Dynamics Of Grassland Net Primary Productivity And Driving Factor Analysis In The Northern Six Provinces In China

Grasslands are one of the largest ecosystems in China,accounting for nearly 40%of total land covers.Grasslands have many ecological functions,such as wind prevention,sand fixation,soil and water conservation,climate adjustment and air clearness.Grasslands also play an important role in maintaining ecological banlance,regional economy and humanity history.As grasslands are mainly distributed in the arid and semi-arid regions,the northern six provinces,namely Inner Mongolia,Shaanxi,Shanxi,Gansu,Ningxia,and Xinjiang,have rich grassland resources,which are the important production base of animal husbandry,and provide material guarantee for the herdsman.The northern grasslands are also important ecological barrier.However,with the global climate change and intensification of human activities,structures and functions of grasslands have been changing accordingly.Therefore,assessing the effects of climate change on the spatiotemporal distribution and dynamics of grasslands do not only help to understand the effects of climate change on grassland structures and functions,but provide guidelines for grassland ecosystems protection and operating management.In this research,the spatiotemporal dynamics of annual and monthly grassland coverage and net primary productivity(NPP)in the northern six provinces during the rencent 30 years(1986-2015)were estimated based on climate data,remote sensing data,and filed observation data using model simulations.The correlations analysis were also conducted to revela how the climate factors have controlled grassland NPP.Results showed that:(1)the mean annual temperature(MAT)and mean annual precipitation(MAP)of the grassland area was 3.76?and 289.93 mm,respectively.The spatial pattern of MAT increased from east to west,and the MAT changed from semi-humid to arid from east to west.In the recent 30 years,MAT increased at a rate of 0.06?/a,and 95.87%of regions showed an increasing trend.These regions were mainly distributed in the south of Xinjiang province,southwestern Tianshan area,and most parts of Qinghai.MAP increased in 87.07%of regions with a increasing rate of 1.75 mm/a,which were mainly located in the south of Xinjiang Kunlun mountains,the central of Qinghai province and the south of Shanxi.(2)The monthly changes of temperature and precipitation showed a parabolic trend,which gradually increase from January to August,and reached the peak in July and August,and decreased from September to December,reflecting the synchronous of rain and heat over the same period.Temporally,regions showing rising monthly temperature and precipitation was larger than that showing the declining trend except for temperature in December and precipitation in March.The climate in the northern grassland regions was changing from warm-dry to warm-wet.(3)In the recent 30 years,grassland coverage and NPP showed similar spatial pattern with higher in the northwest and lower in the southeast.The average NPP of grasslands was 170.63gC/m~2�a,The annual average NPP of each grassland followed an order:slope grassland(324.76 g C/m~2�a)>medow(209.79 g C/m~2�a)>plain grassland(169.59 g C/m~2�a)>alpine subalpine meadow(36.22 g C/m~2�a)>desert grassland(118.27 g C/m~2�a).The average grassland coverage was 40.02%,and followed an order of slope grassland(50.88%)>medow(50.30%)>plain grassland(43.07%)>alpine subalpine meadow(40.78%)>desert grassland(25.59%).Areas of grassland coverage change correspondingly with that of NPP changes,showing the grassland coverage can well indicate the change of grassland NPP.(4)At the annual scale,the grassland coverage and vegetation NPP both showed an overall upward trend in the whole study period.The annual average growth rate of grassland coverage was 0.07,and the average growth rate of grassland NPP was 1.58gC/m~2�a.the highest increasing trend of NPP was in the slope grassland(5.42 gC/m~2�a),which was followed by meadow(2.45 gC/m~2�a),plain grassland(1.50gC/m~2�a),desert grassland(1.01 gC/m~2�a).The alpine subalpine grassland showed the lowest increase with 0.71 gC/m~2�a.(5)At the monthly scale,change of grassland coverage showed a parabolic trend as that of climate factors,which increased from January to August,peaked in July and August,and then decreased gradually.Expect in January,grassland coverage in all the other months showed an increasing trend in the recent 30 years,which increased slowly in most regions.Regions showing higher increasing rate was observed in the southeast of Qinghai and south of Shannxi.(6)The correlation analysis showed that,51.22%of regions showed positive correlation between grassland NPP and temperature,among which 12%and 19%of regions showed extremely significant and significant correlations,respetively.Regions showing extremely significant and significant negative correlations between grassland NPP and temperature account for 6.55%and 11.99%,respetively.By contrst,77.58%of regions showed positive correlation between grassland NPP and precipitation,among which 14.27%and 18.31%of regions showed extremely significant and significant correlations,respetively.Regions showing extremely significant and significant negative correlations between grassland NPP and temperature account for 5.78%and4.84%,Regions showing extremely significant and significant negative correlations between grassland NPP and precipitaiton account for 1.67%and 1.03%,respetively.At the monthly cale,the correlation coefficients between grassland NPP and temperature and precipitation were 0.83 and 0.87,respectively,indicating that grassland growth is mainly affected by dynamic of precipitation.(7)NPP of each grassland type responded diffenretly to temperature and precipitation,showing lagged phenomenon.The lagged effects of temperature and precipitation on NPP of different grassland types were generally delayed for one month.Responses of of grassland NPP to temperature and precipitation in the three preceding month were the weakest.The highest correlation was in sloping grassland with 0.92,while the lowest correlation was in desert grassland with 0.71 between NPP and temperature in the three preceding month.Meanwhile,the highest correlation was in alpine subalpine with 0.94,and the lowest correlation was in desert grassland with 0.78between NPP and precipitation in the three preceding months...