| The Hulunbuir Steppe is located in the semi-arid climate zone of China,the ability to withstand interference is very limited.The grassland ecosystem is easily degraded by unreasonable disturbance of human activities.In recent years,with the rapid development of mining activities,the stability of grassland ecological system has been under threading seriously.How to make scientific and reasonable strategic decisions on grassland mining development,ecological protection and construction depends on a deep understanding of grassland ecosystem stability and its driving mechanism.For one thing,due to the complexity of ecosystem,the effective research method of system stability is still descriptive evaluation,lacking of dynamic and process research.For another,the research on the driving mechanism of grassland degradation mostly focuses on the correlation between plants and some factors,but lacks of the systematicness,hierarchy,configuration among factors and multi factor synergy between plants and its below ground habitat(BGH)from the perspective of system science.In this study,we focused on the coal mining area of Chenqi Basin,Hulunbuir Steppe,Inner Mongolia.Based on the basic theoretical issues of system science,ecosystem ecology and eco geology discussion,the remote sensing data and ecological geological survey were applied,with the following objectives were to be researched:(1)the theory and method of ecosystem stability based on Vegetation Index,(2)the ecosystem stability analysis and unstable area identification,(3)the identification of plant community succession and degradation under the influence of mining activities,(4)the eco geological mechanism of grassland degradation.(1)Spatio-temporal analysis of ecosystem stability based on vegetation indexSystem science,stability and fluctuation theory were employed as theoretical basis,and the vegetation normalization index NDVI data was applied.Based on the analysis of the process of system from stability to instability,three indexes that can characterize the stability of the system were proposed,which were NDVI mean value(?)?mean square deviation(?)and relative fluctuations(?)(?/?).It respectively represents the concentration trend of vegetation spatial distribution and the average state of time series;the dispersion degree of spatial distribution and the fluctuation of time series;and the stable state of vegetation system.By comparing with the background threshold,the higher?value of the system in time-space series indicating more intense of system internal oscillation,and a worse system stability.If the?value continues to rise,it indicates that the system is unstable.On the contrary,it indicates that the stability of the system is enhanced.On this basis,the indicators were calculated based on the NDVI time-series data griding,the macro scale and local scale stability characteristics of ecosystem can be revealed through the spatiotemporal sequence analysis.(2)Ecosystem stability analysis and unstable area identificationAt the macroscale,the vegetation distribution pattern has no obvious affected by mining activities,showing that various disturbance activities on the medium and small scales in the study area have not affected the macro ecosystem pattern,which is also related to the lag and delay effect of the vegetation system on environmental change.On the local scale,the vegetation system is strongly disturbed by mining activities,showing the characteristics of abnormal fluctuation.The natural grassland(background area)was identified in a macrostable state,with the ecosystem structure shows normal fluctuations.The open-pit mining regions were identified as serious disturbance regions,with the grassland replaced by the bare-land.The surface subsidence caused by underground mining,characterized by ground surface deformation and fracture,has mild disturbance on the grassland ecosystem.However,the surface collapse,characterized by the collapse pit,has constantly caused the grassland ecosystem instability.The artificial grassland system in the surface collapse regions has poor stability and has been prone to destabilization in the face of severe natural conditions.(3)Plant community succession and degradation identification under the influence of mining activitiesThrough the study of the plant communities characteristics of the areas affected by the coal mining activities,the results showed that:(1)the number of species around the mining area decreased significantly,and the dominance of the original dominant species decreased,while the dominant position of the low and trample resistant Carex duriuscula increased,and the Artemisia frigida and Potentilla acaulis appeared in large numbers,which indicated that the grassland was moderate degradation.(2)In the surface subsidence area,the species number of plant community decreased by 41.0%,the coverage decreased by 9.6%,but the dominant plant composition of the community is similar to that of the natural grassland,and the community structure is similar to that of the natural grassland,the grassland was mild degradation.(3)In the surface collapse area,the plant community was completely destroyed,the number of community species decreased by 52.5%the coverage decreased by 74.2%,the native species Leymus chinensis declined significantly,and the dominant species Stipa baicalensis disappeared,which indicated that the grassland was severe degeneration.