| Grasslands are among the largest biomes in the world,accounting for nearly 25%of the land surface.Grasslands have many ecological functions,such as wind prevention,sand fixation,soil and water conservation,climate adjustment and air clearness.Grasslands also contribute to food security by providing food for ruminants,which are sources of meat and milk for human consumption.Due to the largest distribution,grasslands play a key role in balancing the concentrations of global atmospheric greenhouse gases,reducing the effects of green-house gases through carbon storage and sequestration.With the global climate change and intensification of human activities,structures and functions of grasslands have been changing accordingly.Assessing the effects of climate change on the spatiotemporal distribution and dynamics of grasslands do not only help to understand the interactions of global change and grassland ecosystems,but provide baselines for grassland carbon cycle and global terrestrial carbon cycle assessment.In this dissertation,the spatiotemporal distribution and variations of grassland net primary productivity(NPP),carbon storage,soil respiration(Rs),heterotrophic respiration(Rh)and net ecosystems productivity(NEP)in China,North America,Europe,and Australia were assessed and compared in 1981-2010.The correlations between the NPP of each grassland type and climate factors were also evaluated to reveal the responses of grasslands to climate change.Then,the spatiotemporal distribution and variations of grasslands,as well as NPP and NEP at global and continental scales in the past 100 years were quantitatively assessed by using the modified CSCS,the segmentation model and the one-compartment model based on humidity index-K.Their future trends under different climate scenarios were also predicted.Finally,the global grassland degradation was conducted by selecting the NPP and grass coverage as indicators.The relative roles of climate change and human activities in grassland degradation was quantitatively assessed and spatialized based on three NPPs-potential NPP,actual NPP and HANPP.This work achieved the following outcomes:1.Quantitative assessment of the spatiotemporal dynamics of grassland productivities in the four typical regionsThe spatiotemporal and temporal dynamic of grassland NPP,carbon storage,Rs,Rh and NEP in China,North America,Europe,and Australia were assessed and compared in 1981-2010.Subsequently,the correlations between the NPP of each grassland type and climate factors were evaluated to reveal the responses of grassland ecosystems to climate change.The results showed that:(1).North America,which has the largest area of grassland ecosystems,exhibits maximum grassland NPP of 4225.30± 215.43 Tg DW·yr-1,whereas Europe,which has the least area of grasslands among the four regions,exhibits minimum grassland NPP of 928.95 ± 24.68 Tg DW·yr-1.Grassland NPP presented an increasing trend in China and Australia,but decreasing in Europe and North America from 1981 to 2010;(2).The largest carbon storage was in North America with 145.25 Pg C,while the least storage of 36.42 Pg C in China.Carbon storage in Europe and Australia is 45.46 and 52.38 Pg C,respectively.More than 95%of carbon was stored in soil;(3).The highest Rs was found in Australia with 5.72±0.62 Pg C·yr-1,while the lowest Rs was in Europe with 1.39±0.05 Pg C·yr-1.By contrast,Rs is 2.13 ± 0.07 and 5.55± 0.18 Pg C·yr-1 in China and North America,respectively.As to the Rh,the maximum value of 2.96±0.09 Pg C·yr-1 was found in North America,and the minimum value was in Europe with 0.73 ±0.02 Pg C·yr-1.In China and Australia,Rh is 1.12 ± 0.03 and 2.92 ± 0.28 Pg C·yr-1,respectively.In 1981-2010,Rs and Rh showed an overall increasing trend in China,Europe and Australia,but a decreasing trend in North America;(4).The highest and lowest NEP was in Europe and Australia with-11.92±9.22 and-1176.03 ±61.73 Tg C·yr-1,respectively.The grassland NEP in China and North America was-22.46±21.45 and-682.73±48.90 Tg C·yr-1,respectively.The grassland NEP showed an overall increasing trend in Australia,but a decreasing trend in the other three regions;(5).Grassland NPP is positively correlated to mean annual precipitation,but demonstrates notable differences with mean annual temperature.2.Quantitative assessment of spatiotemporal distribution of global grassland ecosystems and driving factors analysisThe spatiotemporal distribution of grasslands at global and continental scales in the past 100 years were quantitatively assessed by using the modified CSCS.Their future trends under different climate scenarios were also predicted.In addition,the shift distances and directions of different grassland types were analyzed.The results showed that:(1).The area of global grassland is(5100.21 ± 59.06)×104 km2.Among the five grassland types,the largest