bluuebunny/crossref_metadata_2025_split huggingface dataset:
Antialiasing conditions in the delay‐time Radon transform
Authors: Wang Yanghua 
Published: March 2003

ABSTRACTThe 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

ABSTRACTA 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

AbstractWe 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

AbstractThe 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

AbstractWe 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

AbstractThe 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

AbstractThe 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

SummaryLitter 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

SummaryThe 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

SummaryAn 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

SummaryA 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

SummarySmall 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

SummarySoybean (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

SummaryAnnual 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

SummaryData 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.