As a widely used reconstruction algorithm in quantum state tomography, maximum likelihood estimation tends to assign a rank-deficient matrix, which decreases estimation accuracy for certain quantum states. Fortunately...As a widely used reconstruction algorithm in quantum state tomography, maximum likelihood estimation tends to assign a rank-deficient matrix, which decreases estimation accuracy for certain quantum states. Fortunately, hedged maximum likelihood estimation （HMLE） [Phys. Rev. Lett. 105 （2010）200504] was proposed to avoid this problem. Here we study more details about this proposal in the two-qubit case and further improve its performance. We ameliorate the HMLE method by updating the hedging function based on the purity of the estimated state. Both performances of HMLE and ameliorated HMLE are demonstrated by numerical simulation and experimental implementation on the Werner states of polarization-entangled photons.展开更多
Effective management of water resources,especially groundwater,is crucial and requires a precise understanding of aquifer characteristics,imposed stresses,and the groundwater balance.Simulation-optimization models pla...Effective management of water resources,especially groundwater,is crucial and requires a precise understanding of aquifer characteristics,imposed stresses,and the groundwater balance.Simulation-optimization models plays a vital role in guiding planners toword sustainable long-term aquifer exploita-tion.This study simulated monthly water table variations in the Kashan Plain over a ten-year period from 2008 to 2019 across 125 stress periods using the GMS model.The model was calibrated for both steady-state and transient conditions for the 2008–2016 period and validated for the 2016–2019 period.Results indicated a 4.4 m decline in groundwater levels over the 10-year study period.Given the plain's location in a arid climatic zone with limited effective precipitation for aquifer recharge,the study focused on ground-water extraction management.A modified two-point hedging policy was employed as a solution to mitigate critical groundwater depletion,reducing the annual drawdown rate from 0.44 m to 0.31 m and conserving 255 million cubic meters(mcm)of water annually.Although this approach slightly decreased reliability(i.e.the number of months meeting full water demands),it effectively minimized the risk of severe droughts and irreparable damages.This policy offers managers a dynamical and intelligent tool for regulating groundwater extraction,balancing aquifer sustainability with agricultural and urban water requirements.展开更多
This paper employs the PPO(Proximal Policy Optimization) algorithm to study the risk hedging problem of the Shanghai Stock Exchange(SSE) 50ETF options. First, the action and state spaces were designed based on the cha...This paper employs the PPO(Proximal Policy Optimization) algorithm to study the risk hedging problem of the Shanghai Stock Exchange(SSE) 50ETF options. First, the action and state spaces were designed based on the characteristics of the hedging task, and a reward function was developed according to the cost function of the options. Second, combining the concept of curriculum learning, the agent was guided to adopt a simulated-to-real learning approach for dynamic hedging tasks, reducing the learning difficulty and addressing the issue of insufficient option data. A dynamic hedging strategy for 50ETF options was constructed. Finally, numerical experiments demonstrate the superiority of the designed algorithm over traditional hedging strategies in terms of hedging effectiveness.展开更多
This study examines the dynamic connectedness and hedging opportunities between CSI300(China Security Index 300)and copper,gold,PTA(purified terephthalic acid),and soybean in China from January 09,2008,to June 30,2023...This study examines the dynamic connectedness and hedging opportunities between CSI300(China Security Index 300)and copper,gold,PTA(purified terephthalic acid),and soybean in China from January 09,2008,to June 30,2023.A TVP-VAR and cDCC-FIAPARCH modeling framework was used for the empirical investigation.The results show that the total connectedness index can effectively capture cross-asset information transmission in China’s financial markets.Copper returns are the dominant volatility transmitters,while CSI300,gold,and soybean returns are net recipients.The Russian-Ukraine war reinforced the safe-haven role of gold.Finally,investors with CSI300 long positions may benefit from prioritizing gold for hedging,while those with CSI300 short positions profit more from allocating gold to PTA.Portfolio managers and investors can use the findings to track the dynamics of systemic risk and adjust their long/short positions when investing in China's stock and commodity markets.展开更多
