The Qinghai-Tibet Plateau(QTP)possesses the largest areas of permafrost in the midand low latitude regions on the earth and many large lakes in the permafrost area.Based on a comprehensive investigation around certain...The Qinghai-Tibet Plateau(QTP)possesses the largest areas of permafrost in the midand low latitude regions on the earth and many large lakes in the permafrost area.Based on a comprehensive investigation around certain typical lakes,this study found that although the presence of lakes formed different ranges of unfrozen zones in permafrost,the heating effect of lake water on surrounding permafrost is limited to a small extent.The temperature of permafrost around the lake is closely related to the distance to the lake and the ice content of the permafrost.Around lakes are ice-rich permafrost zones and permafrost temperature in this area is significantly lower than that far away from the lake,which indicates that the existence of lakes in the QTP has special effect on the permafrost distribution.Based on the monitoring results,this study presents the typical distribution pattern of the permafrost around large lakes and discusses the reasons for the distribution pattern.Due to the huge area of lakes and the significant impact of lakes on permafrost distribution,it is suggested to re-estimate the total permafrost area and underground ice storage in the QTP.展开更多
As significant evidence of permafrost degradation,thermokarst lakes play an important role in the permafrost regions by regulating hydrology,ecology,and biogeochemistry.In the Sources Area of the Yellow River(SAYR),pe...As significant evidence of permafrost degradation,thermokarst lakes play an important role in the permafrost regions by regulating hydrology,ecology,and biogeochemistry.In the Sources Area of the Yellow River(SAYR),permafrost degradation has accelerated since the 1980s,and numerous thermokarst lakes have been discovered.In this paper,we use Sentinel-2 images to extract thermokarst lake boundaries and perform a regional-scale study on their geometry across the permafrost region in the SAYR.We also explored the spatiotemporal variations and potential drivers from the perspectives of the permafrost,climate,terrain and vegetation conditions.The results showed that there were 47,518 thermokarst lakes in 2021 with a total area of 190.22×106 m^(2),with an average size of 4,003.3 m^(2).The 44,928 ponds(≤10,000 m^(2))predominated the whole lake number(94.1%)but contributed to a small portion of the total lake area(28.8%).With 2,590 features(5.9%),small-sized(10,000 to 100,000 m^(2))and large-sized lakes(>100,000 m^(2))constituted up to 71.2%of the total lake area.Thermokarst lakes developed more significantly in warm permafrost regions than in cold permafrost areas;74.1%of lakes with a total area of 119.6×106 m^(2)(62.9%),were distributed in warm permafrost regions.Most thermokarst lakes were likely to develop within the elevation range of 4,500~4,800 m,on flat terrain(slope<10°),on SE and S aspects and in alpine meadow areas.The thermokarst lakes in the study region experienced significant shrinkage between 1990 and 2021,characterized by obvious lake drainage;the lake numbers decreased by 5418(56.1%),with a decreasing area of 58.63×106 m^(2)(49.0%).This shrinkage of the thermokarst lake area was attributable mainly to the intensified degradation of rich-ice permafrost thawing arising from continued climate warming,despite the wetting climatic trend.展开更多
Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)an...Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)and soil moisture(SM)conditions on a land surface energy and water simulation in the permafrost region in the Tibetan Plateau(TP)using the Community Land Model version 5.0(CLM5.0).The results indicate that the default initial schemes for ST and SM in CLM5.0 were simplistic,and inaccurately represented the soil characteristics of permafrost in the TP which led to underestimating ST during the freezing period while overestimating ST and underestimating SLW during the thawing period at the XDT site.Applying the long-term spin-up method to obtain initial soil conditions has only led to limited improvement in simulating soil hydrothermal and surface energy fluxes.The modified initial soil schemes proposed in this study comprehensively incorporate the characteristics of permafrost,which coexists with soil liquid water(SLW),and soil ice(SI)when the ST is below freezing temperature,effectively enhancing the accuracy of the simulated soil hydrothermal and surface energy fluxes.Consequently,the modified initial soil schemes greatly improved upon the results achieved through the long-term spin-up method.Three modified initial soil schemes experiments resulted in a 64%,88%,and 77%reduction in the average mean bias error(MBE)of ST,and a 13%,21%,and 19%reduction in the average root-mean-square error(RMSE)of SLW compared to the default simulation results.Also,the average MBE of net radiation was reduced by 7%,22%,and 21%.展开更多
