survivorship, but at the high temperature, survivorship was significantly reduced under elevated CO2. Similarly, elevated CO2 had a stronger negative effect on adult moth weight when combined with the high‐temperature treatment. A possible explanation for these results is that the high temperature accelerated insect development to such an extent that the larvae did not have sufficient feeding time to compensate for the poorer quality of the foliage.

The frequency with which interactions between CO2 and temperature affected both plant and insect performance in this study highlights the need for caution when predicting the effects of future climate change on plant–insect interactions from single‐factor experiments.
Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification
Authors: Carnol Monique , Hogenboom Laure , Jach M. Ewa , Remacle Jean , Ceulemans Reinhart
Published: March 2003

The control of soil nitrogen (N) availability under elevated atmospheric CO2 is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO2 on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO2 (ambient + 400 µmol mol−1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N2O emissions were significantly higher in soils from the elevated CO2 treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO3– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO2 treatment in the field when incubated in the laboratory under elevated CO2 (increase of 100%), but there was no effect when incubated under ambient CO2. Net nitrate production of the soil originating from the ambient CO2 treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO2 on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO2 through higher soil CO2 concentrations. Alternatively, lower microbial NH4 assimilation under elevated CO2 might explain the higher net nitrification. We conclude that elevated atmospheric CO2 has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO2 concentrations compared to atmospheric concentrations.