| It is front-banner issue that predicting the dynamic of Net primary production(NPP)under global climate change which causes a series of environmental problems.NPP is not only the representation of carbon sink of ecosystems,but also the main index of plant's activity and ecosystem process.Plant phenology is the most sensitive biological indicator that represents the response of environmental conditions to climate change.Grassland is the second largest vegetation coverage of the terrestrial ecosystem,which plays an important role of terrestrial ecosystem: exchange of matter and energy,regulation global climate change,food security and ecology security.It is sensitive to climate change for the relative fragile system.The phenology changes of natural grassland vegetation are closely related to the global change process.It not only intuitively represents the trend of regional climate change,but also affects climate.In this study,we focus on the eastern part of northern temperate grassland in China,and analyzed the dynamics of phenology and NPP under climate change.It supported reasonable utilization grassland resource.In this study,we used 19 GCMs data under IPCC CMIP5 three type of emission scenarios which were calibrated by using the local meteorological stations data.So the reconstructed data was created from 2016 to 2100.We analyzed future climate change trend(2016?2100)based on the measured data(1961?2015).We simulated the phenology of study area by using these data(time span 140a).We also calculated annual NPP from 1961?2100 through Biome-BGC model after calibrating by using optimized algorithm.Lastly,we comprehensively analyzed the relationship of climate change,phenology and NPP.The main results are as follows:(1)We analyzed the trend of climate change.The results showed that the average temperature increased 0.32 ?/10 a from 1961 to 2015,and the growth rate has accelerated since1980.The total annual precipitation did not change significantly in past 55 years,but the variability increased during the year.Using the improved Thornthwaite model to calculate the wetness index over period,it showed the future climate will be partial to dry-warming.Hurst exponential analysis showed that the above three climatic indicators tended to have strong persistence in the future.The climatic analysis showed that the temperature will rise by 0.7 ?,2.5 ? and 5.4 ? respectively in the RCP2.6,RCP4.5 and RCP8.5 scenarios at the end of the21 st century,and the annual precipitation will increase by 20 mm,51mm and 91 mm respectively.The results increase in evapotranspiration is weak,so the overall climate tends to be warm and dry,and the degree is RCP8.5> RCP4.5> RCP2.6.(2)We used the cumulative Growing season index(GSI)model to correct the accumulated threshold of grassland for each meteorological site based on MODIS phenology data,and then calculated the annual SOS from 1961 to 2100.The model performance is good to simulate long term annual SOS.The result showed that SOS postponed from the southeast to northwest Geographically,mainly relation to local temperature.The higher the temperature,the SOS earlier.In the time series,most of the SOS were becoming earlier trend as rate of 1.8d/10 a in the average of whole study area and the fast rate is 5d/10 a.Under future climate change,SOS change is not significant in RCP2.6.SOS is earlier as a rate of 1.1d/10 a in RCP4.5,and 2.6d/10 a in RCP8.5.(3)We analyzed the sensitivity of Biome-BGC model,sieved the parameters which were sensitive to NPP simulation,then use the climbing algorithm to optimize the model parameters based on MODIS NPP data.After calibration,we simulated grassland NPP from1961?2100.It was found that there was no significant change in grassland NPP from 1961 to2015,which indicated that the current climate change did not have a substantial impact on grassland productivity.Referring to the present grassland degradation,human activities were the main effect that leading to the decline of grassland NPP.Although the future climate tends to dry-warm,grassland NPP showed an increasing trend consulting the model result.This may be due to increased precipitation in the future climate.(4)We simulated NPP of each site in study area from 1961 to 2100 by using calibrated Biome-BGC model previous.The result exhibited that NPP showed no significant trend in1961?2015.The average NPP is 145 g C/m2,range-7%?6% in AR,185 g C/m2,,range-9%?12%in SA,263 g C/m2,range-9%?11% in SH.It indicated that grassland production was not affected by climate change profoundly in the past 55 year.Refer to the situation of grassland degradation,the human activity mainly cause grassland NPP decline.Under future climate change scenario,grassland NPP showed increasement trend,the increasing rate is RCP8.5>RCP4.5>RCP2.6,respectively 3.1,1.6,0.08 g C/m2,which mainly leaded by precipitation increasement.For this appearance,the study area lack of rain water,the rising temperature cause short of water.We comprehensively analyzed the impact of climate change on SOS and grassland NPP.It is confirmed that precipitation is the main factor to determine NPP,and the temperature has negative effect on NPP in areas where annual mean temperature lower than 10 ?.The reason was that these grasslands are in relatively drier areas,and the increase in temperature leads to insufficient water in the growing season.And most areas were not sensitive to temperature,the main reason may be that the area of heat and water are concentrated in the summer(June,July,August),grassland NPP mostly accumulated in this period,while the annual average temperature increase,because the temperature is relatively high,so the relative temperature increase is not large.There was no significant correlation between SOS and NPP,and only some sites showed significant relationships.The weak advance of SOS leads to the extension of the growing season was not the main reason for NPP,and climate change that caused SOS changes was the primary consideration for NPP.The RUE showed no significant monotonous trend in the past 55 years.but exhibited significant decline under future climate change.Before we have analyzed that NPP and temperature is rising in the future,hence the RUE decline indicated that temperature rising retrained the NPP increasing as expecting.Main innovation of this study:(1)We rebuilt 2016?2100 daily meteorological based on meteorology stations climate data and CMIP5 GCMs data with applying downscaling model.Then we analyzed climate change processing from 1961 to 2100 under three scenarios.We also calculated annual SOS from 1961?2100 using accumulated GSI.To provide some reference and data support for the future policy of agriculture and animal husbandry in the research area(2)We applied Biome-BGC model on grassland which mainly using in woody area former.We analyzed SA of each parameter for applying Biome-BGC on grassland NPP simulation.Then we optimized the key parameters for each station modeling,ensuring the modeling accuracy match to long time series NPP study.Lastly simulated each station NPP from 1961?2100.(3)We composed short step climate change to drive model and analyzed the influence of temperature and precipitation on grassland NPP.This method is more sophisticated than previous study.We calculated 1961?2100 NPP of grassland by using calibrated process model We also analyzed how climate change effected SOS and NPP based one above previous study.Summary the effects of climate change on grassland productivity.In summary,we simulated long term phenology and annual NPP base on restructured climate data and analyzed relationship of them. |
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