The Species Diversity Pattern And Coexistence Of Evergreen And Deciduous Woody Plants In An Evergreen Broad-leaved Forest

As the zone vegetation in the subtropical area,evergreen broad-leaved forest(EBLF)is rich in species diversity and includes several global biodiversity hotspots.Researches about the bidoversity pattern and species coexistence in the EBLF are prevalent.Ecologists have studied the topic in different aspects,such as species,phylogeny and plant functional traits.However,they rarely conduct the relevant reserach at the life form level,especially for the evergreen and deciduous tree species,which are the two major life form compositions in EBLFs.Evergreen and deciduous plants are two types of species with distinct phenological characteristics and growth strategies.It is valuable for analyzing their species diversity characteristics separately,and detecting the mecahnisms of their coexistence in EBLFs.Specifically,the study may provide a new angle on the understanding of generation and maintenance of speices diversity in EBLFs;it is helpful in predicting the community responses to the global change based on the life form;such work will provide some necesary theoretical and experimental data underpinning the practice of biodiversity protection and restoration as well.According to the result of investigation in the autumn of 2010,152 woody plants whose diameters at breast height(DBH)≥ 1 cm were recorded in the 20 ha stem-mapped EBLF dynamic plot in Tiantong National Forest Park,consisted of 79 deciduous and 73 evergreen tree species.Compared to deciduous plants,evergreen species were slightly lower in species richness but substantially higher in abundance and basal area.In this study,we focus on the evergreen and deciduous tree species in the EBLF plots and attempt to answer the following two questions:(1)what are the characteristcs of diversity patterns of evergreen and deciduous trees in Tiantong EBLF?(2)How do these two life forms coexist in the Tiantong plot?Therefore,the species abundance distributions and β diversity patterns of each life form were compared and the potential reasons were discussed.Besides,we also tried to recognize the ecological processes influencing the coexistence of these two life forms in various spatial scales and different ontogentic stages.The main results and conclusions of this study are listed as follows:(1)Empirical cumulative distribution function(ECDF)was utilized to describe the species abundance distribution(SAD)of evergreen and deciduous species,whereas the differences of SAD between the two life forms were not significant according to the result of Kolmogorov-Smirnov test(K-S test).Three types of models,ie.,statistic model(log-normal model and log-series model),niche model(broken-stick model and niche preemption model)and neutral theory model(metacommunity zero-sum multinomial distribution model and Volkov model),were used to fit the SAD of each life form.With reference to the results of K-S test and AIC values,the best fitting model was the neutral theory model,and the niche model showed the poorest fitting of the data.Thus we could conclude that evergreen and deciduous trees had similar SAD pattern,although they are different in species richness and abundance.However,the model fitting results were commonly deermed as the necessary but insufficient condition in understanding the forming mechanism of biodiversity.Hence we could only preliminarily conclude that neutral process had a major effect on the generation of biodiversity pattern of either evergreen and deciduous plants,whereas the possible contributions made by other processes,such as niche differentiations,could not be excluded and measured by this method merely.(2)We calculated the beta-diversity index of evergreen and deciduous plants in Tiantong plot,and further partitioned it into two parts:species replacement and richness/abundance difference.The environmental variables(topographic factors and edaphic factors)and spatial structure variables(PCNM)were employed in the distance-based redundancy analysis(db-RDA)calculating the contribution rates of each variable on the beta-diversity index and the two beta-diversity components for each life form,in order to determine the relative importantce of niche and dispersal.The beta-diversity of deciduous plants was higher than that of evergreen species.Species replacement process was more important than richness/abundance difference process in determining the beta-diversity of both life forms indicating the discrepancy in species of either evergreen or deciduous plants among quadrats were the main source of their beta-diversity.Dispersal process had more effect on the beta-diversity pattern of evergreen and deciduous plants,although niche differentiation and neutral dispersal process influenced such pattern simultaneously.This is in line with the result(1),which stressed the significance of neutral process in shaping the diversity patterns of evergreen and deciduous trees.Moreover,by virtue of the low overall interpretation degree of variation of beta-diversity of deciduous plants by both niche and dispersal processes,pure random processes such as the present of forest gaps might be more important in generating the diversity pattern of deciduous plants.(3)Mosaics of evergreen and deciduous trees that are characteristic of EBLF are thought to arise from habitat filtering,but empirical evidence for this is limited.We test this assertion asking(i)whether environmental heterogeneity explains the distribution of deciduous and evergreen trees;(ii)which are the most important environmental variables;and,(iii)does their importance change with scale?We used data from a 20-ha individual-mapped EBLF in spatial point pattern analyses testing the scale of aggregation within,and segregation between,the two life forms.We used a heterogeneous Poisson process model to remove the effects of environmental heterogeneity,predicting segregation would disappear if the mosaics were due to habitat filtering alone.Finally we tested the relative importance of theoretically important environmental variables using multivariate regression trees at three spatial scales(10,20 and 50 m grid cells).We found evergreen and deciduous trees were aggregated at scales below 125 m and 60 m respectively and mutually exclusive at scales<120 m.Evidence of any spatial segregation between the life forms was removed at all scales after controlling for environmental heterogeneity.Only soil phosphorous concentrations contributed to spatial patterns at all scales,with values greater than 0.27-0.30 g·kg-1 favouring deciduous species.Our study is consistent with habitat filtering creating the observed mosaics of evergreen and deciduous tree species,but micro-habitat heterogeneity contributed even at scales below 20 m.Soil phosphorus availability appears to be the major environmental variable maintaining these mosaic patterns at hillslope scales in EBLF.(4)In order to deliberate on the ecological processes influencing the coexistence of evergreen and deciduous tree speices in different ontogenetic stages,we divided each of the two life forms into three ontogenetic stages(sapling,juvenile and mature)in the light of their DBH.We analyzed the spatial association and habitat preference within and between each ontogenetic stage of the two life forms using spatial point pattern analysis and spatial autocorrelation regression(SAR)analysis.We found that these two life forms in distinct life stages presented a spatial repulsion at relative small scales,and with the developing of individuals of evergreen and deciduous plants the segregated spatial scales increased.Whereas the segregation situation disappeared while the habitat heterogeneities were removed by heterogeneous Poisson process model.The SAR result denoted that albeit the preference of either evergreen or deciduous plants would transform along with the growth of their individuals,the two life forms still performed divergent preference in certain environmental elements.Combinng with these findings,habitat filtering was still the main process mediating the coexistence of the two life forms even for evergreen and deciduous individuals in distinct ontogenetic stages.In summary,evergreen and deciduous trees in Tiantong EBLF possessed similar mechanism in determining their diversity patterns:their species diversity patterns were influenced by both niche and neutral process,whereas the neutral process represented by dispersal process was more important.As to the coexistence of the two life forms,habitat filtering could be the main promoting force behind the pattern,in spite of the differences in spatial scales and ontogenetic stages.