Community Assembly Mechanism Cross Succession Stages In The North Tropical Area In Nonggang,South China

Community assembly as the core issue of ecology,which plays a crucial role in understanding the mechanisms of species coexistence and biodiversity maintenance.Many ecologists have been devoted to revealing the mechanism of community assembly,and to explore the effect of different ecological processes on community assembly.To date,there still no consistent conclusions were obtained about the community assembly mechanism.The Karst ecosystem as a fragile ecosystem,which is difficult to recover once it is destroyed.Therefore,exploring the community assembly process in different succession stages and examing the effects of different ecological processes on the community assembly process could have a better understanding for the mechanism of vegetation community assembly in karst area,and has certain guiding significance for vegetation restoration and reconstruction in this area.In the present study,substitution of space for time was used to select the five typical succession stages in the karst vegetation on the north tropical area of Nonggang,i.e.grass,shrub,young forest,middle-aged forest and old-aged forest community.By sample survey,we analyzed the species composition and structure,species diversity,pedigree diversity,changes of functional traits,pedigree signal and functional diversity of the community from species,phylogenetic to functional traits,and examine the effect of ecological processes such as diffusion restriction,random action,environmental filtration and similarity restriction on community assembly in each succession stage,with aim to provide basis theoretical for revealing the community assembly mechanism and provide guidance for vegetation restoration and reconstruction in this region.The main conclusions as following:(1)The plant composition is rich and diversity in the tropical karst area on the north of Nonggang.A total of 567 plants were observed through the quadrat survey,which belonged to 109 families and 335 genera.Among them,the species in the grass-stage community was 116,which belonging to 39 families and 92 genera;the species in the shrub-stage community was 181,which belonged to 67 families and 134 genera;the species in the young forest stage was 268 species,which belonged to 81 families and186 genera;the species in middle-aged forest stage was 220,which belong to 73 families and 156 genera;the species in old forest stage is 247,which belong to of 78 families and 171 genera.In the vertical layered structure of the community,the difference of species composition of the herb layer is the largest,whereas the difference of species composition in the shrub layer and the tree layer is relatively small.Species diffusion in early succession has a strong influence on the species composition and structure of the community;while species diffusion in later succession is neutralized by factors such as habitat filtering and similarity limitation,the diffusion limitation has a weaker influence on the community species composition and structure.(2)The composition and maintenance of the vegetation community were affected by a variety of ecological processes.As the succession progresses,species diversity and phylogenetic diversity increased gradually,and a clear history of phylogenetic evolution was observed during the community assembly process.In the early succession stage,the phylogenetic structure of the vegetation community display as phylogenetic aggregation,indicating that the coexisting species was closely related to each other and has similar niches within the community.Whereas the phylogenetic structure of the vegetation community in the later succession stage display as phylogenetic divergence,indicating that the coexisting species is far away to each other within the community.The vegetation community in early succession stage was greatly affected by human disturbance and other abiotic factors,and environmental filtering was the dominated factors during community assembly process.However,the community assembly in the later succession stage was mainly affected by similarity restrictions owing to the limited environmental resources and fierce competition among species.Moreover,the vertical layered structure among different communities was affected by different ecological processes and had a different phylogenetic performances.(3)At the level of functional traits,there was a different changing pattern among functional traits in the succession process.For instance,the specific leaf area displays decreased first and then increased trend as the succession progresses,while the dry leaf matter content,leaf thickness,and chlorophyll content shows a increased first and then decreased as the succession progresses.The leaf area shows an overall upward trend while the overall leaf tissue density shows an overall downward trend as the succession progresses.Furthermore,the specific leaf area,dry leaf matter content,leaf thickness,chlorophyll content and leaf tissue density in the grass stage displays as trait divergence,whereas the chlorophyll content dispalys as trait aggregation.The six functional traits all diplay as trait aggregation from shrub stage to the old-aged forest stage.Furthermore,the community trait structure in the grass stage was mainly affected by similarity limitation,while the environmental filtering is the main force that affects the community trait structure from the shrub stage to the old-adged forest stage.Besides,the phylogenetic history does not affected by all functional traits,our results indicated that only chlorophyll content,specific leaf area and dry leaf matter content had a significcant phylogenetic signals among the six functional traits.(4)The change of functional diversity index in community was often related to the utilization strategies of species to environmental resources.In the process of succession,there was a significant changed for the function richness index,function dispersion index and Rao Q index(P<0.05),whereas the functional evenness index and function divergence index exerts no significant change(P>0.05).In the early succession stage,the trait niche space of coexisting species in the community was far away,and the degree of resource utilization in the ecosystem is high.In the later succession stage,the coexisting species within the community were competing for environmental resources,thus the plant tend to obtain more environmental resources by the adjusting functional trait strategies.The community assembly was dominated by similarity limitation,and the community functional diversity is dispersed.Overall,community assembly was a complex process,and different ecological processes have different roles in different succession stages.Environmental filtering and similarity limitation were the main ecological processes affecting community assembly.Our results could provide a basis theoretical for exploring the mechanism of community assembly in the northern tropical karst area succession processon Nonggang,and provides a new insight for vegetation restoration and reconstruction in this region.