In this paper,an updated vegetation map of the permafrost zone in the Qinghai-Tibet Plateau(QTP) was delineated.The vegetation map model was extracted from vegetation sampling with remote sensing(RS) datasets by decis...In this paper,an updated vegetation map of the permafrost zone in the Qinghai-Tibet Plateau(QTP) was delineated.The vegetation map model was extracted from vegetation sampling with remote sensing(RS) datasets by decision tree method.The spatial resolution of the map is 1 km×1 km,and in it the alpine swamp meadow is firstly distinguished in the high-altitude areas.The results showed that the total vegetated area in the permafrost zone of the QTP is 1,201,751 km^2.In the vegetated region,50,260 km^2 is the areas of alpine swamp meadow,583,909 km^2 for alpine meadow,332,754 km^2 for alpine steppe,and 234,828 km^2 for alpine desert.This updated vegetation map in permafrost zone of QTP could provide more details about the distribution of alpine vegetation types for studying the vegetation mechanisms in the land surface processes of highaltitude areas.展开更多
The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tangg...The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes,the interaction between surface energy budget and freeze-thaw processes.The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations.Annual average net radiation(R_(n))for 2010 was 86.5 W m^(-2),with the largest being in July and smallest in November.Surface soil heat flux(G_(0))was positive during warm seasons but negative in cold seasons with annual average value of 2.7 W m^(-2).Variations in R_(n) and G_(0) were closely related to freeze-thaw processes.Sensible heat flux(H)was the main energy budget component during cold seasons,whereas latent heat flux(LE)dominated surface energy distribution in warm seasons.Freeze-thaw processes,snow cover,precipitation,and surface conditions were important influence factors for surface energy flux.Albedo was strongly dependent on soil moisture content and ground surface state,increasing significantly when land surface was covered with deep snow,and exhibited negative correlation with surface soil moisture content.Energy variation was significantly related to active layer thaw depth.Soil heat balance coefficient K was>1 during the investigation time period,indicating the permafrost in the Tanggula area tended to degrade.展开更多
Zonag, Kusai, Hedin Noel and Yanhu Lakes are independent inland lakes in the Hoh Xil region on the Qinghai-Tibet Plateau. In September2011, Zonag Lake burst after the water level had increased for many years. Floods f...Zonag, Kusai, Hedin Noel and Yanhu Lakes are independent inland lakes in the Hoh Xil region on the Qinghai-Tibet Plateau. In September2011, Zonag Lake burst after the water level had increased for many years. Floods flowed through Kusai and Hedin Noel Lakes into Yanhu Lake; since then, the four small endorheic catchments merged into one larger catchment. This hydrological process caused the rapid shrinkage of Zonag Lake and continuous expansion of Yanhu Lake. In this study,based on satellite images, meteorological data and field investigations, we examined the dynamic changes in the four lakes and analyzed the influencing factors. The results showed that before 2011, the trends in the four lake areas were similar and displayed several stages. The change in the area of Zonag Lake corresponded well to the change in annual precipitation(AP), but the magnitude of the change was less than that of a non-glacier-fed lake. Although increased precipitation was the dominant factor that caused Zonag Lake to expand, increased glacier melting and permafrost thawing due to climate warming also had significant effects. After the 2011 outburst of Zonag Lake, due to the increasing AP and accelerating glacier melting, the increases in water volume of the three lakes were absorbed by Yanhu Lake, and Yanhu Lake expanded considerably. According to the rapid growth rates in water level and lake area, Yanhu Lake is likely to burst in 1-2 years.展开更多
The surface energy budget over the Qinghai-Tibet Plateau(QTP)and the Arctic significantly influences the climate system with global consequences.The performances of 30 selected Coupled Model Intercomparison Project Ph...The surface energy budget over the Qinghai-Tibet Plateau(QTP)and the Arctic significantly influences the climate system with global consequences.The performances of 30 selected Coupled Model Intercomparison Project Phase 6(CMIP6)models were evaluated based on six sites in the QTP and Arctic.The simulation results for latent heat flux(LE)were more accurate in the QTP,where the correlation coefficient and root mean square error(RMSE)were 0.9 and 30 W m−2,respectively.The results for sensible heat flux(H)were more accurate in the Arctic,the correlation coefficient and RMSE were 0.8 and 24 W m−2,respectively.Furthermore,the multiple models mean results revealed that the surface energy flux had seasonal variation and regional differences over the QTP and the Arctic.In the QTP,H was the lowest in winter,increased in spring,and reached the maximum in summer.However,the transitional changes in spring and autumn were not apparent in the Arctic,mainly due to seasonal net radiation difference between the two places.LE was affected by precipitation and surface soil moisture content.This work is important for understanding land-atmosphere interactions and useful for improving the accuracy of land surface models simulations.展开更多
