Study On Spatial-temporal Changes Of Fraction Of Absorbed Photosynthetically Active Radiation In Qinling-Daba Mountains

The Fraction of Absorbed Photosynthetically Active Radiation?FPAR?is a basic physiological variable describing the process of energy exchange between vegetation canopy and atmosphere,reflects the part of photosynthetically active radiation that absorbed by vegetation.It's not only an important input parameter for water cycle,carbon cycle,ecological model and crop growth model,but also one of the key climate parameters for Global Terrestrial Observing System?GTOS?and Global Climate Observation System?GCOS?to characterize climate change.Therefore,the quantitative monitoring of regional or global vegetation FPAR is of great significance to the study of climate change and terrestrial ecosystem.As a transitional zone between North and south climate in China,Qinling-Daba Mountains is an important geo-ecological boundary in China.It's not only of great ecological and environmental significance,but also of rich vegetation resources with its unique geographical location,complex natural environment and long civilized history.Under the background of current climate change and rapid economic development,this paper takes Qinling-Daba Mountains as a test area to study the spatial and temporal characteristics of vegetation FPAR,and to explore the impact factors of vegetation FPAR,to provide a reference for regional vegetation change research and environmental protection.Based on the remote sensing data of MODIS FPAR,LAI,land cover,and DEM,population/GDP kilometer grid data,meteorological data from 2006 to 2015,linear regression analysis was used to study pixel scale spatial-temporal changes characteristic of FPAR in Qinling-Daba Mountains with the help of ArcGIS,ENVI,ANUSPLIN,MRT and R software.The main impact factors of vegetation FPAR was chosen based on GeoDetector and random forest regression model.Finally,the relationship between the impact factors and vegetation FPAR was studied quantitatively by correlation analysis and spatial autocorrelation.The main conclusions are as follows:?1?From 2006 to 2015,High values of FPAR took on in the center and southeast,and low values in the northwest and northeast of study area,and the spatial changes of FPAR increased in the central region and decreased in the surrounding area in Qinling-Daba Mountains.During last decade,the FPAR of vegetation,overall,appeared a significant increasing trend with a rate being 4.8%·10 a^-1?P<0.05?and the most outstanding vegetation was crop?P<0.05?in different types in the study area.Among four seasons,the FPAR increased significantly in winter?P<0.05?,and broadleaved forest also increased significantly in winter,and broadleaf forest had the highest proportion of FPAR increasing area,while needleleaf forest had the lowest proportion of FPAR increasing area.?2?The annual average FPAR of vegetation in the seven zones showed an increasing trend,and Hanjiang River,Qinling Mountains and Southwest Henan province increased significantly?P<0.05?.Among of seven zones,the area proportion of FPAR increased significantly in Southwest Henan province was the highest,followed by Qinling Mountains,Haidong-Gannan and Minshan Mountain-Qionglai areas showed a decreasing trend of more than 25%,and environmental protection needed to be strengthened.Moreover,the vegetation FPAR time series curve of seven zones has the rule that the coefficient of skewness decreases gradually from west to east.?3?In the process of exploring the main impact factors based on GeoDetector and random forest regression model,the interpretation of random forest regression model was 95%.Compared with GeoDetector,finally chosen that leaf area index,annual average temperature,altitude,annual precipitation,vegetation type,population density,GDP,distance from city and construction land and vegetation coverage as the main factors influencing the spatial-temporal changes of vegetation FPAR in Qinling-Daba Mountains.?4?The relationship between natural factors and vegetation FPAR is as follows:Firstly,FPAR decreased with altitude and presented obvious vertical zonality along mountain;Secondly,annually,the correlation between vegetation FPAR and climate factors showed that vegetation FPAR was more correlated with annual average temperature than that with annual precipitation on the all vegetation,and needleleaf forest and broadleaf forest FPAR had a higher correlation with annual average temperature than other types,nevertheless,grasslands and crops were influenced by annual precipitation more than average temperature;Finally,FPAR was positively correlated with leaf area index and vegetation coverage in Qinling-Daba Mountains,the increase of leaf area index means vegetation have more photosynthetic tissue area for photosynthesis,and the change of vegetation coverage is also an important reason for the change of vegetation FPAR.?5?The impact of socio-economic factors on vegetation FPAR is as follows:with the increase of population density,GDP,with the decrease of distance from urban and construction land,the overall FPAR of vegetation shows a downward trend.The socio-economic factors in the early period?2006-2010?have a greater impact on vegetation FPAR,and the later period?2010-2015?has weakened.