Study On The Ecological Patterns Of Farming-Pastoral Zones In Semi-Arid Regions

The semi-arid area agricultural-pastoral ecotone is both a typical ecological critical zone and an important ecological barrier in China.Good ecological environment is not only the basis and guarantee of regional economic development,but also the ecological safety barrier of Yellow River,Haihe River Basin,and Beijing-Tianjin-Hebei.The natural attributes of the hydrological cycle in the semi-arid area determine the basic pattern of the ecosystem,which natural ecology is the grassland vegetation supported by groundwater.The spatial pattern and landscape changes of the ecosystem reflect the pattern evolution of groundwater spatial distribution.Grassland ecology represents the natural attributes of the ecosystem in the semi-arid area and is the ecological foundation of the area.Grassland ecological safety not only concerns the pastoral area,but also extremely affects the ecological quality as well as safety and stability of the entire region.Due to the development of irrigated agriculture in the semi-arid grassland,a unique agricultural-pastoral ecotone has formed,and the Xiliaohe plain belongs to the typical area where an inverse relationship exists between grassland farming and animal husbandry.As the groundwater exploitation intensity and the expansion of irrigated area increasing,the cultivated area in the Xiliaohe Plain has surpassed the grassland in last ten years.This reversal of the ecological pattern has led to a substantial decline in the natural attributes of regional ecosystem,depletion of water and land resources,and decline in renewable capacity,leaving significant threat to the economic society and ecological safety of the region.First of all,the grassland area of the Xiliaohe Plain has been continuously compressed and the area of grassland is less than 33%of the Xiliaohe Plain area at present.This has directly led to the disappearance of vegetation that is adaptable to niches,and the decline in species diversity of grassland vegetation.Secondly,large-scale well irrigation has led to an overall decline in groundwater level,and the grassland around the irrigation area was also affected,resulting in the grassland vegetation succession.These facts obviously did not meet the national policy of the construction of ecological civilization.It is necessary to do researches in new ecological patterns of agricultural-pastoral areas that can ensure ecological safety.and propose a reasonable proportion of cultivated land and grassland,which is a far-reaching and significant ecological safety issue.In the Guiding Opinions of the Ministry of Agriculture on the Adjustment of Agricultural Structures in the Agriculture-pastoral Ecotone in North China,the need for adjusting the agricultural structure is clearly stated.In the 13th Five-Year Plan for Eco-environmental protection in the Inner Mongolia Autonomous Region,it is also clearly stated that "For key ecological function areas with less human activities and good ecological conditions,the protection of natural vegetation and the scientific restoration and control of ecologically degraded areas should be carried out".Therefore,there is an urgent need to establish a regional ecological structure evaluation method in the semi-arid agricultural-pastoral zone in the aim of ensuring the natural attributes of the ecosystem.Take the Xiliaohe Plain as an example;the proportion of ecological pattern structure in agricultural-pastoral ecotone is analyzed.In terms of the structural contradictions faced by hydrology and water resources-ecology-economy society development in the construction of water ecological civilization in the semi-arid area,the study of the rational ecological pattern of the semi-arid agricultural-pastoral ecotone is a top priority in Horqin Grassland status research,which aims at protecting the natural attributes of the grassland ecosystem.Firstly,the protection and reconstruction of the Horqin Grassland depends on the deep understanding of the existing grassland vegetation.Secondly,as a vegetation symbol of the Horqin Grassland,it is necessary to conduct a comprehensive and in-depth study on the existing grassland vegetation communities.Thirdly,as a driving factor,the decisive influence of groundwater level regulation and management on the vegetation community structure of grassland must be recognized.Lastly,on the basis of existing grassland,the adjustment of the structure of agriculture and pasture,and groundwater regulation,the proposals of the reconstruction and protection of the grassland in the Xiliaohe plain are made.The main research results are as follows:(1)A systematic and in-depth study on the succession history of grassland vegetation communities in the Xiliaohe PlainIn order to study the natural attributes of the Xiliaohe Plain,the textual research data(as of the 1980s)on all kinds of vegetation species composition in the Xiliaohe plain area were systematically sorted out as the background for natural ecology.In the early 1980s,there were 917 species of various types of plants in the Xiliaohe Plain,belonging to 108 families and 412 genera.Through the GIS map processing of the vegetation distribution in the Xiliaohe Plain,the distribution range,area,number of patches,soil types and species composition were analyzed.The vegetation area is 48930.07km~2,and the species density is about 19 species/thousand km~2,forming 25 mainstream communities.The distribution of vegetation communities in the Xiliaohe Plain is extremely regional,only 128 plant species were discovered over the entire territory,accounting for 14%of the total number of species;and 85.3%of the species were scattered on 50%area.This means that as the area of grassland decreases,the vegetation diversity also decreases at almost the same level,leading to many species that are adaptable to local habitat disappear.It can also be inferred that the species diversity of Horqin Grassland can be satisfactorily protected if half the grassland area can be maintained.This also implies that if the goal of restoring the natural grassland area to 50%of its land area is set,by waking up the residual seeds in the soil,some species may be regenerated to increase the existing species diversity.