| This study uses the scale data of 2000-2015 NDVI,air temperature,precipitation,solar radiation,vegetation type,and get each month of the study period in Chongqing city vegetation NPP model inversion based on CASA(NPP),by linear regression and correlation analysis,in response to the characteristics and variation of NPP in Chongqing city NPP vegetation and climatic factors between the detailed research and analysis.The conclusions are as follows:1)During the past 16 years,the annual variation of NPP in Chongqing was mainly reflected in the slow rising trend,the average annual increase rate of NPP was 0.7022/a.The peak value appears in 2003,and 2008,Valley value in 2001,2006,2009.Therefore,the inter annual variation of vegetation NPP in 16 years can be divided into the following 5 stages,includes two rising stages,were 2001-2003 and 2006-2008.Followed by two stages of decline,were 2000-2001 and 2003-2006,2008-2015 is a stable period of changing trend.There was no obvious extreme weather in Chongqing during the peak year,the climate condition is good,the temperature,rainfall and solar radiation and other climatic factors are evenly distributed,which is suitable for the growth of vegetation,so that the NPP value of these years reached a relatively high level.And the valley value appeared the year was mainly affected by summer drought weather,so vegetation photosynthesis efficiency is reduced,thus reducing the NPP value,such as 2001 and 2006.Seasonal variation of vegetation NPP in 2000-2015,the seasonal average NPP from high to low was summer(228.09gC/m2)> spring(133.72gC/m2)> autumn(117.33gC/m2)> winter(51.94gC/m2).At the same time,the NPP value in winter and summer showed a rising trend,and the rising rate was 1.01gC/m2 and 0.0078gC/m2,and the spring autumn winter three seasons showed decreased,the decrease rate were 0.216gC/m2,0.168gC/m2.NPP reached the highest value in summer,accounting for 43% of the annual NPP value.Mainly in the summer light,water,heat the same period,to achieve the best growth conditions of vegetation,so that the maximum productivity of vegetation in the summer.Spring rainfall is abundant,and the light condition is good,its vegetation NPP is second only to summer.In autumn and winter,the area of Chongqing city has a lot of cloud and fog,and the light condition is poor,the NPP value of the vegetation is low,especially in winter,the NPP value is only 1/3 in spring,1/4 in summer.During the period of 2000-2015,the monthly variation of vegetation NPP was a parabola change,and the main productivity of the vegetation concentrated in May to the year of NPP,and the NPP amount was about 65% of the total NPP in the year of.2)The NPP values of different types of vegetation were different,from high to low,respectively,broad leaved forest(804.35gC/m2)> conifer mixed forest(719.07gC/m2)> coniferous forest(650.26gC/m2)> shrub(544.37gC/m2)> grassland(362.44gC/m2)> arable land(342.58gC/m2).In addition to the decrease or increase of NPP value caused by climate change,the different vegetation types are different because of their different adaptability to the environment in NPP,the leaf area of different vegetation types is different,it will affect the efficiency of absorbing solar radiation,and thus affect the NPP value of this type of vegetation.3)The spatial and temporal variation characteristics of NPP in Chongqing,NPP in Northeast Chongqing is generally high,in Chengkou,Wuxi vegetation NPP is the highest,because of the mountainous area in the northeast of Chongqing,the urban development area is small,the forest cover density is wide,the human interference factors are relatively few,and the annual rainfall is abundant.Secondly,in the southeast of Chongqing,NPP has remained at a high level,second only to the northeast area of Chongqing,the Nanchuan and Qianjiang counties were higher NPP.The main city and the surrounding areas of the overall NPP is low,due to the region's industrial and agricultural development,urban construction area is larger,the greater the impact of human activities.In the analysis of NPP in different seasons,the spatial distribution of NPP in different seasons in Chongqing is consistent with the spatial distribution of annual NPP value,it is mainly due to the overall scope of Chongqing is small,in the range of latitude and longitude,climate change has little impact on the local climate of Chongqing.4)The correlation between NPP and climatic factors(temperature,precipitation,radiation),in 2000-2015,the annual average temperature,annual rainfall and annual total solar radiation showed fluctuation,the annual average temperature and annual precipitation showed a rising trend,and the annual solar radiation is decreased,but the overall range is smaller.The average annual temperature is kept in the range of 17-18 ?,the annual precipitation is maintained in the range of 1100-1200 mm,and the annual total solar radiation is kept up and down in 4000MJ/m2.The extreme values of each climatic factor mainly appear in the typical years of extreme weather,such as 2001,2006.The correlation between vegetation NPP and temperature,precipitation and radiation in Chongqing is from large to small: Correlation(NPP-precipitation)> Correlation(NPPradiation)> correlation(NPP-temperature).From the perspective of inter annual variation,there was a positive correlation between NPP and precipitation and solar radiation in Chongqing,while NPP was negatively correlated with temperature.The correlation coefficient NPP and the average annual temperature,annual precipitation,annual total solar radiation were 0.2,0.5,0.05,so the rainfall influence on vegetation NPP in large,in the process of vegetation growth,adequate water is particularly important,and the temperature effect on the growth of vegetation is relatively small.5)Analysis of correlation between vegetation NPP and climatic factors from seasonal scale,In spring,NPP is related to the temperature and precipitation had positive relationship,and solar radiation values show a negative correlation between the temperature in spring,began to pick up,promote the growth of vegetation,and with the increase of rainfall,rain heat over the same period,so that the rapid rise of vegetation NPP;In summer,the average temperature is above 25?,the individual year reaches to the temperature of 30?,the summer rainfall is abundant,the rainfall is in the range of 243.66-655.13 mm,the average solar radiation is 1467.51MJ/m2.NPP and temperature,the solar radiation value was negatively correlated,the smaller the correlation between NPP and temperature between the temperature has little influence on the vegetation growth status in summer,but the solar radiation value between NPP and larger,and were a significant positive correlation with precipitation,the correlation between vegetation NPP and rainfall,solar radiation and temperature decreasing;In autumn,the rainfall is between 193.99-408.25 mm,the average temperature is 18.37 ?,the temperature change is small,the average solar radiation is about 811.99 MJ/m2.autumn sunshine hours decreased,and the temperature and solar radiation is essential for the growth of vegetation and climatic conditions,thus affecting the vegetation NPP,Therefore,NPP is positively correlated with temperature and solar radiation,due to excessive rain in autumn,when the excess water will affect the growth of vegetation state,there was a negative correlation between NPP and precipitation in autumn;In winter,the average temperature is 8.09 ?,the range of fluctuation is small,and the temperature is relatively high in winter.Affected by the fog in winter in Chongqing City,sunshine time is very small,the average winter solar radiation amount was 619.75 MJ/m2,the rainfall in the 37.97-86.84 mm,so,there is a positive correlation between NPP and air temperature and solar radiation.6)This study used CASA model to estimate the NPP value of simple correlation analysis and the NPP data of MOD17A3 products,NPP products and the results of this study have high correlation,it proves that the results of this study have higher reliability. |
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