| Mechanisms of species coexistence has always been the core topic of ecology research.Increasing number of studies have shown that in a community niche differentiation and stochastic process form the species diversity together.In a forest community,due to the differing capability of dispersal,colonization and resource-acquisition strategies across strata,the mechanisms of community assembly for overstorey and understorey shall be different.However,the current studies on the mechanisms of community assembly usually treat the community as a whole,and it remains unclear whether the two different forces drive the community assembly in differing manners in different strata.In this study,all stems>1 at DBH were individually investigated in a 4.84 ha field,which is located in Tiantong National Forest Park,Zhejiang Province.Three functional traits,mean leaf area(MLA),specifc leaf area(SLA),leaf dry matter content(LDMC)were measured.Four null models of point pattern were adopted to generate the null communitys for whole-community,overstorey strata and understorey strata,to respectively present random placement,habitat filtering,dispersal limitation,and the joint effect of habitat filtering and dispersal limitation.The main conclusion is as follows.(1)At whole-community level,the observed species richness deviated significantly from that predicted by random placement and environmental filtering(p<0.05),while unsignificantly from dispersal limitation and the joint effect of environmental filtering and dispersal limitation(p>0.05).The observed species richness in overstorey deviated significantly from that predicted by random placement,dispersal limitation,and the joint effect of environmental filtering and dispersal limitation(11.5%),with which the probability density curve and the medians are deviated significantly from the observed ones.The observed species richness of the understorey wss best predicted by the jointeffect of environmental filtering and dispersal limitation(12.8%),but less predicted by the random placement(35.5%),environmental filtering(22.3%),dispersal limitation(16.5%).The results indicate that the species richness of whole-community is mainly affected by the joint effect of environmental filtering and dispersal limitation,the species richness of overstore stratum is mainly affected by environmental filtering,the species richness of the understorey stratum is mainly affected by the joint effect of environmental filtering and dispersal limitation while dispersal limitation explained more than environmental filtering.(2)Analysis based on the trait dispersion shows that the observed values of mean,skewness,variance,and SDNND(standard deviation of the nearest neighbor distance)of the MLA at the whole-community are closest to the dispersal limitaion(p>0.05),range and kurtosis is closest to the kernel density curve of the joint effect of environmental filtering and dispersal limitation(p>0.05).Range and variance of SLA are closest to the prediction of the joint effect of environmental filtering and dispersal limitation.The observed LDMC deviates most slightly from the prediction of environmental filtering(p>0.05),and variance and skewness are closest to the prediction of the joint effect of environmental filtering and dispersal limit(p> 0.05);the observed kurtosis is closest to the prediction of random placement(p> 0.05).This result furtherly proves that the assembly of whole-community is mainly affected by the joint effect of environmental filtering and dispersal limitation.(3)The observed mean and skewness of the MLA of overstorey stratum are closest to the prediction of joint effect of environmental filtering and dispersal limitation process(p >0.05);the observed range,variance and kurtosis are closest to the predictions of environmental filtering(p >0.05).This result shows that overstorey stratum is mainly affected by the joint effect of environmental filtering and dispersal limitation process,butenvironmental filtering plays a bigger role.(4)The observed mean,skewness and kurtosis of MLA of understorey stratum are closest to the prediction of the joint effect of environmental filtering and dispersal limitation(p> 0.05);The observed mean,variance,kurtosis and SDNND deviate most slightly from the predictions of environmental filtering(p>0.05);The observed range deviate most slightly from the prediction of the joint effect of environmental filtering and dispersal limitation;the observed mean of LDMC deviates most slightly from the prediction of the joint effect of environmental filtering and dispersal limitation(p>0.05);However,the observed range,skewness and kurtosis are closest to the predictions of random placement(p>0.05).The results further indicate that the assembly of understorey stratum is mainly affected by the joint effects of environmental filtering and dispersal limitation.In summary,our study reveals that environmental filtering,dispersal limitation and random placement have shown relative importance in the coexistence of species in whole-community and across strata,from the aspects of species richness and leaf functional trait dispersion.Among them,the joint effect of environmental filtering and dispersal limitation is the main mechanism maintaining the species diversity in whole-community level.In contrast,the maintenance of the species diversity at overstorey stratum is mainly affected by the environmental filtering process,and the species diversity of understorey stratum is mainly affected by the joint effect of habitat filtering and dispersal limitation,but the role of dispersal limitation seems more prominent.This study dicerned the different community assembly mechanisms in different strata in subtropical evergreen broad-leaved forests,which helps to understand the mechanisms of community assembly and species-coexistence,and also laid a theoretical basis for species diversity protection and vegetation restoration in this region. |
