Spatiotemporal Evolution Of Extreme Climate In Beijing-Tianjin-Hebei And Its Impact On Vegetation Net Primary Productivity

Compared with the mean state,extreme climate events are more abrupt and destructive,and have a greater impact on the ecological environment.In recent years,the frequent occurrence of extreme climate events has brought serious impacts on the ecological environment of Beijing-Tianjin-Hebei.Studying the response mechanism of vegetation Net Primary Productivity(NPP)to extreme climate events in this region is not only beneficial to the protection of ecological environment,but also can provide scientific basis for coping with extreme climate events.Based on the meteorological station data in Beijing-Tianjin-Hebei and surrounding areas,NDVI(Normalized Difference Vegetation Index)data and Vegetation type data from 1980 to 2018,RClim Dex model and improved CASA(Carnegie-Ames-Stanford Approach)model were used to construct extreme climate indices and NPP data sets.Then combined methods such as wavelet analysis,Hurst index and correlation analysis to analyze the spatiotemporal characteristics of extreme climate and NPP,and explore the impact of extreme climate on NPP on different time scales.The main conclusions are as follows:(1)From 1980 to 2018,the extreme temperature indices in Beijing-Tianjin-Hebei showed an upward trend of high-temperature indices and a downward trend of lowtemperature indices.Most stations of high-temperature indices showed an upward trend,while those of low-temperature indices showed a downward trend.Except for consecutive wet days(CWD),all other extreme precipitation indices showed an upward trend.The stations with a significant upward trend were mostly concentrated in the northwest and central regions of Beijing-Tianjin-Hebei.The cycles of extreme climate events ranged from 1 to 7 years.In the future,Beijing-Tianjin-Hebei would maintain such a state of continuous warming,as for the extreme precipitation indices,only the number of heavy precipitation days(R10mm),the number of heavy precipitation days(R20mm)and annual total wet-day precipitation(PRCPTOT)would increase.(2)Spatio-temporal evolution characteristics of NPPThe average annual NPP of the Beijing-Tianjin-Hebei growing season from 2000 to2018 showed an increasing trend.The high-value areas of NPP were mostly distributed in the northern of Beijing-Tianjin-Hebei,while the low-value areas were mostly distributed in the northwest and eastern coastal areas of Beijing-Tianjin-Hebei,with certain seasonal differences.In the future,the trend of NPP in most areas of BeijingTianjin-Hebei region would be anti-continuous.(3)The influence of extreme climate index on NPPAt the interannual scale,cool nights(TN10P),diurnal temperature range(DTR),max 1-day precipitation amount(RX1day)and max 5-day precipitation amount(RX5day)were positively correlated with NPP,while other extreme climate indices were negatively correlated with NPP.At the annual scale,extreme temperature extreme value indices,RX1 day and RX5 day were positively correlated with NPP,while the other indices were negatively correlated with NPP.After eliminating the seasonal influence,warm days(TX90P)and warm nights(TN90P),extreme temperature indices,RX1 day and RX5 day had a positive effect on NPP,while cool days(TX10p),TN10 p and DTR had a negative effect on NPP.The impact of extreme climate events on NPP had a one-month lag.