| As an important ecological barrier and economic zone in China,the Yellow River Basin plays an important role in China’s economic and social development and ecological security.However,affected by climate change and large-scale and high-intensity human development and construction activities for a long time,the holistic and systematic ecological degradation of the Yellow River Basin is prominent,and the ecological security situation is not optimistic.In this context,how does the ecosystem of the Yellow River Basin evolve?Have the key drivers changed?How about coordinated development with urbanization?The exploration of these problems is of great significance for promoting the ecological protection and high-quality development of the Yellow River Basin and building a solid national ecological security barrier.Given that,taking the cascade effect of ecosystem structure,vigor,resilience and service as the baseline,538 districts and counties in the Yellow River Basin as the research unit,this thesis quantified the ecosystem service supply capacity,explored the temporal and spatial evolution of watershed ecosystem health(EH),identified the key factors affecting EH and the change characteristics over time,clarified coupling mechanism and spatial differences between urbanization and ecosystem,and then put forward differentiated suggestions for the coordinated development.This study aims to provide theoretical support and decision-making basis for environmental protection and high-quality development strategy of the Yellow River Basin.The main conclusions are as follows:(1)The evolution characteristics of land use pattern in the Yellow River Basin were analyzed from the perspectives of land use spatial distribution,structural characteristics,quantity change and transfer trajectory.Land use type in the Yellow River Basin is dominated by cultivated land and grassland with obvious agglomeration characteristics:cultivated land and built-up land are concentrated in the plain area of the middle and lower reaches,while the ecological conservation land such as grassland and forest land is mainly distributed in the mountainous and hilly areas of the upper and middle reaches.During the study period,the conversion of land use types was frequent:the net loss of cultivated land reached 2.547 million hm~2in 20years,mainly occupied by grassland and built-up land.The behavior of turning in and turning out of grassland was frequent,but it was mainly turning out.Unused land mainly flows to forestland and grassland;Build-up land is the largest transferable type,mainly occupying cultivated land and grassland.The area of water and wetland is small and relatively stable.Among them,the mutual conversion of land use types in the upper reaches was the most active,the middle reaches was relatively stable than the upper reaches,while the transfer of land use types in the lower reaches was simple.The migration trajectory of LUCC gravity center has obvious directionality:cultivated land gradually shifted to high-quality agricultural areas in the eastern plain.The gravity center of unused land showed an"inverted V"migration characteristic.The gravity center of built-up land gradually shifted to the downstream Central Henan and Shandong Peninsula,and the spatial expansion pattern showed that the independent development of cities gradually developed to the convergence of multi-center space.(2)Supply capacity of key ecosystem services in the Yellow River Basin was quantified by using In VEST,RUSLE and RWEQ models.The food provision service increased nearly twice in the past 20 years,reaching 325.9 million tons by 2020.The overall distribution pattern is high in the south and low in the north,high in the east and low in the west.The food provision capacity per unit area of lower reaches is the largest but the range of the low value area is obviously enlarged.Affected by the interannual variation of rainfall,the water yield fluctuated significantly during the study period,with the spatial distribution of decreasing from the southern mountainous and the eastern coastal area to the northwest inland desert.In the upper reaches,the water yield reached 30,678.1 billion m~3but the average water depth was low.In the lower reaches,the water yield was 10.786 billion m~3,but the average water depth was the highest,while in the middle reaches,the average water depth was the lowest,basically stable at about 25mm.Total carbon storage decreased by 0.496billion tons during the 20 years,and the spacial distribution was highly correlated with land use types.In the upper reaches,carbon storage was the highest and showed an upward trend,while the total amount and mean value of carbon storage in the middle and lower reaches showed a downward trend,resulting in reduced carbon storage in the whole basin.The total amount of soil conservation increased by 5.723.8billion tons,with the most obvious increase in the upper reaches.Soil conservation in the lower reaches was low,which basically stabled at about 300 million tons.The total amount of wind prevention and sand fixation increased by 108,190 tons,with the most remarkably increase in the Loess Plateau.In the upper reaches,the total amount and mean value of wind prevention and sand fixation were the largest,while the lowest in the lower reaches showed a slight decrease.The output of nitrogen and phosphorus showed a decreasing trend and the spatial distribution was basically consistent.The output of nitrogen and phosphorus in mountainous areas was significantly lower than that in plain areas.The total and mean value of nitrogen and phosphorus emissions in the upper reaches increased slightly,but decreased in the middle and lower reaches with the decrease of cultivated land.The increase of vegetation coverage improved the overall habitat quality of the Yellow River Basin,and its average value increased by 0.011 in the past 20 years,but the habitat quality still degraded in some areas.