Analysis Of Grassland NPP And Phenology In Response To Climate Change In Xinjiang

The analysis of vegetation phenology and net primary productivity in response to climate change has been one of the core issues of global change.Grassland ecosystem in Xinjiang is extremely fragile and sensitive to climate and environmental changes.Hence,it has a unique importance to investigate grassland net primary productivity and its response to climate change in Xinjiang.In this paper,the grassland of Xinjiang is selected as the research object,base on the field observation data,meteorological data and remote sensing data,spatial distribution of grassland vegetation phenology and net primary productivity(NPP)within different years,and the change of grassland vegetation phenology in response to climate change,effects of human activities and climate change on grassland and the interaction between grassland net primary productivity and phenology are analyzed.The results show that,1)The implemented interpolation method in ANUSPLIN software is the most suitable method for interpolation of temperature and precipitation in Xinjiang.The values of MAE,MRE and RMSE for precipitation are 7.29 mm,2.17 and 12.14 mm,respectively.And the values of MAE,MRE and RMSE for temperature are 2.17?,0.45 and 2.15?,respectively.During the period from 2001 to 2014,the annual mean temperature and precipitation in Xinjiang are increased with the rising rates of 0.34? / 10a(P <0.01)and 3.1mm / 10a(P <0.01),respectively.2)The standard AMSR2 snow depth product overestimates(underestimates)snow depth when snow depth thinner(thicker)than 30 cm,with an overall increased estimation error(root mean squared error)as snow depth increase;Our improved algorithm shows better accuracy in snow depth estimation,especially,the improved algorithm based on brightness temperature data of AMSR2_A shows higher accuracy with the smaller mean error and root mean square error of 2.5 cm and 6.9 cm,respectively.3)The perennial mean vegetation phenology in Xinjiang grassland shows an obvious regional differences from low altitude to high altitude.With an increasing elevation gradient of 1000 m,the magnitude of spring phenology delays 13 d,autumn phenology advances 7d,the growing season gradually shortens 20 d.The spring phenology is within the days of 100~136,autumn phenology is within the days of 256~291 and the growing season is within the days of 122~190.The advancement of grassland spring phenology shows an increasing trend of 0.11d/a,while the autumn phenology shows delaying trend of 0.14d/a,and the corresponding growth season length is prolonged to 0.25d/a.4)Increased temperature in spring will lead to an early spring phenology,yet the increased temperature in January will not conducive to grassland spring green-up.And the precipitation in spring will not conducive to grassland spring green-up in higher altitude areas,such as alpine meadow,mountain meadow and temper meadow grassland.Increased precipitation in January will not conducive to grassland spring green-up in most regions of Xinjiang(except high altitude areas).Thick snow on March and April is not conducive to grassland spring green-up,and it has no obvious relation on other period.The delayed snow melt will cause the delay of grassland spring green-up.The increase in temperature from May to October is conducive to the delay of autumn phenology,however,the precipitation in September will accelerate the advancement of grassland autumn phenology.5)CASA model is an appropriate method for reflecting the grassland NPP_A in Xinjiang.The average NPP_A of Xinjiang grassland is 102.49 g C m-2yr-1 in 2001-2014.From the aspect of spatial distribution pattern of NPP_A in Xinjiang: the grassland NPP_A of the mountain area is higher than the basin area,and the grassland NPP_A in the northern Xinjiang is higher than the southern Xinjiang.6)The areas of increased grassland NPP_A account for 69.7% of the total grassland area,and the areas of decreased grassland NPP_A account for 30.3% of the total grassland area.The increased grassland NPP_A is attributed to human activities,and the decrease of NPP_A is caused almost equally by both the climate change and human activities.For the three different types of grassland,the area of grassland degradation and recovery are almost equal(50% each),climate and human activities play a similar role for those grasslands(about 25% each);desert grassland restoration area is 82.7%,which 70.1% of them are caused by human activities.However,the degraded area of meadow is 67.7%,and the grassland degradation area caused by human activities accounted for 49.5%.Among the 32.3% meadow restoration area,the climate change induces recovery area accounted for 26.1%.7)The relationship between grassland phenology and NPP_A in Xinjiang is more complicated,and the grassland green-up has a significant effect on the NPP_A in spring.There is a certain relationship between the Grassland autumn phenology and autumn NPP_A,but not obvious.Prolonged grassland growing season will increase the NPP_A in spring.