bluuebunny/crossref_metadata_2025 huggingface dataset:
Resistivity anomaly imaging by probability tomography
Authors: Mauriello Paolo , Patella Domenico
Published: March 2003

Probability tomography is a new concept reflecting the inherently uncertain nature of any geophysical interpretation. The rationale of the new procedure is based on the fact that a measurable anomalous field, representing the response of a buried feature to a physical stimulation, can be approximated by a set of partial anomaly source contributions. These may be given a multiplicity of configurations to generate cumulative responses, which are all compatible with the observed data within the accuracy of measurement. The purpose of the new imaging procedure is the design of an occurrence probability space of elementary anomaly sources, located anywhere inside an explored underground volume. In geoelectrics, the decomposition is made within a regular resistivity lattice, using the Frechet derivatives of the electric potential weighted by resistivity difference coefficients. The typical tomography is a diffuse image of the resistivity difference probability pattern, that is quite different from the usual modelled geometry derived from standard inversion.
Model‐based shear‐wave velocity estimation versus empirical regressions
Authors: Jørstad Arild , Mukerji Tapan , Mavko Gary
Published: March 2003

Modelling of AVO signatures for reservoir characterization requires VS estimation from other available logs when shear‐wave data are not available. We tested various models for predicting VS from P‐wave velocity, porosity and shale volume measured in well logs. Effective medium models which characterize the pore space in terms of ellipsoidal inclusions were compared with statistical VP–VS regressions. The inclusion models were calibrated by non‐linear minimization of the difference between model‐predicted velocities and actual measured velocities. The quality of the VS prediction was quantified in terms of the rms error by comparison with shear‐wave data in wells where both VP and VS were measured. The linear regressions were found to be more robust and the rms error in the prediction was comparable to effective medium model‐based predictions.
Interpretation of shallow refraction seismic data by reflection/refraction tomography
Authors: Ditmar Pavel , Penopp Jan , Kasig Rainer , Makris Jannis
Published: March 2003

A new algorithm for tomographic inversion of traveltimes of reflected and refracted seismic waves is developed. The inversion gives interface configurations and velocity distributions in layers. The important features of the algorithm are: (a) the inclusion of shot time delays in the list of unknown parameters; (b) the regularization is applied in such a way that the most probable model is characterized by the similarity of neighbouring interfaces. As the problem under consideration is non‐linear, several iterations are necessary in order to obtain the final model. In the case of a very inexact initial model, a ‘layer‐by‐layer’ inversion strategy is recommended as a first inversion step. The inversion program is supplied with a user interface, thanks to which one can: (a) pick interactively and identify seismic traveltimes; (b) build and edit depth/velocity models; and (c) display calculated traveltime curves and compare them with picked traveltimes as well as with the original seismic sections. The efficiency of the inversion software developed is illustrated by a numerical example and a field example in which shallow seismic data are considered. Application to wide‐aperture reflection/refraction profiling (WARRP) data is also possible.
2D finite‐difference viscoelastic wave modelling including surface topography
Authors: O. Hestholm Stig , Ruud Bent
Published: March 2003

We have pursued two‐dimensional (2D) finite‐difference (FD) modelling of seismic scattering from free‐surface topography. Exact free‐surface boundary conditions for the particle velocities have been derived for arbitrary 2D topographies. The boundary conditions are combined with a velocity–stress formulation of the full viscoelastic wave equations. A curved grid represents the physical medium and its upper boundary represents the free‐surface topography. The wave equations are numerically discretized by an eighth‐order FD method on a staggered grid in space, and a leap‐frog technique and the Crank–Nicholson method in time.

