Vegetation Phenology Analysis Based On Eddy Covariance Technology

Plant phenology information can reveal the growth and development of plants and their relationship with the environment.At the same time,the study of plant phenology under the background of global change can serve for the study of energy and matter exchange between vegetation and atmosphere.As a new method for phenological research,vorticity correlation technology can obtain continuous phenological information,which is of great significance for accurately describing the process of matter and energy exchange between earth and air,and provides a data basis for studying phenological change.Based on the data(2011?2015?2018)of flux observation station in Fengxian University Town,Shanghai,this paper conducted trend and statistical analysis of Gross Primary Productivity(GPP)in different years,refined threshold criteria based on threshold method to extract phenological information in this area,and compared with the phenological information extracted by Normalized Vegetation Index(NDVI),screened out the appropriate threshold index.Based on the selected threshold index,the trend of plant phenology was analyzed.The main conclusions of this paper are as follows:(1)In 2011,Start of Season(SOS)was extracted from late March to early June and EOS was extracted from late September to early November.With the increase of threshold,SOS was gradually delayed,End of Season(EOS)was gradually advanced,Growing Season Length(GSL)was gradually shortened.The phenological information extracted from GPP=4 was very close to 30%GPPmax,and the phenological information extracted from GPP=2 was very close to 15%GPP_max.(2)The SOS of vegetation in Fengxian District of Shanghai was mainly concentrated in March,April and early May,and 72.42%of the area was located in78d-94d in spring.EOS is mainly concentrated in October and November of autumn and December of winter.83.34%of the region's autumn phenology lies in 292d-311d.The SOS of Shanghai Fengxian University Town in the study area was 102d.EOS is319.68d.The phenological growth node extracted by 10%GPPmax is closest to the remote sensing phenology,which is 97d for SOS and 316d for EOS.(3)The annual distribution of GPP in 2011,2015 and 2018 all showed a single-peak curve,with the trend of increasing in spring,reaching the peak in summer,decreasing in autumn and reaching the lowest value in winter.The Photosynthetically Active Radiation(PAR)in spring and summer of 2018 was significantly higher than that in 2011,and the mean value of GPP was 0.88g·C·m^-2·d^-1higher than that in 2011,indicating that photosynthesis,vegetation carbon sequestration and carbon sink function were enhanced in 2018.(4)The results of extracting phenological information from 2015 and 2018 data by 10%GPP_maxthreshold showed that SOS and EOS were 105d and 315d respectively in 2015.In 2018,SOS was 88d and EOS was 318d.From 2011 to 2015,SOS was delayed by 8 days,EOS was advanced by 1 day and GSL was shortened by 9 days.Between 2015 and 2018,SOS was brought forward by 19 days,EOS by two days and LOS by 21 days.The delay of SOS in 2015 May be due to missing GPP data in spring.From the perspective of long time scale,SOS,EOS and LOS were advanced by 9days,delayed by 2 days and extended by 11 days from 2011 to 2018.SOS,EOS and LOS were advanced,delayed by 2 days and extended by 11 days in the study area.The study area as a whole showed the characteristics of advanced SOS,delayed EOS,and prolonged GSL.