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排序方式: 共有192条查询结果,搜索用时 21 毫秒
91.
This paper introduces a series of 11 studies on the relationships between large Western cities (Paris, London, Brussels, Vienna, Barcelona, Athens, New York, Providence) and their surrounding territories over a long historical time period. The concept of hinterland is introduced to designate a rural territory structured by its function of supplying the city with food, fuel, water and other material. The papers question the usefulness of this concept in the current globalized world, where cities are often considered as simple nodes of a network of worldwide trade exchanges, but where new citizen aspirations for reconnecting urban and rural territories are emerging. 相似文献
92.
This paper provides information about the distribution, structure, and ecology of the world's largest alpine ecosystem, the Kobresia pygmaea pastures in the southeastern Tibetan plateau. The environmental importance of these Cyperaceae mats derives from the extremely firm turf, which protects large surfaces against erosion, including the headwaters of the Huang He, Yangtze, Mekong, Salween, and Brahmaputra. The emphasis of the present article is on the climate-driven evolution and recent dynamics of these mats under the grazing impact of small mammals and livestock. Considering pedological analyses, radiocarbon datings, and results from exclosure experiments, we hypothesize that the majority of K. pygmaea mats are human-induced and replace forests, scrub, and taller grasslands. At present, the carrying capacity is increasingly exceeded, and reinforced settlement of nomads threatens this ecosystem especially in its drier part, where small mammals become strong competitors with livestock and the removal of the turf is irreversible. Examples of rehabilitation measures are given. 相似文献
93.
Karoline Diehl Sabine Wurzler 《Atmospheric environment (Oxford, England : 1994)》2010,44(36):4622-4628
The effects of bacteria acting as immersion ice nuclei were investigated in numerical sensitivity studies and compared to the efforts of other ice nuclei such as mineral dust and soot particles. An adiabatic air parcel model was employed simulating convective situations with different initial aerosol particle distributions. The maximum fractions of active ice nuclei were based on field measurements of the proportioning of atmospheric aerosol particle types in continental and marine air masses. Recent field measurements of bacteria concentrations in cloud water and in snow samples were used. From the concentrations in bulk samples the concentration in mean sized cloud droplets was estimated. Immersion freezing was described based on laboratory measurements to constrain the freezing fraction versus temperature. The results indicated that the effects of diminutive amounts of bacteria on ice formation in convective clouds, while being significantly less than the effects of mineral dust particles, might be comparable to the expected effects of soot particles acting as ice nuclei. It can be predicted that bacterial ice nuclei would have to be enriched by at least 104 times reported concentrations in cloud water in order to equate to the impact of mineral dust ice nuclei present in 20–25% of all cloud droplets. 相似文献
94.
Sabine Braun Christian Schindler 《Environmental pollution (Barking, Essex : 1987)》2010,158(9):2954-2963
For a quantitative estimate of the ozone effect on vegetation reliable models for ozone uptake through the stomata are needed. Because of the analogy of ozone uptake and transpiration it is possible to utilize measurements of water loss such as sap flow for quantification of ozone uptake. This technique was applied in three beech (Fagus sylvatica) stands in Switzerland. A canopy conductance was calculated from sap flow velocity and normalized to values between 0 and 1. It represents mainly stomatal conductance as the boundary layer resistance in forests is usually small. Based on this relative conductance, stomatal functions to describe the dependence on light, temperature, vapour pressure deficit and soil moisture were derived using multivariate nonlinear regression. These functions were validated by comparison with conductance values directly estimated from sap flow. The results corroborate the current flux parameterization for beech used in the DO3SE model. 相似文献
95.
96.