In order to demonstrate the capabilities of the surface topography modelling technique, we simulate incident point sources with a sinusoidal topography in seismic media of increasing complexities. We present results using parameters typical of exploration surveys with topography and heterogeneous media. Topography on homogeneous media is shown to generate significant scattering. We show additional effects of layering in the medium, with and without randomization, using a von Kármán realization of apparent anisotropy. Synthetic snapshots and seismograms indicate that prominent surface topography can cause back‐scattering, wave conversions and complex wave patterns which are usually discussed in terms of inter‐crust heterogeneities.
Improving modelling and inversion in refraction seismics with a first‐order Eikonal solver
Authors: Lecomte Isabelle , Gjøystdal Håvar , Dahle Anders , Pedersen Ole Christian
Published: March 2003

A first‐order Eikonal solver is applied to modelling and inversion in refraction seismics. The method calculates the traveltime of the fastest wave at any point of a regular grid, including head waves as used in refraction. The efficiency, robustness and flexibility of the method give a very powerful modelling tool to find both traveltimes and raypaths. Comparisons with finite‐difference data show the validity of the results. Any arbitrarily complex model can be studied, including the exact topography of the surface, thus avoiding static corrections. Later arrivals are also obtained by applying high‐slowness masks over the high‐velocity zones. Such an efficient modelling tool may be used interactively to invert for the model, but a better method is to apply the refractor‐imaging principle of Hagedoorn to obtain the refractors from the picked traveltime curves. The application of this principle has already been tried successfully by previous authors, but they used a less well‐adapted Eikonal solver. Some of their traveltimes were not correct in the presence of strong velocity variations, and the refractor‐imaging principle was restricted to receiver lines along a plane surface. With the first‐order Eikonal solver chosen, any topography of the receiving surface can be considered and there is no restriction on the velocity contrast. Based on synthetic examples, the Hagedoorn principle appears to be robust even in the case of first arrivals associated with waves diving under the refractor. The velocities below the refractor can also be easily estimated, parallel to the imaging process. In this way, the model can be built up successively layer by layer, the refractor‐imaging and velocity‐mapping processes being performed for each identified refractor at a time. The inverted model could then be used in tomographic inversions because the calculated traveltimes are very close to the observed traveltimes and the raypaths are available.
Cross‐hole resistivity tomography using different electrode configurations
Authors: Bing Zhou , Greenhalgh S.A.
Published: March 2003

This paper investigates the relative merits and effectiveness of cross‐hole resistivity tomography using different electrode configurations for four popular electrode arrays: pole–pole, pole–bipole, bipole–pole and bipole–bipole. By examination of two synthetic models (a dipping conductive strip and a dislocated fault), it is shown that besides the popular pole–pole array, some specified three‐ and four‐electrode configurations, such as pole–bipole AM–N, bipole–pole AM–B and bipole–bipole AM–BN with their multispacing cross‐hole profiling and scanning surveys, are useful for cross‐hole resistivity tomography. These configurations, compared with the pole–pole array, may reduce or eliminate the effect of remote electrodes (systematic error) and yield satisfactory images with 20% noise‐contaminated data. It is also shown that the configurations which have either both current electrodes or both potential electrodes in the same borehole, i.e. pole–bipole A–MN, bipole–pole AB–M and bipole–bipole AB–MN, have a singularity problem in data acquisition, namely low readings of the potential or potential difference in cross‐hole surveying, so that the data are easily obscured by background noise and yield images inferior to those from other configurations.
Effective‐medium theories for fluid‐saturated materials with aligned cracks
Authors: Hudson J.A. , Pointer T. , Liu E.
Published: March 2003

There is general agreement between different theories giving expressions for the overall properties of materials with dry, aligned cracks if the number density of cracks is small. There is also very fair agreement for fluid‐filled isolated cracks. However, there are considerable differences between two separate theories for fluid‐filled cracks with equant porosity. Comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples. However, although the crack number density in the laboratory sample is such that first‐order theory is unlikely to apply, expressions correct to second order (in the number density) provide a worse fit. It also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective‐medium theories, which are valid only when this ratio is large. The data show dispersion effects for dry cracks and scattering, neither of which will occur at sufficiently long wavelengths. Data from the water‐saturated samples indicate that the effect of equant porosity is significant, although the two theories differ strongly as to just how significant. Once again, and in spite of the reservations mentioned above, a reasonable fit between theory and observation can be shown.
The seismic response to overpressure: a modelling study based on laboratory, well and seismic data
Authors: Carcione José M. , Tinivella Umberta
Published: March 2003

We investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore‐pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P‐wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Overpressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore‐pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs.
Returning and New Montana Migrants: Socio‐economic and Motivational Differences
Authors: Reichert Christiane von
Published: March 2003

