| In this dissertation, communities succession of arable old land in the northern of Shaanxi province isstudied systematically and quantitatively by inferring from the representative sites of abandoned land ofdifferent stages. Based on the pre-recognitions of communities succession and changes of soil moisture andfertilities, the relations of succession and sites conditions and relative competition ability of main seresspecies are studied fourthly. Finally succession dynamics is discussed in order to provide revegetation,reclamation or restoration with some scientific foundation.1.Overall recognition of vegetation succession after being abandoned of old arable land⑴ Based on the relative good communities similarity recognition or quantitativeclassification using system clustering, again with the overall understand of fields research,quantitative ordination of PO and CA, secondary succession seres of old land with differentstanding condition and abandoned year can be indicated by and large, that is: from 1 to 6 years,annual weeds association→ annual weeds+tuft grass association→ annual weeds+perennialherbage association (or rhizome grass)→from 4 to 15 years, perennial herbage association→perennial herbage+undershrub association→from 10 to 40 years, rhizome tuft grass+undershrubassociation(in south facing slope) or undershrub+perennial herbage association→undershrubassociation or perennial grass+tuft herbage association(in north facing slope). Its correspondingrepresentative communities are: Artemisia scoparia Waldstet kit community→A.scoparia Waldstetkit+Stipa bungeana Trin→A. scoparia Waldstet kit + Artemisia scarorum Ledeb.(or Agropyroncristatum(L.) community)→A. scarorum Ledeb.→A. scarorum Ledeb. + Lespedeza var.dahurica(Laxm.)Schindl → Bothriochloa ischaemum(L.) + L. var.dahurica (Laxm.)Schindl or L.var.dahurica (Laxm.)Schindl + A.scarorum Ledeb.→L. var.dahurica (Laxm.)Schindl or A.scarorum Ledeb.+ Poa sphondylodes Trin.⑵As abandoned year increase, community evenness decrease while dominance indexincrease, species diversity indices SIMPSON,SHANNON,BRILLOUIN and DAHL decrease,but Whittaker index increase steadily.The species patterns of former succession stage belong tologarithmic series distribution, which imply that niche pre-emption is the main factor incontrolling former succession community structure. While that of middle and later stage islognormal distribution, its community structure may be affected together by niche pre-emption,seed invasion stress of near community, interspecies competition and other factors. Besides that,some community patterns are exception, other statistical or theory models are needed.⑶ The value of niche breadth of species in early succession stage are greater than that ofspecies in later stage and dominant species are greater than companion species. PO using fouroverlap indices illuminate an MacAuthor's r- and k- selection continuum, species located on theleft part of axes are r-selection, right relatively k-election, as well, it indicate species competitionability and succession direction. Moristia and Pianka overlap indices are better than percentageand Wanggang indices for the cluster using the first two are ecological reasonable, for example ifthree groups are cluster, the first group are opt to colonization, most species are in earlysuccession stage; the second are gooder competiter species in later stage; the third are relativebalanced between colonization and competition, species relatively in middle stage.2.Changes of soil moisture and fertilizer along succession⑴Mean soil moisture content of 9 measurements tend to decrease as succession go on, whilesoil moisture fluctuation in index of SD. tend to increase, whether unchangeable standingconditions, including slope direction, gradient and elevation, are excluded or not. Furtherpath-way analysis show that the leading factor influencing top and root layer moisture arevegetation coverage, while that of deep layer is abo...
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