Spatiotemporal Pattern Of Vegetation Productivity And Its Driving Forces In The Three Rivers Source Region

This paper takes the Three River Source Region in Qinghai province as the research object,and uses the improved Carnegie-Ames-Stanford approach（CASA）model and Zhou Guangsheng model,combined with the MOD13Q1 NDVI remote sensing vegetation index product and the meteorological data of the same period to estimate the actual vegetation net primary productivity（Net primary productivity,NPP）and climate productivity NPP of Three River Source Region from 2000 to 2018.This paper uses the difference between actual productivity（NPP_A）and climate productivity（NPP_P）to calculate the impact of human activities on NPP（NPP_H）,then analyzes the temporal and spatial characteristics of NPP_A,NPP_P and NPP_H and quantitatively analyzes the contribution characteristics of human activities and climate change for actual NPP changes.Finally,the meteorological data is used to analyze the spatio-temporal characteristics of temperature and precipitation and the correlation between actual NPP and temperature and precipitation to discuss the driving force of temperature and precipitation on vegetation productivity changes.The main conclusions are as follows:（1）From 2000 to 2018,the annual average NPP_A of the Three River Source Region shows a decreasing trend from southeast to northwest on a spatial scale,and the annual average NPP_A is 169.12g C·m^-2·yr^-1.The annual average NPP_A of the Three River Source Region fluctuated increased at a rate of 0.8459 g C·m^-2·yr^-1 per year.The annual average NPP_A of nature reserves is in the order of Experimental Area>Core Area>Buffer Area.The annual average NPP_A of the 18 nature reserves is in the order of Jiangxi>Mako River>Maixiu>Duoke River>Nianbaoyuze>Zhongtiejungong>Baizha>Dongzhong>Tongtianhe>Angsai>Guozongmucha>Animaqing>Dangqu>Xingxinghai>Zhaling Lake Eling Lake>Suga-Qumahe>Yoguzonglie>Geladandong;the change rate of NPP_A in the 18 protected areas are all greater than 0,and the annual average NPP_A shows an increasing trend.（2）From 2000 to 2018,the annual average NPP_P in the Three Rivers Source Region shows a decreasing trend from southeast to northwest on spatially and obviously spatial differentiation.The annual average value of NPP_P is 284.00g C·m^-2·yr^-1,and NPP_P is annual increase at a rate of 0.5571 g C·m^-2·yr^-1.The annual average NPP_P of nature reserves is in the order of Experimental Area>Core Area>Buffer Area.The annual average NPP_P of the 18 nature reserves is in the order of Ma Ko River>Jiangxi>Duoke River>Nianbaoyuze>Baizha>Angsai>Zhongtiejungong>Maixi u>Dongzhong>Tongtianhe>Guozongmucha>Animaqing>Dangqu>Xingxinghai>Geladandong>Zhaling Lake Eling Lake>Suojia-Quma River>Yueguzonglie;Except for Geladandong,the NPP_P of the other 17 protection zones show an increasing trend.（3）From 2000 to 2018,the annual average NPP_Hof the Three Rivers Source Region is 152.77g C·m^-2·yr^-1,and the fluctuations decreased at a rate of-0.2888 g C·m^-2·yr^-1each year,indicating that the negative effects of human activities on vegetation productivity are gradually decreasing;the change rate of NPP_Hin the whole area is negative,and the area where human activities promote NPP growth accounts for57.00%.The annual average NPP_Hof the three major functional areas of the nature reserves is in the order of Experimental Area>Buffer Area>Core Area.The annual average NPP_Hof the 18 nature reserves is in the order of Geladandong>Xingxinghai>Animaqing>Guozongmucha>Dangqu>Angsai>Zhaling Lake Eling Lake>Suojia-Quma River>Baizha>Yueguzonglie>Nianbaoyuze>Tongtianhe>Duoke River>Zhongtiejungo ng>Dongzhong>Mako River>Maixiu>Jiangxi;5 of the 18 protected regions have a negative NPP_Hchange rate,indicating that human activities have promoted the growth of NPP,and the other 13 protected regions have a positive NPP_Hchange rate,indicating that human activities have a negative effect on NPP.（4）From 2000 to 2018,the contribution rate of climate change to NPP_Achanges in most areas of the Three Rivers Source Region is greater than that of human activities.The average contribution rate of climate change was 55.55%,and the average contribution rate of human activities is 44.45%.The leading factor for the increase of NPP_Ain the three major functional areas of nature reserves is climate change,and the leading factor for the decrease of NPP_Ain each functional area is human activities;among the 18 protected areas,the leading factor for the increase of NPP_Ain 13 protected areas is climate change and the proportion is between 38%and 84%.Only in the Geladandong protection zone,the leading factor for the reduction of NPP_Ais climate change,and in the other 17 protection zones,the leading factor for the reduction of NPP_Ais human activities.（5）In 19 years,the annual average temperature in the Three Rivers Source Region shows a fluctuated increasing trend with an annual growth rate of 0.0449℃/yr,while the annual rainfall fluctuated greatly,and the change trend is not significant;the average annual temperature and the annual cumulative rainfall space distribution of the Three Rivers Source Region generally shows a decreasing trend from southeast to northwest.The average partial correlation coefficient between NPP_Aand temperature is 0.2548,the positive correlation area accounts for 83.29%of the study area,and the negative correlation area accounts for 16.71%;the average value of the partial correlation coefficient between NPP_Aand precipitation is 0.1078,the positive correlation area accounted for 59.29 of the study areas,and the negative correlation area accounts for40.71%.The average value of the multiple correlation coefficients between NPP_Aand temperature and precipitation is 0.4536,14.86%of the area has an extremely significant correlation（P≤0.01）,15.99%of the area has a significant correlation（0.01<P≤0.05）,and 69.14%of the area is no significant correlation（P>0.05）.（6）In general,there are obvious regional differences in the effects of temperature and precipitation on vegetation productivity in the Three Rivers Source Region.The change of vegetation productivity in the source area of the Yangtze River in the west is mainly controlled by temperature,and the change of vegetation productivity in the northeastern area of the Yellow River source area is controlled by rainfall,while the vegetation productivity in most areas such as the Lancang River source area and the southeast of the Yellow River source are not controlled by temperature and precipitation,but are related to human activities and other factors.