This paper examines return migrants and new migrants to Montana: Who are they? Why do they move? Do return migrants move for different reasons than new migrants? Data from the 1994–1997 Montana Poll, a representative survey of Montana households, are used. A comparison of socio‐economic differences of return and new migrants shows that the two migrant types are very similar in terms of education, income, and age. This stands in contrast to the findings of others who maintain that return migrants are negatively selected with respect to education. Logistic regressions were employed to identify the effect of age and place ties on reasons for moving. Return migrants and new migrants move to Montana for very similar reasons, with family being the most important primary reason for moving. Moving for lifestyle reasons, such as environmental quality and urban amenities, were found to systematically change with age. This could explain why people return to a place they left earlier in life. While other research on return migration compared return migrants and other migrants who left the same place of origin, this paper offers a comparison of return migrants and other migrants who seek out the same destination. Results from the Montana Poll suggest that the same destination attracts return migrants and new migrants with similar socio‐economic characteristics who move there for very similar reasons.
Effects of climate change on nitrogen dynamics in upland soils. 2. A soil warming study
Authors: Ineson Philip , Benham David G. , Poskitt Janet , Harrison Anthony F. , Taylor Kenneth , Woods Clive
Published: March 2003

A new warming technique has been developed in a field experimental study of the potential effects of climatic change on N leaching from hill land plant/soil systems. Thermocouple compensating cable has been utilized to provide a small cross‐section, flexible, low voltage heating cable, mounted on a framework of stainless steel mesh, to provide uniform heating at the vegetation/soil interface of zero‐tension lysimeters and surrounding turf. We describe a specially designed heat controller capable of maintaining a temperature differential of 3 °C above ambient at a soil depth of 0.8 cm. The equipment raises temperatures down the soil profile and within the grass sward, whilst tracking normal diurnal temperature variation. Results presented here illustrate the efficacy of the warming technique, together with the consequences for the release of nitrate from lysimeters. The responses of soil solution concentrations of nitrate varied markedly between soil types, but showed a significant decrease in the brown earth during the first 5 months of additional heating. This suggests that increased nutrient release is masked by plant uptake in this soil, but the responses in the other two soils were less marked.
The effect of elevated CO2 concentration and soil pH on the relationship between plant growth and rhizosphere denitrification potential
Authors: Hall J. M. , Paterson E. , Killham K.
Published: March 2003

The effect of CO2 concentration on plant growth and the size of the rhizosphere denitrifier population was investigated for ryegrass grown at 3 different soil pH values (pH 4.3, 5.9 and 7.0). Soil microcosms were planted with ryegrass and maintained under constant growth conditions at either ambient (450ppm) or elevated (720ppm) CO2 concentration. At harvest, the rhizosphere soil was collected and subjected to a potential denitrification assay to provide an estimate of the size of the denitrifier population present. Ryegrass dry matter production varied across the pH range studied and contrary to other studies, elevated CO2 concentration did not consistently increase growth. Plant growth was reduced by ≈ 35% and 23% at pH 4.3 and pH 5.9, respectively, under elevated CO2 concentration. At pH 7.0, however, plant growth was increased by ≈ 45% under elevated CO2. Potential denitrification rates within the rhizosphere followed a similar pattern to plant growth in the different treatments, suggesting that plant growth and the size of denitrifier population within the rhizosphere are coupled. This study investigates the relationship between plant growth and rhizosphere denitrification potential, thereby providing an estimate of the size of the denitrifier population under increased CO2 concentration and soil pH.
Effects of short‐term drainage and aeration on the production of methane in submerged rice soil
Authors: Ratering Stefan , Conrad RalF.
Published: March 2003

