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In plant ecology, site is a central concept. A site is the place where a plant species or plant community grows, and the
site provides the set of conditions in which it lives. Within an initially homogeneous parent material, gravity-driven groundwater
flow influences the site conditions through the spatial distribution of nutrients and other relevant chemical agents. Especially
upward seepage may produce and maintain site conditions that are essential for various relatively rare plant species and communities.
Increased attention to upward seepage among ecologists has resulted in cooperation with hydrologists and the emergence of
a discipline of its own – eco-hydrology – on the boundary of two scientific fields, linked by the site concept.
In the Netherlands, a simple classification of water types, based on the groundwater's subsurface history, was applied for
compiling a nationwide geographical database on ecologically relevant upward seepage. Correspondence analyses of this database
with data on plant-species occurrence demonstrate that in poor Pleistocene sandy soils upward seepage explains the occurrence
of some species and communities quite well, whereas in fluvial plains and polder areas with richer clay soils the influence
of seepage is blurred by the importance of soil characteristics. It is concluded, therefore, that plant species may be used
as seepage indicators in rapid assessments and surveys, but that constant awareness of the limitations is required.
Received, July 1998 · Revised, September 1998 · Accepted, October 1998 相似文献
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Formulation of a matrix‐valued force–displacement relationship which can take radiation damping into account is of major importance when modelling unbounded domains. This can be done by means of fundamental solutions in space and time in connection with convolution integrals or by means of a frequency dependent boundary element representation, but for discrete frequencies Ω only. In this paper a method for interpolating discrete values of dynamic stiffness matrices by a continuous matrix valued rational function is proposed. The coupling between interface degrees of freedom is fully preserved. Another crucial point in soil–structure interaction analysis is how to implement an approximation in the spectral domain into a time‐domain analysis. Well‐known approaches for the scalar case are based on the partial‐fraction expansion of a scalar rational function. Here, a more general procedure, applicable to MDOF‐systems, for the transformation of spectral rational approximations into the time‐domain is introduced. Evaluation of the partial‐fraction expansion is avoided by using the so‐called mixed variables. Thus, unknowns in the time‐domain are displacements as well as forces. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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L. de Witte 《Journal of Geodesy》1967,41(1):41-53
When the values of gravity anomalies are given at the geoid, Ag can be calculated at altitude by application of Poisson’s
integral theorem. The process requires integration of Δg multiplied by the Poisson kernel function over the entire globe.
It is common practice to add to the kernel function terms that will ensure removal of any zeroth and first order components
of Δg that may be present. The effects of trancating the integration at the boundary of a spherical cap of earth central half
angle ψo have been analyzed using an adaptation of Molodenskii’s procedure. The extension process without removal terms retains the
correct effects of inaccuracies in the constant term of the gravity reference model used in the definition of Δg. Furthermore,
the effects of ignoring remote zones or unmapped areas in the integration process are very much smaller for the extension
without removal terms than for the commonly used formula with removal terms. For these reasons the Poisson vertical extension
process without removal terms is to be preferred over the extension with the zeroth order term removal. Truncation of this
process at the point recommended for the Stokes integration, namely, the first zero crossing of the Stokes kernel function,
leaves negligible truncation errors. 相似文献
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Chemical interactions between 2-aminobenzothiazole (ABT) and a soil humic acid (HA) extracted from a gleyic planosol were investigated by solid-state 13C CP/MAS NMR spectroscopy. Reactions of the HA with ABT 13C-labelled at the C-2 position were performed in aqueous solution under noncatalysed and enzyme-catalysed conditions. Without enzymic catalysis the amount of ABT-2-13C taken up by soil HA in an oxygen atmosphere was about twice the uptake under argon. In the presence of the oxidative enzyme laccase and air, about three times as much ABT was bound to HA as compared to the uptake in a control experiment with inactivated laccase. The results suggest that oxidation reactions of the humic acid significantly enhance the extent of binding between ABT and HA. The 13C NMR spectra of the HA–ABT adducts indicated that in both noncatalysed and enzyme-catalysed reactions ABT may be bound to humic acid by noncovalent as well as covalent bonds. 相似文献
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Gerhard Graf Sebastian A. Gerlach Peter Linke Wolfgang Queisser Will Ritzrau Annette Scheltz L. Thomsen Ursula Witte 《International Journal of Earth Sciences》1995,84(1):49-58
The sedimentation pattern of organic material in the Greenland-Norwegian Sea is reflected in the surface sediments, although less than 0.5% of the organic matter is buried in the sediment. Maximum fluxes and benthic responses are observed during June and/or August/September, following the pattern of export production in the pelagial zone. The annual remineralization rate on the Vøring Plateau is 3.0 g C m–2 a –1 Freshly settled phytodetritus, as detected by chlorophyll measurements, is rapidly mixed into the sediment and decomposed. It stimulates the activity of benthic organisms, especially foraminifera. The mixing coefficient for this material is D
b=0.2 cm2 d–1, which is two to three orders of magnitude higher than that estimated from radiotracer methods. The effect on the geological record, however, is likely to be small. Chlorophyll-containing particles are at first very evenly distributed on the seafloor. After partial decomposition and resuspension, a secondary redistribution of particles occurs which can result in the formation of a high accumulation area, with an up to 80-fold increase in the sedimentation rate by lateral advection. This is mainly due to physical processes, because biodeposition mediated by benthic animals increases sedimentation by only a factor of two or three. 相似文献
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J Rullktter D Leythaeuser B Horsfield R Littke U Mann P.J Müller M Radke R.G Schaefer H.-J Schenk K Schwochau E.G Witte D.H Welte 《Organic Geochemistry》1988,13(4-6)
Four shallow boreholes were drilled in the Hils syncline, northern Germany, in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen-type-II-bearing source rock. The holes penetrated the carbonceous Lias shales (Posidonia shale, Lower Toarcian) and part of the adjacent Dogger α and Lias δ mudstones. The maturity of the organic matter in the cores recovered ranges from immature (0.48% R̄0) to overmature 1.45% R̄0) due to location of the Hils syncline in the vicinity of the Vlotho Massif, which is deep-seated intrusive body. Facies variations of the Lias within the short geographical distances in the study area are negligible.Organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the Lias shales (kerogen, bitumen etc.) and on the evidence of a uniform initial composition of these sediments in the study area. Dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations.About 50% of the initial kerogen was transformed into oil, gas and inorganic compounds during the vitrinite reflectance increase from 0.48 to 0.88% R̄o and only marginally more during the maturity increase from 0.88 to 1.45% R̄o. Only a small portion of the generated material remained in the source rock even at a relatively early stage of generation (0.68% R̄o). Expulsion efficiency of oil plus gas reached a value of 86% at the end of the main generation stage (0.88% R̄o). 相似文献
19.