Most terrestrial models synchronously calculate net primary productivity(NPP)using the input climate variable,without the consideration of time-lag effects,which may increase the uncertainty of NPP simulation.Based on...Most terrestrial models synchronously calculate net primary productivity(NPP)using the input climate variable,without the consideration of time-lag effects,which may increase the uncertainty of NPP simulation.Based on Normalized Difference Vegetation Index(NDVI)and climate data,we used the time lag cross-correlation method to investigate the time-lag effects of temperature,precipitation,and solar radiation in different seasons on NDVI values.Then,we selected the Carnegie-Ames-Stanford approach(CASA)model to estimate the NPP of China from 2002 to 2017.The results showed that the response of vegetation growth to climate factors had an obvious lag effect,with the longest time lag in solar radiation and the shortest time lag in temperature.The time lag of vegetation to the climate variable showed great tempo-spatial heterogeneities among vegetation types,climate types,and vegetation growth periods.Based on the validation using eddy covariance data,the results showed that the simulation accuracy of the CASA model considering the time-lag effects was effectively improved.By considering the time-lag effects,the average total amount of NPP modeled by CASA during 2001-2017 in China was 3.977 PgC a^(−1),which is 11.37%higher than that of the original model.This study highlights the importance of considering the time lag for the simulation of vegetation growth,and provides a useful tool for the improvement of the vegetation productivity model.展开更多
Climate warming leads to vast changes in the land cover types and plant biomass in the northern high-latitude regions.The overall trend is of shrubland and tree lines moving northwards,while changes in different land ...Climate warming leads to vast changes in the land cover types and plant biomass in the northern high-latitude regions.The overall trend is of shrubland and tree lines moving northwards,while changes in different land cover types and vegetation growth in response to climate change are largely unknown.Here,we selected land areas with latitudes higher than 50°N as the study area.We compared the land cover type changes and explored relationships between the normalized difference vegetation index(NDVI)values of different land cover types,air temperature,and precipitation during 1982-2015 based on dynamic grid.The results indicated that forest and shrubland areas increased as a large area of grassland shifted to forest and shrubland.The snow/ice,tundra and grassland largely have decreased from 1982 to 2015.Although approximately 277.3×103 km2 of barren land(6.2%of the total barren land area in 1982)changed to tundra,the tundra area still decreased because some tundra shifted to forest and grassland.The NDVI values of tundra significantly increased,but the shrubland showed a decreasing trend.Temperature in the growing season(June to September)showed the largest positive correlation coefficients with the NDVI values of forest,tundra,grassland,and cropland.However,due to shrubification processes and plant mortality in shrubland areas,the shrubland NDVI showed negative relationship with annual temperature but positively correlated with monthly t.Taken together,although there is large room for improvement of the land cover type data accuracy,our results suggested that the land cover types in high-latitude regions changed significantly,while the NDVI values of the different land cover types showed different responses to climate change.展开更多
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences(XDA23060703)the National Natural Science Foundation of China(41671068)the State Key Laboratory of Cryospheric Science(SKLCS-ZZ-2023)。
文摘The Qinghai-Tibet Plateau(QTP)possesses the largest areas of permafrost in the midand low latitude regions on the earth and many large lakes in the permafrost area.Based on a comprehensive investigation around certain typical lakes,this study found that although the presence of lakes formed different ranges of unfrozen zones in permafrost,the heating effect of lake water on surrounding permafrost is limited to a small extent.The temperature of permafrost around the lake is closely related to the distance to the lake and the ice content of the permafrost.Around lakes are ice-rich permafrost zones and permafrost temperature in this area is significantly lower than that far away from the lake,which indicates that the existence of lakes in the QTP has special effect on the permafrost distribution.Based on the monitoring results,this study presents the typical distribution pattern of the permafrost around large lakes and discusses the reasons for the distribution pattern.Due to the huge area of lakes and the significant impact of lakes on permafrost distribution,it is suggested to re-estimate the total permafrost area and underground ice storage in the QTP.
