Mechanism Of Impact Of Fine Roots Functional Traits And Species Richness On Fine Root Production In Subtropical Forests

Fine roots(≤2mm)play important roles in carbon allocation and nutrient cycling in forest ecosystems.The phylogenetic variations of fine root traits,which are related to plant growth and development as well as to physiological and ecological processes.Characterizing patterns of variation in plant traits across species and environmental gradients is critical for understanding performance of species in ecosystems.One-dimensional pattern of variation has been demonstrated in leaf traits,which is known as the leaf economic spectrum.However,it is unclear whether such a spectrum exists for root traits.The mechanism of how tree species diversity affects fine roots p.roduction in forest ecosystems,are not fully understood.In this study,for roots of 16 tree species from subtropical forest,we determined nine morphological,anatomical and chemical root traits.We further evaluated pairwise and multiple-trait relationships by Pearson’s correlation and principle component analysis including phylogenetic contrasts.We used sequential soil coring and ingrowth core method to measure fine root production and their distribution in the soil profile within 60 tree clusters that consist of one to four tree species(Choerospondias axillaris,Cyclobalanopsis glauca,Lithocarpus glabe and Pinus massoniana)in subtropical forests,and then further partition the diversity effects(NE)into three components based on trait-independent complementarity(TICE),dominance(DE)and trait-dependent complementarity effects(TDCE).This study aimed to:(1)examine how tree species and sampling methodology affect the anatomical,morphological and nutrient traits of fine roots;(2)determine whether phylogenetic signals affect fine root trait relationships and influence comparison of root traits between the branch order-based and diameter-based cut-off sampling categories;(3)to examine how tree species diversity affect biomass partitioning and fine root production;(4)to detangle contribution of vertical niche partition(complementarity effect)and species identity(selection effect)to relationship between biodiversity and fine root production and(5)to resolve various determine factors on the the fine root production and diversity effects.The main results are listed as followed:(1)Tree species and sampling methodology significantly affected fine root traits(p<0.05).Mean root diameters,root tissue density(RTD)and carbon-to-nitrogen ratio were the lowest in the first-order sampling category and highest in the≤2 mm sampling category.The reverse pattern was found for specific root length(SRL),specific root area(SRA)and nitrogen concentration.(2)Morphological traits showed significant phylogenetic signals;however,nutrient traits did not reflect phylogenetic conservatism.Phylogenetic factors influenced correlations between traits for the first-order root conomics spectrum.(3)The principle components analysis revealed a multi-dimensional pattern of variation in fine root traits among the 16 coexisting subtropical forest tree species.Moreover,species within the same phylogenetic clades tended to have similar root trait syndromes.(4)The magnitude of biomass partitioning to belowground increased with species diversity.Fine root production increased with tree species and functional trait diversity.(5)The NE in production mostly resulted from TICE.At non-growing season,in most case TICE on biomass were positive and became stronger with richness increased,but the case was contrary at growing season.(6)As tree species richness increased,soil space was more fully occupied by fine roots in terms of vertical heterogeneity.(7)Under different diversity levels and species composition,belowground functional traits had the greatest impact on fine root prodution and diversity effects,and aboveground functional traits also had a certain impact,while soil properties factors had less significant effect on them.This study demonstrated that functional traits of fine roots are multidimensional for subtropical tree species and closely linked to phylogeny.Species in different phylogenetic clades possess diverse root trait syndromes for efficient resource acquisition.Morphological traits of first order roots showed a much stronger phylogenetic signal than those of roots≤2mm(traditionally defined fine roots).The findings improve understanding of root trait strategies in response to environmental changes.Fine root production increased with tree species and this increase was attributed to spatial niche complementarity,and phenology temporal difference and aboveground functional traits also played a role.