| Functional traits of absorptive fine roots in woody plants play key roles in carbon allocation and nutrients cycles in ecosystems.During past two decades,the linkage between absorptive fine root functional traits and belowground ecological processes has become a hot issue in root ecology.Accumulating evidences suggest that root functional traits are influenced by species evolution,growth form,climate and edaphic conditions and mycorrhizal association,however,the relationship between root functional traits and tree age is still poorly understood.To address this key question,we select juvenile(< 15 years)and adult(60-100 years)trees of four temperate commercial tree species with different root morphological characteristics(thinand thick-root species)and mycorrhizal infection types(arbuscular(AM)or ectomycorrhizal(EM)infection),including Pinus koraiensis(EM,thick-root species),Larix gmelinii(EM,thinroot species),Fraxinus mandshurica(AM,thin-root species)and Phellodendron amurense(AM,thick-root species).In order to study the relationship between tree age and the composition(number,biomass and length)and functional traits(including root anatomy,morphology,architecture,tissue chemistry,mycorrhizal infection and physiological characteristics)of absorptive fine roots,the intact root branches(including the first five orders)were excavated;we also investigated the response of typical absorptive fine root(first order root)functional traits of juvenile and adult trees to increased soil nitrogen(N)availability by N addition.The main objective of this study is to reveal the mechanism for the changes of nutrient foraging strategies of absorptive fine roots associated with tree age increasing.Our results showed that:(1)The proportions of absorptive fine root number in each root order remained stable from juvenile to adult trees among four tree species.Across all species,the proportions of absorptive fine root number of the first to third order roots were all more than 70% in both juvenile and adult trees,this suggested that the absorptive fine roots were mainly composed of the first to third order roots from juvenile to adult trees across all tree species.From juvenile to adult trees,the proportions of absorptive fine root biomass in all species increased significantly(10.7%),the proportions of absorptive fine root length in all species had an increasing trend(1.3%),but not significant.This suggests that the relative absorptive capacity of absorptive fine roots is not affected by tree age,but juvenile trees increase their absorptive efficiency by consume less C.Regardless of tree age,the proportions of absorptive fine root biomass and length of thin-root species were significantly higher than those of thick-root species.Principal component analysis(PCA)and pearson’s correlation analysis showed that the proportion of absorptive fine root biomass was mainly affected by the proportion of each order root to the total fine root(the first to fifth order)biomass and the diameter of each order root;the proportion of absorptive fine root length was mainly affected by the proportion of each order root to the total fine root(the first to fifth order)length,the specific root length and branch ratio of each order root,but neither were not related to the proportion of absorptive fine root number of each order root.(2)Tree age had weak effects on functional traits of the first to third order roots in four tree species.From juvenile to adult trees,the cortical thickness to stele radius ratio and mycorrhizal colonization rate of the first to third order roots were generally reduced in all tree species,but were not significant.The former has an average decrease of 14.5%,ranging from0.9%(second order root of L.gmelinii)to 36.5%(first order root of P.koraiensis);the latter has an average decrease of 11.0%,ranging from-6.1%(second order root of P.koraiensis)to27.2%(second order root of L.gmelinii).The root tissue density of the first to third order roots tended to increase in all tree species,with a significant increase of 23.7% and 9.3% in P.koraiensis and L.gmelinii,respectively.There were no consistent changes in other functional traits of absorptive fine roots between tree age.Whether juvenile or adult trees,the correlation between functional traits of the first to third order roots could be integrated into two orthogonal dimensions,the cooperative dimension(root diameter-specific root length axis)and the conservative dimension(root tissue density-root N concentration axis).The cooperative dimension describes the shift in root resource acquisition strategies from “do it yourself” to“outsourcing”,while the conservative dimension describes the shift from slow to fast resource acquisition strategies,which could explain 80.3%,73.8% and 64.3% of root functional trait variation of the first to third order roots,respectively.This shows that there is a multidimensional economic space among the functional traits of absorptive fine root in woody plant,but it is not affected by tree age.(3)The responses of functional traits of typical absorptive fine roots to N addition were consistent in four tree species both juvenile and adult trees.N concentration of typical absorptive fine roots increased by 78.7%,C concentration and C:N decreased by 7.8% and46.6%,respectively.The cortical thickness,the cortical thickness to stele radius ratio and mycorrhizal colonization rate of typical absorptive fine roots decreased,while the stele diameter of typical absorptive fine roots increased.The diameter and branching intensity increased,while specific root length and tissue density decreased.When the tree species were divided according to root morphological characteristics and mycorrhizal infection types,the mycorrhizal colonization rate of thin-root species significantly decreased by 14.6% and branching intensity significantly increased by 31.1% following N addition.The mycorrhizal colonization rate and branching intensity of thick-root species did not change significantly,the mycorrhizal infection types had no effect on them.The response of other typical absorptive fine root traits to N addition was not affected by root morphological characteristics and mycorrhizal infection types.Redundancy analysis(RDA)showed that soil nutrient availability(soil total N concentration,total C concentration,C:N,ammonium and nitrate)explains more than 71% of the variation in functional traits of typical absorptive fine roots in juvenile or adult trees.Soil total N concentration,total C concentration and ammonium were significantly positively correlated with N concentration of typical absorptive fine roots,and negatively with C:N,but weakly with other traits.Overall,our results confirmed that the absorptive fine roots were mainly composed of the first to third order roots in four tree species and were not affected by tree age.The proportion of absorptive fine root biomass increased significantly from juvenile to adult trees,while the proportion of absorptive fine root length did not change significantly.Tree age had a weak impact on the change of functional traits of absorptive fine roots.There is a multi-dimensional economic space among the functional traits of absorptive fine root in woody plant,the cooperative dimension(root diameter-specific root length axis)and the conservative dimension(root tissue density-root N concentration axis),which were not affected by tree age.The response trend of functional traits of typical absorptive fine roots to N addition in the four tree species was also not limited by tree age,and only the N concentration,C concentration and C:N of typical absorptive fine roots were significantly changed following N addition.The results of this study can not only provide insights for better understanding the variations in functional traits of absorptive fine roots in woody plants,but also important to understand the changes in root nutrient acquisition strategies during tree growth and development. |
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