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71.
Suellen Lynn Michael L. Morrison Amy J. Kuenzi Jennifer C. C. Neale Benjamin N. Sacks Robin Hamlin Linnea S. Hall 《西北部美国博物学家》2011,58(4)
The Truckee River in California and Nevada is subject to diverse water regimes and a corresponding variety of flow rates. Original riparian vegetation has been altered by these variable flow rates and by a variety of human uses resulting in loss of native riparian vegetation from its historic extent. We conducted bird surveys along the Truckee River during spring 193 to (1) determine relationships between birds and the present vegetation; (2) determine the importance of different vegetation types to sensitive bird species that have declined recently in the western United States due to competition from exotic plant species, cowbird ( Molothrus ater ) parasitism, reduction in nesting habitat, or other unidentified reasons; and (3) establish a monitoring program and collect baseline data for future comparisons. The most frequently detected bird species throughout the study was the Brown-headed Cowbird. The greatest number of bird species (98 of 116) was found in the native mixed willow ( Salix spp.) riparian scrub vegetation type. We recommend protecting the remaining native riparian vegetation types for bird habitat along the Truckee River. 相似文献
72.
Vernal pools are seasonal pools occurring in Mediterranean-type climates within which grow concentric zones of vegetation. We studied two vernal pools that lie within an Artemisia tridentata/Festuca idahoensis shrub-steppe landscape in the Channeled Scabland of eastern Washington to determine the relationship between vegetation zonation and soil characteristics. Abundant plant species in the pools include Elymus cinereus, Poa scabrella, Lomatium grayi, Allium geyeri, Eleocharis palustris, Epilobium minutum, Myosurus aristatis, Deschampsia danthonioides, and Psilocarphus oregonus . We surveyed topography, measured plant species frequency and cover to describe the vegetation zones, and used Sorenson's index of percent to similarity to verify our designation of plant zones as communities. In one pool we described soil profiles and sampled soils throughout the growing season according to plant communities. We analyzed soils for pH; electrical conductivity; sodium, calcium, and magnesium ions; sodium absorption ratio; particle size; organic carbon; and water matric potential. ANOVA tests of soil characteristics and topography among plant communities showed that only differences in topography are statistically significant. There are, however, trends in particle size, some soil chemical parameters, and soil moisture potential among plant communities along the topographic gradient. Electrical conductivity decreased with increasing dryness of the soil through spring and summer. Seasonal changes in soil moisture potential showed that shallower soils in the centers of pools were wetter during the wet season and drier during the dry season than are deeper soils. These changes in moisture may be the most important influence on vegetation distribution within the vernal pools. 相似文献
73.
Benjamin M. Rau Dale W. Johnson Jeanne C. Chambers Robert R. Blank Annmarie Lucchesi 《西北部美国博物学家》2011,69(4)
Quantifying root biomass is critical to an estimation and understanding of ecosystem net primary production, biomass partitioning, and belowground competition. We compared 2 methods for determining root biomass: a new soil-coring technique and traditional excavation of quantitative pits. We conducted the study in an existing Joint Fire Sciences demonstration area in the central Great Basin. This area is representative of a shrub (sagebrush) ecosystem exhibiting tree (pinyon and juniper) encroachment. The demonstration area had a prescribed burn implemented 4 years prior to our study, and we sampled both control and burned plots. The samples were stratified across 3 microsites (interspace, under shrub, and under tree) and 4 soil depths (0–8, 8–23, 23–38, and 38–52 cm) to determine the effects of plant life form and burning on root biomass. We found that estimates of total root biomass were similar between quantitative pits and our soil cores. However, cores tended to show a more even distribution of root biomass across all microsites and depths than did pits. Overall, results indicated that root biomass differs significantly among microsites and soil depths and that the amount of root biomass at a given depth differs among microsites. Burning reduced root biomass in our study by 23% and altered the spatial distribution of root mass. 相似文献
74.
Chapman HN Fromme P Barty A White TA Kirian RA Aquila A Hunter MS Schulz J DePonte DP Weierstall U Doak RB Maia FR Martin AV Schlichting I Lomb L Coppola N Shoeman RL Epp SW Hartmann R Rolles D Rudenko A Foucar L Kimmel N Weidenspointner G Holl P Liang M Barthelmess M Caleman C Boutet S Bogan MJ Krzywinski J Bostedt C Bajt S Gumprecht L Rudek B Erk B Schmidt C Hömke A Reich C Pietschner D Strüder L Hauser G Gorke H Ullrich J Herrmann S Schaller G Schopper F Soltau H Kühnel KU Messerschmidt M 《Nature》2011,470(7332):73-77
X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded. It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction 'snapshots' are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source. We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes. More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (~200?nm to 2?μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes. This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage. 相似文献
75.