Emission rates of CH4 were measured in microcosms of submerged soil which were planted with rice. Drainage of the rice microcosms for 48 h resulted in drastically decreased CH4 emission rates which only slowly recovered to the rates of the undrained controls. Drainage also resulted in drastically increased sulphate concentrations which only slowly decreased to nearly zero background values after the microcosms were submerged again. The mechanisms responsible for the decrease of CH4 production by aeration were investigated in slurries of a loamy and a sandy Italian rice soil. Incubation of the soil slurries under anoxic conditions resulted first in the reduction of nitrate, sulphate and ferric iron before CH4 production started. Incubation of the soil slurries for 48 h under air resulted in immediate and complete inhibition of CH4 production. Although the soil slurries were then again incubated under anoxic conditions (N2 atmosphere), the inhibition of CH4 production persisted for more than 30 days. The redox potential of the soil increased after the aeration but returned within 15 days to the low values typical for CH4 production. However, the concentrations of sulphate and of ferric iron increased dramatically after the aeration and stayed at elevated levels for the period during which CH4 production was inhibited. These observations show that even brief exposure of the soil to O2 allowed the production of sulphate and ferric iron from their reduced precursors. Elevated sulphate and ferric iron concentrations allowed sulphate‐reducing and ferric iron‐reducing bacteria to outcompete methanogenic bacteria on H2 as common substrate. Indeed, concentrations of H2 were decreased as long as sulphate and ferric iron were high so that the Gibbs free energy of CH4 production from H2/CO2 was also increased (less exergonic). On the other hand, concentrations of acetate, the more important precursor for CH4, were not much affected by the short aeration of the soil slurries, and the Gibbs free energy of CH4 production from acetate was highly exergonic suggesting that acetotrophic methanogens were not outcompeted but were otherwise inhibited. Aeration also resulted in increased rates of CO2 production and in a short‐term increase of N2O production. However, these increases were < 10% of the decreased production of CH4 and did not represent a trade‐off in terms of CO2 equivalents. Hence, short‐term drainage and aeration of submerged paddy fields may be a useful mitigation option for decreasing the emission of greenhouse gases.
Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings
Authors: JANSSENS IVAN. A. , CROOKSHANKS MEG. , TAYLOR GAIL. , CEULEMANS REINHART
Published: March 2003

In this study, we investigated the impact of elevated atmospheric CO2 (ambient + 350 μmol mol–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO2, root production measured by root in‐growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N‐ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO2 efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO2 root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.
Crop water availability in early agriculture: evidence from carbon isotope discrimination of seeds from a tenth millennium BP site on the Euphrates
Authors: Araus J. L. , Febrero A. , Catala M. , Molist M. , Voltas J. , Romagosa I.
Published: March 2003

The analysis of carbon isotope discrimination (Δ) in crop plant remains from archaeological sites may help to assess water availability for early agriculture. This study presents the analysis of Δ in seeds of naked wheat (Triticum aestivum/durum), lentil (Lens orientalis/culinaris), and flax (Linum sp.) found at the archaeological site of Tell Halula in the valley of the Middle Euphrates (Syria). This Neolithic site is the oldest in this region of the Fertile Crescent where the cultivation of domesticated plants has been reported, with seed remains ranging from 9550 to 8465 BP. Most of the seeds analysed showed Δ values greater than 16 ‰, reaching 20 ‰ for some samples of flax. For wheat, Δ values were much higher than those reported in present‐day (1996) durum wheat crops cultivated under rainfed conditions in north‐west Syria under environments with somewhat higher rainfall than Tell Halula. Similarly, grains of present‐day (1997) barley cultivated in the archaeological site also showed lower values than those found in archaeological kernels. An empirical relationship between Δ of mature kernels and total precipitation (plus irrigation where applicable) from heading to maturity (r2 = 0.82, n = 11) was established for durum wheat, currently cultivated in different environments of the Mediterranean basin. The resulting relationship was applied to the data on Δ of wheat fossil kernels from Tell Halula to estimate the accumulated water inputs during the time (about 6 weeks) the kernels were produced. Calculated water inputs for wheat during early agriculture were (over 110 mm) at least 5 times higher than current‐day rainfall accumulated in Tell Halula during the same phenological period. These results strongly suggest that early agriculture wheat was cultivated at Tell Halula under much wetter conditions than are currently to be found in the area. The presence of flax and its very high Δ values also support this conclusion. Whether such humid conditions during cultivation were due to moister conditions prevailing at this time, by planting in alluvial areas or by irrigation works is discussed.
Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure
Authors: Catovsky S. , Bazzaz F. A.
Published: March 2003

