bluuebunny/crossref_metadata_2025 huggingface dataset:
Ray tracing in elliptical anisotropic media using the linear traveltime interpolation (LTI) method applied to traveltime seismic tomography
Authors: Cardarelli Ettore , Antonio Cerreto
Published: March 2003

The linear traveltime interpolation (LTI) method is a suitable ray‐tracing technique for modelling first‐arrival times in isotropic media. LTI is extended to elliptical anisotropic media and applied to a tomographic inversion procedure. A theoretical formulation is first derived and then LTI implementation is discussed in terms of source–receiver arrays and cell size. The method is then combined with the tomographic inversion procedure adopted. The matching of the ray tracing with inversion in elliptical anisotropic media posed a double non‐linear problem. Thus two assumptions were made: the velocity in each cell is uniform and the main directions of anisotropy are known. To take into account the geometrical characteristics of the area under investigation (depth and velocity of the weathering, and thickness of the inner media), cells of varying size were considered. No hypothesis was made on anisotropy weakness.

The algorithm was first tested on synthetic models and then applied to a field survey. On comparing the results of the synthetic models and the field survey with those obtained with a linear raypath approximation, it was found that there were fewer data misfits.
An approach to combined rock physics and seismic modelling of fluid substitution effects
Authors: Johansen Tor Arne , Drottning Åsmund , Lecomte Isabelle , Gjøystdal Håvar
Published: March 2003

The aim of seismic reservoir monitoring is to map the spatial and temporal distributions and contact interfaces of various hydrocarbon fluids and water within a reservoir rock. During the production of hydrocarbons, the fluids produced are generally displaced by an injection fluid. We discuss possible seismic effects which may occur when the pore volume contains two or more fluids. In particular, we investigate the effect of immiscible pore fluids, i.e. when the pore fluids occupy different parts of the pore volume.

The modelling of seismic velocities is performed using a differential effective‐medium theory in which the various pore fluids are allowed to occupy the pore space in different ways. The P‐wave velocity is seen to depend strongly on the bulk modulus of the pore fluids in the most compliant (low aspect ratio) pores. Various scenarios of the microscopic fluid distribution across a gas–oil contact (GOC) zone have been designed, and the corresponding seismic properties modelled. Such GOC transition zones generally give diffuse reflection regions instead of the typical distinct GOC interface. Hence, such transition zones generally should be modelled by finite‐difference or finite‐element techniques.

We have combined rock physics modelling and seismic modelling to simulate the seismic responses of some gas–oil zones, applying various fluid‐distribution models. The seismic responses may vary both in the reflection time, amplitude and phase characteristics. Our results indicate that when performing a reservoir monitoring experiment, erroneous conclusions about a GOC movement may be drawn if the microscopic fluid‐distribution effects are neglected.
Resistivity of partially saturated Triassic Sandstone
Authors: Taylor Steve , Barker Ron
Published: March 2003

The variation in resistivity with saturation of poorly cemented Triassic Sandstone samples from a site in the English Midlands has been measured. The measurements were obtained using an adapted four‐electrode technique, which utilizes conductive gelling agents between electrodes, avoiding the need to sputter electrodes directly on to samples – a difficult process with such friable samples. The measurements provide important information regarding the way in which resistivity varies with saturation in the Triassic Sandstone. The resulting variation in the observed resistivity versus saturation curves indicates the presence of significant pore‐scale variation between samples. Measurements have also been conducted on fully saturated samples. These indicate significant variation in the matrix conductivity between samples. The results have important implications for field‐scale monitoring of the unsaturated zone.
Iterative resolution estimation in least‐squares Kirchhoff migration
Authors: Fomel Sergey , Berryman James G. , Clapp Robert G. , Prucha Marie
Published: March 2003

We apply iterative resolution estimation to least‐squares Kirchhoff migration. Reviewing the theory of iterative optimization uncovers the common origin of different optimization methods. This allows us to reformulate the pseudo‐inverse, model resolution and data resolution operators in terms of effective iterative estimates. When applied to Kirchhoff migration, plots of the diagonal of the model resolution matrix reveal low illumination areas on seismic images and provide information about image uncertainties. Synthetic and real data examples illustrate the proposed technique and confirm the theoretical expectations.
A new method to discriminate between a valid IP response and EM coupling effects
Authors: Xiang Jianping , Jones N.B. , Cheng Daizhan , Schlindwein F.S.
Published: March 2003

