Coupling Relationship Between Soil Hydrothermal Environment And Alpine Meadow Changes Based On Remote Sensing-Taking The Yellow River Source Area As An Example

As an important part of the Chinese"water tower",the alpine ecosystem of the Yellow River source area plays an irreplaceable role in water conservation,climate regulation,and safeguarding the livelihood of farmers and herdsmen.However,with global warming,the continuous warming of the Yellow River source area has led to significant changes in permafrost,which profoundly affects the sensitive and fragile alpine ecosystem.Ecosystem change in the alpine zone is a complex process of interaction and interdependence between the near-surface atmosphere-vegetation and permafrost environments,and its response to climate warming and its induced degradation of permafrost is also a continuous process on a long time scale.In-depth understanding of the impact of permafrost environmental changes on alpine ecosystems and their interactions in the context of climate warming is of great scientific and practical significance for regional ecological protection management and ecological governance.At present,the research on permafrost hydrothermal environmental change is mostly centered on permafrost engineering or local short-term observation,and most of the research on the relationship between permafrost environment and alpine vegetation is in the stage of qualitative description and lacks of quantitative analysis,especially the quantitative analysis of the long time series on the regional scale is even more lacking.Therefore,this paper simulates and analyzes the changing law of climate,permafrost hydrothermal environment and alpine grassland vegetation in the Yellow River source area during the past 40 years based on meteorological stations and multi-source remote sensing data,and quantitatively identifies the influence of climate and permafrost environmental factors on alpine grassland vegetation degradation,and estimates the contribution rate of the main influencing factors,and at the same time,also analyzes the relationship between alpine grassland vegetation in the Yellow River source area under the multifactors coupled effect of water,temperature,and human activities through constructing a structural equation model.Meanwhile,the degradation of alpine grassland in the Yellow River source area was analyzed by constructing a structural equation model for the coupling of moisture,temperature and human activities.The main research results are as follows:During the past 40 years,the air temperature,surface temperature,precipitation,soil moisture and other hydrothermal elements in the source area of the Yellow River have shown different degrees of increase,and the whole is in the trend of warming and humidification.Among them,the annual average temperature increase rate was0.525°C/10a,and the temperature increase rate in the growing season was 0.445°C/10a,which was 1.5 times of the warming rate in the same period in the Tibetan Plateau;the surface temperature increase rate was 0.31°C/10a;the annual precipitation and the precipitation increase rate in the growing season were 14.33 mm/10a and 9.3 mm/10a,respectively;the surface soil water moisture increase rate was 0.5×10^-3 m³/m³ yr^-1.The changes of each hydrothermal element have regional variability in space,which shows that the cold and dry northwestern part is significantly warm and humid,while the warm and humid southeastern part shows a trend of warm and dry,which is the most significant in the warm season.40 years of rapid warming have made the perennial permafrost of the source area of the Yellow River be in a tendency of rapid thawing or disappearance.The percentage of perennial permafrost area decreased from 69.4%in1980s to 46.1%in 2010s,and more than 23%of the perennial permafrost degraded to seasonal permafrost in the past 40 years;meanwhile,the freezing depth is continuing to become shallower（0.015m/10a）,the thawing depth is continuing to become thicker（0.033m/10a）,and the thawing intercalation layer is increasing（0.037m/10a）,and the annual melting duration is also increasing（1.17d/10a）.With climate warming and significant changes in the permafrost environment,the grassland vegetation in the Yellow River source area shows localized improvement and increased degradation,and the ecological environment as a whole is in a deteriorating situation.Although the average NDVI in the Yellow River source area showed an increasing trend at the rate of 0.0034/10a,accompanied by the advancement of the greening period,the delay of the yellowing period,and the lengthening of the growing season,the vegetation degradation of different degrees still occurred in the area with the proportion of 30.91%since the 1880s,and the proportion of degraded area increased from 51.47%to 60.70%in the 40 years,among which the alpine meadow grassland had the largest proportion of degraded area（the largest proportion of degraded area）.The proportion of degraded vegetation area increased from 51.47%to 60.70%in 40 years,among which the degraded area of alpine meadow grassland was the largest（53.53%）.Through statistical analysis,it was found that the degradation of alpine meadows in the source area of the Yellow River mainly occurred in the seasonal permafrost area（43.67%）and the degraded area that had been degraded from perennial permafrost to seasonal permafrost in the past 40 years（35.97%）,and that the degradation of alpine meadows was most significant in the seasonal permafrost area below 4,300 m above sea level,and between 4,700 and 4,900 m above sea level,as well as the sunny slopes of 12°and above.The increase of temperature and its induced permafrost degradation,the increase of thawing depth,the prolongation of thawing duration,and the localized supersaturation of surface soil moisture are all the influencing factors for the degradation of grassland in the Yellow River source area.By quantitatively estimating the contribution of each influence factor to NDVI changes in different permafrost zones,it was found that in the perennial permafrost zone,the prolongation of thawing duration and its enhanced surface soil moisture were the dominant factors of alpine grassland vegetation degradation;in the seasonal permafrost zone,the excessive temperature and excessive depth of permafrost thawing were the dominant factors of alpine grassland vegetation degradation.In the degraded alpine grassland in the Yellow River source area,10.43%of the area is mainly contributed by temperature with negative effect,9.4%of the area is mainly contributed by surface soil moisture with negative effect,9.05%is contributed by depth of permafrost thawing with negative effect,7.11%is contributed by duration of permafrost thawing with negative effect,and the contribution of the other factors with negative effect is not significant or dominated by positive effect.According to the results of quantitative analysis of structural equation modeling,climate change and its induced changes in the near-surface permafrost hydrothermal environment comprehensively explained 79.4%of the changes in grassland vegetation,and the complex natural environmental conditions in the Yellow River source area to a large extent indirectly influenced the growth of grassland vegetation by changing the local regional moisture and temperature conditions under the influence of surface microtopography,which in turn made the grassland vegetation growth in the Yellow River source area form a temporal and spatial differentiation pattern at a smaller The pattern of spatial and temporal differentiation was formed on a smaller spatial scale.After considering the inclusion of human activities,it was found that the combination of moisture,temperature changes and human activities explained 80.3%of the changes in grassland vegetation in the Yellow River source area,and the positive effect of ordered human activities on vegetation growth in the Yellow River source area was generally greater than the negative effect.In summary,under the background of global warming,more than 1/5 of the perennial permafrost in the source area of the Yellow River from 1981 to 2020underwent significant degradation with rapid warming.At the same time,the degradation of nearly 1/3 of the alpine grassland in the Yellow River source area has increased,and the ecological environment is deteriorating.The rapid warming and the significant changes in the elements of the permafrost environment caused the ecological pattern of increased degradation of alpine grassland.A structural equation model integrating natural environmental conditions,moisture conditions,temperature conditions,and human activities was finally proposed to explore and quantitatively assess the relationship between multi-factor coupling and alpine grassland degradation.The results of this paper can provide support for ecological environmental protection and management of river disruption,soil erosion,grassland ecosystem deterioration,land sanding or desertification.Meanwhile,recognizing and understanding the influence of permafrost environmental changes on alpine vegetation and the interaction relationship in the context of climate change can provide scientific references for promoting and facilitating the sustainable development of the region and the ecological protection and high-quality development of the Yellow River Basin.