(2)Survey and Study on the Status of Vegetation Communities in the Natural Grassland in the Xiliaohe PlainSince 1980s,along with the development of irrigated areas,the grassland area of the Horqin Grassland has declined from 48,900 km~2 to 22,400 km~2 in 2016,including a large number of artificial grasslands and succession degraded grasslands.The natural grassland area is less than 10,000 km~2,and seriously fragmented.The existing relatively complete contiguous area(6,702.89 km~2)is the subject of current research.Many field surveys were conducted in order to collect a great deal of first-hand information.The species diversity decreased from 917 species to 245 species,belonging to 19 communities,and the population density decreased from about 19 species/103km~2 to less than 12 species/103km~2.The shrinkage of the Horqin Grassland area has led to the extinction of large amount of species,and the decline in diversity has been faster than the decrease in grassland area.In addition to the great changes in the types of vegetation communities,comparing the historical and current composition of the vegetation communities,it can be seen that the species composition of the same community in the 1980s has changed greatly,mainly reflected in the reduction of plant species that make up the community.Combined with historical survey and analysis,it is inferred that when the area of natural grassland is reduced to 50%(approximately 24,500 km~2),it is the turning point for drastic reduction of vegetation diversity.(3)Analysis of vegetation community successionThe degradation of primary grassland comes from two aspects:one is the development of irrigation areas;land use occupied a large number of grasslands.The other is with the decline in groundwater level,which forced changes in grassland vegetation communities,appears succession species.By comparing the vegetation communities in 1980s and the current year,the succession of natural grassland vegetation communities in Xiliaohe Plain was analyzed from the vegetation community area,the evolution of vegetation species diversity,the evolution of vegetation community types,and the species composition of vegetation communities.Species from different succession stages were found according to the analysis,which reflects the degradation and shrinkage of the Horqin Grassland.(4)Evaluation of Regional Ecological Structure in the Agricultural-pastoral EcotoneAccording to the principle of restoration ecology,based on the existing natural grassland vegetation communities,spatial topography is expanded by retreating and recovering groundwater latent flow field,increasing groundwater level,and the advantage of the strong growth of the community.According to the succession history of grassland vegetation communities in the Xiliaohe Plain,it is inspired that vegetation species diversity may be strengthened when the grassland area recovers to more than 24,500 km~2.The methods of spatial topology approach:1)Using landscape ecology method to establish a preliminary relationship between the proportion of grassland and cultivated land,propose the principle of maintaining the minimum total proportion of natural attributes;2)Based on the profound understanding of the succession analysis of grassland vegetation communities in the Xiliaohe Plain,adjust and combine ecological area:3)Considering the continuity of the ecological landscape,ensure natural ecological landscape connectivity and prevent fragmentation,and further adjust and combine ecological area.Corresponding analyze the rationality of ecological pattern that is supported by groundwater underflow field through critical burial depth of groundwater that recharges vegetation.(5)Study on the Ecological Pattern of the Agricultural-Pastoral Zone in the Xiliaohe Plain Supported by GroundwaterThe existing natural grassland vegetation serves as a gene bank for grassland ecological restoration,and expands follow the principle of on the basis of natural grassland.The "source-sink" theory method in landscape ecology is used to carry out the adjustment and optimization of the agricultural-pastoral ecological pattern in the Xiliaohe Plain.The MODCYCLE model was used to establish a simulation model of the water cycle in the Xiliaohe Plain,which simulated the groundwater flow field in the Xiliaohe plain from 2001 to 2014.By superimposing the contour maps of groundwater level in the expansion areas of the natural grasslands and the Xiliaohe plain,the rationality of supporting the ecological pattern of the agricultural-pastoral area by the groundwater underflow field has been analyzed.According to the groundwater level values<3m,3-4m,and>4m,the groundwater conditions in the expansion area are divided into support,basic support,and no support.The short-term,mid-term,and long-term goals of the reasonable ecological pattern in the ecotone between agricultural-pastoral areas in the Xiliaohe Plain under different groundwater conditions were analyzed,and a set of regional ecological structure assessment methods was established to ensure the natural attributes of ecosystems.The long-term goal shows that after the adjustment of the ecological pattern,the natural grassland area is expected to recover to 22,455.75 km~2,21,000 km~2 for irrigated grassland,and 13,753 km~2 for farmland.And the ratio of structure of agriculture and pasture is reduced from the current 1.04 to 0.32.(6)Rationality of the Ecological Pattern of the Xiliaohe PlainUse source-sink method to do the spatial expansion of existing species communities is driven by groundwater underflow field,and existing plant communities expand and replicate with certain groundwater rules as the groundwater level rises.According to the local characteristics of the grassland vegetation communities in the Xiliaohe Plain,species diversity will change in two areas that have been "invaded" by the existing community's strong expansion:one is some species cannot adapt to new habitats and mutate or extinct.The other is the seeds of local species that remain in the soil or dormant in the newly recovered area are stimulated to regenerate after the groundwater conditions are restored.Finally,the newly restored grassland forms a new type of community that is neither different from the current situation nor is different from previous ones.The species diversity of vegetation generally shows a trend of gradual increase,which will greatly improve the natural attributes of the grassland.(7)proposal of the reconstruction and protection of the grassland in the Xiliaohe plainA proplsal for reconstruction and protection of grassland in the West Liaohe Plain is put forward based on adjusting the structure of agriculture and animal husbandry and regulation of groundwater."returning farmland","restoring water" to "restoring grass":adjusting the area of agricultural irrigation,restoring the groundwater level and restoring grassland ecology,the three links are indispensable.