Research On The Response Of The GPP To Extreme Climate Events In Yangtze River Basin

Terrestrial vegetation is a key component of the Earth 's carbon cycle.Vegetation fixes carbon in the form of organic carbon through photosynthesis.This capability can be measured by Gross Primary Productivity(GPP)of vegetation.GPP plays an important role in the research of terrestrial carbon cycle,change and storage.The dynamic changes of GPP in time and space are also deeply affected by climate.Facing the challenge of global warming,scientifically understanding the response of vegetation dynamics to extreme climate change has become one of the hotspots in current research.As the largest watershed in China,the Yangtze River Basin is rich in vegetation types and diverse climates across regions.It is a typical and complex ecological region.At the same time,the river basin has an important socio-economic status.The mechanism is of great significance.The research area in this paper is the whole area of the Yangtze River Basin,and the research duration is from 1982 to 2016.In order to refine the spatial and temporal differentiation characteristics of the research elements in the whole watershed,the whole watershed is divided into semi-arid plateau sub-arid zone,semi-humid plateau sub-humid zone,temperate humid plateau,semi-humid zone,northern subtropical humid zone and central subtropical humid five climate zones.Taking into account the phenological attributes of vegetation,May-October each year is defined as the growing season of vegetation in the study area,and the spatial-temporal changes of vegetation GPP and extreme climate events in the study area and corresponding studies were carried out based on this.First,the improved MODIS algorithm was used to reconstruct the vegetation GPP in the study area,and the temporal changes were analyzed at the global and climatic zoning scales,and the spatial mean distribution characteristics of the river basin vegetation GPP were studied at the time scales of the whole year and the growing season.At the same time,the extreme climate data set of the river basin was established to summarize the temporal and spatial dynamic changes of extreme climate in the study area.On this basis,to quantify the extreme climate spatiotemporal response characteristics of the GPP in the vegetation growing season of the watershed,the correlation analysis and lag analysis between the two research elements were done respectively.Mainly include the following research contents:(1)The characteristics and trends of spatial-temporal changes of vegetation GPP in the Yangtze River Basin are sorted out.(1)The annual average GPP in the watershed showed a significant upward trend from 1982 to 2016,and there was a difference in the rising range between different climatic zones.The central subtropical humid area was the smallest.(2)During the growing season,the time change of GPP is consistent with the whole year,that is,it increases in fluctuations,and the increasing trend is not significant in the temperate humid and semi-humid areas of the plateau,but it increases significantly in other climatic regions.At the inter-and growing-season levels,there also exhibits similar characteristic.The low-value pixels of the spatial mean are concentrated in the plateau climatic area,and the subtropical humid area is the main distribution area of the high-value pixels;(3)For the spatial trend distribution of GPP,the same similar distribution trend occurs on the interannual and growing season scales.From the overall view of the whole watershed,there are far fewer pixels in the main trend of GPP in each time scale than the increasing trend.The downward trend mainly occurs in the central and eastern humid and semi-humid plateau regions,the northwestern part of the north subtropical humid region,and the southwest part of the central subtropical humid region.In the Sichuan Basin of the central subtropical humid area,the southeast of the hilly mountains in the south of the Yangtze River,and the northern plain of the middle and lower reaches of the Yangtze River,the spatial distribution trend of GPP has increased significantly.(2)Explore the temporal and spatial differentiation of extreme climate events in the Yangtze River Basin.The characterization indicators of extreme climate events adopt the definition of the World Panel on Climate Change Detection and Indicators(ETCCDMI).At the same time,combined with the characteristics of the data available in the study area,10 indicators were selected,including 4 relative temperature indicators describing the number of cold and hot days,4 extreme value indicators describing extremely high and low temperatures,and 2 short-term abnormalities The precipitation index of precipitation.(1)In the interannual time change of extreme climate indicators,the early 21 st century contributed more extreme temperature events,the cold indexes decreased significantly,the frequency of low temperature events decreased significantly,the warm index increased significantly,and the frequency of high temperature events improved.During the study period,the whole basin showed a warm development,and the extreme precipitation increased in an insignificant trend.,with the maximum value occurred in 1998.The time change of the climate indexes during the growing season is the same as that of the whole year.(2)Each climatic zone and the whole area also showed a similar spatiotemporal change trend.The change of relative temperature indexes were the closest to the whole area,and significant regional differences appeared on the extreme value indexes.The indexes representing extreme high temperature events had the largest annual mean value in the humid north subtropical region.The northern part of this region is also the area where the warming indexes of the rising cells gathers.The eastern part of the humid area shows a downward trend.From the northwest inland to the east coast,from the plateau sub-cold zone to the subtropical humid zone,extreme precipitation shows a tendency to increase.In the central subtropical humid area,the annual average value of the largest precipitation index often appears.On the contrary,the minimum value of the annual average value of extreme precipitation almost comes from the semi-arid area of the plateau sub-cold zone.At the scale of the whole area and each subarea,there is no significant change in extreme precipitation events.(3)Discuss the spatial and temporal response characteristics of GPP to extreme climate in the growing season of the Yangtze River Basin.(1)The overall primary productivity of vegetation in the study area during the growing season is generally more affected by extreme temperatures.The warming trend has brought a significant increase in GPP in the growing season.As far as the climate zones are concerned,the temperate humid and semi-humid areas of the plateau have a special performance.Unlike other zones,extreme precipitation events in this area show a tendency to inhibit vegetation GPP in the growing season.In addition,the overall correlation of the watershed shows that the cold indexes inhibit the growth season GPP.While the warm indexes,extreme value indexes,and precipitation indexes promote the growth season GPP.(2)The spatial correlation between the extreme climate indexes and the growth season GPP in the study area is also presents similar characteristics.First of all,the extreme correlation indexes distribution areas are mainly in the plateau sub-cold zone and the eastern part of the subtropical humid zone.These areas are also the areas where the warm climate indexes promote the growth of GPP in the growing season,and the positive role of extreme precipitation events is also concentrated in these areas.The cold indexes have a positive effect on the GPP in the growing season in the eastern part of the eastern Sichuan-Tibet region and the western part of the sub-tropical zone in the temperate and semi-humid areas of the plateau.(3)The timely and rapid response of vegetation to extreme climate occurs in the Yangtze River Basin.In the growing season,once an extreme climate event occurs,the vegetation GPP will respond to this event within a month.The regional differences exist in the duration of the hysteresis effects.The upstream area is controlled by precipitation for up to 2 months,but it is only affected by extreme temperatures for 1 month.The opposite is true in the middle and lower reaches.Extreme precipitation events continue to affect vegetation for one month,while extreme temperatures can continue to affect them for two months.