Effects Of Competition-colonization Trade-off On Species Coexistence In Forest Plant Communities

The competition-colonization trade-off(CCTO)is one of the important hypotheses of coexistence theory.This hypothesis was proved mathematically by assuming that species trade off in their competitive and colonization abilities and follow strict competitive hierarchies.This hypothesis has been extensively studied and examined in simple life-history systems with homogenous habitats,such as microbial communities and herbaceous plant communities.However,in habitat-heterogeneous forest communities,species life history is complex,and there are interspecific and intraspecific responses to heterogeneous habitat.Under this condition,it’s hard to perform with mathematical derivation,and empirical studies lack of long-term data,leaving the effects of CCTO on forest woody plant coexistence unknown.Model simulations enable the study of multi-species coexistence in heterogenenous habitats,thereby providing fundamental reference for empirical studies.In addition,recent advances in forest monitoring networks have provided an appropriate platform to investigate the role of CCTO in species coexistence in woody plant communities.This study is based on tree census data from four forest plots(0.84 million individuals),as well as the environmental data and functional traits data.We explore the effects of CCTO on species coexistence comprehensively with model simulations,census data analysis,and two-year field transplantation of 4158 seedlings.First,key parameters in the simulation are set based on the environmental and species information of natural forests,habitat differentiation and intraspecific competitive ability variation are considered to explore the impact of CCTO on woody plant coexistence.Second,in natural forest communities,we estimate competitive ability by the survival rate under high neighboring density and assess colonization ability using recruitment distance,to examine the relationship between competitive and colonization abilities of tree species during non-seedling stages.Finally,by comparing the differences in seedling survival among different species,analyzing the impacts of neighbors and environmental factors,the role of CCTO in seedling establishment and the seedling establishment mechanism are examined.The main research findings are as follows:1.When there is habitat differentiation in heterogeneous habitats,the impact of CCTO on coexistence differs from scenarios without habitat differentiation.When there is no habitat differentiation among species,CCTO increases the number of coexisting species(P < 0.001).However,when habitat differentiation exists,introducing CCTO significantly decreases the number of coexisting species(P < 0.001).These results suggest that CCTO may not be the key mechanism of maintenance of species diversity in woody plant community when habitat differentiation exists.2.Under the conditions of CCTO,assuming intraspecific competitive ability follows Gaussian distribution along habitat,the impact of intraspecific competitive ability variation on woody plant coexistence is related to habitat differentiation.When there is CCTO but no habitat differentiation,the responses of intraspecific competitive ability to heterogeneous habitats hinder species coexistence significantly(P < 0.001).When both CCTO and strong habitat differentiation exist,intraspecific competitive ability variation reduces competitive exclusion,leading to improved coexistence(P <0.001).However,as the degree of habitat differentiation decreases,this positive effect gradually decreases and turns into a negative effect.This indicates that only when the degree of habitat differentiation is high,the intraspecific competitive ability variation contributes to woody plant coexistence in CCTO.3.The correlation between competitive and colonization abilities of tree species varies among different forest plots.The negative correlations are only observed in two subtropical forest plots,Heishiding and Baishanzu( =-0.20,P < 0.05; =-0.43,P < 0.05).However,the absolute values of these correlation coefficients are smaller than 0.5.And no strong correlations are found between species functional traits and competitive or colonization abilities(|| < 0.40 or P > 0.05).Consistent with simulation results,this indicates that CCTO is not the primary maintenance mechanism of species diversity in forest communities.What’s more,previous method using a single trait as a substitute for competitive and colonization abilities is not applicable in our forest plant communities.4.Dispersers are expected to have low survival rate in area with no conspecifics due to negative effects from heterospecific competitors under CCTO.Although seedlings of dispersers suvive better in area with conspecifcics(P < 0.05;P < 0.01),seedlings are not affected by competitive exclusion from heterospecifics.Instead,seedling survival is primarily influenced by heterospecifics positively,and environmental factors related to light,topography,and soil properties.This suggests that the seedling establishment in natural communities is not driven by CCTO,but rather by environmental filtering and density dependence simultaneously.In conclusion,the model simulations and empirical tests together suggest that the competition-colonization trade-off is not the major diversity maintenance mechanism in habitat-heterogeneous forest woody plant communities in this study.The habitat heterogeneity and the response of species to heterogeneous habitats affect the effects of competition-colonization trade-off on species coexistence.Overall,these findings fill a gap regarding the role of competition-colonization trade-off in habitat-heterogeneous forest plant communities,providing new evidence and insights for the improvement of the theoretical framework of species coexistence.