Study On Weed Community Assembly Of Urban Wasteland In Shanghai

The reflecting mechanisms of urbanization processes and urban biodiversity are the key scientific issues of urban ecology,plenty of ecologists solved issues by selecting birds,insects as the indicated species.In this study,due to the characteristics of weed species such as high sensitivity,strong plasticity and short life cycle,weed community that randomly distributed in urban wasteland habitat was applied as our research objects.24 sites with a size of 10m×10m were established in urbanization gradients,and total 596 plots with a size of 1m×1m were extracted from the sites to do the community investigations,environmental factor tests and leaf trait experiments.Meta statistical analysis were applied to distinguish the contribution of deterministic?habitat factors and biotic-interaction?and nondeterministic factors?dispersal limitation and disturbance?on biodiversity distribution patterns and community assembly for three facets?taxonomic,functional and phylogenetic?,and their shifts among urbanization gradients and at altered scales.Our study could clarify the reflecting mechanisms to the urbanization processes,improve urban vegetation research systems,and guide urban biodiversity conservations and managements.The main results are as follows:1)114 species belonging to 91 genera and 31 families were recorded in our study.Rao's diversity index were selected to quantify alpha,beta and gamma diversity for three facets?taxonomic,phylogenetic and functional? at plot scales,the results indicated that the alpha diversity of weed community for each facet significantly decreased with increasing urbanization,as well as the beta and gamma diversity showed“reverse U”distribution pattern.The functional diversity are much lower than taxonomic and phylogenetic diversity,which showed stronger filtering processes of urbanization on functional traits.The generalized linear regression indicated that the beta diversity was the key component forming the gamma diversity.In addition,alpha diversity was significantly negative correlated to the urbanization score,beta and gamma diversity were significantly positive correlated to the wasteland succession years and average annual temperature,and were significantly negative correlated to the soil hydrolytic nitrogen.2)At plot scales,the explanation of variables represented habitat factors,biotic interactions,dispersal limitation and disturbance were 26.70±15.38%,and the nondeterministic factors were the primary underlying mechanisms;The wasteland succession years had significantly positive relationship correlated to the contributions of dispersal limitation and biotic interactions,and the average annual temperature had significantly negative relationship correlated to the contributions of dispersal limitation.At site scales,the explanation of all the variables was 11.58%,the scale effect existed considering the different effects of dispersal limitation on species composition between plot and site scales.Kolmogorov-Smirnov test were applied to fit observed and predicted evenness curves,the results indicated that Zipf model the best species abundance distribution model,which illustrated that the“long tail effect”distribution patterns of species abundance of weed community,indicated that the ecology niche theory considering nondeterministic factors could be the possible reason.3)MPD_SES and MNTD_SES were selected to represent phylogenetic structures of weed community in urban wastelands,the results indicated that the phylogenetic structures were overdispersal with increasing urbanization,and the main driver was colonization processes.In plot scales,the explanation of all the variables to the phylogenetic structure were 29.99±16.71%(MPD_SES)/25.79±16.43%(MNTD_SES),which was mainly effected by the nondeterministic factors?dispersal limitation,disturbance?,too.The average annual temperature and annual total precipitation had significantly positive relationship correlated to the contributions of dispersal limitation and biotic interactions on the MPD_SES,and the urbanization score had significantly positive relationship correlated to the contributions of biotic interaction on MNTD_SES.At site scales,the explanation of all the variables to the MPD_SES was 39.98%,and the deterministic factors were the primary underlying mechanisms.The scale effect existed considering the different effects of habitat filters and dispersal limitation between plot and site scales.4)MFD_SES and MNFD_SES were selected to represent functional structures of weed community in urban wastelands,the results indicated that the functional structures first clustered then overdispersal with increasing urbanization.In plot scales,the explanation of all the variables to MFD_SES/MNFD_SES were 31.22±18.32%/33.20±16.20%,which was mainly effected by the nondeterministic factors.The wasteland succession years and average annual temperature had significantly positive relationship correlated to the contributions of dispersal limitation and biotic interactions on the MFD_SES.At site scales,the explanation of all the variables to the MNFD_SES was 51.25%,and the nondeterministic factors were the primary underlying mechanisms.The scale effect existed considering the different effects of habitat filters between plot and site scales.The results of RLQ indicated that species with larger species leaf area and procumbent appeared more frequently in urban area.Our study indicated that in plot scales,the community assembly was similar in three facets,the nondeterministic factors were the key underlying mechanisms,the deterministic factors played a second role in species sorting processes.In site scales,the species composition could not be explained by the variables;the deterministic factors are the mainly processes affected phylogenetic structures;in contrast,nondeterministic factors are the mainly processes affected functional structures,although deterministic factors played an important role,too.Our study also showed that the phylogenetic and functional structures at site scales are more sensitive than species composition,which could reflect the effects of urbanization on weed community better.