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Abstract
Global temperature increases and precipitation changes are both expected to alter alpine ecosystem structure and processes. In this paper, we reviewed the recent climate changes observed and the global change researches on the Tibetan Plateau. Firstly, we found that the mean annual temperature and precipitation (data from 75 meteorological stations, where all daily precipitation data are available) have increased since 1971, there were 0.5˚C and 0.7˚C per decade increase in annual and winter temperature, respectively, on Tibetan Plateau, and changes in precipitation were found both less spatially and temporally consistent. Secondly, we reviewed the climate change researches on the Tibetan Plateau published between 2000 and 2019 mainly focused on plant growth and ecosystem carbon balance which including plant phenology, plant productivity, plant diversity, exchanges in ecosystem carbon and soil organic carbon. Findings and insights from these studies have been very useful to understand how the alpine ecosystem processes respond to climate change. However, the effects of temperature increase on plant growth and ecosystem carbon balance are differ depending on the study sites and warming methods and periods, and the effects of precipitation changes are sparse.References
Zheng D, Zhang QS, Wu SH.Mountain Geoecology and Sustainable Development of the Tibetan Plateau. Kluwer Academic, Norwell, MA.2000. https://doi.org/10.1007/978-94-010-0965-2
Li C, Su F, Yang D, Tong K, Meng F, Kan B. Spatiotemporal variation of snow cover over the Tibetan Plateau based on modis snow product, 2001 - 2014. Int J Climatol 2018; 38:708-728. https://doi.org/10.1002/joc.5204
Kang SC, Xu YW, You QL, Flügel W, Yao TD. Review of climate and cryospheric change in the Tibetan Plateau. Environ Res Lett 2010; 5:1-8. https://doi.org/10.1088/1748-9326/5/1/015101
Yang YH, Mohammat A, Feng JM, Zhou R, Fang JY. Storage, patterns and environmental controls of soil organic carbon in China. Biogeochemistry 2007; 84:131-141. https://doi.org/10.1007/s10533-007-9109-z
Xu ZF, Hu TX, Wang KY, Zhang YB, Xian JR. Short-term responses of phenology, shoot growth and leaf traits of four alpine shrubs in a timberline ecotone to simulated global warming, Eastern Tibetan Plateau, China. Plant Spec Biol 2009; 24:27-34. https://doi.org/10.1111/j.1442-1984.2009.00229.x
Tang YH, Wan SQ, He JS, Zhao X. Foreword to the special issue: looking into the impacts of global warming from the roof of the world. J Plant Ecol 2010; 3:229-229.
Liu XD, Chen BD. Climatic warming in the Tibetan Plateau during recent decades. Int J Climatol 2000; 20:1729-1742. https://doi.org/10.1002/1097- 0088(20001130)20:143.0.CO;2-Y
Piao SL, Cui MD, Chen AP, Wang XH, Ciais P, Liu J, et al. Altitude and temperature dependence of change in the spring vegetation green-up date from 1982 to 2006 in the QinghaiXizang Plateau. Agr Forest Meteorol 2011; 151:1599-1608. https://doi.org/10.1016/j.agrformet.2011.06.016
Zheng GH, Yang YT, Yang DW, Dafflon B, Lei HM, Yang HB. "Satellite-based simulation of soil freezing/thawing processes in the northeast Tibetan Plateau." Remote Sens Environ 2019; 231:111269-111269. https://doi.org/10.1016/j.rse.2019.111269
Duan A, Xiao Z. Does the climate warming hiatus exist over the Tibetan Plateau? Rep 2015, 5(1):13711. https://doi.org/10.1038/srep13711
Yang B, He MH, Shishov V, Tychkov L,Vaganov E, Rossi S, et al. New perspective on spring vegetation phenology and global climate change based on Tibetan Plateau tree-ring data. Proc Natl Acad Sci 2017;114(27):6966. https://doi.org/10.1073/pnas.1616608114
Luo YQ, Melillo J, Niu SL, Beier C, Clark J, Classen A, et al. Coordinated approaches to quantify long-term ecosystem dynamics in response to global change. Glob Change Biol 2011; 17:843-854. https://doi.org/10.1111/j.1365-2486.2010.02265.x
Aamir L, Sana I, Zhang YJ, Xin YQ, Zhou L, Zhou Q. Review on global change status and its impacts on the Tibetan Plateau environmen. J Plant Ecol 2019;(6):6.
