| Vegetation is an important component of the terrestrial ecosystem.It is not only an important medium for the global material and energy cycle,but also plays a vital role in regulating the global carbon balance and slowing global warming.The size of the vegetation NPP reflects the vegetation growth status,and represents the quality and structure of an ecosystem.The factors that affect the change of vegetation NPP are complex and diverse.Among them,climate change and human activities are the most important factors.The agricultural pastoral ecotone of Northern?APENC?is a typical transition zone of agricultural and pastoral system.It has a special geographical location,fragile ecological environment,and is sensitive to the disturbance of climate change and human activities.Further research of the temporal and spatial changes of vegetation NPP and its relationship with climate factors in this region can not only reveal the interaction mechanism between climate change and vegetation change,but also provide a reference for improving regional ecological environment.This article is based on VEGETATION-NDVI data,meteorological data and vegetation type data,etc.,with reference to MODIS-NPP and other data,the CASA model is used to estimate and verify the NPP in the study area.Theil-Sen Median trend analysis,Mann-Kendall significance test?MK?,partial correlation analysis,residual analysis and other methods explore the characteristics of vegetation NPP multi-year spatial and temporal changes,and study the impact of climate change and human activities on vegetation NPP.The following conclusions are obtained:?1?Comparing the NPP estimated by the CASA model with the existing research and MODIS-NPP data,it is found that although the NPP estimation results are different from the verification data,they are all within a reasonable range.According to the correlation analysis results at the pixel scale,it is found that the estimated NPP has a good correlation with the MOD17A3 product data,indicating that the CASA model is suitable for the estimation of vegetation NPP in the APENC.?2?During the study period,the NPP of vegetation in the study area has significant spatial and temporal heterogeneity,and varies with vegetation types.In general,from1998 to 2016,the annual average NPP of the study area was characterized by high in the southwest and northeast and low in the middle,and high in summer and low in winter.The annual average value of NPP is mainly concentrated in 200-500gC·m-2·a-1,the annual average value and annual total of NPP of different vegetation types are obviously different.The annual average value of deciduous broadleaf forest is the highest,and the annual total of grassland is the highest.Deciduous broadleaf forest had the highest net primary productivity in July,and grassland had the largest total NPP in July.The average NPP of vegetation in the study area showed an increasing trend in both interannual and seasonal changes.The significant upward trend in 2012 and 2013was particularly significant.The NPP growth rate in summer was the fastest,followed by autumn and spring,and almost no change in winter.Deciduous broadleaf forest have the strongest increasing trend and grasslands have the weakest.The area where vegetation NPP increased was significantly larger than the area where vegetation NPP decreased.The frequency of mutations was mainly one,the time of mutations was mostly in 2012 and 2013,and the type of mutations was mainly improved significantly.The area with a significant increase in NPP accounted for 32.07%of the total area,it mainly distributed in the southern part of the central and western parts of the study area and the sporadic area in the northeast plain.The area with a significant reduction in NPP accounted for 0.85%of the total area,and it was scattered on the northeast edge of the study area.There are many regions with one mutation in NPP,followed by three mutations.?3?The distribution of meteorological elements?temperature and precipitation?in the study area also has significant spatial and temporal differences,and shows an increasing trend during the study period.Spatially,the annual precipitation is high in the southeast and low in the northwest,while the annual average temperature distribution is high in the south and low in the north,high in the east and low in the west.In terms of time,precipitation and temperature have the characteristics of high rain and heat.Summer has the most precipitation and the average temperature is the highest.During the period of the study area,the annual precipitation generally shows an increasing trend,and the average annual temperature in most areas also shows an upward trend.The rate of change of precipitation in autumn is the largest,and the rate of average temperature increase in summer is the fastest.?4?There is a positive correlation between vegetation NPP and precipitation in the APENC,but it has a weak negative correlation with annual average temperature.The average annual NPP of 97.70%pixels is positively correlated with annual precipitation,especially in the southern Ningxia Hui Autonomous Region and the southeastern Inner Mongolia Autonomous Region.The negative correlation pixels account for 50.49%,which are relatively concentrated on the southeast and southwest edges of the study area,the partial correlation of NPP and average temperature is lower than that of precipitation partial correlation,and both partial correlations show significant spatial differences.In different seasons,the vegetation NPP changes in the study area responded differently to changes in meteorological elements.In spring?March,April and May?,vegetation is most sensitive to changes in water and heat.Spring precipitation has the greatest effect on promoting NPP in the study area,followed by summer,autumn and winter;the area with the most positive correlation between NPP and temperature changes is also spring,followed by autumn,summer and winter.In winter,vegetation in most areas stops growing,and the area where vegetation continues to grow and organic matter continues to accumulate only accounts for 15.27%of the total area.?5?During the study period,the positive contribution of climate change and human activities to vegetation was significantly greater than its negative effect.Climate change,human activities,and the combined effect of climate change and human activities accounted for 59.28%,2.68%,and 38.04%of the total area restored,respectively.The three areas that caused vegetation degradation accounted for 2.87%,89.96%,and 7.17%of the total degraded area,respectively.In addition,different vegetation types have different degrees of restoration and degradation caused by climate change,human activities,and their combined effects. |
PDF Full Text Request