The Relationship Between Vegetation Phenology And Net Primary Productivity (NPP) On The Loess Plateau And Its Response To Climate Change

With the impacts of global warming,the vegetation phenology has changed significantly,and the arid area(drylands)ecosystem is facing more and more challenges.As one of the regions with the most serious soil erosion and the weakest ecological environment in the world,the changes of vegetation phenology on the Loess Plateau have a profound impact on vegetation productivity.Therefore,understanding the response law of vegetation phenology and net primary productivity on the Loess Plateau to climate change and the relationship between them is helpful for our in-depth understanding and accurate prediction of regional carbon cycle change.This is very important for the sustainable development of the fragile Loess Plateau to the global dryland ecosystem.Taking the vegetation in the Loess Plateau as the research object,this paper makes a systematic analysis on the present situation and change of climate on the Loess Plateau from 1982 to 2015,and analyzes the spatio-temporal dynamic characteristics of extreme climate events(e.g.drought)by the Standardized Precipitation Evapotranspiration Index(SPEI)of different time scales in the past 30 years.Based on the reconstruction of GIMMS NDVI3g remote sensing data of long time series,the maximum ratio method was used to extract the phenological indexes of the beginning time of spring phenological growing season(The start of season,SOS),the ending time of autumn phenological growing season(The end of season,SOS)and the length of growing season(The length of season,LOS)on the Loess Plateau.At the same time,the net primary productivity(NPP)was estimated based on CASA model.Based on the full verification and evaluation of the precision of the simulation results,the temporal and spatial changes of phenology and NPP were analyzed,and the response characteristics of plant phenology and NPP to different climatic factors and drought were revealed.The discussion on the relationship between vegetation phenological changes and vegetation net primary productivity,and the changes of phenology and NPP under future climate change scenarios with different carbon emission levels were predicted.The main conclusions are listed below:(1)In the past 30 years,the climate of the Loess Plateau shows a trend of warm and humid gradually,but there are significant differences in different regions and different seasons.The southeast of the Loess Plateau is warm and dry year by year,and the northwest is a mixed distribution of warm and humid,or warm and dry.Through the analysis of the seasonal change of climate,it is found that the climate is mainly warm and dry in spring and summer,while warm and humid in autumn and winter.(2)The start of the phenological growth season(SOS)in the southeast of the Loess Plateau is earlier than that in the northwest.The length of the growing season(LOS)in the southeast is also longer than that in the northwest.However,the end of the growing season(EOS)in southeast is slightly later than that in northwest,and the occurrence time is relatively close.On the whole,the Loess Plateau shows the trend of SOS delayed,EOS delayed and LOS prolonged.Among them,EOS showed significant prolonged,with a rate of 0.1 day/year.SOS is mainly affected by temperature,while EOS is mainly affected by precipitation and sunshine hours.The LOS is prolonged with the increaseing temperature in the southeast of the Loess Plateau,in contrast,the LOS is shortened for the increase in temperature in the northwest.(3)The pattern of NPP in the Loess Plateau is mainly high in the southeast and low in the northwest,and overall decrease with the rate of 0.22gC/m2·a but the trend is not significant.In different seasons,except for the gradually increaseed in spring,NPP showed a decreasing trend in other seasons,and the decrease of summer NPP was the most obvious.The areas with an increasing trend of annual NPP are mainly cluster on the Loess Plateau of northern Shaanxi,where the conversion of farmland to forests is intensive.The reduced areas are mainly distributed in the Ordos Plateau and Guanzhong Plain.The annual average NPP in the northwest of the Loess Plateau is mainly caused by the increase of temperature and the decrease of precipitation or SPEI,while the NPP in the southeast is increased by the increase of sunshine hours and temperature.In spring,NPP is sensitive to air temperature and sunshine hours,and is less affected by precipitation or SPEI.Precipitation,SPEI and sunshine hours are the key factors instead of temperature for autumn NPP in the Loess Plateau.(4)Due to the advance of SOS and the delay of EOS,NPP increased in spring and autumn.The prolongation of vegetation growth time weakens the availability of water or nutrients for summer,which leads to the decrease of summer NPP.Phenological changes have different effects on NPP in different climatic conditions of the Loess Plateau.In the eastern part of the Loess Plateau with better hydrothermal conditions,LOS and NPP are mainly positively correlated.However,in the northwest of the Loess Plateau,where the hydrothermal conditions are relatively poor,there is mainly a negative correlation between LOS and NPP.(5)In the area where the average annual precipitation more than 400mm in the Loess Plateau,and the annual average temperature from 4 to 12?,the advance of SOS,the delay of EOS and the prolongation of LOS are all beneficial to the increase of vegetation NPP in this area.In the region where the average annual precipitation is less than 400mm and the average annual temperature from 0 to 10?,the NPP in this area decreases due to the shortening of the length of the growing season LOS.In the area where the average annual temperature is more than 12?,LOS is prolonged and NPP is decreased.Below 2000m above sea level,NPP is mainly affected by the delay of SOS,and NPP decreases with the increase of altitude.Above 2000m above sea level,SOS was delayed and NPP increased with the increase of altitude.(6)Under the climate change scenario of low carbon emission RCP4.5,the climate of the Loess Plateau will show a warm and humid trend in the next 30 years.Under the climate change scenario of high carbon emission RCP8.5,the climate of the Loess Plateau shows a warm and dry trend.Under different scenarios,the future vegetation growth season length LOS of the Loess Plateau is significantly prolonged.Under the RCP8.5 climate change scenario,the vegetation phenology was significantly delayed in autumn,and the NPP increased significantly in autumn.