Klein JA, Harte J, Zhao XQ. Experimental warming causes large and rapid species loss, dampened by simulated grazing, on the Tibetan Plateau. Ecol Lett 2004; 7:1170-1179. https://doi.org/10.1111/j.1461-0248.2004.00677.x
Kato T, Tang YH, Gu S, Hirota M, Du M, Li YN, et al. Temperature and biomass influences on interannual changes in CO2 exchange in an alpine meadow on the QinghaiTibetan Plateau. Glob Change Biol 2006; 12:1285-1298. https://doi.org/10.1111/j.1365-2486.2006.01153.x
Luo CY, Xu GP, Chao ZG, et al. Effect of warming and grazing on litter mass loss and temperature sensitivity of litter and dung mass loss on the Tibetan Plateau. Glob Change Biol 2010; 16:1606-1617. https://doi.org/10.1111/j.1365-2486.2009.02026.x
Li GY, Liu YZ, Frelich LE, Sun SC. Experimental warming induces degradation of a Tibetan alpine meadow through trophic interactions. J Appl Ecol 2011a; 48:659-667. https://doi.org/10.1111/j.1365-2664.2011.01965.x
Liu YZ, Mu JP, Niklas KJ, Li GY, Sun SC. Global warming reduces plant reproductive output for temperate multiinflorescence species on the Tibetan Plateau. New Phytol 2012; 195:427-436. https://doi.org/10.1111/j.1469-8137.2012.04178.x
Wang SP, Duan JC, Xu GP, Wang YF, Zhang ZH, Rui YC, et al. Effects of warming and grazing on soil N availability, species composition, and ANPP in an alpine meadow. Ecol 2012; 93:2365-2376. https://doi.org/10.1890/11-1408.1
Li J, Shi J, Zhang DD, Yang B, Fang K, Pak HY. Moisture increase in response to elevated high-altitude warming evidenced by tree-rings on the southeastern Tibetan Plateau. Clim Dyn 2017;48(1-2):649-660. https://doi.org/10.1007/s00382-016-3101-z
Zong N, Geng SB, Duan C, Shi PL, Chai X, Zhang XZ. The effects of warming and nitrogen addition on ecosystem respiration in a Tibetan alpine meadow: The significance of winter warming. Ecol Evol 2018; 8. https://doi.org/10.1002/ece3.4484
Jiang YB, Fan M, Zhang YJ. Effect of short-term warming on plant communityfeatures of alpine meadow in Northern Tibet. Chin J Ecol 2017;36: 616-622.
IPCC. Climate change 2007: summary for policymakers. Valencia, Spain. 2007.
Wang B, Bao Q, Hoskins B, Wu GX, Liu YM. Tibetan plateau warming and precipitation changes in East Asia. Geophys Res Lett 2008; 35:L14702. https://doi.org/10.1029/2008GL034330
Piao SL, Ciais P, Huang Y, Shen ZH, Peng SS, Li JS, et al. The impacts of climate change on water resources and agriculture in China. Nature 2010; 467:43-51. https://doi.org/10.1038/nature09364
Gautam R, Hsu N C, Lau K M, Tsay S C, Kafatos M. Enhanced pre-monsoon warming over the HimalayanGangetic region from 1979 to 2007. Geophys Res Lett 2009;36: L07704. https://doi.org/10.1029/2009GL037641
Zhao L, Wu X, Wang Z, Sheng Y, Fang HB, Zhao YH, et al. Soil organic carbon and total nitrogen pools in permafrost zones of the Qinghai-Tibetan Plateau. Entific Reports 2018;8(1):3656. https://doi.org/10.1038/s41598-018-22024-2
Klein JA, Harte J, Zhao XQ. Experimental warming, not grazing, decreases rangeland quality on the Tibetan Plateau. Ecol Appl 2007; 17:541-557. https://doi.org/10.1890/05-0685
