bluuebunny/crossref_metadata_2025_split huggingface dataset:
Differential phosphorus and nitrogen effects drive species and community responses to elevated CO2 in semi‐arid grassland
Authors: Grünzweig José M. , Körner Christian 
Published: December 2004

Summary

Productivity of dryland communities is often co‐limited by water and nutrients. Since atmospheric CO2 enrichment induces water savings by plants, elevated CO2 and nutrients could interact to reduce growth limitation, irrespective of the direct influence of CO2 on photosynthesis. We studied CO2 effects in model communities from the semi‐arid Negev of Israel with 17 mostly annual C3 species at three CO2 concentrations and three nutrient treatments.
Community biomass increased at elevated (440 and 600 µL L−1) compared to pre‐industrial CO2 (280 µL L−1) by 34% on average in the low‐nutrient control, by 45% in the high P and by 50% in the high NPK treatment. Less evapotranspiration at elevated CO2 increased soil water content by 30–40% on average. Significant CO2–fertilization interactions indicated that plant responses to CO2 enrichment were constrained by nutrient availability.
Responses of biomass and water‐use efficiency (dry‐matter accumulation per cumulative evapotranspiration) to CO2 enrichment were non‐linear and were saturated at 440 µL L−1 at low nutrient and high P supply. CO2 effects were further increased up to 600 µL L−1 only under full NPK fertilization.
The overall CO2 effect on biomass depended on the differential response of plant functional groups, with the P‐dependent legume response dominating at low nutrient and high P supply, and the N‐dependent grass response dominating at high NPK. With the exception of grasses, species responded differently to combinations of CO2 enrichment and nutrient addition, even within functional groups.
Biomass production was co‐limited by CO2 and nutrients in this semi‐arid seasonal community, with both effects possibly mediated by water availability. Nutrient losses associated with desertification will thus diminish potential gains in biomass due to elevated CO2. Growth stimulation by CO2 enrichment beyond close‐to‐current concentrations will only be seen under nutrient‐rich conditions in semi‐arid and possibly other drought‐stressed grasslands.
Simulated carbon uptake for a canopy of two broadleaved tree species with contrasting leaf habit
Authors: Dungan R. J. , Whitehead D. , McGlone M. , Allen R. B. , Duncan R. P. 
Published: December 2004

SummaryWe used an environmentally driven, process‐based model to investigate the relative benefits of deciduous and evergreen leaves, in terms of net annual canopy carbon uptake, for a canopy of two tree species with contrasting leaf habit: deciduous Fuchsia [Fuchsia excorticata(J. R. Forst. and G. Forst) L. f.]; and annual‐evergreen Wineberry (Aristotelia serrataJ. R. Forst. and G. Forst, W. R. B. Oliv.).Parameters for the model were derived from a year of measurements at a field site with a maximum leaf area index (L) of 5·4 m2 m−2, of which Wineberry and Fuchsia comprised 4·9 and 0·5 m2 m−2, respectively. Net annual C uptake (ANET; daily photosynthesis minus respiration at night) was 930 g C m−2, of which Wineberry and Fuchsia contributed 850 and 80 g C m−2, respectively. Canopy respiration at night (RN) accounted for 29 and 18% of daily uptake for Wineberry and Fuchsia, respectively. The model was used to simulate the effects of an evergreen phenology for both species. This increasedANETrelative to that estimated using the observed phenology by only 1% for Wineberry, and 4% for Fuchsia. This was attributable to increased net C uptake during winter with the evergreen phenology being offset by increasedRN, and reducedANETduring summer.This comparison was repeated for sites spanning the natural range of Wineberry and Fuchsia across New Zealand. In terms of net C balance, the relative advantage of an evergreen phenology decreased with average winter temperature for both species, to the point where a deciduous phenology resulted in a higher value forANETfor Fuchsia at sites where frosts limited winter uptake.Our simulations suggest that there is no clear advantage, in terms of annual C balance, for a deciduous or evergreen phenology at sites with relatively mild winters. In contrast, at sites where frost‐induced photoinhibition limits photosynthesis for overwintering leaves, a deciduous phenology results in a higher value ofANETfor species, such as Fuchsia, with leaves that are very active photosynthetically during spring and summer. The lack of deciduousness in the New Zealand flora may be due to the low number of tree species with such photosynthetic characteristics, and this may be attributable to the generally poor nutrient availability in forest soils.
Gender effects on the post‐facilitation performance of two dioecious Juniperus species
Authors: Verdú M. , Villar‐Salvador P. , García‐Fayos P. 
Published: December 2004