基金supported by the Natural Science Foundation of Qinghai Province,China(No.2021-ZJ940Q)the Open Project of State Key Laboratory of Plateau Ecology and Agriculture,Qinghai University(No.2022-ZZ-02)。
文摘As significant evidence of permafrost degradation,thermokarst lakes play an important role in the permafrost regions by regulating hydrology,ecology,and biogeochemistry.In the Sources Area of the Yellow River(SAYR),permafrost degradation has accelerated since the 1980s,and numerous thermokarst lakes have been discovered.In this paper,we use Sentinel-2 images to extract thermokarst lake boundaries and perform a regional-scale study on their geometry across the permafrost region in the SAYR.We also explored the spatiotemporal variations and potential drivers from the perspectives of the permafrost,climate,terrain and vegetation conditions.The results showed that there were 47,518 thermokarst lakes in 2021 with a total area of 190.22×106 m^(2),with an average size of 4,003.3 m^(2).The 44,928 ponds(≤10,000 m^(2))predominated the whole lake number(94.1%)but contributed to a small portion of the total lake area(28.8%).With 2,590 features(5.9%),small-sized(10,000 to 100,000 m^(2))and large-sized lakes(>100,000 m^(2))constituted up to 71.2%of the total lake area.Thermokarst lakes developed more significantly in warm permafrost regions than in cold permafrost areas;74.1%of lakes with a total area of 119.6×106 m^(2)(62.9%),were distributed in warm permafrost regions.Most thermokarst lakes were likely to develop within the elevation range of 4,500~4,800 m,on flat terrain(slope<10°),on SE and S aspects and in alpine meadow areas.The thermokarst lakes in the study region experienced significant shrinkage between 1990 and 2021,characterized by obvious lake drainage;the lake numbers decreased by 5418(56.1%),with a decreasing area of 58.63×106 m^(2)(49.0%).This shrinkage of the thermokarst lake area was attributable mainly to the intensified degradation of rich-ice permafrost thawing arising from continued climate warming,despite the wetting climatic trend.
基金the National Natural Science Foundation of China(Grant No.U20A2081)West Light Foundation of the Chinese Academy of Sciences(Grant No.xbzg-zdsys-202102)the Second Tibetan Plateau Scientific Expedition and Research(STEP)Project(Grant No.2019QZKK0105).
文摘Accurate initial soil conditions play a crucial role in simulating soil hydrothermal and surface energy fluxes in land surface process modeling.This study emphasized the influence of the initial soil temperature(ST)and soil moisture(SM)conditions on a land surface energy and water simulation in the permafrost region in the Tibetan Plateau(TP)using the Community Land Model version 5.0(CLM5.0).The results indicate that the default initial schemes for ST and SM in CLM5.0 were simplistic,and inaccurately represented the soil characteristics of permafrost in the TP which led to underestimating ST during the freezing period while overestimating ST and underestimating SLW during the thawing period at the XDT site.Applying the long-term spin-up method to obtain initial soil conditions has only led to limited improvement in simulating soil hydrothermal and surface energy fluxes.The modified initial soil schemes proposed in this study comprehensively incorporate the characteristics of permafrost,which coexists with soil liquid water(SLW),and soil ice(SI)when the ST is below freezing temperature,effectively enhancing the accuracy of the simulated soil hydrothermal and surface energy fluxes.Consequently,the modified initial soil schemes greatly improved upon the results achieved through the long-term spin-up method.Three modified initial soil schemes experiments resulted in a 64%,88%,and 77%reduction in the average mean bias error(MBE)of ST,and a 13%,21%,and 19%reduction in the average root-mean-square error(RMSE)of SLW compared to the default simulation results.Also,the average MBE of net radiation was reduced by 7%,22%,and 21%.
