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431.
Morgan JA LeCain DR Pendall E Blumenthal DM Kimball BA Carrillo Y Williams DG Heisler-White J Dijkstra FA West M 《Nature》2011,476(7359):202-205
Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO(2) may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO(2)-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO(2) also leads to higher plant biomass, and therefore greater transpirational surface. Furthermore, although warming is predicted to favour warm-season, C(4) grasses, rising CO(2) should favour C(3), or cool-season plants. Here we show in a semi-arid grassland that elevated CO(2) can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600?p.p.m.v. CO(2) increased soil water content (SWC), 1.5/3.0?°C day/night warming resulted in desiccation, such that combined CO(2) enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO(2) favoured C(3) grasses and enhanced stand productivity, whereas warming favoured C(4) grasses. Combined warming and CO(2) enrichment stimulated above-ground growth of C(4) grasses in 2 of 3?years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO(2)-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected. 相似文献
432.
Williams K Christensen J Pedersen MT Johansen JV Cloos PA Rappsilber J Helin K 《Nature》2011,473(7347):343-348
433.
Atmospheric oxidation capacity sustained by a tropical forest 总被引:2,自引:0,他引:2
Lelieveld J Butler TM Crowley JN Dillon TJ Fischer H Ganzeveld L Harder H Lawrence MG Martinez M Taraborrelli D Williams J 《Nature》2008,452(7188):737-740
Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NO(x) = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO(x) air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment. 相似文献
434.
Terrestrial planet formation is believed to have concluded in our Solar System with about 10 million to 100 million years of giant impacts, where hundreds of Moon- to Mars-sized planetary embryos acquired random velocities through gravitational encounters and resonances with one another and with Jupiter. This led to planet-crossing orbits and collisions that produced the four terrestrial planets, the Moon and asteroids. But here we show that colliding planets do not simply merge, as is commonly assumed. In many cases, the smaller planet escapes from the collision highly deformed, spun up, depressurized from equilibrium, stripped of its outer layers, and sometimes pulled apart into a chain of diverse objects. Remnants of these 'hit-and-run' collisions are predicted to be common among remnant planet-forming populations, and thus to be relevant to asteroid formation and meteorite petrogenesis. 相似文献
435.
Leighton Vaughan Williams 《Journal of forecasting》2000,19(6):505-513
This paper investigates the performance of professional racetrack forecasting services in the UK. The racetrack market is chosen for investigation because it offers a clear and well‐defined set of costs and returns. As such, it provides a useful perspective from which to consider the efficiency of professional forecasting performance. Five services are selected based on their reported success in a period prior to the sample selection. The results of this study indicate that all the services showed evidence of net pre‐tax profits, notably for more strongly recommended tips, but this would have required quite high stakes to cover the costs of accessing the information. Statistical tests of significance were unable, however, to confirm the evidence of profitability at conventional levels of acceptance. Taking into account the transactions costs of using these services, it is even more difficult to confirm the existence of abnormal returns. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
436.
研究了解答者之间的横向公平关切特征对众包竞赛发包方知识共享激励的影响。在构建公平关切下解答者效用函数的基础上,建立并求解了无知识共享激励模型(NKS)和知识共享激励模型(KS),用理论分析和数值仿真方法研究了公平关切敏感度对发包方最优知识共享激励程度、解答者最优私有解答努力程度及知识共享努力程度、众包项目绩效以及双方期望经济收益的影响,并与无知识共享激励进行了对比。研究表明:解答者公平关切心理能降低实施共享知识激励机制的门槛,并提升发包方最优知识共享激励程度;解答方知识共享努力与公平关切正相关,但私有解答努力与之负相关;众包项目绩效、发包方绝对经济收益与公平关切的关系取决于解答努力绩效转化率和知识共享努力绩效转化率之间的比值;知识共享激励机制能实现发包方和解答者双方经济利益的共赢,且其经济价值随公平关切敏感度增大而提高,并受到参赛人数的调节作用。 相似文献
437.
438.
Carbon isotope discrimination and water relations of oak hybrid populations in southwestern Utah 总被引:1,自引:0,他引:1
The evergreen oak Quercus turbinella and the deciduous Q. gambelii form natural hybrids in southwestern Utah and northern Arizona. Hybrid individuals also are found in northern Utah in a region where only Q. gambelii currently exists, indicating that Q. turbinella has recently retreated southward. Our objectives were to (1) examine the ecophysiology of parental taxa and hybrids under natural conditions in southeastern Utah, and (2) investigate the level of integration between leaf carbon isotope discrimination (a synthetic gas exchange trait) and structural and chemical traits of leaves in morphologically variable hybrid populations. Leaf length, width, mass-to-area ratio (LMA, g m -2 ), and nitrogen concentration (N, g g -1 ) within 2 hybrid populations near New Harmony, Utah, were highly intercorrelated. Variation within the hybrid populations spanned mean values for these traits observed in parental taxa from adjacent ""pure"" populations of each species. Carbon isotope discrimination (Δ), an integrated measure of the ratio of intercellular to ambient CO 2 concentration, ranged from 16.1‰ to 19.6‰ within the 2 hybrid populations and was positively correlated with leaf nitrogen concentration and negatively correlated with LMA; individuals in hybrid populations with leaves resembling Q. gambelii had the highest leaf Δ and N concentrations and lowest LMA compared with leaves from plants that resembled Q. turbinella . CO 2 uptake is limited by stomatal conductance and possibly by mesophyll resistance to a greater extent in Q. turbinella phenotypes than in intermediate or Q. gambelii phenotypes. δD of stem xylem water (an indication of active rooting depth) and predawn water potential during the peak monsoon period in August were not correlated to leaf Δ values within the hybrid populations. Several individuals that were morphologically similar to Q. turbinella in the hybrid populations maintained high predawn water potentials and derived moisture from winter recharge that presumably was taken from deep soil layers. Apparently, a few adult individuals of the Q. turbinella phenotype in hybrid populations accessed water from deep in the soil profile, which enabled them to avoid summer drought. Reduced monsoonal activity may have been an important, but not the single, determinant of Q. turbinella s retreat from northern Utah during the recent Holocene. 相似文献
439.
Cooper GM Coe BP Girirajan S Rosenfeld JA Vu TH Baker C Williams C Stalker H Hamid R Hannig V Abdel-Hamid H Bader P McCracken E Niyazov D Leppig K Thiese H Hummel M Alexander N Gorski J Kussmann J Shashi V Johnson K Rehder C Ballif BC Shaffer LG Eichler EE 《Nature genetics》2011,43(9):838-846
To understand the genetic heterogeneity underlying developmental delay, we compared copy number variants (CNVs) in 15,767 children with intellectual disability and various congenital defects (cases) to CNVs in 8,329 unaffected adult controls. We estimate that ~14.2% of disease in these children is caused by CNVs >400 kb. We observed a greater enrichment of CNVs in individuals with craniofacial anomalies and cardiovascular defects compared to those with epilepsy or autism. We identified 59 pathogenic CNVs, including 14 new or previously weakly supported candidates, refined the critical interval for several genomic disorders, such as the 17q21.31 microdeletion syndrome, and identified 940 candidate dosage-sensitive genes. We also developed methods to opportunistically discover small, disruptive CNVs within the large and growing diagnostic array datasets. This evolving CNV morbidity map, combined with exome and genome sequencing, will be critical for deciphering the genetic basis of developmental delay, intellectual disability and autism spectrum disorders. 相似文献