Characterizing The Turning Points In Ecosystem Functioning And Their Causes Over The Arid And Semi-Arid Regions Of Northern China Based On Time Series RUE

Ecosystem functioning(EF)refers to an attribute related to the performance of the entire ecosystem,and is the result of one or more ecological processes.Under the influence of strong external interference(such as extreme events)or slow but continuous changes(such as increased grazing/human pressure),ecosystem functioning may change suddenly,of which sudden degradation may have serious ecological and economic consequences.Located in the hinterland of Eurasia,the arid and semi-arid region in northern China(abbreviated as ASARNC)is a typically vulnerable area of ecological environment,which is extremely sensitive to climate change.Studies have shown that there are signs of the transition from drought to moisture in northern China in recent years.With the implementation of large-scale vegetation restoration and reconstruction measures,vegetation ecosystem in arid and semi-arid areas has changed significantly.However,it is not clear whether the ecosystem function of the region changed abruptly and how that change was affected by natural and anthropogenic factors in the past 20 years.In this thesis,the rain use efficiency(RUE)is used as the indicator of ecosystem functioning,and the monthly rain use efficiency(RUE)data was estimated based on the MODIS NDVI time series data and precipitation data from 2000 to 2019.The Breaks For Additive Season and Trend(BFAST)family algorithms are introduced to detect Turning Point Occurrence Index(TPOI),the timing and types of turning points(TPs)of ecosystem functioning in the arid and semi-arid regions of northern China.The standard precipitation evapotranspiration index(SPEI)is calculated.The relationship between abrupt change of ecosystem functioning and drought in different time and space is clarified by analyzing the SPEI characteristics of the month when the turning point occurred and the prior one to four months.Correspondence analysis(CA)was used to reveal the impact of dry and wet events on the timing and types of turning points.The information of land use change frequency and types,population density and GDP change in the study area was examined,and the possible relationship between the TPs of ecosystem functioning and human factors in the past 20 years was discussed.The main findings are summarized as follows:(1)From 2000 to 2019,ecosystem functioning in arid and semi-arid areas of northern China changed significantly(p<0.1).TPs was detected in 63.2%of the pixels(about 1.71×10~6 km~2).The regions with high mutation occurrence index(TPOI)are mainly distributed in the central part of Northeast Songnen Plain,the west and southeast of Hulunbuir of Inner Mongolia,Baotou City,north of Hohhot,northwest of Ulanqab,the junction of southern Alxa League and Gansu Province,Altay region and central Tacheng region of northern Xinjiang,and central Tibet.The main type of ecosystem functioning break was Type 6(RUE decrease with positive break).In 66.5%of the detected break types,RUE trend did not change significantly before and after TPs,indicating the EF were stable.The TPs occurred mainly in 2004,2013,2014 and 2015.From the monthly scale,the number of TPs was the largest in October,followed by November.Based on Sen trend analysis method and BFAST01model,the time series NDVI and precipitation data are analyzed.It is found that the TPs in RUE time series are mostly the result of the joint action of NDVI and precipitation,and the TPs occurring only in NDVI or precipitation may not lead to the TPs of RUE.(2)The mothly SPEI values calculated on the scale of 1,3 and 12 months when the TPs occurred were significantly different.SPEI-01(1 month)had the largest number of pixels in drought(accounting for 55.6%of the total pixels),followed by SPEi-03(3 months)(accounting for 31%).SPEI-12(12 months)was the least(22.2%),and the TPs of EF was more associated with short-term drought.The break detection of SPEI-12 time series and Sen trend analysis showed that the central and eastern parts of the study area changed in the direction of wetness,where was disturbed by drought events.However,the western region changed towards drought direction and experienced moist disturbance during 2000–2019.The corresponding analysis between the break types of EF and SPEI level showed that the position break(Type 3,Type 6)and Type 8 were strongly correlated with the continuous wetting before the break,while the negative break(Type 5)was strongly correlated with the persistent drought event before the mutation.(3)From 2000 to 2019,there was little difference in the proportion of the number of TPs in different times of land use change,which may be related to the absolute dominance of grassland area(70.40%)and the small land use change area in the study area.The analysis of land use type change shows that the positive break and Type 8 in the study area may be related to vegetation restoration,while the difference of break types detected in the vegetation degradation area is not obvious,which may be related to the negative break being more affected by natural factors such as drought.Combined with the spatial statistical analysis of the raster data of GDP and population density,it is found that 95.36%of the abrupt changes occur in areas with GDP growth and 65.33%in areas with population density increase.In the areas with reduced population pressure,the change trend of ecosystem functioning was increased after the TPs occurred.However,more negative mutations were detected in the regions where population pressure increased,and the change trend of ecosystem functioning tended to decline after the break.In conclusion,ecosystem functioning in arid and semi-arid areas of northern China has undergone sudden changes during 2000–2019,and there is a certain relationship between the time and types of TPs,dry and wet states,the number and type of land use changes,population density growth and GDP growth.The types of TPs that improved ecosystem functioning(Type 3,Type 6,and Type 8)were associated with continued wetting or decreased population density,while those that deteriorated ecosystem functioning(with negative break,such as Type 4,Type 5)were associated with continued drought or increased population density.Our research furthers the understanding of how and why TPs of EF occur and provides fundamental data for the conservation,management,and better decision-making concerning dryland ecosystems in China.