基金supported by the National Natural Science Foundation of China (Grant No.41101055)the Hundred Talents Program of the Chinese Academy of Sciences granted to Tonghua Wu (Grant No.51Y251571)the “National Basic Research Program of China (973 Program)” (Grant No.2010CB951402)
文摘In this paper,an updated vegetation map of the permafrost zone in the Qinghai-Tibet Plateau(QTP) was delineated.The vegetation map model was extracted from vegetation sampling with remote sensing(RS) datasets by decision tree method.The spatial resolution of the map is 1 km×1 km,and in it the alpine swamp meadow is firstly distinguished in the high-altitude areas.The results showed that the total vegetated area in the permafrost zone of the QTP is 1,201,751 km^2.In the vegetated region,50,260 km^2 is the areas of alpine swamp meadow,583,909 km^2 for alpine meadow,332,754 km^2 for alpine steppe,and 234,828 km^2 for alpine desert.This updated vegetation map in permafrost zone of QTP could provide more details about the distribution of alpine vegetation types for studying the vegetation mechanisms in the land surface processes of highaltitude areas.
基金supported by the National Natural Science Foundation of China(Grant Nos.42071093,41671070)the National Key Research and Development Program of China(2020YFA0608500)+1 种基金the State Key Laboratory of Cryospheric Science(SKLCS-ZZ-2020)the National Natural Science Foundation of China(Grant Nos.41941015,42071093,41690142,41771076,41601078,and 41571069)。
文摘The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes,the interaction between surface energy budget and freeze-thaw processes.The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations.Annual average net radiation(R_(n))for 2010 was 86.5 W m^(-2),with the largest being in July and smallest in November.Surface soil heat flux(G_(0))was positive during warm seasons but negative in cold seasons with annual average value of 2.7 W m^(-2).Variations in R_(n) and G_(0) were closely related to freeze-thaw processes.Sensible heat flux(H)was the main energy budget component during cold seasons,whereas latent heat flux(LE)dominated surface energy distribution in warm seasons.Freeze-thaw processes,snow cover,precipitation,and surface conditions were important influence factors for surface energy flux.Albedo was strongly dependent on soil moisture content and ground surface state,increasing significantly when land surface was covered with deep snow,and exhibited negative correlation with surface soil moisture content.Energy variation was significantly related to active layer thaw depth.Soil heat balance coefficient K was>1 during the investigation time period,indicating the permafrost in the Tanggula area tended to degrade.
基金supported by the Science and Technology Project of Ecological Civilization Construction of Beautiful China (No. XDA23060703)the Hundred Talents Program of the Chinese Academy of Sciences (No. 51Y551831)+1 种基金the National Natural Science Foundation of China (No. 41671068, 41571075)the State Key Laboratory of Cryosphere Sciences (No. SKLCS-ZZ2019)
文摘Zonag, Kusai, Hedin Noel and Yanhu Lakes are independent inland lakes in the Hoh Xil region on the Qinghai-Tibet Plateau. In September2011, Zonag Lake burst after the water level had increased for many years. Floods flowed through Kusai and Hedin Noel Lakes into Yanhu Lake; since then, the four small endorheic catchments merged into one larger catchment. This hydrological process caused the rapid shrinkage of Zonag Lake and continuous expansion of Yanhu Lake. In this study,based on satellite images, meteorological data and field investigations, we examined the dynamic changes in the four lakes and analyzed the influencing factors. The results showed that before 2011, the trends in the four lake areas were similar and displayed several stages. The change in the area of Zonag Lake corresponded well to the change in annual precipitation(AP), but the magnitude of the change was less than that of a non-glacier-fed lake. Although increased precipitation was the dominant factor that caused Zonag Lake to expand, increased glacier melting and permafrost thawing due to climate warming also had significant effects. After the 2011 outburst of Zonag Lake, due to the increasing AP and accelerating glacier melting, the increases in water volume of the three lakes were absorbed by Yanhu Lake, and Yanhu Lake expanded considerably. According to the rapid growth rates in water level and lake area, Yanhu Lake is likely to burst in 1-2 years.
基金supported by the National Key Research and Development Program of China:[Grant Number 2020YFA0608502]the National Natural Science Foundation of China:[Grant Number 42071093]+2 种基金the Natural Science Foundation of Gansu Province:[Grant Number 22JR5RA054]the State Key Laboratory of Cryospheric Science:[Grant Number SKLCS-ZZ-2022]the National Natural Science Foundation of China:[Grant Number 41941015,41671070].