Frans Witte Ole Seehausen Jan H. Wanink Mary A. Kishe-Machumu Marjolein Rensing Tijs Goldschmidt 《Aquatic Sciences - Research Across Boundaries》2013,75(2):169-183
During the past decades, major anthropogenic environmental changes occurred in Lake Victoria, including increased predation pressure due to Nile perch introduction, and decreases in water transparency and dissolved oxygen concentrations due to eutrophication. This resulted in a collapse of the haplochromine cichlids in the sub-littoral waters of the Mwanza Gulf in 1986–1990, followed by a recovery of some species in the 1990s and 2000s, when Nile perch densities declined. We studied two data sets: (1) haplochromines from sand and mud bottoms in the pre-collapse period; (2) haplochromines from sub-littoral areas during the pre-collapse, collapse and recovery periods. Water over mud is murkier and poorer in oxygen than water over sand, and differences in haplochromine communities in these natural habitats during the pre-collapse period may predict the effects of anthropogenic eutrophication during the collapse and recovery periods. In the pre-collapse period, haplochromine densities over sand and mud did not differ, but species richness over sand was 1.6 times higher than over mud bottoms. Orange- and white-blotched colour morphs were most common at the shallowest sand station. More specifically, insectivores and mollusc-shellers had higher numbers of species over sand than over mud, whereas for mollusc-crushers no difference was found. Laboratory experiments revealed that mollusc shelling was more affected by decreased light intensities than mollusc crushing. During the pre-collapse period, spawning occurred year-round in shallow areas with hard substrates and relatively clear water. In deeper areas with mud bottoms, spawning mainly occurred during months in which water clarity was high. No effects of hypoxia on spawning periods were found. It follows that clearer water seems to support differentiation in feeding techniques as well as year-round spawning, and both may facilitate species coexistence. Water clarity is also known to be important for mate choice. These observations may explain why, since the decline of Nile perch, haplochromine densities have recovered, the numbers of hybrids increased and species diversity in the current eutrophic sub-littoral waters has remained 70 % lower than before the environmental changes. 相似文献
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Unsaturated hydraulic properties of xerophilous mosses: towards implementation of moss covered soils in hydrological models
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Bernard R. Voortman Ruud P. Bartholomeus Peter M. van Bodegom Harm Gooren Sjoerd E. A. T. M. van der Zee Jan‐Philip M. Witte 《水文研究》2014,28(26):6251-6264
Evaporation from mosses and lichens can form a major component of the water balance, especially in ecosystems where mosses and lichens often grow abundantly, such as tundra, deserts and bogs. To facilitate moss representation in hydrological models, we parameterized the unsaturated hydraulic properties of mosses and lichens such that the capillary water flow through moss and lichen material during evaporation could be assessed. We derived the Mualem‐van Genuchten parameters of the drying retention and the hydraulic conductivity functions of four xerophilous moss species and one lichen species. The shape parameters of the retention functions (2.17 < n < 2.35 and 0.08 < α < 0.13 cm?1) ranged between values that are typical for sandy loam and loamy sand. The shapes of the hydraulic conductivity functions of moss and lichen species diverged from those of mineral soils, because of strong negative pore‐connectivity parameters (?2.840 < l < ?2.175) and low hydraulic conductivities at slightly negative pressure heads (0.016 < K0 < 0.280 cm/d). These K0 values are surprisingly low, considering that mosses are very porous. However, during evaporation, large pores and voids were air filled and did not participate in capillary water flow. Small K0 values cause mosses and lichens to be conservative with water during wet conditions, thus tempering evaporation compared to mineral soils. On the other hand, under dry conditions, mosses and lichens are able to maintain a moisture supply from the soil, leading to a higher evaporation rate than mineral soils. Hence, the modulating effect of mosses on evaporation possibly differs between wet and dry climates. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献