Unlike the current extensive literature,which discusses which assets can avoid the risks caused by the COVID-19 pandemic,this study examines whether the characteristics of different assets affect the extreme risk tran...Unlike the current extensive literature,which discusses which assets can avoid the risks caused by the COVID-19 pandemic,this study examines whether the characteristics of different assets affect the extreme risk transmission of the COVID-19 crisis.This study explores the effects of COVID-19 pandemic-related risk factors(i.e.,pandemic severity,pandemic regulations and policies,and vaccination-related variables)on the risk of extreme volatility in asset returns across eight assets.These eight assets belong to the following classes:virtual,financial,energy,commodities,and real assets.To consider the different possible aspects of the COVID-19 impact,this study adopts both empirical methods separately,considering variables related to the pandemic as exogenous shocks and endogenous factors.Using these methods,this study enabled a systematic analysis of the relationship between the features of different asset types and the effects of extreme risk transmission during the COVID-19 crisis.The results show that different types of asset markets are affected by different risk factors.Virtual and commodity assets do not exhibit extreme volatility induced by the COVID-19 pandemic.The energy market,including crude oil,is most affected by the negative impact of the severity of the pandemic,which is unfavorable for investment at the beginning of the pandemic.However,after vaccinations and pandemic regulations controlled the spread of infection,the recovery of the energy market made it more conducive to investment.In addition,this study explains the differences between the hedging characteristics of Bitcoin and gold.The findings of this study can help investors choose asset types systematically when faced with different shocks.展开更多
A semantics-based model is proposed to enable weakened hedges, such as ＂more or less＂ and ＂roughly＂ in the context of linguistic multi-criteria decision making. First, the resemblance relations are defined based o...A semantics-based model is proposed to enable weakened hedges, such as ＂more or less＂ and ＂roughly＂ in the context of linguistic multi-criteria decision making. First, the resemblance relations are defined based on the semantics of terms on the domain. Then, the hedges can be represented after the upper and loose upper approximations of a linguistic term are derived. Accordingly, some compact formulae can be derived for the semantics of linguistic expressions with hedges. Parameters in these formulae are objectively determined according to the semantics of original terms. The proposed model presents a more natural way to express the decision information under uncertainties and its semantics is clear. The proposed model is clarified by solving the problem of evaluation and selection of sustainable innovative energy technologies. Computational results demonstrate that the model can deal with various uncertainties of the problem. Finally, the model is compared with existing techniques and extended to the case when the semantics of terms are represented by trapezoidal fuzzy numbers.展开更多
Hedging, referring to the use of various lexical and syntactic features modifying and mitigating propositions and claims, is extensively employed in effective academic writing. However, the use of hedges seems to be q...Hedging, referring to the use of various lexical and syntactic features modifying and mitigating propositions and claims, is extensively employed in effective academic writing. However, the use of hedges seems to be quite problematic for lots of Chinese ESL learners. In view of the situation, this study explores the use of hedges in academic abstracts by a group of undergraduate English majors in a university in Hainan Province. After analyzing data from a corpus and interviews, several problems have been found in the use of hedging devices of these students, and relevant implications have been drawn to the teaching of hedging in the context, which can help the students produce more effective academic writing and to achieve greater university success.展开更多
This paper investigates the evolutionary change in the OF(OF) of hedges in different moves of EMRAs(EMRAs).Two corpora are established for longitudinal study,with Corpus A consisting of EMRAs from 1990-1994 and Corpus...This paper investigates the evolutionary change in the OF(OF) of hedges in different moves of EMRAs(EMRAs).Two corpora are established for longitudinal study,with Corpus A consisting of EMRAs from 1990-1994 and Corpus B from 2005-2009.Generally,no significant change has been found in overall use of hedges.However,apart from move 6,7 and 10,we have found some interesting changes in the other moves.On the one hand,the OF of hedges in move 1,move 3,move 4 and move 5 shows a significant increase.On the other hand,the OF of hedges in move 2,8,9 and move 11 show a significant decrease.展开更多