文摘The surface energy budget over the Qinghai-Tibet Plateau(QTP)and the Arctic significantly influences the climate system with global consequences.The performances of 30 selected Coupled Model Intercomparison Project Phase 6(CMIP6)models were evaluated based on six sites in the QTP and Arctic.The simulation results for latent heat flux(LE)were more accurate in the QTP,where the correlation coefficient and root mean square error(RMSE)were 0.9 and 30 W m−2,respectively.The results for sensible heat flux(H)were more accurate in the Arctic,the correlation coefficient and RMSE were 0.8 and 24 W m−2,respectively.Furthermore,the multiple models mean results revealed that the surface energy flux had seasonal variation and regional differences over the QTP and the Arctic.In the QTP,H was the lowest in winter,increased in spring,and reached the maximum in summer.However,the transitional changes in spring and autumn were not apparent in the Arctic,mainly due to seasonal net radiation difference between the two places.LE was affected by precipitation and surface soil moisture content.This work is important for understanding land-atmosphere interactions and useful for improving the accuracy of land surface models simulations.
基金National Natural Science Foundation of China,No.42161058The State Key Laboratory of Cryospheric Science,No.SKLCS-ZZ-2022The West Light Foundation of the Chinese Academy of Sciences。
文摘Most terrestrial models synchronously calculate net primary productivity(NPP)using the input climate variable,without the consideration of time-lag effects,which may increase the uncertainty of NPP simulation.Based on Normalized Difference Vegetation Index(NDVI)and climate data,we used the time lag cross-correlation method to investigate the time-lag effects of temperature,precipitation,and solar radiation in different seasons on NDVI values.Then,we selected the Carnegie-Ames-Stanford approach(CASA)model to estimate the NPP of China from 2002 to 2017.The results showed that the response of vegetation growth to climate factors had an obvious lag effect,with the longest time lag in solar radiation and the shortest time lag in temperature.The time lag of vegetation to the climate variable showed great tempo-spatial heterogeneities among vegetation types,climate types,and vegetation growth periods.Based on the validation using eddy covariance data,the results showed that the simulation accuracy of the CASA model considering the time-lag effects was effectively improved.By considering the time-lag effects,the average total amount of NPP modeled by CASA during 2001-2017 in China was 3.977 PgC a^(−1),which is 11.37%higher than that of the original model.This study highlights the importance of considering the time lag for the simulation of vegetation growth,and provides a useful tool for the improvement of the vegetation productivity model.
基金the National Key Research and Development Program of China(2020YFA0608501,2019YFA0607003)the National Natural Science Foundation of China(41941015,32061143032,41961144021)+3 种基金the State Key Laboratory of Cryospheric Science(SKLCS-ZZ-2021)the West Light Foundation of the Chinese Academy of SciencesZhang Wen-Xin acknowledged grants of Swedish Research Council FORMAS 2016-01201 and Swedish National Space Agency 209/19This work was also supported by Russian Fund for Basic Research grant N18-05-60080 and State topic N AAAA-A16-116032810095-6.
文摘Climate warming leads to vast changes in the land cover types and plant biomass in the northern high-latitude regions.The overall trend is of shrubland and tree lines moving northwards,while changes in different land cover types and vegetation growth in response to climate change are largely unknown.Here,we selected land areas with latitudes higher than 50°N as the study area.We compared the land cover type changes and explored relationships between the normalized difference vegetation index(NDVI)values of different land cover types,air temperature,and precipitation during 1982-2015 based on dynamic grid.The results indicated that forest and shrubland areas increased as a large area of grassland shifted to forest and shrubland.The snow/ice,tundra and grassland largely have decreased from 1982 to 2015.Although approximately 277.3×103 km2 of barren land(6.2%of the total barren land area in 1982)changed to tundra,the tundra area still decreased because some tundra shifted to forest and grassland.The NDVI values of tundra significantly increased,but the shrubland showed a decreasing trend.Temperature in the growing season(June to September)showed the largest positive correlation coefficients with the NDVI values of forest,tundra,grassland,and cropland.However,due to shrubification processes and plant mortality in shrubland areas,the shrubland NDVI showed negative relationship with annual temperature but positively correlated with monthly t.Taken together,although there is large room for improvement of the land cover type data accuracy,our results suggested that the land cover types in high-latitude regions changed significantly,while the NDVI values of the different land cover types showed different responses to climate change.
