The cultural context of forest degradation in adjacent Purépechan communities, Michoacán, Mexico     

Martha A Works  Keith S Hadley 《The Geographical journal》2004,170(1):22-38

Recent changes in the cultural and forest landscapes of the Meseta Purépecha in Michoacán, Mexico as a result of forest degradation underscore the complexity of forest change processes in the tropical highlands of Latin America. Differences in community perception and forest structure and composition between the furniture-making and lumber-producing towns of Pichátaro and Sevina, Purépechan indigenous communities located amidst pine forests on Michoacán's volcanic plateau illustrate the dynamics of this process. We base our comparisons on interviews and field measures of forest structure. Our results show dramatic changes in the forests and cultural landscapes of both communities during the past decade. Following high regional timber exports during the early 1990s, Sevina shifted from a self-sufficient to a timber importation community. By comparison, communal forests and individual parcels in Pichátaro continue to provide wood for approximately 300 wood shops. Field data and forest stand maps confirm the perception of forest degradation in both communities. While Pichátaro has maintained a larger and more diverse forest base to date, stand structure data indicate selective harvesting has led to a shift in dominance toward the less economically desirable pine species and oak. Deforestation and degradation of Sevina and Pichátaro's community forests are symptomatic of both the Meseta Purépecha and Mexico in general. Current forest conditions in both communities justify local, regional, and national concerns regarding declining biodiversity and sustainable economies.  相似文献   

Marine geochemistry—2: scavenging redux     

Edward D. Goldberg  Minoru Koide  Kathe Bertine  Vern Hodge  Martha Stallard  Darko Martincic  Nevenka Mikac  Marco Branica  Jamal K. Abaychi 《Applied Geochemistry》1988,3(6)

Inorganic surfaces sorb dissolved and particulate phases from seawater onto organic films from a variety of sources. Heavy metals such as Au, Pt, Mn and Cd come to the coatings primarily from particulate phases. This observation confirms a previous finding of this laboratory for the uptake of U, Pu and Po. With depth in the open ocean there are greater contributions from the dissolved phases. Microbial and photochemical processes can enrich surfaces in some metals through reduction reactions. Copper and Mn enrichments in the films are attributed to the photo-reductions of Cu(II) to Cu(I) and manganese oxides to Mn(II), respectively, while the uptake of Mo is a consequence of the reduction, mediated by organisms, of molybdate to Mo(V) or Mo(IV). Cadmium, on the other hand, appears to be bioaccumulated in the films.  相似文献   

Salt marsh cliff stability in the oosterschelde     

Martha M. Van Eerdt 《地球表面变化过程与地形》1985,10(2):95-106

Salt marsh cliff erosion in the Oosterschelde, due to basal scour and small-scale failure, was monitored during a two-year period using reference stakes. The composite marsh cliffs have a cantilever profile. Their stability is evaluated from beam failure analysis. A model is proposed, in which the cantilever weight is taken as the motive force; the major resistive force is the tensile strength. By substituting height, undermining width and soil mechanical properties of the cliff in the model, one can identify the cliffs that are likely to fail. Salt marsh cliffs, which combine a large tensile strength due to roots at the top of the profile with a large compressive stress at the cliff base due to the sandy texture of the subsoil, display the highest resistance to beam failure. The critical cliff dimensions, observed in the field, correspond with the values calculated from the proposed model of beam failure.  相似文献   

A Computer Model for Simulating Water Quality and Quantity in a Wellfield in an Alluvial Aquifer     

Martha W. Gilliland  Quang M. Nguyen 《Ground water》1987,25(2):151-159

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Response of L-shaped rigid foundations embedded in a uniform half-space to traveling seismic waves     

Martha Surez  Javier Avils  Francisco J. Snchez-Sesma 《Soil Dynamics and Earthquake Engineering》2002,22(8)

A simplified indirect boundary element method is applied to compute the impedance functions for L-shaped rigid foundations embedded in a homogeneous viscoelastic half-space. In this method, the waves generated by the 3D vibrating foundation are constructed from radiating sources located on the actual boundary of the foundation. The impedance functions together with the free-field displacements and tractions generated along the soil–foundation interface are used to calculate the foundation input motion for incident P, S and Rayleigh waves. This is accomplished by application of Iguchi's averaging method which, in turn, is verified by comparison with results obtained rigorously using the relation between the solutions of the basic radiation (impedance functions) and scattering (input motions) problems. Numerical results are presented for both surface-supported and embedded foundations. It is shown how the seismic response of L-shaped foundations with symmetrical wings differs from that of enveloping square foundations. The effects of inclination and azimuth of the earthquake excitation are examined as well. These results should be of use in analyses of soil–structure interaction to account for the traveling wave effects usually overlooked in practice.  相似文献   