Global extreme hydrological events pose considerable challenges to the sustainable development of human society and river ecology.Land use/cover change(LUCC)is a visible manifestation of human activity and has caused ...Global extreme hydrological events pose considerable challenges to the sustainable development of human society and river ecology.Land use/cover change(LUCC)is a visible manifestation of human activity and has caused substantial alterations in extreme hydrological regimes across rivers worldwide.The Jinsha River lies upstream of the Yangtze River and its hydrological variability has had profound socioeconomic and environmental effects.In this study,we developed Hydrological Simulation Program–FORTRAN(HSPF)and land-use simulation models of the entire watershed to simulate the effects of LUCC on hydrological extremes and quantify the inter-relationships among them.The main land-use changes between 1995 and 2015 were those associated with cropland,forest land,and grassland.Between 2015 and 2030,it is estimated that the coverage of forest land,grassland,construction land,and unused land will increase by 0.64%,0.18%,69.38%,and 45.08%,respectively,whereas that of cropland,water bodies,and snow-and ice-covered areas will decline by 8.02%,2.63%,and 0.89%,respectively.LUCC has had irregular effects on different hydrological regimes and has most severely altered stream flows.The responses of hydrological extremes to historical land-use change were characterized by spatial variation.Extreme low flows increased by 0.54%–0.59%whereas extreme high flows increased by 0%–0.08%at the lowest outlet.Responses to future land-use change will be amplified by a 0.72%–0.90%reduction in extreme low flows and a 0.08%–0.12%increase in extreme high flows.The hedging effect caused by irregular changes in tributary stream flow was found to alleviate the observed flow in mainstream rivers caused by land-use change.The extreme hydrological regimes were affected mainly by the net swap area transferred from ice and snow area to forest(NSAIF)and thereafter to cultivated land(NSAIC).Extreme low flows were found to be positively correlated with NSAIF and NSAIC,whereas extreme high flows were positively correlated with NSAIC and negatively correlated with NSAIF.展开更多
Through the application of the VAR-AGARCH model to intra-day data for three cryptocurrencies(Bitcoin,Ethereum,and Litecoin),this study examines the return and volatility spillover between these cryptocurrencies during...Through the application of the VAR-AGARCH model to intra-day data for three cryptocurrencies(Bitcoin,Ethereum,and Litecoin),this study examines the return and volatility spillover between these cryptocurrencies during the pre-COVID-19 period and the COVID-19 period.We also estimate the optimal weights,hedge ratios,and hedging effectiveness during both sample periods.We find that the return spillovers vary across the two periods for the Bitcoin–Ethereum,Bitcoin–Litecoin,and Ethereum–Litecoin pairs.However,the volatility transmissions are found to be different during the two sample periods for the Bitcoin–Ethereum and Bitcoin–Litecoin pairs.The constant conditional correlations between all pairs of cryptocurrencies are observed to be higher during the COVID-19 period compared to the pre-COVID-19 period.Based on optimal weights,investors are advised to decrease their investments(a)in Bitcoin for the portfolios of Bitcoin/Ethereum and Bitcoin/Litecoin and(b)in Ethereum for the portfolios of Ethereum/Litecoin during the COVID-19 period.All hedge ratios are found to be higher during the COVID-19 period,implying a higher hedging cost compared to the pre-COVID-19 period.Last,the hedging effectiveness is higher during the COVID-19 period compared to the pre-COVID-19 period.Overall,these findings provide useful information to portfolio managers and policymakers regarding portfolio diversification,hedging,forecasting,and risk management.展开更多
