Phenological Change Of Grassland And Its Response To Climatic Factors In Recent 30 Years Of China

As an indicator of global climate change,vegetation phenology plays an important role in regional water conservation,biodiversity conservation and ecosystem balance.China is rich in grassland resources,with natural grassland of about 400×10~6 hm~2.Grassland ecosystem is very sensitive to climate change.Therefore,studying the phenological characteristics of grassland in China in the past 30 years is helpful to understand the dynamic changes of grassland ecosystem,and which can also be used as an evaluation index to study the sustainable development of grassland,and provides reference for the protection and rational utilization of grassland resources in China.Based on the data of GIMMS NDVI3g from 1986 to 2015 and the data of climate observation stations in the same period,the time series of NDVI was reconstructed by Savitzky-Golay filtering method.The phenological parameters of grassland in China were extracted by dynamic threshold method from the beginning of growing season(SOS),the end of growing season(EOS),and the length of growing season(LOS).Taking 1998 as the time dividing point,the temporal evolution trend and spatial differentiation of the three phenological characteristics extracted at different time scales were analyzed.Based on the method of sen+M-K,on the basis of defining the spatial and temporal pattern of temperature and precipitation in China,the response of phenology and climatic elements of grassland vegetation was evaluated and discussed based on partial correlation analysis method.The main conclusions are as follows:(1)In the past 30 years,the average annual growth period of grassland in China began from the late April to late may and ends from late September to early October.The length of the growing season is concentrated between 130-170 days,In addition to grasslands in northern Xinjiang and central Inner Mongolia,LOS showed a gradually shortened change from southeast to northwest.The six grassland types are arranged according to the length of the phenological growth season,that is desert grassland<alpine and sub-alpine plain grassland<alpine and sub_alpine meadow<plain grassland<meadow<slope grassland.(2)From 1986 to 2015,the phenology of grassland vegetation in China has fluctuated.Taking 1998 as the demarcation point,and the trend of SOS changed from general advance to postponement,EOS trend changed from general delay to advance,and LOS trend changed from generally increasing to shortening.Among them,the phases of phenological changes of alpine and sub_alpine meadow,alpine and sub-alpine plain grassland,slope grassland and desert grassland are more obvious.The trend of grassland LOS in China is dominated by grassland SOS.(3)From 1986 to 2015,the annual mean temperature of grassland in China increased significantly(P<0.001),which was 0.0497 C/a,especially in spring and summer.Annual precipitation showed no obvious increasing trend(P>0.1),but spring precipitation showed a more obvious increasing trend.Influenced by the strong El Nio in 1998,the annual mean temperature and annual precipitation of grassland in China reached the highest value in 30 years in 1998,and the abrupt change of temperature and precipitation in 1998 was obtained through the analysis of the abrupt change point of climate,which accorded with the characteristics of grassland phenological change,and further illustrated the response of climate change to phenological change.(4)There are seasonal differences in the relationship between grassland and climatic factors in China,overall,temperature was the main driving factor affecting SOS in grassland,and the influence range was the largest in the month of returning green,which showed a weakening trend with the advance of pre-season time;Precipitation is the dominant factor affecting grassland EOS,and which also is the strongest in the first month before yellow wilt.When the pre-season time is long,the influence range of temperature gradually increases.The grassland type with fragile ecosystem is more sensitive to environmental change,and its phenological change responds significantly to climate change compared with the grassland type with stable ecosystem.