Seedling Dynamics And The Influencing Factors In A Deciduous Broad-Leaved Forest In The Qinling Mountains Of The Subtropical-Temperate Trasitional Zone,North-Central China

The survival of species seedlings in forest ecosystems is closely related to various biotic and abiotic factors.Studying the dynamics of seedlings helps to understand the renewal status of forest communities.Exploring the effect of key factors on seedling survival can help us to understand species coexistence mechanisms.Based on the 135 seedling censuses stations in the 25 ha forest dynamic monitoring plot(QL FDP)in the Foping National Nature Reserve,this study was conducted to surveythe long-term dynamicsof seedlings by examining the species composition,quantativecharacteristics and spatio-temporal patternsof the seedlings in the Qinling 25 ha dynamic plot over five years(2015-2019).Torus-translation method was used to testspecies-habitat associations at seedling,juvenile and adult stage.And the generalized linear mixed effect models(GLMMs)were used to test the impact of neighbor density and habitat factors on seedlingsby analysing the negative density dependence(NDD)effect.The main findings are as follows:(1)A total of 11,408 seedlings belonging to 69 taxa were recorded,of which 65 could be identified to species.They are belonging to 42 genera and 24 families.The composition of species was basically the same during monitoring period and the dominant species were stable.There are 17 species with 100-1000 individual seedlings,comprised 56.28% of the total seedlings.Among them,the number of emergent seedlings reached 6280,and there were 53 taxa had recruitments.We found that more emergent seedlings appeared in May-July than August-September over every year.The number of dead seedlings was 6,929,of which the death of new emergent seedlings was much larger than that of old seedlings.The death of new emergentseedlings was 4,469,comprised 74.14% of the total number of new seedlings.The peak of seedling death roughly coincides with the newly increased peak.Due to the increase and death of seedlings,the number of seedlings fluctuates smoothly.This shows that the species renewal situation in Qinling 25 ha dynamic plot is in a stable and fluctuating state.(2)The QL FDP was divided into five types of habitats: high-slope,low-slope,ridge,low-valley and high-valley according to the terrain.Habitats have a significant impact on seedling composition and numeric characteristics.Among the 5 habitats,there are 67 taxaand 5,924 seedlings in the high-valley habitat.And the lowest is the high slope,with a total of 248 seedlings belonging to27 taxa.We used Torus-translation method to conduct species-habitat association and found that 38 species in the seedling stage were significantly associated with at least one type of habitat.There were 26 species with significant positive associations and 34 species with significant negative associations.Species that showed significant association with one habitat type at the seedling stage(38),followed by the juvenile(25),the least at the adult(17),and most of them were negatively associated with habitats.The species-habitat association of numerous species at the seedling stage cannot be over to the juvenile or adult.Only three species can maintain their habitat association from seedlings to adult.(3)The generalized linear mixed effect model(GLMMs)was used to explore the effect of neighborhooddensity,terrain,soil and other habitat factors on seedling survival.It was found that biotic factors play a major role in the study of community level.The seedlings survival probability decreased significantly with the increase of conspecific seedling density and conspecific adult;it also increased with interspecific seedlings density.It shows that significant NDD effects on seedling dynamics and survival.Studies at different seedling age levels have found that biotic and abiotic factors both affected the seedlings survival probability,but the role of neighborhooddensity is more significant.In summary,the regeneration status of seedlings is in a stable and fluctuating state in the Qinling 25 ha dynamic monitoring plot,and there appeared the steady seedling banks for many species.At seedling stage,habitat-based niche occurred.There were niche differences between different species,and this differentiationvariedacross their life stages.And conspecific neighbors also could drive seedling survival pattern.We conclude that both habitat heterogeneity and negative density dependent mortality shape the species coexistence at seedling stage in the Qinling plot,but conspecific negative density dependence effect plays a more important role.