Effects Of Phonological Changes On Net Ecosystem Productivity Of Different Vegetation Types In The Northern Hemisphere

Based on the data processing of 797 site-years observed at 112 sites in the northern hemisphere in the international flux observation network(FLUXNET),five phenological indicators including the beginning of the carbon uptake period(CUP BDOY),the ending of the carbon uptake period(CUP EDOY),net carbon uptake period(CUP),the day of the maximum daily net ecosystem productivity(NEPmax DOY)and the maximum daily net ecosystem productivity(NEPmax)were extracted.Using univariate linear regression analysis,analysis of variance and multiple comparisons,we presented an analysis of phenological changes of evergreen needleleaf forests(ENF),evergreen broadleaf forests(EBF),deciduous broadleaf forests(DBF),mixed forests(MF),open shrublands(OSH)and grasslands(GRA)within the scope of the northern hemisphere.Besides,we also analyzed both interannual and spatial relationships between annual net ecosystem production(NEP)and these phonological indicators.This paper aims to understand the change trends of phenological indicators and annual NEP of different vegetation types in the northern hemisphere,quantify the effects of phenological changes on annual NEP,and reveal the effects of phenological changes on carbon sequestration of different vegetation types in the background of global changes.The main results are as follows:(1)Among the six vegetation types in the northern hemisphere,except for MF,CUP length of other vegetation types showed a trend of extension year by year.The NEPmax DOY of ENF and MF showed a trend of delay year by year,while the NEPmax DOY of EBF,DBF and GRA showed a trend of advancement year by year.The NEPmax of ENF,DBF,MF and GRA increased year by year,and the NEPmax of OSH decreased year by year.Except for EBF,the annual NEP of other vegetation types increased year by year.(2)In the different years of the same site,when CUP BDOY,CUP EDOY and CUP length change the same number of days,the annual NEPanomalies of DBF changes more than that of ENF,the annual NEP anomalies of MF is between that of ENF and DBF.The annual NEP anomalies of GRA and OSH are less sensitive to the anomalies of CUP BDOY,CUP EDOY and CUP length compared with forests.The annual NEP anomalies of forests are more sensitive to the anomalies of NEPmax than that of GRA and OSH.The anomalies of CUP length and NEPmax of each vegetation type affect the anomaly of annual NEP jointly.(3)In the study of the spatial relationships between phenology and annual NEP,the changes of CUP BDOY,CUP EDOY,CUP length and NEPmax of forests at different sites have more significant effects on annual NEP than those of non-forest ecosystems.In forests,the changes of CUP and NEPmax of the same types of forests(ENF,EBF and DBF)at different sites have a greater impact on annual NEP than those of MF,and the changes of NEPmax at different sites of evergreen forests have a greater impact on annual NEP than those of deciduous forests.(4)The effect of phenological changes between different sites on the annual NEP has a larger R2 and a higher slope than that of interannual phenological anomalies in the same site on the annual NEP.Therefore,the effect of phenological changes caused by spatial phenological changes on annual NEP is larger than that caused by interannual phenological anomalies.The following conclusions can be drawn:(1)Under the background of global change,CUP,NEPmax and the annual carbon sequestration of most vegetation types increased year by year.(2)Interannual anomalies of CUP and NEPmax have different effects on carbon sequestration of different ecosystems in the northern hemisphere.(3)The spatial changes of CUP and NEPmax have different effects on carbon sequestration of different ecosystems in the northern hemisphere.(4)The effect intension of interannual and spatial phenological changes on annual NEP are different.