The problem of discrimination between a valid induced polarization (IP) response and electromagnetic (EM) coupling effects is considered and an effective solution is provided. First, a finite dimensional approximation to the Cole‐Cole model is investigated. Using the least‐squares approach, the parameters of the approximate model are obtained. Next, based on the analysis of overvoltage, a finite dimensional structure of the IP model is produced. Using this overvoltage‐based structure, a specific finite dimensional approximation of the Cole‐Cole model is proposed. Summarizing the analysis of the finite dimensional IP model, it is concluded that the proposed IP model, which fits the field data much better than the traditional Cole‐Cole model, is essentially an RC‐circuit. From a circuit‐analysis point of view, it is well known that an electromagnetic effect can be described by an RL‐circuit. The simulation results on experimental data support this conception. According to this observation, a new method to discriminate between a valid IP response and EM coupling effects is proposed as follows: (i) use a special finite dimensional model for IP–EM systems; (ii) obtain the parameters for the model using a least‐squares approach; (iii) separate RC‐type terms and RL‐type terms – the first models the IP behaviour, the latter represents the EM part. Simulation on experimental data shows that the method is very simple and effective.
The global carbon sink: a grassland perspective
Authors: Scurlock J. M. O. , Hall D. O.
Published: March 2003

The challenge to identify the biospheric sinks for about half the total carbon emissions from fossil fuels must include a consideration of below‐ground ecosystem processes as well as those more easily measured above‐ground. Recent studies suggest that tropical grasslands and savannas may contribute more to the ‘missing sink’ than was previously appreciated, perhaps as much as 0.5 Pg (= 0.5 Gt) carbon per annum. The rapid increase in availability of productivity data facilitated by the Internet will be important for future scaling‐up of global change responses, to establish independent lines of evidence about the location and size of carbon sinks.
Growth dynamics and genotypic variation in tropical, field‐grown paddy rice (Oryza sativa L.) in response to increasing carbon dioxide and temperature
Authors: MOYA TOLENTINO B. , ZISKA LEWIS H. , NAMUCO OFELIA S. , OLSZYK DAVE.
Published: March 2003

While previous studies have examined the growth and yield response of rice to continued increases in CO2 concentration and potential increases in air temperature, little work has focused on the long‐term response of tropical paddy rice (i.e. the bulk of world rice production) in situ, or genotypic differences among cultivars in response to increasing CO2 and/or temperature. At the International Rice Research Institute, rice (cv IR72) was grown from germination until maturity for 4 field seasons, the 1994 and 1995 wet and the 1995 and 1996 dry seasons at three different CO2 concentrations (ambient, ambient + 200 and ambient + 300 μL L–1 CO2) and two air temperatures (ambient and ambient + 4 °C) using open‐top field chambers placed within a paddy site. Overall, enhanced levels of CO2 alone resulted in significant increases in total biomass at maturity and increased seed yield with the relative degree of enhancement consistent over growing seasons across both temperatures. Enhanced levels of temperature alone resulted in decreases or no change in total biomass and decreased seed yield at maturity across both CO2 levels. In general, simultaneous increases in air temperature as well as CO2 concentration offset the stimulation of biomass and grain yield compared to the effect of CO2 concentration alone. For either the 1995 wet and 1996 dry seasons, additional cultivars (N‐22, NPT1 and NPT2) were grown in conjunction with IR72 at the same CO2 and temperature treatments. Among the cultivars tested, N‐22 showed the greatest relative response of both yield and biomass to increasing CO2, while NPT2 showed no response and IR72 was intermediate. For all cultivars, however, the combination of increasing CO2 concentration and air temperature resulted in reduced grain yield and declining harvest index compared to increased CO2 alone. Data from these experiments indicate that (a) rice growth and yield can respond positively under tropical paddy conditions to elevated CO2, but that simultaneous exposure to elevated temperature may negate the CO2 response to grain yield; and, (b) sufficient intraspecific variation exists among cultivars for future selection of rice cultivars which may, potentially, convert greater amounts of CO2 into harvestable yield.
Transferring soils from high‐ to low‐elevation forests increases nitrogen cycling rates: climate change implications
Authors: Hart Stephen C. , Perry David A.
Published: March 2003