Yu HY, Luedeling E, Xu JC. Winter and spring warming result in delayed spring phenology on the Tibetan Plateau. Proc Natl Acad Sci 2010; 107:22151-22156. https://doi.org/10.1073/pnas.1012490107
Shen MG. Spring phenology was not consistently related to winter warming on the Tibetan Plateau. Proc Natl Acad Sci 2011; 108:E91-E92. https://doi.org/10.1073/pnas.1018390108
Jochner S, Sparks TH, Laube J, Menzel A. Can we detect a nonlinear response to temperature in european plant phenology? Int J Biometeorol 2016. https://doi.org/10.1007/s00484-016-1146-7
Chen H, Zhu QA, Wu N, Wang YF, Peng CH. Delayed spring phenology on the Tibetan Plateau may also be attributable to other factors than winter and spring warming. Proc Natl Acad Sci 2011; 108:E93-E93. https://doi.org/10.1073/pnas.1100091108
Ge Q, Wang HJ, Rutishauser T, Dai JH. Phenological response to climate change in China: a meta-analysis. Glob Chang Biol 2015;21(1):265-274. https://doi.org/10.1111/gcb.12648
Li N, Wang GX, Yang Y, Gao YH, Liu GS. Plant production, and carbon and nitrogen source pools, are strongly intensified by experimental warming in alpine ecosystems in the Qinghai-Tibet Plateau. Soil Biol Biochem 2011b; 43:942- 953. https://doi.org/10.1016/j.soilbio.2011.01.009
Gao MD,Piao SL,Chen AP, Yang H, Janssens LA.Divergent changes in the elevational gradient of vegetation activities over the last 30 years. Nature Commun 2019;10(1):2970. https://doi.org/10.1038/s41467-019-11035-w
Duan JC. Studies of effects of climate change and grazing on plant and plant community on the alpine meadow of the Qinghai-Tibetan plateau. Chinese Academy of Sciences 2012.
Piao SL, Fang JY, Zhou LM, Ciais P, Zhu B. Variations in satellite-derived phenology in China's temperate vegetation. Glob Change Biol 2006b; 12:672-685. https://doi.org/10.1111/j.1365-2486.2006.01123.x
Rustad LE, Campbell JL, Marion GM, Norby RJ, Mitchell MJ, Hartley AE, et al. A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming. Oecologia 2001; 126:543-562. https://doi.org/10.1007/s004420000544
Arft AM, Walker MD, Gurevitch J, Alatalo JM, Wookey PA. Responses of tundra plants to experimental warming: Metaanalysis of the international tundra experiment. Ecol Monogr 1999; 69:491-511. https://doi.org/10.2307/2657227
Wang JW, Fu G, Zhang GY, Shen ZX. The Effect of Higher Warming on Vegetation Indices and Biomass Production is Dampened by Greater Drying in an Alpine Meadow on the Northern Tibetan Plateau. J Resour Ecol 2017;8(1):105-112. https://doi.org/10.5814/j.issn.1674-764x.2017.01.013
Xu W, Zhu MY, Zhang ZH, Ma ZY, Liu HY, Chen LT, et al. Experimentally simulating warmer and wetter climate additively improves rangeland quality on the Tibetan Plateau. J Appl Ecol 2018;55(3):1486-1497. https://doi.org/10.1111/1365-2664.13066
Walker MD, Wahren CH, Hollister RD, Henry GH, Ahlquist LE, Alatalo JM. Plant community responses to experimental warming across the tundra biome. Proc Natl Acad Sci 2006; 103:1342-1346. https://doi.org/10.1073/pnas.0503198103
Beier C, Beierkuhnlein C, Wohlgemuth T, Penuelas J, Emmett B, Körner C, et al. Precipitation manipulation experiments - challenges and recommendations for the future. Ecol Lett 2012; 15:899-911. https://doi.org/10.1111/j.1461-0248.2012.01793.x
