Spatial Patterns Of Plant Diversity In A Subtropical Forest And The Underlying Mechanisms For Its Maintenance

The underlying mechanisms for species diversity maintenance are the core issues in community ecology and biodiversity science.Under global climate change with a rapid loss of biodiversity,the study of diversity maintenance and community assembly has even more significant implications both in theory and practice.For the current hypotheses aiming to decipher biodiversity maintenance mechanism,both niche theory and neutral theory focus on the use and assessment of the role of habitat heterogeneity in the theoretical models.As a combination of multiple ecological factors,topographic factors are the fundamental elements for the microclimate formation in a forest community and the major source of habitat heterogeneity.However,in a subtropical mountain forest community,it still remains unclear to what extent and via what ecological processes topography plays a role in species diversity maintenance and community assembly.A 10-hm~2 permanent plot was set up in KangheNature Reserve of Guangdong province for data collection of topographic and soil factors,tree attributes,i.e.,spatial location,diameter at breast height(DBH)and height of tree individuals in the subtropical evergreen broadleaved forest.Pertinent to the characteristics of field ecology data,a number of nonparametric multivariate statistical methods,i.e.,Kruskal-Wallis test,Spearman rank correlation analysis,analysis of similarity(ANOSIM),and multi-response permutation procedures(MRPP),wereused to test for differences or similaritiesof species composition,stand structureandstand biomass along topographic gradients.The responseof species to topography and the environmental filtering effect of topographywas determined usingindicator species analysis(ISA)based onplot groups classified by k-means clustering and multivariate regression tree(MRT).Habitat modelling was constructed forthe indicator speciesusingnonparametric multiplicative regression(NPMR).Spatial patterns of the dominant species and their association with topographic variables were analyzed usingspatial statistical models.Canonical correspondence analysis(CCA)was used to quantitatively describe the variation of species distribution interpreted by topographic and soil factors,and the spatial autocorrelation of the environmental variables was tested by Moran's I coefficient.The major driver of diversity maintenance were assessed and tested by variation partitioning,combined with topographic,soil,and spatial variables represented by vector obtained from principal coordinates of neighbor matrices(PCNM).The results are as follows:(1)The distribution of species diversity showed a clear topographic gradient pattern,and the spatial heterogeneity of species distribution was the result of topographic effect.The influence of elevationwas the largest among topographic factors,with the increasing of elevation,Shannon-Wienerdiversityindex,Pielou's evenness index,andvariation in taxonomic distinctness were decreased,while the number of stems and basal area were increased.With the increase of elevation,difference between plot groups,species similarity decreased,and beta diversity increased.(2)Species composition showed specific response to the differences in topography.Species composition of plot groupscategorized byelevation,slope and aspect,respectively,were significantly different,and different topographic groups had significant indicator species.Characteristic species were most abundant in the high elevation and low elevation area,indicating that habitat filtering induced by elevationhad maximal effects.Seedling trees comprised the most indicator species,indicating the highest degree of habitat specialization for young trees.(3)The main dominant species,Castanopsis carlesii and Schima superba,showed significantaggregated distribution.With the increase of DBH,the degree of aggregation decreased,and their spatial distribution had significant association with topographic factor.C.carlesii andS.superba showed significant negative interspecific and intraspecific associations.Heterogeneous Thomas process fitted the pattern well,showing that dispersal plays an important role in the formation of the pattern.(4)Stand biomass was significantly correlated with topography,and slopewas the factor with highest correlation.Thepotential direct incident radiation(PDIR)calculated from aspect is a good predictor for the spatial variation of biomass.There was no significant correlation between stand biomass and species richness,while biomass wasnegatively correlated with diversity index.As a major contributor to stand biomass,the basal area of adult treeshada significantly positive correlation with stand biomass.(5)The resultsfrom Canonical correspondence analysisshowed that elevation and slopeof the topographic factors,and available Manganese,total Potassium,total Phosphorus,and pH of thesoil factors had great explanatory powersforthe spatial variability of species distribution.All thesix factors exhibited significant spatial autocorrelation.Analysis using variation partitioning showed that 63.8%and 62.6%of the spatial variation cannot be explained by topographyplus space or soil plusspace variables,and space was more powerful in explaining the variability of species distribution.For tree seedlings,the explanatory power of spatial variable was the greatest,while for adult trees,soil was most powerful.The distribution of species diversity,community structure,and biomass were significantly influenced by topographic gradients,and the species showed specific adaptations to the variations in topography,demonstrating the topographic controls on the community assembly process,and the topographic factors were the major source of environmental heterogeneity in montane forest community.Both patterns of dominant species and the main environmental factors exhibited significant spatial structure,and the spatial variable was the major driver for the community assembly process.The results showed that the ecological process of both niche theory and neutral theory played significant roles in community assembly and biodiversity maintenance of the subtropical evergreen broadleaved forest.However,the effect of biotic process in the community,such as intraspecific and interspecific competitions,on biodiversity maintenance still warrants further examination.