Extreme weather conditions occur at an increasing rate as evidenced by higher frequency of hurricanes and more extreme precipitation and temperature anomalies. Such extreme environmental conditions will have important...Extreme weather conditions occur at an increasing rate as evidenced by higher frequency of hurricanes and more extreme precipitation and temperature anomalies. Such extreme environmental conditions will have important implications for all living organisms through greater frequency of reproductive failure and reduced adult survival. We review examples of reproductive failure and reduced survival related to extreme weather conditions. Phenotypic plasticity may not be sufficient to allow adaptation to extreme weather for many animals. Theory predicts reduced reproductive effort as a response to increased stochasticity. We predict that patterns of natural selection will change towards truncation selection as environmental conditions become more extreme. Such changes in patterns of selection may facilitate adaptation to extreme events. However, effects of selection on reproductive effort are difficult to detect. We present a number of predictions for the effects of extreme weather conditions in need of empirical tests. Finally, we suggest a number of empirical reviews that could improve our ability to judge the effects of extreme environmental conditions on life history [Current Zoology 57 （3）： 375-389, 2011].展开更多
The phenomenon of tree waves （hedges and ribbons） formation within the alpine ecotone in Altai Mountains and its response to observed air temperature increase was considered. At the upper limit of tree growth Siberi...The phenomenon of tree waves （hedges and ribbons） formation within the alpine ecotone in Altai Mountains and its response to observed air temperature increase was considered. At the upper limit of tree growth Siberian pine （Pinus sibirica） forms hedges on windward slopes and ribbons on the leeward ones. Hedges were formed by prevailing winds and oriented along winds direction. Ribbons were formed by snow blowing and accumulating on the leeward slope and perpendicular to the prevailing winds, as well as to the elevation gradient. Hedges were always linked with microtopography features, whereas ribbons were not. Trees are migrating upward by waves and new ribbons and hedges are forming at or near tree line, whereas at lower elevations ribbons and hedges are being transformed into dosed forests. Time series of high-resolution satellite scenes （from 1968 to 2OLO） indicated an upslope shift in the position ribbons averaged 155＋26 m （or 3-7 m yr^-1） and crown closure increased （about 35%-90%）. The hedges advance was limited by poor regeneration establishment and was negligible. Regeneration within the ribbon zone was approximately 2.5 times （5060 vs 2120 ha^-1） higher then within the hedges zone. During the last four decades, Siberian pine in both hedges and ribbons strongly increased its growth increment, and recent tree growth rate for 50 year-old trees was about twice higher than those recorded for similarly-aged trees at the beginning of the 20^th century. Hedges and ribbons are phenomena that are widespread within the southern and northern Siberian Mountains.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos 11574291,61108009 and 61222504
文摘As a widely used reconstruction algorithm in quantum state tomography, maximum likelihood estimation tends to assign a rank-deficient matrix, which decreases estimation accuracy for certain quantum states. Fortunately, hedged maximum likelihood estimation （HMLE） [Phys. Rev. Lett. 105 （2010）200504] was proposed to avoid this problem. Here we study more details about this proposal in the two-qubit case and further improve its performance. We ameliorate the HMLE method by updating the hedging function based on the purity of the estimated state. Both performances of HMLE and ameliorated HMLE are demonstrated by numerical simulation and experimental implementation on the Werner states of polarization-entangled photons.
文摘Effective management of water resources,especially groundwater,is crucial and requires a precise understanding of aquifer characteristics,imposed stresses,and the groundwater balance.Simulation-optimization models plays a vital role in guiding planners toword sustainable long-term aquifer exploita-tion.This study simulated monthly water table variations in the Kashan Plain over a ten-year period from 2008 to 2019 across 125 stress periods using the GMS model.The model was calibrated for both steady-state and transient conditions for the 2008–2016 period and validated for the 2016–2019 period.Results indicated a 4.4 m decline in groundwater levels over the 10-year study period.Given the plain's location in a arid climatic zone with limited effective precipitation for aquifer recharge,the study focused on ground-water extraction management.A modified two-point hedging policy was employed as a solution to mitigate critical groundwater depletion,reducing the annual drawdown rate from 0.44 m to 0.31 m and conserving 255 million cubic meters(mcm)of water annually.Although this approach slightly decreased reliability(i.e.the number of months meeting full water demands),it effectively minimized the risk of severe droughts and irreparable damages.This policy offers managers a dynamical and intelligent tool for regulating groundwater extraction,balancing aquifer sustainability with agricultural and urban water requirements.