distribution is the tropical savannas with(2010.05± 108.32)× 104 km2,while the least area is in typical grassland with(414.21 ± 19.00)× 104 km2,the area of the tundra&alpine steppe,desert grassland,and temperate humid grassland is(1442.78 ± 85.73),(780.84 ± 13.16)and(452.32± 32.26)× 104 km2,respectively;(2).In the past 100 years,the area of global grasslands decreased from 5175.73 to 5102.16 × 104 km2,in which the tundra&alpine steppe decreased the most with 192.35× 104 km2,while the area of the desert grassland,the typical grassland and the temperate humid grassland declined by 14.31,34.15 and 70.81 × 104 km2,respectively.The distribution of tropical savannas increased by 238.06 × 104 km2;(3).The area of global grassland would continue to decrease during the following decades.The most decrease would occur in the RCP8.5 scenario with 516.55 ×104 km2,while the least decrease of 405.84× 104 km2 would take place in the RCP2.6 scenario.In RCP4.5 and 6.0 scenarios,grassland distribution would shrink by 503.74 and 482.02×104 km2,respectively;(4).Asia has the largest distribution of grasslands with(1940.62 ± 48.14)×104 km2,while the minimum distribution is in Europe with(201.52± 12.95)× 104 km2,the grassland area in Africa,North America,South America and Oceania is(1007.72± 24.14),(1065.10± 53.19),(397.39 ± 7.19)and(487.85 ± 47.31)÷104 km2,respectively;(5).In the past 100 years,the area of grassland decreased in Asia,Europe and North America but increased in other continents.In the following decades,the grassland area would continue to decrease in Asia and North America but increase in Africa and South America.In Europe,grassland area would increase obviously in the RCP8.5 scenario,but change slightly in the other scenarios.Grassland would shrunk in the next decades in Oceania.(6).In the past 100 years,in the northern hemisphere,the mean center of temperate humid grassland shifted towards northwest,while all the other grassland types moved towards northeast.The longest shift distance was found in typical grassland with 633.11km.In the southern hemisphere,the mean centers of desert grassland and tropical savanna shifted towards southwest and southeast,while all the other grassland types were estimated to move northwards.The longest shift distance of 1289.75 km occurred in the typical grassland.In the next decades,grassland types would move the longest distance in the RCP8.5 scenario.There are more discrepancies in the shift directions in RCP2.6 scenario among GCMs.3.Quantitative assessment of the spatiotemporal dynamic of global grassland NPP and driving factors analysisThe spatiotemporal distribution and dynamics of grassland NPP were quantitatively assessed by using the segmentation model based on humidity index-K.The correlations between the grassland NPP and climate factors were also analyzed to reveal the sensitivity of grassland NPP to climate factors.The results showed that:(1).The average global grassland NPP is 26.09±0.44 Pg DW·yr-1.In the five grassland types,the maximum NPP was found in the tropical savannas with 14.08±0.86 Pg DW-yr-1,the second highest was the tundra&alpine steppe with 5.88±0.36 Pg DW·yr-11,the minimum NPP was in the typical grassland with 1.59± 0.06 Pg DW·yr-1,the NPP of desert grassland and temperate humid grassland savannas was 2.47±0.02 and 2.07±0.12 Pg DW·yr-1,respectively;(2).In the past 100 years,global grassland NPP showed an overall increasing trend,which increased by 745.32 Tg DW.yr-1 in 1920s-1990s.In the following decades,grassland NPP would exhibit different changing trends under different scenarios.In the RCP2.6 scenario,grassland NPP would not change obviously.In the RCP4.5 scenario,grassland NPP would increase slightly.In the RCP6.0 scenario,grassland NPP would increase after 2050s.In the RCP8.5 scenario,grassland NPP would increase sharply.In 2070s,grassland NPP would increase by 2.88%,4.45%,5.70%,and 12.35%,respectively;(3).NPP of the tundra&alpine steppe and temperate humid grassland decreased gradually in the entire study period,while NPP of the desert grassland and the typical grassland showed larger fluctuation,and would present various changing trend in different scenarios.NPP of the tropical savanna increased gradually in 1920s-2070s,and would increase by 53.61%in the RCP8.5 scenario and 23.76%in the RCP2.6 scenario;(4).In the six continents,the maximum grassland NPP is in Asia,which occupies 30.73%of total grassland NPP.The minimum grassland NPP is found in Europe,which occupies 4.40%of total grassland NPP.Grassland NPP in Africa,North America,Oceania and South America take up to 27.69%,17.26%?10.64%and 9.29%of total grassland NPP,respectively.(5).In the past 100 years,grassland NPP in Asia,Africa,Oceania and South America showed an overall increasing trend,while in Europe and North America,grassland NPP decreased.In the following decades,grassland NPP would decrease in Asia and Oceania and increase sharply in Africa and South America.In Europe and North America,grassland NPP would change slightly.