Austin AT. Differential effects of precipitation on production and decomposition along a rainfall gradient in Hawaii. Ecol 2002; 83:328-338. https://doi.org/10.1890/0012- 9658(2002)083[0328:DEOPOP
Chivers MR, Turetsky MR, Waddington JM, Harden JW, McGuire AD. Effects of experimental water table and temperature manipulations on ecosystem CO2 fluxes in an Alaskan rich fen. Ecosyst 2009; 12:1329-1342. https://doi.org/10.1007/s10021-009-9292-y
Wu ZT, Dijkstra P, Koch GW, Peuelas J, Hungate BA. Responses of terrestrial ecosystems to temperature and precipitation change: a meta-analysis of experimental manipulation. Glob Change Biol 2011b; 17:927-942. https://doi.org/10.1111/j.1365-2486.2010.02302.x
Cox PM, Betts RA, Jones CD, Spall SA, Totterdell LJ. Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature 2000; 408:750-750. https://doi.org/10.1038/35047138
Trumbore S. Carbon respired by terrestrial ecosystems - recent progress and challenges. Glob Change Biol 2006; 12:141-153. https://doi.org/10.1111/j.1365-2486.2006.01067.x
Zhang XZ, Shen ZX, Fu G. A meta-analysis of the effects of experimental warming on soil carbon and nitrogen dynamics on the tibetan plateau. Appl Soil Ecol 2015. https://doi.org/10.1016/j.apsoil.2014.11.012
Luo CY, Xu GP, Wang YF, Wang SP. Effects of grazing and experimental warming on DOC concentrations in the soil solution on the Qinghai-Tibet Plateau. Soil Biol Biochem 2009; 41:2493-2500. https://doi.org/10.1016/j.soilbio.2009.09.006
Wu YB, Tan HC, Deng YC, Wu J, Xu XL, Wang YF, et al. Partitioning pattern of carbon flux in a Kobresia grassland on the Qinghai-Tibetan Plateau revealed by field 13C pulselabeling. Glob Change Biol 2010; 16:2322-2333 https://doi.org/10.1111/j.1365-2486.2009.02069.x
Shi FS, Chen H, Chen HF, Wu Y, Wu N. The combined effects of warming and drying suppress CO2 and N2O emission rates in an alpine meadow of the eastern Tibetan Plateau. Ecol Res 2012; 27:725-733. https://doi.org/10.1007/s11284-012-0950-8
Lin XW, Zhang ZH, Wang SP, Hu YG, Xu GP, Luo CY, et al. Response of ecosystem respiration to warming and grazing during the growing seasons in the alpine meadow on the Tibetan Plateau. Agr Forest Meteorol 2011; 151:792-802. https://doi.org/10.1016/j.agrformet.2011.01.009
Rui YC, Wang SP, Xu ZH, Wang YF, Chen CR, Zhou XQ, et al. Warming and grazing affect soil labile carbon and nitrogen pools differently in an alpine meadow of the Qinghai-Tibet Plateau in China. J Soil Sediment 2011; 11:903-914. https://doi.org/10.1007/s11368-011-0388-6
Rui YC, Wang YF, Chen CR, Zhou XQ, Wang SP, Xu ZH, et al. Warming and grazing increase mineralization of organic P in an alpine meadow ecosystem of Qinghai-Tibet Plateau, China. Plant Soil 2012; 357:73-87. https://doi.org/10.1007/s11104-012-1132-8
Fu G, Shen ZX, Zhang XZ, Zhou YT. Response of soil microbial biomass to short-term experimental warming in alpine meadow on the Tibetan Plateau. Appl Soil Ecol 2012; 61:158-160. https://doi.org/10.1016/j.apsoil.2012.05.002
Ma ZL, Zhao WQ, Liu M, Liu Q. Effects of warming on microbial biomass carbon and nitrogen in the rhizosphere and bulk soil in an alpine scrub ecosystem. Chinese Journal of Applied Ecology 2019.