We assessed the potential impact of global warming resulting from a doubling of preindustrial atmospheric CO2 on soil net N transformations by transferring intact soil cores (0–15 cm) from a high‐elevation old‐growth forest to a forest about 800 m lower in elevation in the central Oregon Cascade Mountains, USA. The lower elevation site had mean annual air and soil (10‐cm mineral soil depth) temperatures about 2.4 and 3.9 °C higher than the high‐elevation site, respectively. Annual rates of soil net N mineralization and nitrification more than doubled in soil transferred to the low‐elevation site (17.2–36.0 kg N ha–1 and 5.0–10.7 kg NO3––N ha–1, respectively). Leaching of inorganic N from the surface soil (in the absence of plant uptake) also increased. The reciprocal treatment (transferring soil cores from the low‐ to the high‐elevation site) resulted in decreases of about 70, 80, and 65% in annual rates of net N mineralization, nitrification, and inorganic N leaching, respectively. Laboratory incubations of soils under conditions of similar temperature and soil water potential suggest that the quality of soil organic matter is higher at the high‐elevation site. Similar in situ rates of soil net N transformations between the two sites occurred because the lower temperature counteracts the effects of greater substrate quantity and quality at the high elevation site. Our results support the hypothesis that high‐elevation, old‐growth forest soils in the central Cascades have higher C and N storage than their low‐elevation analogues primarily because low temperatures limit net C and N mineralization rates at higher elevations.
Seasonal and annual respiration of a ponderosa pine ecosystem
Authors: Law Beverly E. , Ryan Michael G. , Anthoni Peter M.
Published: March 2003

The net ecosystem exchange of CO2 between forests and the atmosphere, measured by eddy covariance, is the small difference between two large fluxes of photosynthesis and respiration. Chamber measurements of soil surface CO2 efflux (Fs), wood respiration (Fw) and foliage respiration (Ff) help identify the contributions of these individual components to net ecosystem exchange. Models developed from the chamber data also provide independent estimates of respiration costs. We measured CO2 efflux with chambers periodically in 1996–97 in a ponderosa pine forest in Oregon, scaled these measurements to the ecosystem, and computed annual totals for respiration by component. We also compared estimated half‐hourly ecosystem respiration at night (Fnc) with eddy covariance measurements. Mean foliage respiration normalized to 10 °C was 0.20 μmol m–2 (hemi‐leaf surface area) s–1, and reached a maximum of 0.24 μmol m–2 HSA s–1 between days 162 and 208. Mean wood respiration normalized to 10 °C was 5.9 μmol m–3 sapwood s–1, with slightly higher rates in mid‐summer, when growth occurs. There was no significant difference (P > 0.10) between wood respiration of young (45 years) and old trees (250 years). Soil surface respiration normalized to 10 °C ranged from 0.7 to 3.0 μmol m–2 (ground) s–1 from days 23 to 329, with the lowest rates in winter and highest rates in late spring. Annual CO2 flux from soil surface, foliage and wood was 683, 157, and 54 g C m–2 y–1, with soil fluxes responsible for 76% of ecosystem respiration. The ratio of net primary production to gross primary production was 0.45, consistent with values for conifer sites in Oregon and Australia, but higher than values reported for boreal coniferous forests. Below‐ground carbon allocation (root turnover and respiration, estimated as Fs– litterfall carbon) consumed 61% of GPP; high ratios such as this are typical of sites with more water and nutrient constraints. The chamber estimates were moderately correlated with change in CO2 storage in the canopy (Fstor) on calm nights (friction velocity u* < 0.25 m s–1; R2 = 0.60); Fstor was not significantly different from summed chamber estimates. On windy nights (u* > 0.25 m s–1), the sum of turbulent flux measured above the canopy by eddy covariance and Fstor was only weakly correlated with summed chamber estimates (R2 = 0.14); the eddy covariance estimates were lower than chamber estimates by 50%.
The photosynthesis – leaf nitrogen relationship at ambient and elevated atmospheric carbon dioxide: a meta‐analysis
Authors: Peterson Andrew G. , Ball J. Timothy , Luo YiqI. , Field Christopher B. , Reich Peter B. , Curtis Peter S. , Griffin Kevin L. , Gunderson Carla A. , Norby Richard J. , Tissue David T. , Forstreuter Manfred , Rey AnA. , Vogel Christoph S. , Participants Cmeal
Published: March 2003