Increased levels of atmospheric CO2 may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO2 influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L–1) and elevated (700 μL L–1) atmospheric CO2. For both individually and competitively grown paper birch seedlings, there was a greater CO2 growth enhancement for seedlings watered less frequently than for well‐watered seedlings. This differential change in CO2 responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO2 only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO2 reduced this sensitivity. Additionally, elevated CO2 improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO2 may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO2 levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.
Predicted disappearance of coral‐reef ramparts: a direct result of major ecological disturbances
Authors: Williams Ernest H. , Bartels PauL. J. , Bunkley‐Williams LucY.
Published: March 2003

Two coral cays near La Parguera, Puerto Rico, have large, exposed coral ramparts composed almost entirely of loose pieces of elkhorn coral Acropora palmata (88% of horizontal transects, 98% of vertical transects). The total volume of elkhorn coral in the ramparts of the two cays was estimated at 3600 and 12 800 m3. The present volume of living elkhorn coral on these two reefs was estimated at 7 and 14 m3 and previous volumes at 11 000 and 34 900 m3. White‐band disease was found on 8.5% of living elkhorn colonies. Lang’s boring sponge Cliona langae covered 10.8% of the total transect area, overgrowing both dead and living corals. White‐band disease and coral‐reef bleaching have drastically reduced the populations of elkhorn coral, thus, skeletal coral materials to replenish the plate ramparts are severely reduced, disrupting the process of forming and maintaining these coral reef ramparts. We predict that the next series of major storms striking these prominent cay ramparts will remove them. These disappearances will represent a quick, obvious and permanent consequence of global disturbances. Loss of cay ramparts will modify the environments on and around Atlantic coral reefs. Ramparts may be similarly lost from Indo‐Pacific reefs. The lack of any other indisputable definitive indicators of long‐term, major disturbances on coral reefs makes the distinct loss of coral‐reef ramparts an important physical sign.
Modelling changes in VOC emission in response to climate change in the continental United States
Authors: . Constable JohN. V. H , Guenther AleX. B. , Schimel David S. , Monson Russell K.
Published: March 2003

The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO2 concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO2 concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO2 concentration enhanced total modelled VOC emission by 81.8% (isoprene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO2 alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene – 11.7%, monoterpenes – 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf‐level adjustments to elevated CO2 (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem‐level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).
Ocean province classification using ocean colour data: observing biological signatures of variations in physical dynamics
Authors: Esaias Wayne E. , Iverson Richard L. , Turpie Kevin
Published: March 2003

We have used satellite colour data to classify ocean environments for monitoring interannual changes in the ocean. The unsupervised classification method is based on our observation that the frequency distributions of Coastal Zone Color Scanner (CZCS) annual pigment means and standard deviations are nonuniform and contain distinct clusters. The frequency distributions are used to objectively determine ocean areas with similar pigment statistical characteristics. A major separation between high variance, high pigment and lower variance, lower pigment waters is observed in terms of global ocean area. The ocean areas determined with our method reflect different bio‐logical responses to variations in ocean physical dynamics. Pigment means and variances around the Joint Global Ocean Flux Study (JGOFS) Time Series stations are used as fiducial characteristics. Hawaii Ocean Time‐series (HOT) station is associated with the low‐variance portion of the global annual pigment distribution characteristic of the central gyres, but shows slightly higher mean and variance than the minima in the central Pacific gyre. The Bermuda Atlantic Time Series (BATS) pigment associations comprise a transitional region between the gyres and high‐variance pigment areas, and circumscribe the HOT pigment associations. Together, these associations encompass 23% (HOT‐like) and 48% (BATS‐like) of the Northern Hemisphere open ocean. The Pacific regions delineated by the JGOFS station pigment‐based patterns are similar to distributions described historically for Pacific zooplankton communities. Interannual variation for the northern hemisphere gyre area is on the order of by 10% for the 11/78–10/81 period.
Canopy transpiration in a chronosequence of Central Siberian pine forests
Authors: Zimmermann Reiner , Schulze Ernst‐ Detlef , Wirth Christian , Schulze Ernst‐Eckart , Mcdonald Kyle C. , Vygodskaya Natascha N. , Ziegler Waldemar
Published: March 2003

