The Synthetical Assessments Of Carbon Sequestration Ability In Temperate Eurasian Steppe Using The Improved Grassland Ecosystem Model BEPS-GESS

The long term co-effect of Climate dynamics and human activities is one of the important perspectives of global change,which has draw huge influence on land ecosystems.Grassland ecosystem is among the widest distributed ecosystems on land surface.It stores large amount of soil carbon and plays essential role in the land carbon water and energy cycling.Meanwhile,grassland ecosystems are also important to the regional/global economy and food security as the substance and energy sources to the agriculture and animal husbandry.Temperate Eurasian Steppe is the world’s largest transition ecosystem with an area of more than 13 × 106 km2.The region is characterized by a temperate continental climate with different degree of aridity.The diversity of the environmental conditions drives the variation in the grass vegetation within the Eurasian Steppe.The latitudinal change of climate with decreasing aridity degree and lengthening of the frost-free period enforce building different zonal types of steppe,which subsequently replace one another from north to south.The zonal steppe types are the following:meadow steppe(forest steppe),true or typical steppe in semi-arid climate,semi-desert steppe in arid climate,and desert grassland in hyper-arid climate.Because of the ecological and physiological characteristics of temperate grassland,the ecosystem is sensitive to the climate change.Meanwhile,the abundant natural resources are easy to be utilized,so the region is also seriously affected by the human beings.The combination of the two factors determines the spatio-temporal dynamics and variation trend of the regional carbon cycling.Land ecosystem model is currently a useful instrument to the global change study.With the global warming,the raising concentration of green house gases and the higher probability of extreme climate events,the application of land ecosystem models is continuously increasing.However,the regional modeling study on Temperate Eurasian Steppe is largely lacked.Besides,because of its human-driven feature,grazing disturbance is difficult to be assessed at large scale.Until now,the grazing impact on carbon cycling of grassland has not been well-evalutated in current land models.This study first developed a new grassland ecosystem model,BEPS-GESS based on a wide used model,BEPS.The new model revised some algorithms of important algorithms(e.g.carbon uptake and vegetation maintain respiration et al.),and coupled the disturbance modules(a statistics-process combined grazing scheme and a fire model based on Glob-FIRM).Thereafter,the model outputs were proved and evaluated against multi-scale of observation datasets,which indicated that BEPS-GESS could(1)improve its simulation ability on grassland than the original BEPS and(2)properly reproduce the fire and grazing dynamics.Then the model was applied to quantitative study(1)the spatio-temporal trend of major regional carbon and water outputs including NPP,NEP,ET and WUE during the past 30 years;(2)the regional influences of grazing and fire disturbance to carbon cycling;(3)the response of the grassland to climate change at different aridity level using the Aridity Index and(4)the current situation,management style and protection of grassland ecosystem by combining the model outputs with multi-aspects of regional politics,economy and society indexes,and then give potential sustainability suggestions.In the end,the model was synthetically evaluated and discussed by importing inter-model comparison,parameter sensitivity analysis and a model decomposition analysis based on "traceable scheme",which will give potential suggestions and directions for the future modeling developments and improvements.This study includes the following major outcomes:1.The synthetical validation And Accuracy evaluation of major outputs of BEPS-GESS.The BEPS-GESS was developed based on a late version of BEPS with parameter and algorithm revisions and new disturbance modules.The model performance was tested and validated against multi-source observation datasets.The results showed that the R2 between model NPP results and field observations was 0.79,which is higher than the BEPS results by 0.08.The comparison of modeled NEP against the flux sites ’observation indicated that the R2 ranges from 0.74 to 0.85 at the 5 sites and the RMSE ranges from 0.4-0.96 gC/m2.by which the R2 is higher by 0.09 and RMSE is lower by 0.23 gC/m2 than the orginal BEPS.The comparison of