(4)In the groundwater drawdown zone,the number of plant community species decreased by 61.0%,the coverage decreased by 32.9%,with the original dominant species declined,and the degradation indicator species became the dominant species,the dominance of meso functional group decreased significantly,which indicated that the grassland was in the stage of moderate to severe degradation.(5)The number of species in the reclaimed grassland decreased by 50.8%,the coverage decreased by 53.6%,the species selected for grassland restoration decline obviously.And the similarity between community structure and natural grassland was low,which indicated the grassland is in the transition stage from artificial grassland to natural grassland,showing the characteristics of moderate degradation.(4)The eco geological mechanism of grassland degradationBased on the synergy of plant community and its BGH succession process,synusia structure of plant roots,and the combination state of ecological factors analysis,the driving mechanism of grassland plant community degradation was revealed.Through the investigation and analysis of plant community root group and?(D),?(18O)stable isotope tracing,the root mass of natural grassland is a double-layer vertical structure,with the root mass of annual and perennial plants at 0-20 cm and 20-40 cm depth,respectively.Moreover,the bottom limit depth of natural grassland BGH was 60 cm.According to the actual statistical situation of different types of sample plots in the field,the configuration map of geo ecological factors was proposed.The three-dimensional space occupied by the survey points of different types in the configuration map reflects the BGH of the corresponding plant communities,which contains the results of the synergistic effect of plant communities and its BGH.The analysis of grassland BGH showed that:(1)The grasslands around the mining regions and the surface subsidence area both have lower ecological factors than those in natural grassland,and the configuration of soil water,salt,organic matter and cation exchange capacity is similar to that in the natural grassland area,with large overlapping areas.The root mass was found a double-layer vertical structure also,but shows a shallow trend.It conducted that the stability of the BGH in these areas were affected by mining activities,but it still maintains stable and its corresponding ecological function.(2)The stable vegetation BGH in the ground collapse area was completely destroyed.The soil ecological factors were significantly lower than that of natural grassland,with the configuration quite different from that of natural grassland.The plant roots were mainly concentrated in the topsoil but also with a double-layer vertical structure.It indicated that the VBGH of the collapse area is changing from instability to new stability,with the root masses of the annual and perennial plants are gradually differentiated in the vertical direction.(3)The soil ecological factors of groundwater drawdown area were significantly lower than natural grassland,with the factors configuration quite different from that of natural grassland.The root mass showed a single layer in the vertical direction.It showed that the soil ecological factors tend to be impoverished and the plant habitat is in the degradation stage,but the fluctuation of the factors is small,which indicates that a new stable BGH has been formed suitable for the degraded species.(4)The soil ecological factors in the reclaimed grassland system were significantly lower than that in the natural grassland,and showed obvious heterogeneity and volatility.In some areas,the vegetation coverage is very low,and the corresponding root mass is mainly distributed in the shallow0-20 cm soil layer,with the root frequency of 75.1%,which indicates that the BGH is unstable,and the plant communities are difficult to settle down and exist stably,which has a trend towards bare land.In some areas with high vegetation coverage,a relatively stable community habitat was formed,and the vertical distribution of root mass was similar to that of natural grassland,with a double-layer vertical structure.The degradation of plant community is closely related to its below ground habitat,and three relationship models between the plant community degradation and its below ground habitat were summarized:(1)when the original macro stability of the vegetation BGH is destroyed but the new macro stability has not yet formed,the species will decline,if they cannot adapt to the changing habitat.(2)If the vegetation BGH instability continues and a new macro stable state has not been formed,it will lead to the decline of the original plants and the formation of new plant communities,and even the development of bare land.(3)If the vegetation BGH can form a new stable state,the plants that can adapt to the change of the environment structure may settle here.The ecosystem stability method proposed in this paper is a supplement of current theories and methods of ecosystem stability research.Moreover,the research on the grassland degradation and its eco-geological mechanism systematically explained the interaction and succession process between plant communities and their BGH in the process of grassland degradation.The results can provide a scientific foundation for the rational utilization and ecological management.On the basis of this study,further studies could be carried out in the applications of the landscape stability method in forest,desert and other complex vegetation systems. |
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