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm2 m−2 (28‐y) to 11.5 cm2 m−2 (67‐y) and decreased again to 7 cm2 m−2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m−2 (28‐y) to 14 kg dry weight m−2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m−2 s−1) and canopy transpiration (1.5 mm d−1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d−1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = − 0.02 + 1.15SA R2) which was determined by stand density and tree sapwood area.
Methane dynamics of a restored cut‐away peatland
Authors: Tuittila Eeva‐Stiina , Komulainen Veli‐Matti , Vasander Harri , Nykänen Hannu , Martikainen Pertti J. , Laine Jukka
Published: March 2003

We measured a cut‐away peatland's CH4 dynamics using the static chamber technique one year before and two years after restoration (rewetting). The CH4 emissions were related to variation in vegetation and abiotic factors using multiple linear regression. A statistical model for CH4 flux with cottongrass cover (Eriophorum vaginatum L.), soil temperature, water level, and effective temperature sum index as driving variables explained most (r2 = 0.81) of the temporal and spatial variability in the fluxes. In addition to the direct increasing effect of raised water level on CH4 emissions, rewetting also promoted an increase of cottongrass cover which consequently increased carbon flux (substrate availability) into the system. The seasonal CH4 dynamics in tussocks followed seasonal CO2 dynamics till mid August but in late autumn CH4 emissions increased while CO2 influxes decreased. The reconstructed seasonal CH4 exchange was clearly higher following the rewetting, although it was still lower than emissions from pristine mires in the same area. However, our simulation for closed cottongrass vegetation showed that CH4 emissions from restored peatlands may remain at a lower level for a longer period of time even after sites have become fully vegetated and colonized by mire plants.
Controls over carbon storage and turnover in high‐latitude soils
Authors: Hobbie Sarah E. , Schimel Joshua P. , Trumbore Susan E. , Randerson James R.
Published: March 2003

Despite the importance of Arctic and boreal regions in the present carbon cycle, estimates of annual high‐latitude carbon fluxes vary in sign and magnitude. Without accurate estimates of current carbon fluxes from Arctic and boreal ecosystems, predicting the response of these systems to global change is daunting. A number of factors control carbon turnover in high‐latitude soils, but because they are unique to northern systems, they are mostly ignored by biogeochemical models used to predict the response of these systems to global change. Here, we review those factors. First, many northern systems are dominated by mosses, whose extremely slow decomposition is not predicted by commonly used indices of litter quality. Second, cold temperature, permafrost, waterlogging, and substrate quality interact to stabilize soil organic matter, but the relative importance of these factors, and how they respond to climate change, is unknown. Third, recent evidence suggests that biological activity occurring over winter can contribute significantly to annual soil carbon fluxes. However, the controls over this winter activity remain poorly understood. Finally, processes at the landscape scale, such as fire, permafrost dynamics, and drainage, control regional carbon fluxes, complicating the extrapolation of site‐level measurements to regional scales.
Detection of vegetation change using reconnaissance imagery
Authors: Shugart Herman H. , French Nancy H. F. , Kasischke Eric S. , Slawski James J. , Dull Charles W. , Shuchman Robert A. , Mwangi Joe
Published: March 2003

Vegetation occurs at its highest elevations on equatorial mountains. Inspection of archival and recent high‐resolution reconnaissance imagery of tropical mountains shows, in all cases, features indicating an increase in the elevation of mountain vegetation zones and an increase in vigour in the high‐elevation vegetation. These changes are consistent with an increased plant performance from increased levels of carbon dioxide in the atmosphere as well as with a warmer or more favourable climate.
Lake diatom response to recent Arctic warming in Finnish Lapland
Authors: Sorvari Sanna , Korhola Atte , Thompson Roy
Published: March 2003

High‐resolution palaeolimnological data from a number of remote and nonpolluted lakes in Finnish Lapland reveal a distinct change in diatom assemblages. This parallels the post‐19th century Arctic warming detected by examination of long‐term instrumental series, historical records of ice cover and tree‐ring measurements. The change was predominantly from benthos to plankton and affected the overall diatom species richness. A particularly strong relationship was found between spring temperatures and compositional structure of diatoms. The change is irrespective of the lake type and catchment characteristics, and is reflected by several other biological indicators, such as chrysophytes and zooplankton, suggesting that entire lake ecosystems have been affected. No corresponding change in the diatom‐inferred lake‐water pH was observed; hence, atmospheric fallout of acid substances cannot have been the driving force for the observed biological change. The mechanism behind the diatom response is unclear, but it may be related to decreased ice‐cover duration, prolonged growing season and increased thermal stability. We postulate that 19th century Arctic warming, rather than acidic or other anthropogenic deposition, is responsible for the recent ecological changes in these high latitude lakes.
Inversion of net ecosystem CO2 flux measurements for estimation of canopy PAR absorption
Authors: Hanan Niall P. , Burba George , Verma Shashi B. , Berry Joseph A. , Suyker Andrew , Walter‐Shea Elizabeth A.
Published: March 2003