基金supported by the Foundation of Key Laboratory of System Control and Information Processing,Ministry of Education,China,Scip20240111Aeronautical Science Foundation of China,Grant 2024Z071108001the Foundation of Key Laboratory of Traffic Information and Safety of Anhui Higher Education Institutes,Anhui Sanlian University,KLAHEI18018.
文摘This paper employs the PPO(Proximal Policy Optimization) algorithm to study the risk hedging problem of the Shanghai Stock Exchange(SSE) 50ETF options. First, the action and state spaces were designed based on the characteristics of the hedging task, and a reward function was developed according to the cost function of the options. Second, combining the concept of curriculum learning, the agent was guided to adopt a simulated-to-real learning approach for dynamic hedging tasks, reducing the learning difficulty and addressing the issue of insufficient option data. A dynamic hedging strategy for 50ETF options was constructed. Finally, numerical experiments demonstrate the superiority of the designed algorithm over traditional hedging strategies in terms of hedging effectiveness.
基金support by the Humanities and Social Science Fund of Ministry of Education of China[Grant Number:23YJC850005]the Federal Ministry of Education,Science and Research(BMBWF)through Austria's Agency for Education and Internationalization(OeAD)[Grant Number:TW 01/2021].
文摘This study examines the dynamic connectedness and hedging opportunities between CSI300(China Security Index 300)and copper,gold,PTA(purified terephthalic acid),and soybean in China from January 09,2008,to June 30,2023.A TVP-VAR and cDCC-FIAPARCH modeling framework was used for the empirical investigation.The results show that the total connectedness index can effectively capture cross-asset information transmission in China’s financial markets.Copper returns are the dominant volatility transmitters,while CSI300,gold,and soybean returns are net recipients.The Russian-Ukraine war reinforced the safe-haven role of gold.Finally,investors with CSI300 long positions may benefit from prioritizing gold for hedging,while those with CSI300 short positions profit more from allocating gold to PTA.Portfolio managers and investors can use the findings to track the dynamics of systemic risk and adjust their long/short positions when investing in China's stock and commodity markets.
文摘Unlike the current extensive literature,which discusses which assets can avoid the risks caused by the COVID-19 pandemic,this study examines whether the characteristics of different assets affect the extreme risk transmission of the COVID-19 crisis.This study explores the effects of COVID-19 pandemic-related risk factors(i.e.,pandemic severity,pandemic regulations and policies,and vaccination-related variables)on the risk of extreme volatility in asset returns across eight assets.These eight assets belong to the following classes:virtual,financial,energy,commodities,and real assets.To consider the different possible aspects of the COVID-19 impact,this study adopts both empirical methods separately,considering variables related to the pandemic as exogenous shocks and endogenous factors.Using these methods,this study enabled a systematic analysis of the relationship between the features of different asset types and the effects of extreme risk transmission during the COVID-19 crisis.The results show that different types of asset markets are affected by different risk factors.Virtual and commodity assets do not exhibit extreme volatility induced by the COVID-19 pandemic.The energy market,including crude oil,is most affected by the negative impact of the severity of the pandemic,which is unfavorable for investment at the beginning of the pandemic.However,after vaccinations and pandemic regulations controlled the spread of infection,the recovery of the energy market made it more conducive to investment.In addition,this study explains the differences between the hedging characteristics of Bitcoin and gold.The findings of this study can help investors choose asset types systematically when faced with different shocks.