(4).Grassland NPP is more controlled by MAP at the global scale.4.Quantitative assessment of the spatiotemporal dynamic of global grassland NEP and driving factors analysisThe spatiotemporal dynamics of grassland NEP in the past 100 years were quantitatively assessed by one-compartment model based on humidity index-K.Their future trends under different climate scenarios were also predicted.The correlations between grassland NEP and climate factors were also analyzed to reveal the sensitivity of grassland NEP to climate factors.The results showed that:(1).The average global grassland NEP was 117.66 ± 173.44 Tg C·yr-1.Among the five grassland types,the NEP of typical grassland is-41.94±32.38 Tg C·yr-1.The highest NEP is in the tundra&alpine steppe with 82.38 ± 108.16 Tg C·yr-1,the average NEP of the tropical savannas is 46.00 ± 39.57 Tg C·yr-1;the minimum NEP of 4.61±7.01 Tg C·yr-1 is found in the desert grassland,the average NEP of the temperate humid grassland is 26.61±27.43 Tg C·yr-1;(2).In the past 100 years,global grassland NEP decreased from 8.40 Tg C·yr-1 in 1920s to-42.91 Tg C·yr-1 in 1990s,implying that global grassland ecosystems transformed from absorbing carbon to releasing carbon.In the following decades,grassland NEP will decrease to-713.50 ± 302.29 Tg C·yr-1 in 2070s in the RCP8.5 scenario.In the RCP2.6 scenario,grassland NEP will increase slightly in 2030s,and will amounted to-166.63 ± 103.14 Tg C·yr-1 in 2070s;In RCP4.5 and RCP6.0 scenarios,grassland NEP will reach 424.51±177.63 and 406.43±167.49 Tg C·yr-1,respectively,in 2070s;(3).In the six continents,grassland in Asia has the strongest carbon storage ability.In the past 100 years,grassland ecosystems in the South America have been a net carbon source,while grasslands in other continents are net carbon sinks.However,all grasslands would become net carbon sources at the end of this century;(4).Grassland NEP is more controlled by MAP at the global scale.5.Remote sensing monitoring of global grassland degradation and driving factors analysisGrassland NPP and vegetation coverage were selected as indexes to monitor the global grassland degradation in 2000-2013,and the individual contribution of climate change and human activities in grassland dynamics were also quantitatively assessed based on three NPPs.The results showed that:(1).At the global scale,1401.01 × 104 km2 of grasslands experienced degradation,occupying 23.90%of global grassland areas.While 3017.24 × 104km2 of grasslands remained unchanged,taking up to 1.47%of total grasslands;(2).Most of regions experienced slight recovery,which occupied 16.30%of total grassland areas,while regions experiencing moderate and significant degradation took up to 15.30%and 2.07%,respectively;(3).In Asia and North America,regions exhibiting slight recovery took up to 17.55%and 23,48%of respective grassland covers,while grasslands experiencing slight degraded distributed the largest in other continents.The largest area of degradation and restoration both occurred in Asia;(4).Climate change was the dominant cause that resulted in 45.51%of degradation,compared with 32.53%caused by human activities.On the contrary,39.40%of grassland restoration was induced by human interferences,and 30.6%was driven by climate change;(5).NPP losses ranged between 1.40 Tg C·yr-1(in North America)and 13.61 Tg C-yr-1(in Oceania)because of grassland degradation.Maximum NPP increase caused by restoration was 17.57 Tg C·yr-1(in North America).Minimum NPP was estimated at 1.59 Tg C-yr-1(in Europe).The roles of climate change and human activities on degradation and restoration were not consistent at the continental level.6.The accuracy tests and error sourcesThe time scale of this study span from 1920s to 2070s,and the spatial scale ranged from regions to continents and to the global scale.The effects of climate change on grassland productivities are quantitatively assessed at multiple spatiotemporal scales.The outcomes of this research enrich the current grassland carbon dataset.However,the modelled results are difficult to be verified because of the large spatiotemporal scales.To reduce the uncertainty of modelled results,a three dimensional sampling-cross validation-comprehensive judgment method was used.The modelled results have been validated by filed observation data,various modelled results,and literatures.The validation tests showed that the modelled results are well in line with the current results.The main error sources lie in:firstly,only precipitation and temperature are input data in the models,other important parameters,such as human activities,fertilization effect of CO2,nitrogen deposition,and plant physio-ecological