Ma ZL, Zhao WQ, Liu M, Liu Q. Responses of soil respiration and its components to experimental warming in an alpine scrub ecosystem on the eastern Qinghai- Tibet Plateau. Sci Total Environ 2018;643(DEC.1):1427-1435. https://doi.org/10.1016/j.scitotenv.2018.06.243
Hu YG, Chang XF, Lin XW, Wang FP, Duan JC, Zhang ZH, et al. Effects of warming and grazing on N2O fluxes in an alpine meadow ecosystem on the Tibetan Plateau. Soil Biol Biochem 2010; 42:944-952. https://doi.org/10.1016/j.soilbio.2010.02.011
Saito M, Kato T, Tang Y. Temperature controls ecosystem CO2 exchange of an alpine meadow on the northeastern Tibetan Plateau. Glob Change Biol 2009; 15:221-228. https://doi.org/10.1111/j.1365-2486.2008.01713.x
Yang YH, Fang JY, Tang YH, Ji CJ, Zheng CY, He JS, et al. Storage, patterns and controls of soil organic carbon in the Tibetan grasslands. Glob Change Biol 2008; 14:1592-1599. https://doi.org/10.1111/j.1365-2486.2008.01591.x
Yang YH, Fang JY, Smith P, Tang YH, He JS. Changes in topsoil carbon stock in the Tibetan grasslands between the 1980s and 2004. Glob Change Biol 2009; 15:2723-2729. https://doi.org/10.1111/j.1365-2486.2009.01924.x
Piao SL, Fang JY, He JS. Variations in vegetation net primary production in the Qinghai-Xizang Plateau, China, from 1982 to 1999. Clim Change 2006a;74:253-267. https://doi.org/10.1007/s10584-005-6339-8
Zhu K, Chiariello N R, Tobeck T, Fukami T, Field CB. Nonlinear, interacting responses to climate limit grassland production under global change. Proc Natl Acad Sci 2016;113(38):201606734. https://doi.org/10.1073/pnas.1606734113
Heng T, Wu JG, Xie SY, Wu MX. The responses of soil C and N, microbial biomass C or N under alpine meadow of Qinghai-Tibet Plateau to the change of temperature and precipitation. Chin Agric Sci Bull 2011;27:425-430.
Klein JA, Harte J, Zhao XQ. Dynamic and complex microclimate responses to warming and grazing manipulations. Glob Change Biol 2005; 11: 1440-1451. https://doi.org/10.1111/j.1365-2486.2005.00994.x
Shen HH, Wang SP, Tang YH. Grazing alters warming effects on leaf photosynthesis and respiration in Gentiana straminea, an alpine forb species.J Plant Ecol 2013; 6(5): 418-427. https://doi.org/10.1093/jpe/rtt010
Wang SP, Meng FD, Duan JC, Wang YF, Cui XY, Piao SL, et al. Asymmetric sensitivity of first flowering date to warming and cooling in alpine plants. Ecology 2014a; 95(12): 3387- 3398. https://doi.org/10.1890/13-2235.1
Wang SP, Wang CS, Duan JC, Zhu XX, Xu GP, Luo CY, et al.Timing and duration of phenological sequences of alpine plants along an elevation gradient on the Tibetan plateau. Agric For Meteorol 2014b; 189-190: 220-228. https://doi.org/10.1016/j.agrformet.2014.01.021
Jing X, Wang YH, Chung H, Mi ZR, Wang SP, Zeng H, et al.No temperature acclimation of soil extracellular enzymes to experimental warming in an alpine grassland ecosystem on the Tibetan Plateau. Biogeochemistry 2014; 117: 39-54. https://doi.org/10.1007/s10533-013-9844-2
Lin XW, Wang SP, Hu YG, Luo CY, Zhang ZH, Niu HS, et al. Experimental Warming Increases Seasonal Methane Uptake in an Alpine Meadow on the Tibetan Plateau. Ecosyst 2015; 18: 274-286. https://doi.org/10.1007/s10021-014-9828-7
Shen ZX, Zhou XM, Chen ZZ, Zhou HK. Response of plant groups to simulated rainfall and nitrogen supply in alpine Kobresia humilis meadow. Acta Phytoecol Sin 2002; 26: 288- 294.
Yang Y, Wang GX, Klanderud K, Yang LD. Responses in leaf functional traits and resource allocation of a dominant alpine sedge (Kobresia pygmaea) to climate warming in the Qinghai-Tibetan Plateau permafrost region. Plant Soil 2011; 349: 377-387. https://doi.org/10.1007/s11104-011-0891-y
Wang JF, Wu QB. Impact of elevated temperature on the heat and water changes of the shallow soil layers in the swamp meadow areas of the Qinghai-Tibetan Plateau. J Lanzhou Univ 2010; 46: 33-39.
Zhao JZ, Liu W, Zhou HK, Zhang Y, Yu L, Xu QM. Effects of Simulated Greenhouse Effect on Growth Characteristics of Kobresia humilis. Acta Bot Bori-Occid Sin 2006; 12: 2533- 2539.
Shi FS, Wu N, Luo P. Effect of temperature enhancement on community structure and biomass of subalpinemeadow in Northwestern Sichuan. Acta Ecol Sin 2008; 28: 5286-5293.