modeled ET against the 4 flux sites’ observation showed that the average R2 is 0.79 and RMSE is 9.96 W/m2.The reults from BEPS and BEPS-GESS is very similar.The modeled grazing activity and fire simulation is validated against multi-year fenced experiments and remote sensing observation product,respectively.The results indicated that BEPS-GESS could catch the seasonal variations of grassland under different livestock pressure,and the model prediction is consistent with the observations(R2=0.71,p<0.01);the consistence between modeled fire area and the L3RJC remote sensing observation reaches 86%,and the fire occurrence is well simulated as well(R2=0.52,p<0.05).2.The spatio-temporal dynamics of NPP and its response to climate in Temperate Eurasian Steppe.During the period of 1982-2008,the annual mean NPP is 120 gC/m2 for the entire region,where the value is 116.9 gC/m2 for Mongol Steppe and 122 gC/m2 for Kazakh Steppe.At the background of regional warming and drying trend,NPP of Mongol Steppe was increasing,and the trend was increasing for Kazakh Steppe during the Soviet period and then went down after its collapse.Regional NPP is significantly positive correlated with mean annual precipitation,while the correlationship beween NPP and mean annual temperature is weak,which indicated that precipitation is the major climate factor to the regional vegetation growth.3.The spatio-temporal dynamics of NEP and its response to climate in Temperate Eurasian Steppe.During the period of 1999-2008,the regional annual NEP is 12.86 gC/m2,where Mongol Steppe was 14.61 gC/m2 and Kazakh Steppe was 4.55 gC/m2.The result indicate that this region was a small carbon sink with sequestration ability of 57 TgC yr-1.The spatio-temporal results showed that because of the climatic "warming and drying" trend,the carbon sequestration ability was getting weak and have a possibility to become a carbon source,especially for the Kazakh Steppe,where NEP was reducing during the decade.Regional NEP is correlated with mean annual precipitation,but have no correlationship with mean annual temperature.grassland under different temperature showed different correlationship between NPP and AGSP.The R2 are very similar(0.265)between the NEP and AGSP at LAGPT and MAGPT regions,while the R2 is much higher(0.507)at the HAGPT region.Althouogh NEP is correlated with AGSP,the correlationship is much weaker,the corresponding R2 were 0.09,0.18 and 0.03 at LGST,MAGPT and HAGPT,respectively.The results indicated that AGSP is the major climate factor for the grassland carbon sequestration,but the AGPT has no direct effect on NEP.4.The spatio-temporal dynamics of WUE in Temperate Eurasian Steppe.During the period of 1982-2008,regional mean annual WUE was 0.55 gC/m2 mm yr,where Mongol Steppe was 0.63 gC/m2 mm yr and Kazakh Steppe was 0.5 gC/m2 mm yr.The regional WUE is increasing,where the WUE in Mongol Steppe increased by 0.0036 gC/m2 mm yr,while in Kazakh Steppe,WUE only increased during the Soviet period with 0.011 gC/m2 mm yr.The WUE trend is mainly caused by the NPP enhancement during the 27 years,for example,in Mongol Steppe,NPP showed significant increasing trend with a rate of 0.96 gC/m2(p<0.001);while in Kazakh Steppe,NPP increased rapidly during the Soviet period(3.78 gC/m2 yr,p<0.005)and then decreased after its collapse(1.56 gC/m2 yr,p<0.05).During the 27 years,ET did not show significant trend,the only observed trend was in China with an enhancement of 0.54 mm/yr(p<0.05).5.The spatio-temporal dynamics of major disturbance factors to regional carbon cycling in Temperate Eurasian Steppe.During the period of 1999-2008,the grazing activity in Temperate Eurasian Steppe annually consumed 1.41 × 1013 gC,while the value was 1.59×1010 gC for fire,respectively.The two disturbances together took up 2.9%of the regional annual NPP.At the decade,the impacts of both disturbances increased significantly(graze activity:R2=0.82,p<0.001,fire:R2=0.51.p<0.05).Herbivore number almost linearly increased,especially for the Kazakh Steppe.6.Regional assessment and mechanical analysis of climate response to carbon cycling based on the Aridity Index.The regional NPP,LAI,ET and WUE exhibit clear increasing trend from arid zone to humid zone.The highest NPP,LAI and WUE were found in dry and semi-humid sub-region(DSH),while the lowest ones were found in arid sub-region(AR);The highest ET was found in humid sub-region(HU)and the lowest ET was found in AR with 175.65 mm.Over 65%of the region exhibited increasing WUE.This WUE improvement,however,could not indicate the recovery of grasslands because