| Global changing is one of the most challenging environmental problems and it has seriously affected the terrestrial ecosystem carbon and water cycle.Vegetation phenology,which is expected to be one of the most sensitive bio-indicators of the impact of global warming,can directly or indirectly regulates the exchange of carbon(gross primary productivity,GPP)and water(evapotranspiration,ET)between the lower atmosphere and the biosphere.Tibetan Plateau has a typical and unique vegetation composition and climate features,thus sensitive to climate change than other regions.In addition,the shelter function of Tibetan Plateau has an important effect on the ecological and water security in China.Therefore,it is importan to study the influence of climate change on the vegetation phenology and the water use efficiency(WUE)in the Tibetan Plateau.Based on two vegetation indices(MODIS NDVI and SPOT NDVI),we firstly explored the spatiotemporal changes of the start of growing season(SOS),peak of growing season(POS),end of growing season(EOS)and length of growing season(LOS)of the different vegetations in the different ecoregions in the Tibetan Plateau,using four methods(HANT_MaxRatio,HANT_MidPoint,Polyfit_MaxRatio and Polyfit_MidPoint).Secondly,the relationship between climate changes and vegetation phenology was studied by the forward selection from Redundancy analysis(RDA).Thirdly,we analyzed the spatiotemporal patterns of the seasonal and annual GPP,ET and WUE(the ratio of GPP to ET)for different vegetations in the Tibetan Plateau by using the MODIS GPP and ET and the VPM simulated GPP and the RS-PM simulated ET.Finally,we analyzed the patterns and sensibility of interannual variations of GPP,ET and WUE in response to phenology using forward selection from RDA.The results are listed as below.(1)The spatiotemporal distribution of vegetation phenology showed a strong spatial variations.The SOS of the needleleaf forest,broadleaf forest and shrub in the eastern Tibetan Plateau ranged from 120 to 150 day of year(DOY),the EOS could be 300 DOY and the length LOS was more than 180 days.While the SOS of the steppe and desert in the western Tibetan Plateau was later than 180 DOY,the EOS was as early as 250 DOY and the LOS was shorter than 100 days.Generally,the SOS for all vegetation in the Tibetan Plateau showed significant advancing trends(-0.41 days year-1,P<0.01),the EOS delayed significantly(0.47 days year-1,P<0.01),and the LOS increased significantly(0.88 days year-1,P<0.01),while the POS changed insignificantly.Although,the SOS,EOS and LOS of the different vegetations in the whole study area changed significantly,the phenology of the different vegetations in the different ecoregions showed significant difference.The SOS of the desert in the Qaidam montane desert zone(IID2)advanced by-1.15 days year-1(P<0.01)and the average EOS of which delayed by 1.47 days year-1(P<0.01).While the SOS of meadow in the Tibetan Plateau only advanced by-0.24 days year-1(P<0.01)and the EOS delayed by 0.24 days year-1(P<0.01).Moreover,the SOS,EOS and LOS of the meadow in the southern Qinghai high-cold meadow steppe zone(IC1)and the steppe in the Qiangtang high-cold steppe zone(IC2)is statistically insignificant.(2)The Redundancy analysis(RDA)and the partial correlation analysis indicated that the SOS for all vegetations in the Tibetan Plateau was mainly affected by the mean temperature in spring.The mean temperature had the highest explanatory power(22.40%,P=0.05)in the conditional term effects on SOS,while the explanatory power of the accumulative precipitation of the spring was only 0.30%(P>0.1).While the POS for all vegetations was significantly correlated with the accumulative precipitation of summer(20.70%,P=0.05),the EOS was significantly affected by the mean temperature in autumn(47.10%,P<0.01)and the LOS was significantly affected by the annual mean temperature and accumulative precipitation(47.80%and 12.40%,P<0.05).However,the phenology of the different vegetations in the different ecoregions were affected by different climatic factors.For example,the phenology of the meadow and steppe in the western Tibetan Plateau were mainly affected by the mean temperature,while the SOS and EOS of needleleaf forest,broadleaf forest and shrub in the eastern Tibetan Plateau were significantly affected by the mean temperature and the accumulative precipitation.Moreover,the SOS of evergreen broadleaf forest were mainly affected by the accumulative precipitation in spring(27.40%,P<0.05)and the LOS of desert in the Tibetan Plateau was only affected by annual accumulative precipitation significantly.