Effects Of Land Cover And Phenological Changes On The Gross Primary Productivity In The Mu Us Desert

Climate change and human activities are important driving factors for the interdecadal changes in vegetation productivity,and clarifying their joint effects on vegetation productivity is essential for the sustainable management of ecosystems.Climate change may lead to changes in vegetation phenology,which in turn affects vegetation productivity.We aimed to examine how changes in vegetation cover and phenology affect the trend of gross primary productivity(GPP)in the Mu Us desert during the first two decades of the 21th century.We used the vegetation photosynthesis model(VPM)in combination with remote sensing data(MODIS)to simulate GPP dynamics during 2005-2018.Eddy covariance(EC)measurements of GPP(GPP_flux)were used to parameterize and validate the VPM model.The“derivation and threshold”approach was used to determine the start(SOS)and end of the growing season(EOS),as well as the length of growing season(LOS)for each year.Ordinary least squares(OLS)regression and the Mann-Kendall analysis meathod(M-K)with the Sens'median slpoe was used to examine the variations in temperature,normalized differences vegetation index(NDVI),and GPP over time.OLS,multiple,and partial correlation analyses were used to test the relationships among temperature,NDVI,phenology,and GPP.(1)Modeled GPP well captured the dynamics of GPP_flux,whereas the MODIS product(MOD17A2H)significantly underestimated GPP_flux in the temperate semi-arid shrubland.(2)NDVI,annual maximum NDVI(NDVI_max),and annual GPP all showed an increasing trend during2005-2018,indicating the role of vegetation recovery in promoting GPP in the temperate semi-arid shrubland.SOS showed an advancing trend(21 d 14 yr^-1)and EOS showed a delaying trend(15 d 14 yr^-1),and therefore both SOS and EOS contributed to the increasing trend of LOS(36 d 14 yr^-1)in the temperate semi-arid shrubland.Besides,increases in LOS were consistent with the rising trend of spring(March-May),autumn(September-November),and annual mean air temperature.(3)Annual GPP was enhanced by earlier SOS(2.94 g C m^-2 yr^-1 for 1day advance in SOS),later EOS(3.33 g C m^-2 yr^-1 for 1day delay in SOS),and greater LOS(6.44 g C m^-2 yr^-1 for 1day increase in SOS),and the effect of LOS on annual GPP was significant(6.44 g m^-2 yr^-1),but weaker than that of SOS(11.39g m^-2yr^-1)in the temperate semi-arid shrubland.(4)Increases in vegetation cover and growing season length explained 79%and 57%of the variations in GPP in the temperate semi-arid shrubland,respectively.Increases in vegetation cover played a more important role than growing season extension in promoting GPP.(5)The annual mean value of NDVI in the Mu Us Desert increased by about 0.007 per year on average(P<0.05),and the annual mean value of NDVI mainly between 0.2 and 0.5,GPP increased significantly by about 9.75g C m^-2yr^-1,and the annual mean GPP value was mainly between 200-300 g C m^-2 yr^-1.The SOS was advanced by 2.62 days per year on average,EOS was significantly delayed by4.32 d/yr,and the LOS showed a significant prolongation trend of 6.98 d/yr in the Mu Us Desert;SOS was mainly concentrated in the 110th-170th day,and EOS was mainly concentrated in the 280th-295th day,the LOS time span was 100-252 days.(6)In the Mu Us Desert,the delay of EOS significantly increased GPP in 54.7%of areas,the extension of LOS significantly increased GPP in 81%of areas,and the increase of NDVI significantly increased GPP in 52%of areas.