The fractional absorption of photosynthetically active radiation (fPAR) is frequently a key variable in models describing terrestrial ecosystem–atmosphere interactions, carbon uptake, growth and biogeochemistry. We present a novel approach to the estimation of the fraction of incident photosynthetically active radiation absorbed by the photosynthetic components of a plant canopy (fChl). The method uses micrometeorological measurements of CO2 flux and incident radiation to estimate light response parameters from which canopy structure is deduced. Data from two Ameriflux sites in Oklahoma, a tallgrass prairie site and a wheat site, are used to derive 7‐day moving average estimates of fChl during three years (1997–1999). The inverse estimates are compared to long‐term field measurements of PAR absorption. Good correlations are obtained when the field‐measured fPAR is scaled by an estimate of the green fraction of total leaf area, although the inverse technique tends to be lower in value than the field measurements.

The inverse estimates of fChl using CO2 flux measurements are different from measurements of fPAR that might be made by other, more direct, techniques. However, because the inverse estimates are based on observed canopy CO2 uptake, they might be considered more biologically relevant than direct measurements that are affected by non‐physiologically active components of the canopy. With the increasing number of eddy covariance sites around the world the technique provides the opportunity to examine seasonal and inter‐annual variation in canopy structure and light harvesting capacity at individual sites. Furthermore, the inverse fChl provide a new source of data for development and testing of fPAR retrieval using remote sensing. New remote sensing algorithms, or adjustments to existing algorithms, might thus become better conditioned to ‘biologically significant’ light absorption than currently possible.
Growth and carbon stock change in eucalypt woodlands in northeast Australia: ecological and greenhouse sink implications
Authors: Burrows W. H. , Henry B. K. , Back P. V. , Hoffmann M. B. , Tait L. J. , Anderson E. R. , Menke N. , Danaher T. , Carter J. O. , . McKeon G. M
Published: March 2003

Data from 57 permanent monitoring sites are used to document the growth in woody vegetation and estimate the carbon sink in 27 M ha of eucalypt woodlands (savannas), contained within c. 60 M ha of grazed woodlands in Queensland (northeast Australia). The study sites are shown to be representative of the environment and structure of the eucalypt woodlands in the defined study area. Mean basal area increment for all live woody plants in 30 long‐term sites, with an average initial basal area of 11.86 ± 1.38 (SE) m2 ha−1, was 1.06 m2 ha−1 over a mean 14 years timeframe. The majority of the measurement period, commencing between 1982 and 1988, was characterized by below‐average rainfall. The increase in live tree basal area was due primarily to growth of existing trees (3.12 m2 ha−1) rather than establishment of new plants (0.25 m2 ha−1) and was partly offset by death (2.31 m2 ha−1). A simple but robust relationship between stand basal area and stand biomass of all woody species was developed for the eucalypt dominant woodlands. Analysis of above‐ground carbon stocks in live and standing dead woody plants gave a mean net above‐ground annual carbon increment for all 57 sites of 0.53 t C ha−1 y−1, similar to values estimated elsewhere in world savannas. Published root : shoot ratios were used to infer C flux in woody root systems on these sites. This results in an estimated sink in above‐ and below‐ground biomass of 18 Mt C y−1 over the eucalypt woodlands studied, and potentially up to 35 Mt C y−1 if extended to all grazed woodlands in Queensland. It is suggested that introduction of livestock grazing and altered fire regimes have triggered the change in tree‐grass dominance in these woodlands. Thus, change in carbon stocks in the grazed woodlands of Queensland is identified as an important component of human‐induced greenhouse gas flux in Australia, equivalent in magnitude to c. 25% of the most recently published (1999) total estimated national net emissions. The latter inventory takes into account emissions from land clearing, but does not include the sink identified in the present study. This sequestration also represents a small but significant contribution to the global terrestrial carbon sink.
Vegetation and climate controls on potential CO2, DOC and DON production in northern latitude soils
Authors: Neff Jason C. , Hooper David U.
Published: March 2003