基金The National Natural Science Foundation of China(No.61273209)the Scientific Research Foundation of Graduate School of Southeast University(No.YBJJ1528)the Scientific Innovation Research of College Graduates in Jiangsu Province(No.KYLX15-0191)
文摘A semantics-based model is proposed to enable weakened hedges, such as ＂more or less＂ and ＂roughly＂ in the context of linguistic multi-criteria decision making. First, the resemblance relations are defined based on the semantics of terms on the domain. Then, the hedges can be represented after the upper and loose upper approximations of a linguistic term are derived. Accordingly, some compact formulae can be derived for the semantics of linguistic expressions with hedges. Parameters in these formulae are objectively determined according to the semantics of original terms. The proposed model presents a more natural way to express the decision information under uncertainties and its semantics is clear. The proposed model is clarified by solving the problem of evaluation and selection of sustainable innovative energy technologies. Computational results demonstrate that the model can deal with various uncertainties of the problem. Finally, the model is compared with existing techniques and extended to the case when the semantics of terms are represented by trapezoidal fuzzy numbers.
文摘Hedging, referring to the use of various lexical and syntactic features modifying and mitigating propositions and claims, is extensively employed in effective academic writing. However, the use of hedges seems to be quite problematic for lots of Chinese ESL learners. In view of the situation, this study explores the use of hedges in academic abstracts by a group of undergraduate English majors in a university in Hainan Province. After analyzing data from a corpus and interviews, several problems have been found in the use of hedging devices of these students, and relevant implications have been drawn to the teaching of hedging in the context, which can help the students produce more effective academic writing and to achieve greater university success.
文摘This paper investigates the evolutionary change in the OF(OF) of hedges in different moves of EMRAs(EMRAs).Two corpora are established for longitudinal study,with Corpus A consisting of EMRAs from 1990-1994 and Corpus B from 2005-2009.Generally,no significant change has been found in overall use of hedges.However,apart from move 6,7 and 10,we have found some interesting changes in the other moves.On the one hand,the OF of hedges in move 1,move 3,move 4 and move 5 shows a significant increase.On the other hand,the OF of hedges in move 2,8,9 and move 11 show a significant decrease.
基金National Key Research and Development Program of China,No.2021YFC3201004。
文摘Global extreme hydrological events pose considerable challenges to the sustainable development of human society and river ecology.Land use/cover change(LUCC)is a visible manifestation of human activity and has caused substantial alterations in extreme hydrological regimes across rivers worldwide.The Jinsha River lies upstream of the Yangtze River and its hydrological variability has had profound socioeconomic and environmental effects.In this study,we developed Hydrological Simulation Program–FORTRAN(HSPF)and land-use simulation models of the entire watershed to simulate the effects of LUCC on hydrological extremes and quantify the inter-relationships among them.The main land-use changes between 1995 and 2015 were those associated with cropland,forest land,and grassland.Between 2015 and 2030,it is estimated that the coverage of forest land,grassland,construction land,and unused land will increase by 0.64%,0.18%,69.38%,and 45.08%,respectively,whereas that of cropland,water bodies,and snow-and ice-covered areas will decline by 8.02%,2.63%,and 0.89%,respectively.LUCC has had irregular effects on different hydrological regimes and has most severely altered stream flows.The responses of hydrological extremes to historical land-use change were characterized by spatial variation.Extreme low flows increased by 0.54%–0.59%whereas extreme high flows increased by 0%–0.08%at the lowest outlet.Responses to future land-use change will be amplified by a 0.72%–0.90%reduction in extreme low flows and a 0.08%–0.12%increase in extreme high flows.The hedging effect caused by irregular changes in tributary stream flow was found to alleviate the observed flow in mainstream rivers caused by land-use change.The extreme hydrological regimes were affected mainly by the net swap area transferred from ice and snow area to forest(NSAIF)and thereafter to cultivated land(NSAIC).Extreme low flows were found to be positively correlated with NSAIF and NSAIC,whereas extreme high flows were positively correlated with NSAIC and negatively correlated with NSAIF.