Estimation of leaf photosynthetic rate (A) from leaf nitrogen content (N) is both conceptually and numerically important in models of plant, ecosystem, and biosphere responses to global change. The relationship between A and N has been studied extensively at ambient CO2 but much less at elevated CO2. This study was designed to (i) assess whether the A–N relationship was more similar for species within than between community and vegetation types, and (ii) examine how growth at elevated CO2 affects the A–N relationship. Data were obtained for 39 C3 species grown at ambient CO2 and 10 C3 species grown at ambient and elevated CO2. A regression model was applied to each species as well as to species pooled within different community and vegetation types. Cluster analysis of the regression coefficients indicated that species measured at ambient CO2 did not separate into distinct groups matching community or vegetation type. Instead, most community and vegetation types shared the same general parameter space for regression coefficients. Growth at elevated CO2 increased photosynthetic nitrogen use efficiency for pines and deciduous trees. When species were pooled by vegetation type, the A–N relationship for deciduous trees expressed on a leaf‐mass basis was not altered by elevated CO2, while the intercept increased for pines. When regression coefficients were averaged to give mean responses for different vegetation types, elevated CO2 increased the intercept and the slope for deciduous trees but increased only the intercept for pines. There were no statistical differences between the pines and deciduous trees for the effect of CO2. Generalizations about the effect of elevated CO2 on the A–N relationship, and differences between pines and deciduous trees will be enhanced as more data become available.
The effects of free‐air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth
Authors: Wechsung G. , Wechsung F. , Wall G. W. , Adamsen F. J. , Kimball B. A. , Pinter JR. P. J. , Lamorte R. L. , Garcia R. L. , Kartschall TH.
Published: March 2003

Spring wheat [
Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO2 concentrations, 550 μmol mol–1 for high‐CO2 plots, and 370 μmol mol–1 for control plots, using a Free‐Air CO2 Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO2, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split‐plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in‐row and inter‐row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO2 and soil water, in this agricultural ecosystem.

Wheat root growth increased under elevated CO2 conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO2 enhancement until anthesis. During the early vegetative growth, root dry mass of the inter‐row space was significantly higher for FACE than for Control treatments suggesting that elevated CO2 promoted the production of first‐order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter‐row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO2 and reduced soil water supports the hypothesis that plants grown in a high‐CO2 environment may better compensate soil‐water‐stress conditions.
Net methane emissions from grazing sheep
Authors: Judd Murray J. , Kellier Francis M. , Ulyatt Marcus J. , Lassey Keith R. , Tate Kevin R. , Shelton I. David , Harvey Michael J. , Walker Carolyn F.
Published: March 2003

Methane emissions from ruminant livestock are responsible for 45 % of New Zealand’s combined CO2‐equivalent greenhouse gas inventory, and arise principally from sheep. Using a flock of 6‐month old sheep (20 ha–1) grazing abundant pasture, we compare micrometeorological measurements of net methane emission rates with measurements from individual sheep based on a sulphur‐hexafluoride tracer technique. Individual sheep emission rates were highly variable and averaged 19.5 ± 4.8 (SD) g CH4 sheep–1 d–1, or 39 ± 9.6 mg CH4 m–2 d–1 on an areal basis. Emission rates were poorly correlated with animal live weight or dry matter intake but represented an average dietary energy loss of 3.6%.

 Methane fluxes from the surface were determined as half hourly averages by a flux‐gradient technique using temperature and methane gradients. Soil methane consumption was measured using chambers and found to be negligible (< 0.09 mg CH4 m–2 d–1) in comparison with the animal contribution. Daily net emission rates averaged 46 mg m–2 d–1 and exhibited a broad peak in the early afternoon which corresponded with animal activity patterns. On average, net emisssion rates were 40% higher during the day than at night. Stable nocturnal conditions led to a separation of the micrometeorological measurements from the methane source and hence highly variable results. Based on two corroborating techniques, the average net methane emission rate was c. 43 mg CH4 m–2 d–1 or 155 kg CH4 ha–1 y–1.
Elevated CO2 increases nitrogen fixation and decreases soil nitrogen mineralization in Florida scrub oak
Authors: Hungate Bruce A. , Dijkstra PauL. , Johnson DalE. W. , Hinkle C. RosS. , Drake BerT. G.
Published: March 2003

We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO2 during the first 14 months of experimental treatment. Elevated CO2 stimulated above‐ground growth, nitrogen mass, and root nodule production of the nitrogen‐fixing vine, Galactia elliottii Nuttall. During this period, elevated CO2 reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO2 did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH4+ immobilized per unit microbial N) measured over a 24‐h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO2 best explains these changes.

 These results demonstrate that atmospheric CO2 concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long‐term feedbacks to the growth responses of plants to elevated CO2. Greater nitrogen fixation and reduced leaching could stimulate nitrogen‐limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant‐available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.
Measurements of gross and net ecosystem productivity and water vapour exchange of a Pinus ponderosa ecosystem, and an evaluation of two generalized models
Authors: Law B. E. , Waring . R. H. , Anthoni P. M. , Aber J. D.
Published: March 2003

