Changes in different land cover areas and NDVI values in northern latitudes from 1982 to 2015

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.

Export of nutrients and suspended solids from major Arctic rivers and their response to permafrost degradation

The rapid warming of the Arctic has led to permafrost degradation,accelerating the transport of terrestrial materials by rivers.The quan-titative assessment of riverine nutrients and total suspended solids(TSS)flux is important to clarify the land-ocean connections in the Arctic.However,much is unknown about the estimates of these components from direct measurements in the Arctic rivers and the response of the components to permafrost degradation.Here,we report the results from the Arctic Great Rivers Observatory(Arctic-GRO)for the six major Arctic rivers(Yenisey,Lena,Ob',Mackenzie,Yukon,and Kolyma)to investigate the riverine exports of TSS,total dissolved nitrogen(TDN),nitrate(NO3-),bicarbonate(HCO3-),total dissolved phosphorus(TDP),and phosphate(PO43-).The results showed that from 2004 to 2017,the annual TSS,TDN,and NO3-exports to the Arctic Ocean were approximately 106,026 Gg,692 Gg,and 130 Gg,respectively,and the HCO3-,TDP,and PO43-exports were approximately 79,092 Gg,32 Gg,and 18 Gg,respectively.There were remarkable variations in component concentrations and fluxes between seasons.More than 80%of the TDN,TDP,PO43,and TSS exports mainly occurred in spring and summer,and a high HCO3-flux was recorded in summer,while a high NO3-flux in some rivers occurred in winter.The active layer thickness was significantly positively correlated with the annual TDN,NO3-,and HCO3-exports.In addition,the HCO3-flux of the six Arctic rivers increased by 247 Gg per year during 2004-2017.The positive relationship between the active layer thickness and river discharge indicates that permafrost degradation accelerated rivefine carbonate,nitrogen,and phosphorus exports.This study demonstrates that riverine exports play an important role both in the Arctic terrestrial and marine ecosystems,and permafrost degradation will likely increase the riverine material exports to the ocean.

Changes in net ecosystem exchange of CO_(2)in Arctic and their relationships with climate change during 2002-2017

Arctic wanning leads to permafrost degradation,which can increase ecosystem respiration and release more greenhouse gas into the at-mosphere.Meanwhile,climate warming also promotes the plant growth and increases carbon assimilation.Presently,it is largely unknown about the carbon budget and their responses to climate change in the Arctic regions.In this study,to investigate the seasonal and annual net ecosystem carbon exchange(NEE),we collected 71 observation stations for net ecosystem exchange(NEE)of CO_(2)in the high latitude permafrost regions during 2002-2017.The results showed that the annual NEE was-8.2±4.1 g CO_(2)m^(-2)d^(-1)for forest,-3.3±2.6 g CO_(2)m^(-2)d^(-1)for shrub,-4.8±4.1 g CO_(2)m^(-2)d^(-1)for grassland,-3.6±3.0 g CO_(2)m^(-2)d^(-1)for wetland and 0.02±0.62 g CO_(2)m^(-2)d^(-1)for tundra,respectively.From 2002 to 2017,the CO_(2)emissions of grassland(carbon source)showed a decreasing trend,and the CO_(2)assimilation of shrub and forest(carbon sink)has been increased.The wetland and tundra are shifting from carbon sources to sinks.There were great variations in temperature sen-sitivities(Q10)of NEE in different seasons,with larger values in winter and lower values in summer.These findings indicate that the Arctic terrestrial ecosystem presently acts as a carbon sink,while there is a possibility that future warming,especially the warming in winter,may decrease the carbon sink of the Arctic terrestrial ecosystem.