| Birds of a feather flock together and there is no exception to conspecific plants in natural forest communities.This aggregated distribution of conspecifics hints that conspecific density may influence strongly on populations and communities.Existing studies have shown that negative interactions will happen among aggregated individuals through resource competition and sharing natural enemies(such as pathogens and herbivores),and strength of the interaction may be positively correlated with conspecific density(namely,Conspecific negative density dependence,CNDD).Certainly,positive interactions may also happen among conspecifics by enjoying favorable habitats and beneficial microorganisms(such as nitrogen-fixing bacteria),and strength of the interaction may be positively correlated with conspecific density(namely,Conspecific positive density dependence,CPDD).If rare species in the community have weaker CNDD or stronger CPDD than abundant species,community compensatory trend(CCT)will emerge and promote species coexistence at community level,and vice versa.Therefore,to what extent and which form(CNDD,CPDD,or totally irrelevant)of conspecific density affect population dynamics and community structures(such as species diversity)is still one of the focal and hot topics in current community ecology research.Although many studies have addressed CNDD,CPDD,and CCT in forest communities,three major gaps still exist.Firstly,most CNDD related studies focused on seedling stage of trees(including seed germination and seedling regeneration),and less attention have been taken beyond seedling stage.For forest tree community,non-seedling stage takes much longer in trees' life history,and may be dominated by different ecological processes.Secondly,existing studies implicitly assumed the structure of CNDD in their parametric models.However,many controversies remain in the structure of CNDD.Thirdly,survival and mortality rates of seeds and seedlings are commonly used to explore the CNDD while less studied are based on plant growth.Actually,CNDD has to affect plants' growth first before it affects plants' survival and morality.To fill these gaps and to search a better answer of whether CNDD,CPDD and CCT exist in the subtropical evergreen broad-leaved forest communities,we carried out experimental and observational research at both seedling stage and beyond seedling stage,respectively,in Tiantong,Zhejiang Province.The seedling experiment was carried out from 2015 to 2017 and included 7 local species,namely as Castanopsis sclerophylla,Cyclobalanopsis gilva,Cyclobalanopsis glauca,Cyclobalanopsis myrsinaefolia,Lithocarpus harlandii,Phoebe sheareri and Schima superba.Two seedling density treatments(low and high)and two types of soil(high-temperature sterilization and non-sterilization)from mother trees treatments were used in the experiment.Beyond the seedling stage,we used non-parametric point pattern analysis to quantify CNDD or CPDD of 67 species as a function of spatial distance based on the 5-year growth rate of 80,273 individuals in 20 ha Tiantong plot.Results of the seedling experiment showed that:(1)the relative growth rate of seedlings under low conspecific density was significantly larger than that of seedlings under high conspecific density,indicating that CNDD exists at the seedling stage of the experimental species;(2)after high-temperature sterilization,the relative growth rate based on biomass of seedlings was significantly higher than that of seedlings with soil samples contained pathogens(P-value < 0.05),indicating that pathogens may mediate the observed CNDD at the seedling stage;Under high conspecific density,however,seedlings with unsterilized soil performed better on the relative growth rate(marginally significant,P-value < 0.1),suggesting that this CPDD could be mediated by benefit soil microbes;(3)there were no significant difference on seedlings' growth after vaccinated pathogens at different distances away from mother trees,indicating that experimental species' growth may not be restricted by distance dependence resulted from pathogens;(4)the strength of CNDD for each experimental species was positively correlated with their relative abundance in the 20-ha Tiantong plot(R2: 0.40;marginal significant,P-value: 0.07).Statistical analyses beyond seedling stage showed that:(1)16 species existed CNDD,8 species had CPDD and other species did not show significant neighhor density dependence;(2)for the 16 and 8 species mentioned above,strength of CNDD or CPDD decreases with spatial distance;(3)there was a negative correlation between strength of CNDD and the relative abundance of species that showed CNDD(R2: 0.17;marginal significantly,P-value: 0.06).It means abundant species have weaker CNDD than rare species;(4)there was no significant correlation between strength of CPDD and the relative abundance of species that showed CPDD.In summary,CNDD existed both in the seedling and non-seedling stages of tree species in the subtropical evergreen broad-leaved forest in Tiantong,and may partially generated by soil pathogens.Meanwhile,CPDD also presented at both stages,and thus the impact of CPDD on populations cannot be ignored in future studies.However,there were lots of interspecific variation on the strength of CNDD and CPDD.Finally,CCT existed in seedling stage while reversed CCT appeared in non-seedling stage,suggesting that dominate ecological processes may be different between seedling and non-seedling stage.Admittedly,there are many deficiencies in our current research.For example,the number of species involved in the seedling experiments is small and the analysis beyond seedling stage only covers a short time scale.This research is just a preliminary attempt to test CNDD,CPDD,and CCT in the subtropical evergreen broad-leaved forest in Tiantong.Current results are still valuable for further understanding the effect of conspecific density on populations and communities,and provide basic data support for future studies on the CNDD,CPDD,CCT and prediction of population dynamics and diversity change. |
PDF Full Text Request