Net ecosystem productivity (NEP), net primary productivity (NPP), and water vapour exchange of a mature Pinus ponderosa forest (44°30′ N, 121°37′ W) growing in a region subject to summer drought were investigated along with canopy assimilation and respiratory fluxes. This paper describes seasonal and annual variation in these factors, and the evaluation of two generalized models of carbon and water balance (PnET‐II and 3‐PG) with a combination of traditional measurements of NPP, respiration and water stress, and eddy covariance measurements of above‐and below‐canopy CO2 and water vapour exchange. The objective was to evaluate the models using two years of traditional and eddy covariance measurements, and to use the models to help interpret the relative importance of processes controlling carbon and water vapour exchange in a water‐limited pine ecosystem throughout the year. PnET‐II is a monthly time‐step model that is driven by nitrogen availability through foliar N concentration, and 3‐PG is a monthly time‐step quantum‐efficiency model constrained by extreme temperatures, drought, and vapour pressure deficits. Both models require few parameters and have the potential to be applied at the watershed to regional scale. There was 2/3 less rainfall in 1997 than in 1996, providing a challenge to modelling the water balance, and consequently the carbon balance, when driving the models with the two years of climate data, sequentially. Soil fertility was not a key factor in modelling processes at this site because other environmental factors limited photosynthesis and restricted projected leaf area index to ∼1.6. Seasonally, GEP and LE were overestimated in early summer and underestimated through the rest of the year. The model predictions of annual GEP, NEP and water vapour exchange were within 1–39% of flux measurements, with greater disparity in 1997 because soil water never fully recharged. The results suggest that generalized models can provide insights to constraints on productivity on an annual basis, using a minimum of site data.
How much do local factors matter for predicting transient ecosystem dynamics? Suggestions from permafrost formation in boreal peatlands
Authors: Camill Philip
Published: March 2003

With rapid climate warming, ecosystems will probably exhibit complex dynamics because local factors and life history attributes of species mediate the effects of regional climate change. To assess the relative importance of local vs. regional processes on permafrost formation in boreal peatlands, I sampled for permafrost and factors affecting its formation in 38 collapse scars across a 4 °C mean annual temperature (MAT) gradient in the discontinuous permafrost zone of northern Manitoba, Canada. Three complimentary approaches were used to model factors important to permafrost formation at both local and regional scales. In the first analysis, a mechanistic, spatial model of permafrost formation was developed as a function of Picea mariana size and proximity. In the second approach, permafrost formation was modelled as a function of two local factors, diameter of Picea mariana trees and emergent organic matter depth, and the regional factor, mean annual temperature (MAT). Finally, published aerial photography data were used to determine whether the proportion of bogs with permafrost changes across a MAT gradient. Results show that permafrost formation in boreal permafrost peatlands is best described as a locally driven process within regional climatic constraints. At local scales of 1–2 meters, the spatial and size distributions of trees controlled the spatial distribution of permafrost. At regional scales, tree size was a significantly better predictor than emergent organic matter or MAT. These results suggest that transient models of discontinuous permafrost based only on climate may poorly predict changes in vegetation and permafrost.
The high‐latitude terrestrial carbon sink: a model analysis
Authors: White A. , Cannell M. G. R. , Friend A. D.
Published: March 2003

A dynamic, global vegetation model, hybrid v4.1 (Friend et al. 1997), was driven by transient climate output from the UK Hadley Centre GCM (HadCM2) with the IS92a scenario of increasing atmospheric CO2 equivalent, sulphate aerosols and predicted patterns of atmospheric N deposition. Changes in areas of vegetation types and carbon storage in biomass and soils were predicted for areas north of 50°N from 1860 to 2100. Hybrid is a combined biogeochemical, biophysical and biogeographical model of natural, potential ecosystems. The effect of periodic boreal forest fires was assessed by adding a simple stochastic fire model. Hybrid represents plant physiological and soil processes regulating the carbon, water and N cycles and competition between individuals of parameterized generalized plant types. The latter were combined to represent tundra, temperate grassland, temperate/mixed forest and coniferous forest. The model simulated the current areas and estimated carbon stocks in the four vegetation types.

It was predicted that land areas above 50°N (about 23% of the vegetated global land area) are currently accumulating about 0.4 PgC y−1 (about 30% of the estimated global terrestrial sink) and that this sink could grow to 0.8–1.0 PgC y−1 by the second half of the next century and persist undiminished until 2100. This sink was due mainly to an increase in forest productivity and biomass in response to increasing atmospheric CO2, temperature and N deposition, and includes an estimate of the effect of boreal forest fire, which was estimated to diminish the sink approximately by the amount of carbon emitted to the atmosphere during fires. Averaged over the region, N deposition contributed about 18% to the sink by the 2080 s. As expected, climate change (temperature, precipitation, solar radiation and saturation pressure deficit) and N deposition without increasing atmospheric CO2 produced a carbon source. Forest areas expanded both south and north, halving the current tundra area by 2100. This expansion contributed about 30% to the sink by the 2090 s. Tundra areas which were not invaded by forest fluctuated from sink to source. It was concluded that a high latitude carbon sink exists at present and, even assuming little effect of N deposition, no forest expansion and continued boreal forest fires, the sink is likely to persist at its current level for a century.
Meeting Europe's climate change commitments: quantitative estimates of the potential for carbon mitigation by agriculture
Authors: Smith Pete , Powlson David S. , Smith Jo U. , Falloon Pete , Coleman Kevin
Published: March 2003

