Temporal And Spatial Distribution Characteristics And Influencing Factors Of Vegetation Net Primary Productivity In The Northern Slope Of Tianshan Mountains

The northern slope of Tianshan Mountains is an important agricultural and animal husbandry development base in Xinjiang.How to realize the coordination and sustainable development of nature,economy and society in the region has become a realistic issue for urgent discussion and research.Net primary productivity?NPP?refers to the difference between the total amount of organic matter fixed by photosynthesis of green plants and the maintenance of autotrophic respiration.It is the result of the interaction between the physiological characteristics of the vegetation itself and the external environment.It is also an important indicator for assessing global or regional ecological security.Therefore,accurately grasping the temporal and spatial evolution trend of NPP is of great theoretical and practical significance for understanding the mechanism of NPP in terrestrial vegetation caused by climate change,topography and land use change,and for scientifically evaluating the process of ecosystem regulation.Based on MODIS remote sensing data,meteorological data,topographic data and land use data from2004 to 2016,this paper uses the CASA model to estimate the vegetation NPP on the northern slope of Tianshan Mountains,and analyses the spatial and temporal distribution characteristics of NPP and its relationship with climatic factors,topographic factors and land use and cover change.The main conclusions of this study are as follows:?1?From 2004 to 2016,the NPP of the northern slope of Tianshan Mountains showed an upward trend,with an annual average of 131.94 gC·m^-2·a^-1 and an interannual slope of 3.20 gC·m^-2·a^-1.The spatial distribution of NPP on the northern slope of Tianshan Mountains shows a trend of high in the West and low in the east.The annual average value of NPP in the western region is the highest,while that in the eastern region is the lowest,the central and western regions are the main NPP supply areas on the northern slope of Tianshan Mountains;The annual average value of NPP in Shihezi City is the highest,while that of Yiwu County and Barikun County is the lowest.The variation of NPP in different seasons was obvious.The NPP was the highest in June and July,and the lowest in January.The NPP was the highest in summer and the lowest in winter.The variation of NPP in different vegetation types was obvious,which are as follows:forest land(554.72 gC·m^-2·a^-1)>cultivated land(385.43 gC·m^-2·a^-1)>grassland(165.56 gC·m^-2·a^-1)>construction land(106.02 gC·m^-2·a^-1)>water area(38.59 gC·m^-2·a^-1)>unused land(31.58 gC·m^-2·a^-1),The growth rate of NPP in forest land was the fastest,while that in unused land was the lowest,and the fluctuation of total NPP in grassland was the largest with season.?2?The climate change varies greatly in different areas of the northern slope of the Tianshan Mountains.The central region has the highest annual average temperature and the lowest in the east;the western region has the highest precipitation,the middle is the second,and the eastern is the lowest.The NPP on the northern slope of the Tianshan Mountains was significantly correlated with temperature and precipitation,accounting for 3.70%and 11.81%,respectively.The effect of precipitation on NPP on the northern slope of Tianshan Mountains was greater than temperature.NPP with cultivated land and construction land has a significantly positive correlation with temperature than precipitation,and other vegetation types NPP are mainly affected by precipitation.Among them,the grassland and precipitation were positively correlated with the largest area,accounting for 17.43%of the total grassland area.?3?Different topographical factors on the northern slope of Tianshan Mountains have different effects on the change trend of NPP.With the increase of altitude,NPP tends to decrease first,then increase,and finally decrease.The high value area of NPP is mainly distributed at an altitude of 2.5-3 km.With the increase of slope,NPP also showed a trend of decrease first,then increase and finally decrease.When the slope is in the range of 15-20 degrees,the average value of NPP reached the maximum;For different slope directions,the variation range of NPP is between 50.61-155.88 gC·m^-2,and the mean value of NPP is the highest on the north slope,and the NPP on the shady slope is generally higher than that on the sunny slope.?4?The transfer of grassland and unused land to the cultivated land on the northern slope of Tianshan Mountains is the main way of land use cover change in recent years.The fluctuation of vegetation NPP is mainly dominated by the transfer to the cultivated land and construction land,transfer out the grassland and unused land.With the development of agriculture and the acceleration of urbanization,the trend of grassland degradation is accelerating.