| The climate in the Ordos arsenic sandstone area of Inner Mongolia is dry and the severe vegetation there is severely destructed.People's extensive planting has caused the soil layer to become thinner and thinner and soil fertility to decline year by year.This has caused more serious soil erosion,resulting in soil erosion,a large area of bare ground and little top-level communities.In 2002,the policy of the conversion of farmland to forest and grassland was initiated.Different types of secondary communities have gradually formed in this area.In order to speed up the restoration of reclaimed farmland,it is necessary for us to understand the law and mechanism of community succession,as well as the community characteristics at each succession stage.This paper performs the community survey and vertical conditions survey in 26 sample plots of the arsenic area from Dongsheng to Zhungeer Banner using the method of spatial sequence instead of time sequence.Indicators like soil moisture and nutrients of soil sample in those sample plots were determined in the laboratory.The clusters are classified and divided by systematic clustering method.DCCA(except trend canonical correspondence analysis)sorting analyzes the law,speed and direction of plant succession on abandoned land in this area.The succession sequence is divided and the community characteristics in different succession stages are studied combining the diversity index.On the basis of understanding the local succession situation,it is necessary to introduce foreign species artificially in order to promote the restoration of the reclaimed farmland.In this study,some suitable plant introduction experiments were carried out where the germination rate,height,overwintering rate,flowering and fruiting situation and other indicators were observed and recorded.The main findings are as follows:(1)A total of 126 species of plants were found in the study area,belonging to 33 families and 83 genera.Among them,Compositae,Gramineae,Leguminosae and Labiatae have a total of 42 genera and 75 species,accounting for 50.60% and 59.52% of the total number of genera and species.(2)After systematic clustering,the 26 plots were classified according to the dominant species and sub-dominant species and were sorted by DCCA along the axis of abandonment years.Basically,the succession stage or sequence of the wasteland can be determined:(5-15 years): annual fored grass community ? annual ford grass + clump grass community ? annual ford grass + perennial grass community(or rhizome grass community)?(12-30 years): perennial grass community ? perennial grass + Small shrub community ?(15-40 years): rhizome primary grass + small shrub community or small shrub + perennial herb community ? small shrub community or perennial herb +primary grass community.The representative communities are in order: Allium ledebourianum(predominant plant in the reclaimed land)? Allium ledebourianum +Setaria viridis ?Artemisia scoparia + Setaria viridis ?Agropyron cristatum + Artemisia scoparia(Dysphania aristata + Leymus chinensis)?Agropyron cristatum + Leymus secalinus ?Stipa bungeana + Lespedeza bicolor ?Lespedeza bicolor + Stipa bungeana? Stipa grandis + Lespedeza bicolor(Artemisia ordosica).(3)According to the trend of the Bray-curtis community distance index in the later and early stages of the succession,the speed of abandonment succession in the Ordos arsenic sandstone area is: the initial succession is relatively fast,and it will become slower and slower in the future until the community reaches a stable state.The average value of the Bray-curtis community distance index calculated in different stages according to the 26 sample plots with different conversion years shows convergence.Therefore,the direction of abandonment succession in the Ordos arsenic sandstone area is shown as a progressive succession.Although the community goes from the initial abandonment state to the zonal vegetation in the top climate state,there will be many selective succession paths and trajectories.And the richer the species,the more alternative paths there are.Between 12 and 18 years,the succession path and succession trajectory of the community are quite different,but in the later stage,they have different paths to the same goal.(4)According to the multivariate ranking,the main environmental factors that affect the succession of farmland in the Ordos arsenic area are slope aspect,soil water content,total nitrogen,alkaline nitrogen,organic matter,etc.;according to the angle between environmental factors and abandonment years,elevation,slope aspect,Slope,total nitrogen,and alkali-hydrolyzed nitrogen are positively correlated with the years of abandonment,and available potassium,total phosphorus,and available phosphorus are negatively correlated with the years of abandonment.According to the analysis of soil nutrient trends,with the increase in the number of years of abandonment,total soil nitrogen,soil organic matter,and soil available nitrogen all show a trend of first increasing,then decreasing and then increasing.At the same time,with the increase in the number of years of wasteland,the plot coverage,Shannon-Wiener diversity index,Margalef richness index and Gleason species richness index increased first and then decreased.In the process of succession,the amount of resources available increases,the diversity and richness increase,and the competition among species in the community becomes increasingly fierce,and then the diversity and richness declines.This is the result of competition between species and other factors.(5)According to the two-year introduction and growth suitability test,annual plants such as Carthamus tinctorius,Sorghum sudanense and Sorghum bicolor 'Dochna',have emerged and grown well.Perennial plants such as Glycyrrhiza uralensis,Anemarrhena asphodeloides,Sophora flavescens,Medicago sativa,Astragalus mongholicus and Vicia sativa,have good germination rate,vigor and overwintering rate.Therefore,the plant succession of the arsenic sandstone area is from the annual weed community with abundant seed resources in the soil to the perennial zonal clump herb community,and then gradually to the drought-tolerant and barren shrub community.The succession direction is progressive succession which shows obvious convergence.The main environmental factors affecting succession are slope aspect,soil water content,total nitrogen,alkali hydrolyzed nitrogen,organic matter and so on.It is recommended that the following plants could be introduced in the Ordos arsenic sandstone area: annual plants such as Carthamus tinctorius,Sorghum sudanense and Sorghum bicolor 'Dochna',are suitable for commercial crops in the arsenic sandstone area which could be used as raw materials for house feeding silage and also could be used as green manure plants to promote the increase of soil organic matter.Perennial plants such as Glycyrrhiza uralensis,Anemarrhena asphodeloides,Sophora flavescens,Medicago sativa,Astragalus mongholicus and Vicia sativa,are suitable as grass species for ecological restoration in this area which can be used to increase the species diversity of this area and then increase the succession speed of the plant community to a higher level. |
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