Under the Kyoto Protocol, the European Union is committed to a reduction in CO2 emissions to 92% of baseline (1990) levels during the first commitment period (2008–2012). The Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere. Thus, land‐use/land‐management change and forestry activities that are shown to reduce atmospheric CO2 levels can be included in the Kyoto targets. These activities include afforestation, reforestation and deforestation (article 3.3 of the Kyoto Protocol) and the improved management of agricultural soils (article 3.4). In this paper, we estimate the carbon mitigation potential of various agricultural land‐management strategies and examine the consequences of European policy options on carbon mitigation potential, by examining combinations of changes in agricultural land‐use/land‐management.

 We show that no single land‐management change in isolation can mitigate all of the carbon needed to meet Europe's climate change commitments, but integrated combinations of land‐management strategies show considerable potential for carbon mitigation. Three of the combined scenarios, one of which is an optimal realistic scenario, are by themselves able to meet Europe's emission limitation or reduction commitments.

 Through combined land‐management scenarios, we show that the most important resource for carbon mitigation in agriculture is the surplus arable land. We conclude that in order to fully exploit the potential of arable land for carbon mitigation, policies will need to be implemented to allow surplus arable land to be put into alternative long‐term land‐use.

 Of all options examined, bioenergy crops show the greatest potential for carbon mitigation. Bioenergy crop production also shows an indefinite mitigation potential compared to other options where the mitigation potential is finite. We suggest that in order to exploit fully the bioenergy option, the infrastructure for bioenergy production needs to be significantly enhanced before the beginning of the first Kyoto commitment period in 2008.

 It is not expected that Europe will attempt to meet its climate change commitments solely through changes in agricultural land‐use. A reduction in CO2‐carbon emissions will be key to meeting Europe's Kyoto targets, and forestry activities (Kyoto Article 3.3) will play a major role. In this study, however, we demonstrate the considerable potential of changes in agricultural land‐use and ‐management (Kyoto Article 3.4) for carbon mitigation and highlight the policies needed to promote these agricultural activities. As all sources of carbon mitigation will be important in meeting Europe's climate change commitments, agricultural carbon mitigation options should be taken very seriously.
Natural nitrogen filter fails in polluted raised bogs
Authors: Lamers Leon P. M. , Bobbink Roland , Roelofs Jan G. M.
Published: March 2003

Raised bogs are among the ecosystems most susceptible to atmospheric nitrogen pollution. Based on global data ranging from pristine to heavily polluted areas, a conceptual model is presented to explain the logistic response of these terrestrial carbon reservoirs to increased airborne nitrogen fluxes.
Sensitivity of three grassland communities to simulated extreme temperature and rainfall events
Authors: White Todd A. , Campbell Bruce D. , Kemp Peter D. , Hunt Chris L.
Published: March 2003

Three grassland communities in New Zealand with differing climates and proportions of C3 and C4 species were subjected to one‐off extreme heating (eight hours at 52.5°C) and rainfall (the equivalent of 100 mm) events. A novel experimental technique using portable computer‐controlled chambers simulated the extreme heating events. The productive, moist C3/C4 community was the most sensitive to the extreme events in terms of short‐term community composition compared with a dry C3/C4 community or an exclusively C3 community. An extreme heating event caused the greatest change to plant community species abundance by favouring the expansion of C4 species relative to C3 species, shifting C4 species abundance from 43% up to 84% at the productive, moist site. This was observed both in the presence and absence of added water. In the absence of C4 species, heating reduced community productivity by over 60%. The short‐term shifts in the abundance of C3 and C4 species in response to the single extreme climatic events did not have persistent effects on community structure or on soil nitrogen one year later. There was no consistent relationship between diversity and stability of biomass production of these plant communities, and species functional identity was the most effective explanation for the observed shifts in biomass production. The presence of C4 species resulted in an increased stability of productivity after extreme climatic events, but resulted in greater overall shifts in community composition. The presence of C4 species may buffer grassland community productivity against an increased frequency of extreme heating events associated with future global climate change.
Oxidation of atmospheric methane in Northern European soils, comparison with other ecosystems, and uncertainties in the global terrestrial sink
Authors: Smith K. A. , Dobbie K. E. , Ball B. C. , Bakken L. R. , Sitaula B. K. , Hansen S. , Brumme R. , Borken W. , Christensen S. , Priemé A. , Fowler D. , Macdonald J. A. , Skiba U. , Klemedtsson L. , Kasimir‐Klemedtsson A. , Degórska A. , Orlanski P.
Published: March 2003

This paper reports the range and statistical distribution of oxidation rates of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land‐use change and other factors on the oxidation rate.

Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log‐normal distribution (log‐mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two‐thirds –‐ closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes > 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration.

Analysis of all available data for CH4 oxidation rates in situ showed similar log‐normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short‐and longer‐term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y−1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to > 100 Tg CH4 y−1). Little or no information is available for many major ecosystems; these should receive high priority in future research.
A proposed CO2‐controlled mechanism of woody plant invasion in grasslands and savannas
Authors: Bond William J. , Midgley Guy F.
Published: March 2003

We propose that elevated CO2 may have a significant positive effect on woody plant success and thus favour tree invasion and thickening in grass‐dominated ecosystems. We note that savanna tree biomass is strongly constrained by disturbance, particularly fire, and that elevated CO2 could act to reduce this constraint. Our argument combines knowledge of tree recovery from injury after grassland fires, with theory about carbon acquisition and carbohydrate storage patterns in C3 woody plants in response to elevated CO2. We propose simply that elevated CO2 will tend to favour regrowth of juvenile trees trapped (sometimes for decades) in the ‘topkill’ zone, thus allowing them to escape more readily from periodic fires as CO2 continues to rise. Little empirical evidence exists to test this hypothesis, even though the process may have important implications for tree/grass codominated ecosystems currently in a dynamic equilibrium.
The role of fire in the boreal carbon budget
Authors: Harden J. W. , Trumbore S. E. , Stocks B. J. , Hirsch A. , Gower S. T. , O'neill K. P. , Kasischke E. S.
Published: March 2003

To reconcile observations of decomposition rates, carbon inventories, and net primary production (NPP), we estimated long‐term averages for C exchange in boreal forests near Thompson, Manitoba. Soil drainage as defined by water table, moss cover, and permafrost dynamics, is the dominant control on direct fire emissions. In upland forests, an average of about 10–30% of annual NPP was likely consumed by fire over the past 6500 years since these landforms and ecosystems were established. This long‐term, average fire emission is much larger than has been accounted for in global C cycle models and may forecast an increase in fire activity for this region. While over decadal to century times these boreal forests may be acting as slight net sinks for C from the atmosphere to land, periods of drought and severe fire activity may result in net sources of C from these systems.
Herbaria century record of increasing eutrophication in Spanish terrestrial ecosystems
Authors: Peñuelas Josep , Filella Iolanda
Published: March 2003

Additional biological evidence is presented for the alteration of biogeochemical cycles by human activities. The leaf δ15N and the concentrations of nutrients in herbarium specimens of 24 species of vascular plants and 3 species of bryophytes collected in northern and eastern regions of Spain have substantially changed throughout the XX century. In the second half of the century, when anthropogenic nitrogen fixation and mobilization started to increase rapidly, leaf δ15N values started to decrease strongly, indicating that additional anthropogenic nitrogen is being retained in Spanish terrestrial ecosystems. The concentration of nutrients in vascular plants did not present any clear pattern, but there were increasing concentrations of N and other nutrients (P, K, and S) in the last decades in bryophytes, which are usually better biomonitors of airborne chemicals than vascular plants. Important consequences for ecosystem structure and functioning such as enhancement of the carbon sink or changes in community biodiversity and species distribution may be expected from this increase in eutrophication.
Parameter estimation in surface exchange models using nonlinear inversion: how many parameters can we estimate and which measurements are most useful?
Authors: Wang Ying‐Ping , Leuning Ray , Cleugh Helen A. , Coppin Peter A.
Published: March 2003

Models of mass and energy exchanges between the biosphere and the atmosphere generally contain a nonlinear dependence between fluxes and model parameters, and thus estimation of these parameters from measurements in a heterogeneous landscape depends on the scale of the observations. The scale‐dependence of a typical surface‐exchange model (the CSIRO Biospheric Model, CBM) is examined using the diurnal variation of hourly fluxes of CO2, latent heat, sensible heat and soil heat. The fluxes were measured using micrometeorological techniques over six sites in a grazing/pasture system in SE Australia during a period of three weeks in 1995. Nonlinear parameter inversion was used to determine model parameters.

