The Relationship Between Land Cover Change And Underlying Surface Climate Factors In China Typical Grasslands

From the 1980s to the beginning of the 21^st century,under natural and human interference,the grassland ecosystems in China experienced complex and profound ecological change.The goal of this thesis project was to conduct systematic and comprehensive scientific research to study and assess the ecological progress and environmental effects.In this project,the three typical grassland ecosystems in China,Madoi alpine grassland in Qinghai Province,Siziwang Banner arid grassland,and Hulunbuir temperate grassland in Inner Mongolia Autonomous Region,were selected as the pilot study areas.Using satellite data and remote sensing,the land cover changes and the underlying surface climatic factors pattern dynamics in the past two decades?1988²011?were monitored and analyzed at a macroscopic scale and in a spatially explicit manner.Results indicated that vegetation in the alpine grassland has recovered since 2000 as the area proportion of grassland raised by approximate 23%.In the arid grassland,the vegetation areas of high coverage grassland and low coverage grassland shifted up and down at the level approximate 42%.Since 2000,the temperate grassland has been detected to degrade more evidently as the area proportion of grassland decreased by approximate16%.In the land-atmosphere interaction system,4 key variables were chosen as indicators and their spatial-temporal pattern were displayed.These indicators with the average results include:?1?normalized difference vegetation index?NDVI??0.26±0.13?,?2?albedo????0.18±0.03?,?3?surface radiation temperature?T_s??306.2±8.2K?,and 4)evapotranspiration?ET??2.03±1.11cm/day?,which assess the structure and function of the coupled land-atmosphere system.Based on an overall comparison,the four kinds of key effects,?1?lag effect,?2?convergence effect,?3?divergence effect,and?4?buffering effect,were defined and systematically summarized to describe the asynchrony in the interaction of the structural and functional variables.A correlation analysis on the 4 pairs of NDVI–?,?–T_s,T_s–ET,and ET–NDVI was performed to calculate the Pearson regression coefficient.The positive and negative correlations are consistent among the three types of grassland.All of the regression results are statistically significant.The correlation analysis results proved the objectivity and the substantiality of the feedback loop in the coupled system between vegetation and regional climate in the theory of eco-climatology.The cases provided empirical evidence to comprehend the general regulation in the land-atmosphere feedback loop.Results determined that the structural and functional variables have a strong correlation on the spatial dimension while they have asynchrony on the temporal dimension.In addition,the causality among the key variables was explored.An innovative quantification method to assess the causality among the 4 key variables was developed using the correlation analysis on different order derivatives of the variable dynamics to measure the effects contributed by each of them.It was concluded that in the grassland ecosystem,the underlying surface with vegetation?1.116±0.639?is more tightly interacting with the land-atmosphere than the layer with non-vegetation?0.495±1.081?.For the different types of grassland ecosystems,they have different coupling characteristics of the land-atmosphere systems.For the temperate and alpine grasslands,with the cooler and wetter climate and the higher vegetation coverage,the internal interactions in the structural?NDVI–??and functional?T_s–ET?subsystems are weaker,while the interactions between the subsystems are stronger.The overall indirect causality in the temperate and alpine grasslands is larger as the feedback response is delayed.On the contrary,for the arid grasslands,with warmer and dryer climate and lower vegetation coverage,the internal interactions in subsystems are stronger,while the interactions between the subsystems are weaker.Its overall direct causality is larger as the feedback response is instantaneous.Through the overall comparison of land cover change among the grasslands,a framework of‘location-structure-function'was summarized to provide a theoretical reference for the regional grassland resource planning and management in the context of climate change adaptation and mitigation.