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Using stable isotopes to quantify water sources for trees and shrubs in a riparian cottonwood ecosystem in flood and drought years 总被引：1，自引：0，他引：1
Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue. 相似文献
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Uneven urban metabolisms: toward an integrative (ex)urban political ecology of sustainability in and around the city 总被引：1，自引：0，他引：1
Innisfree McKinnon Patrick T Hurley Colleen C Myles Megan Maccaroni Trina Filan 《Urban geography》2019,40(3):352-377
ABSTRACTExpanding cities present a sustainability challenge, as the uneven proliferation of hybrid landscape types becomes a major feature of 21st century urbanization. To fully address this challenge, scholars must consider the broad range of land uses that being produced beyond the urban core and how land use patterns in one location may be tied to patterns in other locations. Diverse threads within political ecology provide useful insights into the dynamics that produce uneven urbanization. Specifically, urban political ecology (UPE) details how economic power influences the development decision-making that proliferate urban forms, patterns of uneven access, and modes of decision-making, frequently viewing resource extraction and development through the urban metabolism lens. The political ecology of exurbia, or, perhaps, an exurban political ecology (ExPE), examines the symbolic role nature and the rural have played in conservation and development efforts that produce social, economic, and environmental conflicts. While UPE approaches tend to privilege macroscale dynamics, ExPE emphasizes the role of landowners, managers, and other actors in struggles over the production of exurban space, including through decision-making institutions and within the context of broader political economic forces. Three case studies illustrate the strengths and weaknesses of these approaches, demonstrating the benefits for and giving suggestions on how to integrate their insights into urban sustainability research. Integrated political ecology approaches demonstrate how political-economic processes at a variety of scales produce diverse local sustainability responses. 相似文献
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During the Pliocene–Pleistocene epoch, covering last ∼5.2 Ma of Earth’s history, altogether 34 terrestrial meteoritic impact
craters are known. Most of these craters (29) have diameter ≤10 km, among which 11 craters fall in 1,000 to 100 m range, and
7 are still smaller in dimension and of recent age. The age versus impact-frequency plot shows that the meteoritic impacts
during this time period occurred in discrete intervals but have a periodicity that shows the best possible coincidence with
the ∼425 Ky climatic cycles observed by Fourier analysis and FFT filtering of composite high resolution benthic foraminiferal
δ18O record. This observation is also supported by Monte Carlo test with 71% success where meteoritic impact(s) shows coincidence
with climatic cooling within our error limit. The newly observed climatic–meteoritic cycle may be same with the ∼400 Ky Milankovitch
cycle or it is a different newly understood cycle relating both the climatic variation and meteoritic impact events. 相似文献
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Much of the information in micromorphological thin sections exists at the mesoscopic level. The use of flatbed scanners can improve analysis at this level and aid in preservation of the information. This article describes scanning in both plane and cross‐polarized light and discusses the hardware and software required. © 2002 Wiley Periodicals, Inc. 相似文献
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