Effects of wave passage on the relevant dynamic properties of structures with flexible foundation     

Javier Avils  Martha Surez  Francisco J. Snchez‐Sesma 《地震工程与结构动力学》2002,31(1):139-159

An evaluation of the wave passage effects on the relevant dynamic properties of structures with flexible foundation is presented. A simple soil–structure system similar to that used in practice to take into account the inertial interaction effects by the soil flexibility is studied. The kinematic interaction effects due to non‐vertically incident P, SV and Rayleigh waves are accounted for in this model. The effective period and damping of the system are obtained by establishing an equivalence between the interacting system excited by the foundation input motion and a replacement oscillator excited by the free‐field ground motion. In this way, the maximum structural response could be estimated from standard free‐field response spectra using the period and damping of the building modified by both the soil flexibility and the travelling wave effects. Also, an approximate solution for the travelling wave problem is examined over wide ranges of the main parameters involved. Numerical results are computed for a number of soil–structure systems to identify under which conditions the effects of wave passage are important. It comes out that these effects are generally negligible for the system period, but they may significantly change the system damping since the energy dissipation within the soil depends on both the wave radiation and the diffraction and scattering of the incident waves by the foundation. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

Models of dense stars     

Zdeněk Kopal  Martha B. Shapley 《Astrophysics and Space Science》1968,1(1):112-124

The aim of this paper is to present the results of a construction of five models of composite stellar configurations, consisting of an energy-generating convective core surrounded by source-free envelope in radiative equilibrium, and of sufficient density for the coefficient of absorption to vary approximately as inverse square of the local temperature. The principal characteristic of such models proved to be their very high degree of central condensation; their central densities being 10³–10⁴ times as large as the mean density of the composite configuration. The relevance of such models to the internal structure of subdwarf components of close binary systems with periods less than a day is briefly pointed out.Investigation supported in part by Contract N5 ori-07843 with the Office of Naval Research of the U.S. Navy Department.  相似文献   

Midsummer Energy Balance for the Southern Seas     

Gerd Wendler  Brian Hartmann  Chris Wyatt  Martha Shulski  Henry Stone 《Boundary-Layer Meteorology》2005,117(1):131-148

During a ship voyage from Tasmania to Antarctica in summer 2000/01, radiative and meteorological measurements were continuously made, from which the surface energy budget was calculated. Sea conditions throughout the voyage ranged from open water to broken pack and finally to snow-covered unbroken sea ice in McMurdo Sound. The global radiation increased on average during the trip (to higher latitudes) as we travelled poleward. The net radiation, which was positive (toward the surface) on average, decreased however, mostly due to the increase in surface albedo. For open water, most of the net radiation is used for evapouration (61%), while for broken sea-ice conditions, nearly all energy is used for melting of the sea ice or heating of the ocean (96%). For unbroken snow-covered sea ice, the net radiation lies close to zero, due to the high surface albedo, which reached a mean value of 0.81. The sensible heat flux becomes the largest heat source and nearly all the energy is used for warming of the surface. Finally, a Radarsat image, on which the ship track was visible, was used to compare the ship observations with satellite derived ice types.  相似文献   

Drift mode accelerometry for spaceborne gravity measurements     

John W. Conklin 《Journal of Geodesy》2015,89(11):1053-1070

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Comparison of Enterococcus density estimates in marine beach and bay samples by real-time polymerase chain reaction, membrane filtration and defined substrate testing     

Ferretti JA  Tran HV  Cosgrove E  Protonentis J  Loftin V  Conklin CS  Grant RN 《Marine pollution bulletin》2011,62(5):1066-1072

Currently, densities of Enterococcus in marine bathing beach samples are performed using conventional methods which require 24 h to obtain results. Real-time PCR methods are available which can measure results in as little as 3 h. The purpose of this study was to evaluate a more rapid test method for the determination of bacterial contamination in marine bathing beaches to better protect human health. The geometric mean of Enterococcus densities using Enterolert® defined substrate testing and membrane filtration ranged from 5.2 to 150 MPN or CFU/100 mL and corresponding qPCR results ranged from 6.6 to 1785 CCE/100 mL. The regression analysis of these results showed a positive correlation between qPCR and conventional tests with an overall correlation (r) of 0.71. qPCR was found to provide accurate and sensitive estimate of Enterococcus densities and has the potential to be used as a rapid test method for the quantification of Enterococcus in marine waters.  相似文献