Summary

Plant facilitation usually changes to competition as plants age. In dioecious plants, females should be affected more negatively than males by stressful conditions because of the greater costs of female reproduction.
We investigated the gender effects on the post‐facilitation performance of adult plants of two dioecious Juniperus species from the high mountains of eastern Spain: J. sabina L. acts as a nurse plant for J. communis L. We compared physiological (water potential, carbon isotope discrimination and nitrogen concentration), vegetative (shoot growth) and reproductive (number of male flowers, and number of fruits and seeds) characters of associated and non‐associated plants of both species, to test the hypothesis that this association represents a more stressful condition for females than for males because of the greater costs of female reproduction.
Despite their close phylogenetic relatedness, both species showed a distinct performance pattern after the facilitation phase. Association with the nurse plant reduced the growth and reproductive capacity of both genders in J. communis, the facilitated species. In contrast, the association with J. communis did not affect the fitness of the nurse plant, J. sabina, although in accordance with our hypothesis a gender effect was found on several physiological parameters. Thus J. sabina‐associated females had a more negative water potential and carbon isotope discrimination than the associated males, but there were no differences between genders when growing in isolation.
The consequences of the post‐facilitation interaction between the two long‐lived woody Juniperus species are asymmetrical: harmful for the facilitated species, but harmless for the nurse.
Gender had also asymmetrical consequences on some functional traits of the nurse – but not the facilitated – species.
Mosses mediate grazer impacts on grass abundance in arctic ecosystems
Authors: Van Der Wal R. , Brooker R.W. 
Published: December 2004

Summary

Large herbivores have significant impacts on the structure and function of temperate and tropical ecosystems. Yet herbivore impacts on arctic systems, particularly the mechanisms by which they influence plant communities, are largely unknown.
High arctic vegetation, commonly overlying permafrost soils, is often moss‐dominated with sparse vascular plant cover. We investigated the potential influence of large herbivores on arctic plant communities via their impact on the depth of the moss layer, leading to warmer soils and potentially benefiting vascular plants.
We found that grazer impacts on moss depth, and subsequently soil temperature, may influence vascular plant abundance and community composition because of the observed positive but growth‐form‐specific response of vascular plants to soil warming, promoting grasses in particular.
We propose that the positive association of grasses and large herbivores in arctic moss‐dominated systems results from two simultaneously operating positive feedback loops. First, herbivore grazing and trampling reduces moss layer depth, increasing soil temperatures. Second, grasses benefit directly from grazers as a result of additional nutrients from faeces and urine. Additionally, the tolerance of grasses to grazing may enable grasses to expand despite the losses suffered from herbivory.
Does within‐population variation in egg size reduce intraspecific competition in Atlantic Salmon, Salmo salar?
Authors: Einum S. , Fleming I. A. 
Published: December 2004

Summary

If phenotypes differ in habitat preferences there may be a decrease in intraspecific competition with increasing phenotypic variation, causing heterogeneous populations to perform better than homogeneous ones. Furthermore, if the relative performance of different phenotypes depends on environmental conditions, population heterogeneity may buffer populations against temporal environmental variation.
Here these hypotheses are tested using 10 experimental populations of juvenile Atlantic Salmon (Salmo salar L.) reared across a range of five different water discharges. Each population consisted of fish of identical parental origins to control for genetic differences among populations, but differed in body size heterogeneity owing to differences in egg size distributions.
No effect of population heterogeneity on final body size or dispersal rates was detected. Thus, egg size heterogeneity does not appear to influence the average level of intraspecific competition in Atlantic Salmon.
There was a strong positive effect of water discharge on body size, but no effects of phenotypic heterogeneity on variation in final body size among discharge levels. Thus, the relative growth performance of juveniles arising from different egg sizes may not change sufficiently with environmental changes for egg size variation to create a strong buffering effect against year‐to‐year variation in environmental conditions.
Increase of male reproductive components with size in an animal‐pollinated hermaphrodite, Narthecium asiaticum (Liliaceae)
Authors: Ishii H. S. 
Published: December 2004

Summary

In animal‐pollinated plants, female function is expected to increase with size because increased pollen production leads to more competition for ovules by pollen grains from the same parent, and a large flower number results in increased geitonogamy and, in turn, increased pollen discounting. To date, many studies have supported this prediction.
In Narthecium asiaticum (Maxim), an animal‐pollinated plant, however, male reproductive components in flowers increased with raceme size whereas female reproductive components did not. This was mostly because of the similar trend for middle to upper flowers within racemes.
Height above ground for middle to upper flowers was associated with raceme size, and higher flowers attracted more pollinators. Accordingly, more pollen grains were removed from upper flowers in larger racemes. Furthermore, an increased number of flowers may not increase local mate competition and geitonogamy in this species because only a few flowers function as males at any one time, irrespective of raceme size.
This may be the reason why male reproductive components of upper flowers increased with size, even though N. asiaticum is an animal‐pollinated plant. Male reproductive components of lower flowers did not increase, probably because the height of lower flowers was not associated with raceme size.
Towards a self-consistent approach to palaeomagnetic field modelling
Authors: Khokhlov A. , Hulot G. , Carlut J. 
Published: December 2004