processes are not involved in.Nevertheless,models used in this research grasp the main climate factors that induce grassland occurrence and development.Therefore,the modelled results are capable of reflecting the responses of grassland productivities to climate change.Secondly,the errors of input data.Due to the few meteorological stations,it is a big problem to extrapolate to global scale from limited materials,particularly the precipitation data which showed great heterogeneity.Precipitation is the primary factor that affect grassland productivities,thus,the uncertainty of precipitation would cause the errors.Finally,there are discrepancies in climate data between GCMs due to different algorithm and parameter schemes.As a consequence,the modelled results driven by GCMs showed differences.In this research,we utilized the mean average value of GCMs to reduce the uncertainties.7.The innovations of this research lie in:(1).At present,the estimation of grassland productivities mainly focused on the homogeneous samples.In this research,the grassland NPP,carbon storage,Rs,Rh and NEP in the four typical grassland regions-China,North America,Europe and Australia were quantitatively assessed and compared by using optimized models.The outcomes are helpful to determine the carbon source/sink and storage ability of grassland ecosystems at regional scales.(2).The spatiotemporal distributions and dynamics of grassland area,NPP and NEP were quantitatively assessed at the global and continental scales by using the modified CSCS,segmentation model and one-compartment model based on humidity index-K.Their future trends under different climate scenarios were also projected.The results provide basic data for global grassland carbon cycle,and serve as a guide for regions or periods where lacking adequate collective datasets.In addition,these data can be supportive of the sixth IPCC assessment report.(3).Most of current research about grassland degradation mainly focus on the field observation.The monitoring indexes in different studies led to discrepancies in comparing the results,even in the same region.Furthermore,there was hardly any research about remote sensing monitoring of grassland degradation at larger scales and quantitatively assessment of its driving factors.Therefore,the remote sensing monitoring of grassland degradation was conducted at the global scale.Meanwhile,the relative contribution of climate change and human activities to grassland degradation was quantitatively assessed based on three NPPs-potential NPP,actual NPP and HANPP.The results do not explicit the global grassland degradation status,but determine the respect contribution of driving factors at different regions,and make clear the dominant factors of grassland degradation.The outcomes may also provide valuable suggestions for reasonable adjustment of ecosystem restoration programs and sustainable development of grassland resources,and check the efficiency of the restoration programs.8.SummaryThe vast distribution with complex terrain and climate conditions made grassland productivities showing great spatial heterogeneity.The past 30 years were estimated to be the warmest in the past 800 years.In this period,grassland NPP and Rh in China and Australia both increased.Higher increasing rate of Rh than NPP lead to the overall decrease of NEP in China,while in Australia,grassland NPP increased faster than Rh,which caused the overall increase of NEP.In Europe,NEP decreased due to the decrease of grassland NPP and the increase of Rh.In North America,grassland NEP decreased because of the overall decrease of both NPP and Rh.Grassland ecosystems are more vulnerable and sensitive to climate change.Climate change in the past 100 years has led to the shrunk of grassland globally,especially in the mid/high latitude.The area of tundra&alpine steppe reduced significantly,and were forced to head northward due to significant ascending temperature in the northern hemisphere,while the distribution of tropical savannas expanded at the same time.In the following decades,the area of grassland will continue to decrease,and the NPP will continue to increase,while the grassland ecosystems will release more carbon to the atmosphere.Precipitation is the dominate factor that affect grassland productivities.At the global scale,climate change is the leading factor of grassland degradation,while human activities dominates grassland recovery.The results of this dissertation do not provide insights into understanding the global change and terrestrial carbon cycle,and the roles of climate change and human activities in grassland carbon cycle,but may serve as guidelines for governments to work out the grassland eco-environmental problems,and provide data support for the sixth IPCC assessment report. |
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