Climatic change may influence decomposition dynamics in arctic and boreal ecosystems, affecting both atmospheric CO2 levels, and the flux of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) to aquatic systems. In this study, we investigated landscape‐scale controls on potential production of these compounds using a one‐year laboratory incubation at two temperatures (10° and 30 °C). We measured the release of CO2, DOC and DON from tundra soils collected from a variety of vegetation types and climatic regimes: tussock tundra at four sites along a latitudinal gradient from the interior to the north slope of Alaska, and soils from additional vegetation types at two of those sites (upland spruce at Fairbanks, and wet sedge and shrub tundra at Toolik Lake in northern Alaska). Vegetation type strongly influenced carbon fluxes. The highest CO2 and DOC release at the high incubation temperature occurred in the soils of shrub tundra communities. Tussock tundra soils exhibited the next highest DOC fluxes followed by spruce and wet sedge tundra soils, respectively. Of the fluxes, CO2 showed the greatest sensitivity to incubation temperatures and vegetation type, followed by DOC. DON fluxes were less variable. Total CO2 and total DOC release were positively correlated, with DOC fluxes approximately 10% of total CO2 fluxes. The ratio of CO2 production to DOC release varied significantly across vegetation types with Tussock soils producing an average of four times as much CO2 per unit DOC released compared to Spruce soils from the Fairbanks site. Sites in this study released 80–370 mg CO2‐C g soil C−1 and 5–46 mg DOC g soil C−1 at high temperatures. The magnitude of these fluxes indicates that arctic carbon pools contain a large proportion of labile carbon that could be easily decomposed given optimal conditions. The size of this labile pool ranged between 9 and 41% of soil carbon on a g soil C basis, with most variation related to vegetation type rather than climate.
Solar UV‐B radiation affects below‐ground parameters in a fen ecosystem in Tierra del Fuego, Argentina: implications of stratospheric ozone depletion
Authors: Zaller Johann G. , Caldwell Martyn M. , D. Flint Stephan , Scopel Ana L , Salo Osvaldo E. , Ballaré Carlos L.
Published: March 2003

Stratospheric ozone depletion caused by the release of chlorofluorocarbons is most pronounced at high latitudes, especially in the Southern Hemisphere (including the so‐called ‘ozone hole’). The consequent increase in solar ultraviolet‐B radiation (UV‐B, 280–315 nm) reaching the earth's surface may cause a variety of alterations in terrestrial ecosystems. Most effects might be expected to occur above‐ground since sunlight does not penetrate effectively below‐ground. Here, we demonstrate that solar UV‐B radiation in a fen in Tierra del Fuego (Argentina), where the ozone hole passes overhead several times during the Austral spring, is causing large changes of below‐ground processes of this ecosystem. During the third and fourth year of a manipulative field experiment, we investigated root systems in these plots and found that when the ambient solar UV‐B radiation was substantially reduced, there was a 30% increase in summer root length production and as much as a threefold decrease in already low symbiotic mycorrhizal colonization frequency of the roots compared with plots receiving near‐ambient solar UV‐B. There was also an apparent shift toward older age classes of roots under reduced solar UV‐B. Such large changes in root system behaviour may have decided effects on competition and other ecological interactions in this ecosystem.
Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests
Authors: MacDonald J. A. , Dise N. B. , Matzner E. , Armbruster M. , Gundersen P. , Forsius M.
Published: March 2003

The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha−1 y−1 in northern Norway and Finland to greater than 60 kg N ha−1 y−1 in the Netherlands and Czech Republic. The amount of NO3– leached covers a smaller range, between 1 and 40 kg N ha−1 y−1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P < 0.05) are observed between N input and NO3– leached. The slope of the relationship for those sites where C/N ratio is  ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is > 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO3– leaching also shows a relationship with soil pH, with high rates of NO3– leaching only observed at sites with a pH < 4.5 and N inputs > 30 kg N ha−1 y−1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.