Effects Of Tree Functional Diversity On The Relationship Between Plant Diversity And Production In Herbaceous Stratum

The maintenance mechanism of the relationship between biodiversity and ecosystem function has always been one of the most important issues in ecology.In forest ecosystems,interactions between different life forms have important effects on the relationship between plant diversity and community productivity.At the early stage of forest succession,herbaceous plants have an important influence on tree growth,and conversely,different tree diversity combinations have different inhibition on herbaceous plant productivity.However,studies on how tree functional diversity regulates herbaceous layer diversity and its relationship with productivity are still scarce.Based on the experimental platform for controlling the tree functional diversity and ecosystem functioning relationship in Putuo Mountain,Zhejiang Province,this study explored the interannual dynamics of herbaceous layer species and functional diversity from 2018 to 2021,i.e.,temporal β diversity,and the changing characteristics of herbaceous layer plant diversity-productivity relationship across the gradient of functional diversity and functional composition in the early stage of planting experiment,which was established by the combination of eight tree species,including Sapindus saponaria,Koelreuteria bipinnata,Liquidambar formosana,Sapium pleiocarpum,Neolitsea sericea,Machilus thunbergii,Schima Superba and Cyclobalanopsis glauca.The main results are as follows:(1)The temporal β diversity of herb species composition was not affected by the tree functional diversity,but affected by the tree functional composition.The βdiversity of herb species compositions increased significantly from 2018 to 2019 with the proportion of resource acquisition strategy species(community weighted average of resource acquisition traits)increased in tree composition.The results showed that at the beginning of the planting experiment,the species composition of herbaceous layer changed more dramatically over time in the community with a higher proportion of acquisitive tree species.(2)Tree functional diversity could significantly change the temporal β diversity of herb functional composition.At the beginning of the planting experiment,the community with greater tree function diversity had greater interannual variation of herbaceous layer functional composition.With time goes by,the temporal β diversity of herb functional composition decreased significantly with the increasing functional diversity and proportion of acquisitive trees of tree community,that is,the interannual difference of herbaceous layer functional composition decreased significantly,and the herbaceous layer had higher functional homogeneity.(3)The effect of herb species diversity on productivity changed with time.At the beginning of the experiment,the species diversity of herbaceous layer had a negative effect on productivity,which was significantly enhanced with the increase of tree functional diversity.The positive effect of herb species diversity on productivity decreased with the increasing proportion of acquisitive species of the tree functional composition over time.(4)The relationship between functional diversity and productivity of herbaceous layer did not change with tree functional diversity,but significantly changed with tree functional composition.The results indicated that the higher the proportion of acquisitive species in the tree community,the lower the understory light availability,the stronger the competition among herb species,and the lower the "complementary effect",leading to the weakening relationship between functional diversity and productivity of herb layer.(5)Both functional diversity and functional composition of trees significantly affected the relationship between functional composition and productivity of herbaceous layer,and even changed its direction.As time goes by,the understory light resources availability reduced in the tree community with higher functional diversity and proportion of acquisitive trees,because of the "niche complementary effect" and "selection effect",which led to the decrease of fast growing species,and increase of conservative species in herbaceous layer,resulting in the decrease of the positive effect of the functional composition on productivity in herbaceous layer.In conclusion,this study comprehensively reveal the significant dynamic effects of tree functional diversity on herb species and function composition,higher functional diversity and proportion of acquisitive species in the tree community weaken the positive effect of plant diversity on productivity in herbaceous layer due to the functional homogeneity.This study emphasizes the importance of tree functional diversity on regulating the structure and function of herbaceous layer,which helps our understanding of the relationship between tree diversity and ecosystem function in the early stage of planting experiment.