the NPP even slightly decreased.It was mainly attributed to the reduction of ET over 70%of the region,which is closely related to the rainfall decrease.LAI is well correlated with WUE at the sub-regions with various aridity levels,the R2 ranges from 0.702-0.859(p<0.01),which make it a good proxy to predict WUE.The results also suggested a similar negative spatial correlationship between the WUE and the mean annual precipitation(MAP)at the driest and most humid ends.This regional pattern reflected the different roles of water in regulating the terrestrial ecosystems and the response of grasslands under different aridity levels.7.The sustainability analysis of regional grassland ecosystem.In Kazakh Steppe,both the collapse of Soviet Union and Economic privatization draw huge impacts on regional animal husbandry and grassland production.As the major owner of the grassland,the collapse of Soviet Union caused a breakdown of animal husbandry in Kazakhstan.More than 2/3 of the national livestock lost due to the political event.Meanwhile,the economic privtizaition process caused a continuous reduction of grassland production after 2000.In Mongol Steppe,the different political and economical development leads to a wide divergence of grassland utility and management between Mongolia and Inner Mongolia.In past 30 years,the population,GDP and livestock number in Inner Mongolia is much higher than those in Mongolia,especially after 2000,while the grassland total production in Mongolia is slightly higher than that in Inner Mongolia.The synthetical indexes showed that after entering the 21 century,the proportion of animal husbandry in national economy was decreasing rapidly in every country of the region.However,the impacts of animal husbandry to grassland were enhancing.The obvious examples are Uzbekistan and Inner Mongolia.In Uzbekistan,the ratio of livestock number and grassland total production increased rapidly since 2000,it reached more than 10000 million head/PgC in 2008,which exceeds the mean annual value by 65.8%,which was significantly higher than other countries in the region.The index in Inner Mongolia also showed continuous increasing trend but reached a peak in 2007 with 49 million head/PgC,which exceeds the mean annual value by 85%.According to the different zonal develop situation and grassland resource features,this study gives specific suggestions of sustainability development of grassland ecosystem,and thus expect to offer useful reference to the regional maintainance and protection of grassland ecosystem resource,ecological functions and carbon sequestration ability.The innovations of this study includes:First,this study developed and applied a new grassland ecosystem simulator,BEPS-GESS.The model inherited the advantages of the orginal BEPS and also improved the descriptions of ecosystem processes;by integrating the graze and fire modules,BEPS-GESS could quantify the regional effects of major disturbances to the carbon cycling.The new statistics-process combined grazing scheme has a potential to be applied in a global grazing evaluation.Secondly,this study could fill the gaps of the regional carbon cycling study in Temperate Eurasian Steppe,especially for Kazakh Steppe,where grassland carbon sequestration and carbon-water coupling,its response to climate change and disturbance effects have little been studied before.Thus,this study could offer important model results and statistical basis for the future study.Thirdly,this study incorporated a multi-data combined model validation,evaluation and uncertainty analysis system.This study based on both knowledge of grassland and carbon balance theory,then incorporated multi-source observation dataset(from field study to remote sensing observations),and thus synthetically evaluated the model performance on regional carbon study;In addition,this study first applied the "tracable scheme" to the remote sensing based land model.The analytical model decomposition could facilitate the understanding of the model structure and point out where the major uncertainty comes from,and thus instruct the future direction of model improvement.To sum up,by improving BEPS on grassland ecosystem and combining multi-scale statistics and analytical methods,this study present a synthetical quantitative research on carbon cycling in Temperate Eurasian Steppe.The outputs of this study could contribute new thoughts and solutions for regional carbon cycling evalutation and thus offer important statistical supports and scientific evidences to the regional grassland utility and management assessments.