(3)The GPP and ET estimated from MODIS products and the VPM and RS-PM model followed a distinct east-west transition.Overall,the GPP and ET of the needleleaf forest,broadleaf forest and shrub in the eastern Tibetan Plateau was higher than the steppe and desert in the western Tibetan Plateau.Spatially,the WUE of the vegetation in the IIC2 ecoregion had larger WUE,while the smaller WUE were found in the center of the Tibetan Plateau.Futhermore,the seasonal and annual GPP and ET of the all vegetation in the Tibetan Plateau showed increasing trends,while the WUE changed insignificantly.The annual MODIS GPP and the VPM GPP of the all vegetation in the study area increased by 1.25 and 2.13 gC m-2 year-1(P<0.05),and the annual MODIS ET and VPM ET increased by 2.70(P<0.01)and 2.25 mm year-1(P<0.01),respectively.The seasonal and annual GPP,ET and WUE of the different vegetation in the different ecoregions showed different interannual variations.For example,the GPP of the different vegetation in the study area during spring increased significantly except the steppe.Moreover,the increasing trends of the spring,summer and annual GPP of the vegetation in the IIC2 ecoregions is significant(P<0.05).The annual MODIS GPP of the deciduous broadleaf forest in the IIC2 ecoregion increased by 4.79 gC m-2 year-1(P<0.01),while the annual GPP of the desert in the ID1 ecoregion increased by 0.46 gC m-2 year-1(P<0.01).The the seasonal and annual ET of the needleleaf forest,broadleaf forest and shrub in the eastern Tibetan Plateau increased significantly and the annual MODIS ET deciduous broadleaf forest in the IIC2 ecoregion increased by 6.57 mm year-1(P<0.01).Generally,the seasonal and annual WUE for the majority vegetations in the Tibetan Plateau had no significant changes,while the summer and annual WUE of the needleleaf forest,broadleaf forest,shrub and meadow in the IIC2 ecoregions and the desert in the IID2 and ?D3 ecoregions reduced significantly.(4)The partial correlation analysis indicated that the spring GPP,ET and WUE of the vegetation in the whole Tibetan Plateau showed negative correlation with SOS.the main patterns of spring WUE response to SOS suggest that an increase in spring WUE with an earlier SOS were mainly because the increase in GPP is relatively larger in magnitude compared to that of ET.The spring ET is significantly affected by the SOS,and the explanatory power of the SOS to spring ET is 15.70%(P<0.05).Futhermore,the annual GPP and ET of the all vegetation in the Tibetan Plateau is significantly affected by the LOS,with the explanatory power of the LOS to GPP and ET of 10.30%(P<0.05)and 41.30%(P<0.01).The seasonal and annual GPP,ET and WUE for the different vegetations in the different ecoregions showed different response patterns to phenology.(a)The spring GPP of the needleforest,shrub and desert were mainly affected by the mean temperature and SOS,while the explanatory power of the SOS to spring GPP of broadleaf forest in the Tibetan Plateau is 69.60%(P<0.01)and 61.20%(P<0.01).Moreover,the spring ET of the different vegetations in the Tibetan Plateau were significantly affected by SOS and the spring WUE of the desert were mainly affected by the SOS(21.90%,P<0.01).(b)The summer GPP and ET of most vegetations in the Tibetan Plateau were affected by the mean temperature,while the explanatory power of the POS to summer GPP of broadleaf forest in the Tibetan Plateau is 49.70%(P<0.01)and 42.70%(P<0.01).(c)The autumn GPP and ET of the vegetation in the whole Tibetan Plateau showed significant positive correlation with EOS(P<0.05).The main patterns of autumn WUE response to EOS suggested that an increase in autumn WUE with a delayed EOS were mainly because the increase in GPP is relatively larger in magnitude compared to that of ET.Although the interannual variation of autumn GPP of the most vegetations was dominated by mean temperature of autumn and the interannual variation of autumn ET was dominated by accumulative precipitation of autumn and the autumn WUE was affected by the interaction of mean temperature and accumulative precipitation of autumn.The explanatory power of the EOS to autumn GPP of the steppe and desert in the Tibetan Plateau is62.1%(P<0.01)and 69.7%(P<0.01).Moreover,the autumn GPP of broadleaf forest dominated by EOS.(d)The annual GPP and ET of the most vegetations showed significant positive correlation with LOS.But the annual WUE of the majority vegetation in the eastern and western Tibetan Plateau is negatively correlated to LOS,because the increase in GPP is relatively smaller than the ET result from the increasing LOS,while the annual WUE of the vegetation in the center Tibetan Plateau is positively correlated to LOS,because the increase in GPP is relatively larger in magnitude compared to that of ET result from the increasing LOS.The RDA indicated that the annual GPP of the deciduous broadleaf forest,steppe and desert were dominated by LOS,meanwhile the annual ET of the different vegetations were significantly affected by LOS.In summary,the SOS of the vegetation in the Tibetan Plateau advanced significantly,the EOS delayed significantly and the LOS increased significantly,which are resulted from the increasing temperature and precipitation in the Tibetan Plateau.These changes in vegetation phenology resulted in the increased seasonal and annual GPP and ET.Although,the WUE of vegetation changed insignificanly because the same effect of phenology on GPP and ET,the WUE of the different vegetation in the Tibetan Plateau were significantly affected by vegetation phenology and showed different responsed patterns. |
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