Rocheleau S Kuperman RG Martel M Paquet L Bardai G Wong S Sarrazin M Dodard S Gong P Hawari J Checkai RT Sunahara GI 《Chemosphere》2006,62(4):545-558
The toxicities of 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene (2,4-DNT), and 2,6-dinitrotoluene (2,6-DNT) to terrestrial plants alfalfa (Medicago sativa L.), Japanese millet (Echinochloa crusgalli L.), and perennial ryegrass (Lolium perenne L.) were determined in Sassafras sandy loam soil using seedling emergence, fresh shoot, and dry mass measurement endpoints. A 13-week weathering and aging of energetic materials in soils, which included wetting and drying cycles, and exposure to sunlight of individual soil treatments, was incorporated into the study design to better reflect the soil exposure conditions in the field than toxicity determinations in freshly amended soils. Definitive toxicity tests showed that dinitrotoluenes were more phytotoxic for all plant species in freshly amended treatments based on EC20 values for dry shoot ranging from 3 to 24mgkg(-1) compared with values for TNB or TNT ranging from 43 to 62mgkg(-1). Weathering and aging of energetic materials (EMs) in soil significantly decreased the toxicity of TNT, TNB or 2,6-DNT to Japanese millet or ryegrass based on seedling emergence, but significantly increased the toxicity of all four EMs to all three plant species based on shoot growth. Exposure of the three plant species to relatively low concentrations of the four compounds initially stimulated plant growth before the onset of inhibition at greater concentrations (hormesis). 相似文献
97.
98.
Tausz M Landmesser H Posch S Monschein S Grill D Wienhaus O 《Environmental monitoring and assessment》2007,128(1-3):75-82
In this work we measured a set of antioxidative and photoprotective compounds (chlorophylls, carotenoids, tocopherol, ascorbate
and glutathione), which were suggested previously as stress markers in conifer needles, at two spruce forest sites at different
elevation in Saxony, Germany. Most variables differed significantly between current and 1-year-old needles, but only the content
of the xanthophyll cycle per mg total chlorophyll and the oxidation state of glutathione were significantly different between
the sites. We applied principal component analysis (PCA) to address the question if underlying accumulated variables are similar
to the ones found in spruce needles across Alpine elevation profiles and/or for pines in Mediterranean ecosystems. Four principal
components (accumulated variables, PC) representing 68% of the total variance of the dataset were extracted. PC 1 encompassed
total chlorophyll, lutein, and β-carotene contents, PC 2 combined the epoxidation state of xanthophylls, ascorbate content
and redox state, and glutathione content, PC 3 represented the content of xanthophylls and the redox state of glutathione,
and PC 4 encompassed the content of α-carotene and the epoxidation state of xanthophylls. Only PC 3 was significantly different
between sites. The PCA structure shows many similarities to corresponding findings in studies on spruce in mountain forests
in the Alps and pines in Mediterranean systems. This corroborates the interpretation of PCs as indicative for underlying physiological
processes. However, separation of the two investigated sites by PCs was in the present case study not superior to the separation
by single input variables. 相似文献
99.
The enemy-release hypothesis (ERH) states that species become more successful in their introduced range than in their native range because they leave behind natural enemies in their native range and are thus "released" from enemy pressures in their introduced range. The ERH is popularly cited to explain the invasive properties of many species and is the underpinning of biological control. We tested the prediction that plant populations are more strongly regulated by natural enemies (herbivores and pathogens) in their native range than in their introduced range with enemy-removal experiments using pesticides. These experiments were replicated at multiple sites in both the native and invaded ranges of the grass Brachypodium sylvaticum. In support of the ERH, enemies consistently regulated populations in the native range. There were more tillers and more seeds produced in treated vs. untreated plots in the native range, and few seedlings survived in the native range. Contrary to the ERH, total measured leaf damage was similar in both ranges, though the enemies that caused it differed. There was more damage by generalist mollusks and pathogens in the native range, and more damage by generalist insect herbivores in the invaded range. Demographic analysis showed that population growth rates were lower in the native range than in the invaded range, and that sexually produced seedlings constituted a smaller fraction of the total in the native range. Our removal experiment showed that enemies regulate plant populations in their native range and suggest that generalist enemies, not just specialists, are important for population regulation. 相似文献
100.
In this diary study, we aimed to examine the moderating effects of the following: (i) recovery efforts at work and (ii) detachment from work on the relationship between work‐related flow and energy after work. Specifically, we hypothesized that flow would be beneficial for energy after work when employees failed (versus managed) to recover during work breaks. Additionally, we predicted that when employees experience flow at work, they would be more vigorous (and less exhausted) at the end of the day when they detached from work in the evening compared with days when they failed to detach. The study tracked 83 participants who completed daily surveys over four consecutive days. Results of multilevel analyses indicated that some characteristics of flow, such as absorption and enjoyment, were significantly associated with energy after work. Recovery at work and detachment from work moderated the relationship between flow (specifically the enjoyment component) and after‐work energy. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献