(3)The temporal evolution characteristics and spatial agglomeration effect of county ecosystem health in the Yellow River Basin were systematically evaluated based on the cascade effect of ecosystem structure,vigor,resilience and service.The overall landscape pattern of the Yellow River Basin tended to be complicated,fragmented and dispersed,and the landscape heterogeneity increased during the study period.The ecosystem organization and vigor were increasing,while the ecosystem resilience was decreasing in the Fenwei Valley in the midstream and the Shandong plain in the downstream,indicating that the ability of the ecosystem to resist external disturbance and self-recovery decreased with the intensification of external stress.The comprehensive supply capacity of ecological services improved obviously,but mainly in low,relatively low and medium levels,and the number of counties in the middle-lower reaches of the low-value area increased significantly.The EH level of the districts and counties in the study area mainly concentrated in three grades:poor,relatively poor and general.The overall health level was not high but the trend of improvement was obvious.Among them,the significant improvement areas were concentrated in the Loess Plateau and Sanjiangyuan Nature Reserve with high vegetation coverage in the middle and upper reaches,while the degradation areas were scattered in some districts and counties with better economic development conditions in the middle and lower reaches.The spatial agglomeration characteristics of EH were obvious.However,in recent years,the hot spot area was shrinking,while the cold spot area gradually expanded to the northwest of Henan province.The"polarization"phenomenon of the overall EH status has been further aggravated.(4)A machine learning algorithm was proposed to identify the key factors influencing watershed ecosystem health and reveal the nonlinear relationship and response threshold.Overall,the inhibition of natural factors on watershed EH is weakened,while the stress of human activities increases year by year.The mutation thresholds of land use intensity(LUI),population density(PD)and proportion of built-up land(POB)are 2.5,5000 people/km~2and 50%,respectively.Beyond the threshold range,EH will decline sharply.NDVI and slope are positively correlated with EH,especially when the slope is greater than 5 degrees,its positive promotion effect is significant.In the upper reaches,the impact of human activities on EH is gradually strengthened,but the natural factors are still the main factors.LUI and EH showed an"inverted U"trend.When the average annual temperature is roughly between 8-10℃,the EH decreases significantly.The mutation threshold of PD is approximately 3000 people/km~2.In the middle reaches,EH is obviously influenced by the synergistic mechanism of natural and human activities,showing a linear correlation between factors and EH,and the mutation threshold is not obvious.While in the lower reaches,the human activities are the main factors influencing EH.The mutation thresholds of LUI,PD and POB are 2.8,2500 and 40%,respectively.When the NDVI is lower than 0.7,EH is basically stable.(5)The coupling mechanism and spatial differences of EH and urbanization in538 county units of the Yellow River Basin were analyzed,and the differentiated coordinated development path was proposed.The urbanization level of the Yellow River Basin increased significantly during the study period but with low level and obviously spatial difference.In the downstream,the urbanization is the highest and the fastest,while the spatial difference of the urbanization in the midstream is more prominent than that in the downstream,and the urbanization in the upstream is lower and the growth rate is slow.The overall relationship between watershed urbanization and ecosystem is in the running-in and adaptation stage.The coordination degree in the upper reaches was the lowest and showed a declining trend in the past 10 years,while the coordination degree in the middle and lower reaches increased slowly but the overall trend of improvement is obvious.The spatial agglomeration degree of coordination is continuously strengthened.The high-value clusters are concentrated in the three regions centered on Zhengzhou,Jinan and Ordos,and the spatial scope is gradually expanding.The low-value agglomeration areas are concentrated in most districts and counties of Qinghai Province and northeastern Inner Mongolia in the upper reaches.The low-high and high-low heterogeneous areas are scattered in some districts and counties in Shanxi,Shaanxi,Gansu and Ningxia.The kernel density curve shows that the coordination degree of the districts and counties in the study area is significantly improved but the spatial difference is great.However,this expansion trend has decreased in recent years.The intra-regional and inter-regional differences jointly cause the overall differences of the basin,and the intra-regional differences are the main source of the overall discrepancy,indicating that narrowing the intra-regional differences in the upper,middle and lower reaches,especially in the middle reaches,is the key to improving the overall coordination level of the study area in the future.Finally,according to the ecological environment conditions and economic development status of the upper,middle and lower reaches,differentiated development paths are proposed from three aspects:ecological protection and restoration,characteristic urbanization construction and high-quality coordinated improvement. |