文摘Through the application of the VAR-AGARCH model to intra-day data for three cryptocurrencies(Bitcoin,Ethereum,and Litecoin),this study examines the return and volatility spillover between these cryptocurrencies during the pre-COVID-19 period and the COVID-19 period.We also estimate the optimal weights,hedge ratios,and hedging effectiveness during both sample periods.We find that the return spillovers vary across the two periods for the Bitcoin–Ethereum,Bitcoin–Litecoin,and Ethereum–Litecoin pairs.However,the volatility transmissions are found to be different during the two sample periods for the Bitcoin–Ethereum and Bitcoin–Litecoin pairs.The constant conditional correlations between all pairs of cryptocurrencies are observed to be higher during the COVID-19 period compared to the pre-COVID-19 period.Based on optimal weights,investors are advised to decrease their investments(a)in Bitcoin for the portfolios of Bitcoin/Ethereum and Bitcoin/Litecoin and(b)in Ethereum for the portfolios of Ethereum/Litecoin during the COVID-19 period.All hedge ratios are found to be higher during the COVID-19 period,implying a higher hedging cost compared to the pre-COVID-19 period.Last,the hedging effectiveness is higher during the COVID-19 period compared to the pre-COVID-19 period.Overall,these findings provide useful information to portfolio managers and policymakers regarding portfolio diversification,hedging,forecasting,and risk management.
文摘Extreme weather conditions occur at an increasing rate as evidenced by higher frequency of hurricanes and more extreme precipitation and temperature anomalies. Such extreme environmental conditions will have important implications for all living organisms through greater frequency of reproductive failure and reduced adult survival. We review examples of reproductive failure and reduced survival related to extreme weather conditions. Phenotypic plasticity may not be sufficient to allow adaptation to extreme weather for many animals. Theory predicts reduced reproductive effort as a response to increased stochasticity. We predict that patterns of natural selection will change towards truncation selection as environmental conditions become more extreme. Such changes in patterns of selection may facilitate adaptation to extreme events. However, effects of selection on reproductive effort are difficult to detect. We present a number of predictions for the effects of extreme weather conditions in need of empirical tests. Finally, we suggest a number of empirical reviews that could improve our ability to judge the effects of extreme environmental conditions on life history [Current Zoology 57 （3）： 375-389, 2011].
基金supported by the Russian Science Foundation(grant#14-24-00112)supported by NASA Terrestrial Ecology Program
文摘The phenomenon of tree waves （hedges and ribbons） formation within the alpine ecotone in Altai Mountains and its response to observed air temperature increase was considered. At the upper limit of tree growth Siberian pine （Pinus sibirica） forms hedges on windward slopes and ribbons on the leeward ones. Hedges were formed by prevailing winds and oriented along winds direction. Ribbons were formed by snow blowing and accumulating on the leeward slope and perpendicular to the prevailing winds, as well as to the elevation gradient. Hedges were always linked with microtopography features, whereas ribbons were not. Trees are migrating upward by waves and new ribbons and hedges are forming at or near tree line, whereas at lower elevations ribbons and hedges are being transformed into dosed forests. Time series of high-resolution satellite scenes （from 1968 to 2OLO） indicated an upslope shift in the position ribbons averaged 155＋26 m （or 3-7 m yr^-1） and crown closure increased （about 35%-90%）. The hedges advance was limited by poor regeneration establishment and was negligible. Regeneration within the ribbon zone was approximately 2.5 times （5060 vs 2120 ha^-1） higher then within the hedges zone. During the last four decades, Siberian pine in both hedges and ribbons strongly increased its growth increment, and recent tree growth rate for 50 year-old trees was about twice higher than those recorded for similarly-aged trees at the beginning of the 20^th century. Hedges and ribbons are phenomena that are widespread within the southern and northern Siberian Mountains.
