The Differentiation Of Ecological Processes Driving The Survival Of Woody Seedlings Along A Spatial-temporal Environmental Gradient

Seedling is one of the most crucial and vulnerable stages in the life history of woody plants.The performance of seedling is influenced by a variety of external biotic and abiotic factors,environmental filtering,negative density dependence,hierarchical competition,and niche differentiation were the main ecological processes that impacted its performance.The strength and relative importance of these ecological processes are significantly influenced by temporal dynamics and environmental gradient differentiation along latitude.The relative importance of these ecological processes driving by external biotic and abiotic factors on seedling performance and their differentiation of spatio-temporal are currently unknown.Functional traits can reflect the ecological strategies of plants,so the response mechanism of seedling performance to biotic and abiotic factors can be analyzed using functional traits.By exploring the relative importance of ecological processes driving seedling performance and their temporal dynamics and differentiation along latitudinal gradients and elucidating how functional traits modulate the interspecific variation of ecological processes driving seedling performance,we can predict how seedling performance responds to temporal and spatial changes in the environment.In this study,we focused on the tree seedlings in four large-scale forest dynamic plots(20-ha subtropical mid-mountain moist evergreen broad-leaved forest dynamics plot in Ailao Mountains,Puer(hereinafter to referred as ALS plot),20-ha tropical seasonal rainforest dynamic plot in the Naban River watershed national reserve(hereinafter to referred as NBH plot),20-ha Xishuangbanna tropical rainforest dynamics plot(hereinafter to referred as XSBN plot)and 30-ha tropical wet seasonal evergreen forest dynamic plot in Mo Singto,central Thailand(hereinafter to referred as Mo Singto plot))taken from “101°E’ Forest Transect from Southwest China to the Indo-China Peninsula”.We constructed a Bayesian hierarchy model to explore the driving effects and relative importance of multiple ecological processes(environment filtering,negative density dependence,hierarchical competition,and niche differentiation)on seedling survival and the modulating effects of functional traits on seedling survival.The differentiation of negative density dependence effect driving seedling survival along the latitude environmental gradient was explained by a generalized linear mixed model.The results are as follows:(1)The negative density dependence effect was significant for the survival of tree seedlings during the three monitoring periods(2017.05-2018.05,2018.05-2019.05 and2019.05-2020.05).The soil environment showed significant filtering effect in both the first and second monitoring periods,while there was no significant filtering effect in the third monitoring period.The effect of hierarchical competition was significant in all monitoring periods.Significant niche differentiation was detected in the second monitoring period,but no niche differentiation was detected in the remaining monitoring periods.In general,negative density dependence was the main driving force for the survival of tree seedlings.The effect of hierarchical competition is more important than that of niche differentiation.Interannual variability in the ecological processes that drive seedling survival is clearly related to extreme drought events caused by the 2019-2020 El Ni?o.(2)Functional traits further influence seedling survival by modulate interspecific variation in the ecological processes that drive seedling survival.At the community level,multiple functional traits affect seedling survival by mediating hierarchical competition and niche differentiation among seedlings.Hierarchical competition based on stem dry matter content and chlorophyll content was detected during the first monitoring period,hierarchical competition based on root specific gravity and leaf specific gravity during the second monitoring period,and hierarchical competition based on chlorophyll content during the third monitoring period.Niche differentiation based on multidimensional functional traits was detected only during the second monitoring period.(3)At the species level,functional traits further affected seedling survival by modulate environmental filtering and negative density dependence,and this regulation was influenced by extreme interannual drought events.During the first monitoring period,no extreme drought occurred.At this time,species with smaller leaf area and lower chlorophyll content were more tolerant to shade and had higher survival rate in the shaded environment compared with species which with larger leaf area and higher chlorophyll content.The species with higher leaf dry matter content and smaller stem mass fraction were more tolerant to barren and drought and had higher survival rate in arid and barren environment compared with the species with lower leaf dry matter content and larger stem mass fraction.Species with higher root dry matter content had weaker negative density dependence effect and had higher survival rate in the environment with higher density of conspecific adult neighbors compared with species with lower root dry matter content.At the end of the second monitoring period,obvious drought stress appeared,leading to the disappearance of the regulatory effect of functional traits on environmental filtration in the previous monitoring period.The modulate effect of root dry matter content on negative density dependence changed from negative modulate in the first monitoring period to positive regulation in this period.The hierarchical competition among seedlings changed from the competition for light resources above ground to the competition for groundwater resources.In the third monitoring period,drought stress lasted to 8 months,and the modulate effect of leaf area on shading effect changed from positive modulate in the first monitoring period to negative modulate.Species with smaller leaf area and higher root proportion were more tolerant to barren and drought than those with smaller leaf area and higher root fraction.Species with larger leaf area and lower root fraction had higher survival rate in arid and barren environment than those with larger leaf area and lower root fraction.During the third monitoring period,the modulate effect of functional traits on the negative density dependence disappeared.Extreme drought events caused by El Nino are the main reason for the interannual variation of functional traits in environmental filtering and negative density dependence.(4)The relative importance of environmental filtering and negative density dependence driving seedling survival was significantly different between tropical and subtropical forests.In low-latitude tropical forests,the effect of negative density dependence on seedling survival was more important than environmental filtering,while in high-latitude subtropical forests,the effect of environmental filtering was more important than negative density dependence.(5)The negative density dependence effect driving seedling survival had significant latitudinal differentiation.In the Mo Singto,XSBN and NBH plots,which are located in the tropical region,the density of conspecific adult tree neighbors had a significant negative effect on seedling survival.In the ALS plot,which is located in the subtropical region,the density of conspecific adult tree neighbors had no significant negative effect on seedling survival.In both tropical and subtropical forests,the conspecific seedling neighbor density had no significant effect on seedling survival.In general,the negative density-dependence effect in tropical forests was stronger than in subtropical forests.Our research presents a hierarchical framework for investigating trait-based mechanisms that determine fitness,which is the foundation for scaling population dynamics up to community assembly.This study confirmed significant differences in the relative importance of ecological processes driving seedling survival,with negative density dependence playing a dominant role.It further confirmed that coexisting species modulate the trade-off and synergies of various ecological processes through multidimensional functional traits to promote the equalization of average fitness.It was found that the interannual differentiation of functional traits in modulating ecological processes was significantly correlated with interannual extreme drought events.The negative density dependence effect varied significantly along the latitudinal environmental gradient.However,this study was relatively small in its latitudinal span and had certain limitations.The change pattern of negative density-dependence on the latitudinal gradient needs to be further explored.Future studies should further explore the mechanisms by which the ecological processes driving tree-plant performance respond to temporal and spatial changes in regional climatic change.