| Net primary production(NPP)is not only an important variable for characterizing plant activities,but also a major factor for determining carbon source/sink and regulating ecological processes in ecosystems.It is mainly affected by climate,land use and other environmental factors.The Loess Plateau is located in the semi-arid and semi humid area of China,with serious soil erosion and is a typical ecologically fragile area.NPP changes are sensitive to drought and land use change.Therefore,understanding the temporal and spatial changes of NPP and its quantitative relationship with climate,land use and other environmental factors in the Loess Plateau will help us deeply understand and accurately predict the changes of regional carbon cycle,which is very important for the sustainable development of the Loess Plateau ecosystem.This paper first analyzedthe spatial and temporal variations of NPP,drought and land use types on the Loess Plateau during the 16-year period from 2000 to 2015 by using GLO PEM model of Resources and Environmental Data Platform of Chinese Academy of Sciences,precipitation and air temperature data from meteorological stations and MODIS land use and cover data.On this basis,the contribution of drought and land use to NPP change is quantified,especially the contribution of main land use types and differentdrought severity aggravation or mitigation to NPP change is further refined.The implementation effect of ecological restoration measures such as converting farmland to forest/grass and sand land treatment in Loess Plateau is evaluated by selecting typical areas.Finally,by using the FLUS model,11 natural and socio-economic factors are selected as the driving forces of land use change,and the spatial-temporal pattern of land use in 2025 is simulated under the scenario of future baseline,fast economic development,ecological protection.Based on the average NPP of various land types in 16 years,the change of NPP under different scenarios in the future was predicted,and the impact of different land use types on NPP was quantified.The main conclusions are as follows:(1)The spatial and temporal variation characteristics of NPP,drought and land use types were significantly different in different periods and regions.The annual variation of NPP shows a fluctuating growth trend with obvious periodicity.The average value of NPP was generally low and showed a decreasing distribution from southeast to northwest.Most regions showed a linear increasing trend,while a few regions showed a linear decreasing trend,with a reverse trend of linear change and change rate.On the whole,the average value of SPEI fluctuates,and its fluctuation range gradually decreases,with great difference among provinces.Mild drought and moderate drought are the main forms of drought in the study area.The linear variation trend of drought and NPP was consistent.Land use change is more intense,and the change range and direction of various land types are different.The area of land use change is mainly distributed in the interlaced zone of farmland,forestland and grassland.(2)The coupling effect of precipitation and temperature is the main factor to promote the change of NPP in Loess Plateau.The change of NPP with a total amount of about 2/3 is attributed to the coupling effect of precipitation and temperature.The contribution of the coupling effect of precipitation and temperature to NPP change varies with land use types and regions.Among the six land types,the coupling effect of precipitation and temperature on NPP change of forestland,farmland and grassland was greater.The intensity of NPP reduction was grassland>farmland>forestland,and the intensity of NPP increase was farmland>grassland>forestland.From the perspective of administrative regions,the coupling effect of precipitation and temperature on NPP change in all provinces and cities is relatively high,and the degree of influence is different among different provinces and cities due to the large difference of land use.(3)Drought caused by the coupling effect of precipitation and temperature is an important factor restricting NPP production.About 33%of the NPP change of vegetation was affected by drought.The occurrence and aggravation of drought have strong inhibition on NPP.Compared with severe drought,light drought and moderate drought had a greater impact on NPP because of their more frequency and longer duration.Drought had great influence on NPP change of main ecological land(farmland,forestland and grassland),and the decreasing intensity of NPP was grassland>farmland>forestland,the increasing intensity of NPP was grassland>forestland>farmland.Among them,light drought had a greater impact on farmland NPP,and moderate drought had a greater impact on grassland NPP.From the perspective of administrative regions,NPP changes in all provinces and cities are greatly affected by drought.With the aggravation of drought level,the difference of influence degree between different provinces and cities is gradually increasing.(4)The NPP change in the regions with significant land use change is mainly caused by the land use type transfer caused by human activities(sand land management,returning farmland to forest/grassland,etc.).The transfer of land use type mainly occurred in local areas,presented an aggregative "point" distribution,and dispersed in different areas of the Loess Plateau,some high-value points contribute more than 45%or even more than 60%,which had little influence on the overall NPP change.However,in local areas,it was the dominant factor of NPP change in these areas.The impact of land use change on NPP decrease is weaker than that on NPP increase,which indicates that ecological restoration policy has achieved certain results.The influence of land use change on NPP at provincial and municipal levels is increasing from west to east,and the average contribution to NPP changes is low.In the northwest of Inner Mongolia Autonomous Region and Ningxia Hui Autonomous Region,the effect of sand land management and returning farmland to forest in Shaanxi Province is better.The overall effect of ecological protection in eastern provinces is relatively weak,but some regions are relatively good.The restoration effect of the area of Shanxi Province and Henan province belongs to Taihang mountain area is better,and the ecological protection of some farmland belonging to Fenhe Valley and Henan Province is relatively weak.(5)Under the three simulation scenarios,land use type transfer mainly occurredbetween ecological land(farmland,forestland,grassland,and water area),unused land and ecological land.The spatial distribution pattern of land use was roughly the same,and NPP showed an increasing trend,in which NPP increased the most under the ecological protection scenario(212.45gC/km2).Under the baseline scenario,the cities with NPP growth trend are the most,followed by the fast economic development scenario.Under the ecological protection scenario,the cities with NPP increasing trend are relatively the least,but the high value of NPP growth is the most.The reduction of NPP in cities with decreasing trend is generally low.The average reduction of NPP is the most in the scenario of fast economic development,followed by the baseline scenario,and the relative reduction of NPP is the least in the scenario of ecological protection.The change of NPP caused by different situations and land use transfer reflects the significant difference of ecological protection effect.Among them,under the scenario of ecological protection,the concept of economic and social development has changed,and the effect of ecological restoration is the most significant,which can best protect forestland,grassland,water area,and control sandy land. |
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