Wang B, Sun G, Luo P, Wang M, Wu N. Labile and recalcitrant carbon and nitrogen pools of an alpine meadow soil from the eastern Qinghai-Tibetan Plateau subjected to experimental warming and grazing. Acta Ecol Sin 2011; 31: 1506-1514.
Liu Q, Yin HJ, Chen JS, Zhao CZ, Cheng XY, Wei YY, et al. Belowground responses of Picea asperata seedlings to warming and nitrogen fertilization in the eastern Tibetan Plateau.Ecol Res 2011; 26: 637-648. https://doi.org/10.1007/s11284-011-0824-5
Xu ZF, Tang Z, Wan C, Xiong P, Cao G, Liu Q. Effects of simulated warming on soil enzyme activities in two subalpine coniferous forests in west Sichuan. Chin J App Ecol 2010a; 21: 2727-2733.
Xu ZF, Wan C, Xiong P, Tang Z, Hu R, Cao G, et al. Initial responses of soil CO2 efflux and C, N pools to experimental warming in two contrasting forest ecosystems, Eastern Tibetan Plateau, China. Plant Soil 2010b; 336: 183-195. https://doi.org/10.1007/s11104-010-0461-8
Wu XW, Duffy JE, Reich PB, Sun SC. A brown-world cascade in the dung decomposer food web of an alpine meadow: effects of predator interactions and warming. Ecol Monogr 2011a; 81: 313-328. https://doi.org/10.1890/10-0808.1
Wu XC, Sun H, Yang WQ, Wang KY. Responses of subalpine Betula albo-sinensis soil invertase activity to elevated atmospheric temperature and CO2 concentration in Western Sichuan. Chin J App Ecol 2007; 18: 1225-1230.
Zhao YH, Wei XH, Shen ZX, Sun L, Niu XY. Effect of simulated warming on the reproductive ecology of Carex thibetica Franch. Ecol Environ Sci 2010; 19: 1783-1788.
Yu CQ, Shen ZX, Zhang XZ, Sun W, Fu G. Response of Soil C and N, Dissolved Organic C and N, and Inorganic N to Short-Term Experimental Warming in an Alpine Meadow on the Tibetan Plateau. The Scientific World Journal 2014; 152576. https://doi.org/10.1155/2014/152576
Fu G, Zhang XZ, Zhang YJ, Shi PL, Li YL, Zhou YT, et al. Experimental warming does not enhance gross primary production and above-ground biomass in the alpine meadow of Tibet. J Appl Remote Sens 2015.
Shen ZX, Li YL, Fu G. Response of soil respiration to shortterm experimental warming and precipitation pulses over the growing season in an alpine meadow on the Northern Tibet. Appl Soil Ecol 2015; 90: 35-40. https://doi.org/10.1016/j.apsoil.2015.01.015
Yang Y, Wang GX, Yang LD, Guo JY. Effects of Drought and Warming on Biomass, Nutrient Allocation, and Oxidative Stress in Abies fabri in Eastern Tibetan Plateau. 2013; 32: 298-306. https://doi.org/10.1007/s00344-012-9298-0
Lu XY, Fan JH, Yan Y, Wang XD. Responses of Soil CO2 Fluxes to Short-Term Experimental Warming in Alpine Steppe Ecosystem, Northern Tibet. PLoS One 2013; 8(3): e59054. https://doi.org/10.1371/journal.pone.0059054
Bai W, Wang GX, Liu GS. Effects of elevated temperature on CO2 flux during growth season in an alpine meadow ecosystem of Qinghai-Tibet Plateau. Chin J Ecol 2011;30:1045-1051.
Yang YH, Fang JY, Ma WH, Smith P, Mohanmat A, Wang SP, et al. Soil carbon stock and its changes in northern China's grasslands from 1980s to 2000s. Glob Change Biol 2010; 16: 3036-3047. https://doi.org/10.1111/j.1365-2486.2009.02123.x
Zhang YQ, Tang YH, Jiang J, Yang YH. Characterizing the dynamics of soil organic carbon in grasslands on the Qinghai-Tibetan Plateau. Sci China Ser D 2007; 50: 113-120. https://doi.org/10.1007/s11430-007-2032-2
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