SUMMARYRecent studies of the palaeomagnetic field behaviour over the past 5 Myr rely on statistical analysis of mainly directional data. However, the data are quite sparse and ill-distributed, and directional parameters are non-linear functions of the local field, rendering such statistical analysis non-trivial. Up to now these difficulties have usually been ignored or removed by relying on simplifications (linearization, neglecting internal correlations, etc.) that are unfortunately not justified if the field contains some amount of complexity.The purpose of the present paper is to present a rigorous statistical forward approach to palaeomagnetic field modelling. Starting from a statistical model of the field defined in terms of the statistics of its Gauss coefficients (along the lines pioneered by Constable & Parker 1988), we show how such a model may be exactly tested against any given data set, either on a local regional or a global scale. A method to implement this approach is outlined and examples based on published models are provided.In particular we focus on the treatment of directional data, for which the method is most relevant. The corresponding local probability density functions are derived and shown to be non-Fisherian, which we note may be a significant source of artefacts for standard mean-field modelling. Although the method we propose is already useful in its present state, some slight improvements are possible in order to account for noise in the data better.
On anelliptic approximations for qP velocities in VTI media
Authors: Fomel Sergey 
Published: December 2004

ABSTRACTA unified approach to approximating phase and group velocities of qP seismic waves in a transversely isotropic medium with vertical axis of symmetry (VTI) is developed. While the exact phase‐velocity expressions involve four independent parameters to characterize the elastic medium, the proposed approximate expressions use only three parameters. This makes them more convenient for use in surface seismic experiments, where the estimation of all four parameters is problematic. The three‐parameter phase‐velocity approximation coincides with the previously published ‘acoustic’ approximation of Alkhalifah. The group‐velocity approximation is new and noticeably more accurate than some of the previously published approximations. An application of the group‐velocity approximation for finite‐difference computation of traveltimes is shown.
Inversion of nuclear well‐logging data using neural networks
Authors: Aristodemou Elsa , Pain Christopher , De Oliveira Cassiano , Goddard Tony , Harris Christopher 
Published: December 2004

ABSTRACTThis work looks at the application of neural networks in geophysical well‐logging problems and specifically their utilization for inversion of nuclear downhole data. Simulated neutron and γ‐ray fluxes at a given detector location within a neutron logging tool were inverted to obtain formation properties such as porosity, salinity and oil/water saturation. To achieve this, the forward particle‐radiation transport problem was first solved for different energy groups (47 neutron groups and 20 γ‐ray groups) using the multigroup code EVENT. A neural network for each of the neutron and γ‐ray energy groups was trained to re‐produce the detector fluxes using the forward modelling results from 504 scenarios. The networks were subsequently tested on unseen data sets and theunseeninput parameters (formation properties) were then predicted using a global search procedure. The results obtained are very encouraging with formation properties being predicted to within 10% average relative error. The examples presented show that neural networks can be applied successfully to nuclear well‐logging problems. This enables the implementation of a fast inversion procedure, yielding quick and reliable values for unknown subsurface properties such as porosity, salinity and oil saturation.
Simulation of full‐waveform log in saturated cracked formations using Hudson's approach
Authors: Kazatchenko E. , Markov M. , Mousatov A. 
Published: December 2004

ABSTRACTWe study the propagation of elastic waves that are generated in a fluid‐filled borehole surrounded by a cracked transversely isotropic medium. In the model studied the anisotropy and borehole axes coincide. To obtain the effective elastic moduli of a cracked medium we have applied Hudson's theory that enables the determination of the overall properties as a function of the crack orientation in relation to the symmetry axis of the anisotropic medium. This theory takes into account the hydrodynamic mechanism of the elastic‐wave attenuation caused by fluid filtration from the cracks into a porous matrix.We have simulated the full waveforms generated by an impulse source of finite length placed on the borehole axis. The kinematic and dynamic parameters of the compressional, shear and Stoneley waves as functions of the matrix permeability, crack orientation and porosity were studied. The modelling results demonstrated the influence of the crack‐system parameters (orientation and porosity) on the velocities and amplitudes of all wave types. The horizontally orientated cracks result in maximal decrease of the elastic‐wave parameters (velocities and amplitudes).Based on the fact that the shear‐ and Stoneley‐wave velocities in a transversely isotropic medium are determined by different shear moduli, we demonstrate the feasibility of the acoustic log to identify formations with close to horizontal crack orientations.
The effect of orthorhombic anisotropy and its implication for oil recovery and reservoir exploitation
Authors: Assad Jamal M. 
Published: December 2004