Climate change: High risk of permafrost thaw 总被引:2,自引:0,他引:2
76.
77.
David Marpaung Blair Morrison Mattia Pagani Ravi Pant Benjamin J. Eggleton 《科学通报(英文版)》2014,59(22):2684-2692
A bandstop radio frequency (RF) filter can enhance the capabilities of RF communication systems by removing unwanted signal components such as distortion, or interference. In modem cognitive radio systems, frequency agile filters are desired to block interferers with dynamically changing frequencies and powers. These filters should be tunable over a broad frequency range, and they should be capable of suppressing very strong signals with a high resolution. To simultaneously meet these requirements with traditional RF filters are highly chal- lenging. Microwave photonic (MWP), a technology that primary deals with the generation, distribution, and processing of high speed RF signals using photonic techniques and components, is promising for creating frequency agile RF filters. However, traditional MWP filters are lacking the high resolution and peak suppression exhibited by state-of-the-art RF electrical filters. We recently introduced a new class of MWP notch filter which is free from this limitation. This scheme allowed the creation of anomalously high suppression MWP notch filter from virtually any kind of optical resonance, irrespective of its type (gain or absorp tion), or its magnitude. This enabled, for the first time, simultaneous optimization of the MWP filter resolution, peak attenuation, and frequency tuning range. In this paper, we present the analysis of notch filter response creation using the novel sideband processing technique. We then show the applicability of this technique to a wide range of optical filters. We compare simulated and experimental results of the notch filter response created using three types of optical filter commonly used in MWP signal processing, namely stimulated Brillouin scattering (SBS), an integrated optical ring resonator (ORR), and a phase-shifted fiber Bragg-grating (FBG). 相似文献
78.
Engelbrecht BM Comita LS Condit R Kursar TA Tyree MT Turner BL Hubbell SP 《Nature》2007,447(7140):80-82
Although patterns of tree species distributions along environmental gradients have been amply documented in tropical forests, mechanisms causing these patterns are seldom known. Efforts to evaluate proposed mechanisms have been hampered by a lack of comparative data on species' reactions to relevant axes of environmental variation. Here we show that differential drought sensitivity shapes plant distributions in tropical forests at both regional and local scales. Our analyses are based on experimental field assessments of drought sensitivity of 48 species of trees and shrubs, and on their local and regional distributions within a network of 122 inventory sites spanning a rainfall gradient across the Isthmus of Panama. Our results suggest that niche differentiation with respect to soil water availability is a direct determinant of both local- and regional-scale distributions of tropical trees. Changes in soil moisture availability caused by global climate change and forest fragmentation are therefore likely to alter tropical species distributions, community composition and diversity. 相似文献
79.
Macpherson LJ Dubin AE Evans MJ Marr F Schultz PG Cravatt BF Patapoutian A 《Nature》2007,445(7127):541-545
The nervous system senses peripheral damage through nociceptive neurons that transmit a pain signal. TRPA1 is a member of the Transient Receptor Potential (TRP) family of ion channels and is expressed in nociceptive neurons. TRPA1 is activated by a variety of noxious stimuli, including cold temperatures, pungent natural compounds, and environmental irritants. How such diverse stimuli activate TRPA1 is not known. We observed that most compounds known to activate TRPA1 are able to covalently bind cysteine residues. Here we use click chemistry to show that derivatives of two such compounds, mustard oil and cinnamaldehyde, covalently bind mouse TRPA1. Structurally unrelated cysteine-modifying agents such as iodoacetamide (IA) and (2-aminoethyl)methanethiosulphonate (MTSEA) also bind and activate TRPA1. We identified by mass spectrometry fourteen cytosolic TRPA1 cysteines labelled by IA, three of which are required for normal channel function. In excised patches, reactive compounds activated TRPA1 currents that were maintained at least 10 min after washout of the compound in calcium-free solutions. Finally, activation of TRPA1 by disulphide-bond-forming MTSEA is blocked by the reducing agent dithiothreitol (DTT). Collectively, our data indicate that covalent modification of reactive cysteines within TRPA1 can cause channel activation, rapidly signalling potential tissue damage through the pain pathway. 相似文献
80.
Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas 总被引:1,自引:0,他引:1
Bao Z Weatherspoon MR Shian S Cai Y Graham PD Allan SM Ahmad G Dickerson MB Church BC Kang Z Abernathy HW Summers CJ Liu M Sandhage KH 《Nature》2007,446(7132):172-175
The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 degrees C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications. 相似文献