Analysis of the covariance of the estimates of the parameters and the unexplained residuals of the model showed that a maximum of three or four parameters could be determined independently from the observations for all six sites. Estimates of a key model parameter, jmax, the mean of maximum potential electron transport rate of all leaves within the canopy, was best determined by the measurements of net CO2 flux at all sites examined. Measurements of ground heat flux provide little information about any of the model parameters in CBM.

Because of nonlinearities in the surface exchange model, calculated fluxes will be in error if parameters for the component vegetation types are simply averaged in proportion to their areal fraction. The magnitude of these errors was examined for CBM using a hypothetical land surface consisting of two surface types, each with different parameter values. Predictions of net CO2, latent heat and ground heat fluxes using a linear combination of model parameters for the two surface types were quite similar with those found using optimal estimates of the parameters for the landscape, but were significantly poorer for sensible heat fluxes.
Carbon‐based secondary and structural compounds in Mediterranean shrubs growing near a natural CO2 spring
Authors: Peñuelas J. , Castells E. , Joffre R. , Tognetti R.
Published: March 2003

We studied carbon‐based secondary and structural compounds (CBSSCs) in Myrtus communis, Erica arborea, and Juniperus communis co‐occurring in a natural CO2 spring site and in a nearby control site in a Mediterranean environment. Leaf concentrations of phenolics and CBSSCs, such as lignin, cellulose, and hemicellulose, total nonstructural carbohydrates (TNCs), and lipids were measured monthly (phenolics) and every two months (the other compounds) throughout a year. There was a slight seasonal trend towards maximum concentrations of most of these CBSSCs during autumn–winter and minimum values during the spring season, particularly in Myrtus communis. For most of the CBSSCs and species, there were no consistent or significant patterns in response to the elevated [CO2] (c. 700 μmol mol−1) of the spring site. These results were not due to a dilution effect by increased structural or nonstructural carbon. Therefore, in contrast to many experimental studies of CO2 enrichment, mainly conducted for short periods, there were no greater concentrations of phenolics, and, as in many of these studies, there were neither greater concentrations of the other CBSSCs. These results do not agree with the predictions of the carbon source‐sink hypotheses. Possible causes of this disagreement are discussed. These causes include the complex heterogeneous environmental conditions and the variability of resource availabilities in the field, photosynthetic down‐regulation, and/or the homeostatic and evolutionary nature of organisms. These results suggest evolutionary adaptive responses to changes in CO2. They also suggest caution in attributing increased CBSSC concentrations to elevated [CO2] at long‐term scale in natural conditions, and therefore in their implications for plant–herbivore interactions and for decomposition.
Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems
Authors: Huenneke Laura F. , Anderson John P. , Remmenga Marta , Schlesinger William H.
Published: March 2003

AbstractThe Chihuahuan desert of New Mexico, USA, has changed in historical times from semiarid grassland to desert shrublands dominated byLarrea tridentataandProsopis glandulosa. Similar displacement of perennial grasslands by shrubs typifies desertification in many regions. Such structural vegetation change could alter average values of net primary productivity, as well as spatial and temporal patterns of production. We investigated patterns of aboveground plant biomass and net primary production in five ecosystem types of the Jornada Basin Long‐Term Ecological Research (LTER) site. Comparisons of shrub‐dominated desertified systems and remnant grass‐dominated systems allowed us to test the prediction that shrublands are more heterogeneous spatially, but less variable over time, than grasslands.

We measured aboveground plant biomass and aboveground net primary productivity (ANPP) by species, three times per year for 10 years, in 15 sites of five ecosystem types (three each inLarreashrubland,Bouteloua eriopodagrassland,Prosopisdune systems,Flourensia cernuaalluvial flats, and grass‐dominated dry lakes or playas). Spatial heterogeneity of biomass at the scale of our measurements was significantly greater in shrub‐dominated systems than in grass‐dominated vegetation. ANPP was homogeneous across space in grass‐dominated systems, and in most growing seasons was significantly more patchy in shrub vegetation. Substantial interannual variability in ANPP complicates comparison of mean values across ecosystem types, but grasslands tended to support higher ANPP values than did shrub‐dominated systems. There were significant interactions between ecosystem type and season.

Grasslands demonstrated higher interannual variation than did shrub systems. Desertification has apparently altered the seasonality of productivity in these systems; grasslands were dominated by summer growth, while sites dominated byLarreaorProsopistended to have higher spring ANPP. Production was frequently uncorrelated across sites of an ecosystem type, suggesting that factors other than season, regional climate, or dominant vegetation may be significant determinants of actual NPP.