ABSTRACTResults of an experimental study of shear‐ and compressional‐wave propagation in an orthorhombically anisotropic medium are presented. The experiments were performed on a physical model consisting of two sets of fractures. The first set consisted of orientated rubber inclusions simulating weak material‐filled cracks. The second set consisted of a system of closely spaced parallel fractures simulated by thin plates of epoxy resin, superimposed orthogonally on the first set. Three cases of fracture orientations within the model were identified and studied. Case 1 is analogous to a jointed fracture reservoir with one vertical set of fluid‐filled cracks or fractures and one non‐filled horizontal set. This case is referred to as JFV. Case 2 is analogous to a double fracture reservoir with one horizontal set of fluid‐filled fractures or cracks and one non‐filled vertical set. This is referred to as DFH. Case 3 is analogous to a double fracture reservoir with two vertical sets of fractures or cracks, with only one fluid‐filled. Case 3 is referred to as DFV. A pulse transmission method was performed on all three modelled cases along the three principal axes. A directional variation in the compressional‐ and shear‐wave velocities, as well as distinct shear‐wave splitting, was observed. The elastic constants for each case were determined and differences between them were noted and compared with the controlled results of both layered (transverse isotropy, TI) and vertically fractured (azimuthally anisotropic models, VF) media. The differences in elastic moduli and velocities indicate the potential of recognizing the different fracture orientations and suggest an approach to designing a method of drilling to further enhance oil recovery and reservoir exploitation.
Nitrogen cycling in grazed pastures at elevated CO2: N returns by ruminants
Authors: Allard Vincent , Newton Paul C. D. , Lieffering Mark , Clark Harry , Matthew Cory , Soussana Jean‐François , Gray Yvonne S. 
Published: December 2004

AbstractIn pastures grazed by large herbivores, nutrients cycle both through litter and animal excreta. We compared nitrogen (N) returns from sheep grazing a temperate pasture exposed to ambient or elevated CO2 (475 μmol mol−1) in a FACE (Free Air CO2 Enrichment) experiment established in the spring of 1997. In the spring of 2000 and 2001, we measured the chemical composition of the diet, sheep faeces and of individual plant species before grazing to characterize feed intake and to compare the intake of N to the N produced in faeces. In both years under elevated CO2, leaves of the individual species exhibited lower N concentrations and higher water‐soluble carbohydrate (WSC) concentrations. There was a significantly greater proportion of legume in the diet at elevated CO2 but, together with the changes in chemical composition of individual species, this resulted in diets that had similar N but higher WSC and digestibility for both ambient and elevated CO2. We found that a greater proportion of dietary N was partitioned to urine at elevated CO2, probably because of the higher proportion of legume N in the diet, with possible differences in protein quality. A potentially significant consequence of this change in partitioning is greater N loss through volatilization at higher CO2 levels.
Phenological responses in maple to experimental atmospheric warming and CO2enrichment
Authors: Norby Richard J. , Hartz‐Rubin Jennifer S. , Verbrugge Maria J. 
Published: December 2004

AbstractEvidence that global warming has altered the phenology of the biosphere, possibly contributing to increased plant production in the northern hemisphere, has come from a diversity of observations at scales ranging from the view of the back yard to satellite images of the earth. These observations, coupled with an understanding of the effects of temperature on plant phenology, suggest that future changes in the atmosphere and climate could alter plant phenology with unknown or unpredictable consequences. We assessed the effects of simulated climatic warming and atmospheric CO2enrichment on the spring and autumn phenology of maple trees (Acer rubrumandA. saccharum) growing for four years in open‐top field chambers. CO2enrichment (+300 ppm) had no consistent effects on the timing of budbreak and leaf unfolding in the spring or leaf abscission in the autumn. Warming (+4°C) usually had predictable effects: in two of the three years of assessment, budbreak occurred earlier in warm chambers than in ambient temperature chambers, and leaf abscission always occurred later. The lengthening of the growing season could contribute to increased productivity, although effects of temperature on other physiological processes can concurrently have negative effects on productivity. In 1995, budbreak was unexpectedly delayed in the warmer chambers, apparently the result of advanced budbreak leading to injury from a late‐spring frost. Likewise, there was increased risk associated with longer leaf retention in the autumn: in 1994, leaves in the warm chambers were killed by freezing temperatures before they had senesced. These observations support the premise that global warming could increase the length of the growing season. Phenological responses should, therefore, be part of any assessment of the possible consequences of global change, but our results also suggest that those responses may not always have positive effects on production.
Role of lakes for organic carbon cycling in the boreal zone
Authors: Algesten Grete , Sobek Sebastian , Bergström Ann‐Kristin , Ågren Anneli , Tranvik Lars J. , Jansson Mats 
Published: December 2004

AbstractWe calculated the carbon loss (mineralization plus sedimentation) and net CO2 escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437–48 263 km2). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO2 emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO2 emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO2 between the atmosphere and the boreal landscape.