| Fine roots are the important organs of plants to absorb nutrients and water from soil and play a key role in the carbon cycle in forest ecosystems.Fine roots are mostly defined according to their diameter size(≤2mm),but this ignores the heterogeneity of fine root characteristics.Based on root branch order method,fine roots are grouped into absorbing roots(1-3 orders)and transportive roots(4-5 orders).Investigating functional traits of absorptive roots can reveal plant nutrient acquisition strategies.The functional traits of fine root involve in anatomy(internal organizational structure),morphology(diameter,specific surface area,and specific root length),chemistry(nutrient content such as N and P),and fungal symbiosis(mycorrhizal colonzation).Exploring the differences in fine root functional traits among tree species,the relationships between functional trait indicators,and multidimensional features,could provide scientific basis for studying nutrient acquisition strategies and species coexistence mechanisms of tree species.In this study,we elected 40 common tree species from three secondary forests at different stages(Pinus massoniana-Lithocarpus glaber coniferous broad-leaved mixed forest,Choerospondiasaxillaris deciduous broad-leaved forest,and L.glaberCyclobalanopsis glauca evergreen broad-leaved forest)in Dashanchong National Forest Park,Changsha County,Hunan Province.Complete fine root samples were collected to determine diameter,specific root length,specific surface area,tissue density,nitrogen and carbon content,and mycorrhizal colonzation rate of 1-3 branch order of fine roots.Two-way ANOVA(analysis of variance)was used to compare the differences in fine root functional traits among tree species and root orders as well as their interactive effects.Pearson correlation analysis was employed to examine the relationship between functional traits and the correlation with phylogenetic signals.The multidimensional economical space of fine root functional traits was revealing by using principal component analysis(PCA).The main results of this study are presented as follows:(1)Functional traits of fine roots varied significantly with root orders(p<0.01).Diameter,tissue density,cortical thickness,column length,and carbon content of fine roots increased,while specific root length,specific surface area,and mycorrhizal colonzation rate decreased from root order 1 to 3.Average diameter was 0.314mm for order 1,0.394 mm for order 12,and 0.593 mm for order 3.From root order 1 to 3,specific root length were 81.14,43.60,and 17.60 m/g,and specific surface area were 648.1,421.5,and 237.4 m2/g.Carbon contents in fine roots were 434.6,483.5,and 509.7 g/kg,nitrogen content were 18.95,16.33,and 13.48 g/kg,and carbon:nitrogen ratio were 26.22,35.57,and 47.53 for order 1,2 and 3,respectively.The colonization percentage of mycorrhiza in fine roots were 49.98%for order 1,43.97%for order 2,and 28.2%for order 3.For order 1,2,and 3,cortical thickness of fine roots were 392.3,415.4,and 303.9μm,and pericycle diameter were 180.7,288.4,and 446.2μm,respectively.The results show that the functional traits of fine roots with different root sequences are highly heterogeneous,and it is necessary to study fine roots in root sequences.(2)There are significant differences in functional traits among tree species(p<0.01).The Litseacoreanavar.lanuginosa and Trachycarpus fortunei have the largest fine root diameters,Rhododendron simsii has the largest specific root length and specific root surface area,Rhus chinensis and Zelkova serrata have the highest tissue density,Schima superba and Zelkova serrata have the highest fine root carbon content,Litseacoreanavar.lanuginosa and Quercus fabri have the highest fine root nitrogen content,Phyllostachys edulis has the highest fine root carbon to nitrogen ratio,and Diospyros oleifera has the highest mycorrhizal infestation rate.For different mycorrhizal types tree species arbuscular mycorrhizal(AM),ectomycorrhizal(ECM),nitrogen contents in fine roots of AM tree species(16 g/kg),diameter(0.447 mm),specific root length(48.60 m/g),specific surface area(446.9m2/g),and cortical thickness(202.0μm)were significantly higher than that of ECM tree species.But no significant difference of carbon contents in fine roots was found between AM and ECM tree species(p>0.05).Average diameter of fine roots(0.446 mm)and nitrogen content of fine roots(17.66 g/kg)of tree species are higher than that of shrub species,while specific surface area(507.3 m2/g),specific root length(66.99 m/g),and tissue density(0.399 g/cm~3)of shrub species were significantly higher than that of tree species.No significant difference in carbon and nitrogen contents in fine roots were found between tree and shrub species.The differences in functional traits among tree species reflect the different nutrient acquisition strategies of different tree species,thus achieving the separation of subsurface ecological niches and promoting species coexistence.(3)Mycorrhizal colonization percentage in fine roots of AM tree species was significantly and positively correlated with root diameter and nitrogen content(p<0.05),but was negatively correlated with specific root length,specific surface area,tissue density,and carbon content(p<0.05).Mycorrhizal colonization percentage of ECM tree species was positively correlated with specific root length and specific surface area(p<0.05),but was not correlated to root diameter(p>0.05).It is shown that the AM species provide more space for the bushy mycorrhizal fungi to colonize by increasing the diameter of fine roots,while the ECM species provide more attachment area for the ectomycorrhizal fungi by increasing the specific surface area,and use mycelium to expand the nutrient acquisition area and obtain more nutrients from the soil.(4)Functional traits of fine roots exhibited two dimensions of root economic space.Along collaborative dimension,one end was characterized by fine root diameter with thick fine roots to host mycorrhizal fungi and adopt the "outsourcing" strategy,while another end was indicated by higher specific root length to improve nutrient acquisition efficiency and adopt a "do it yourself" strategy.The second dimension was the fastslow dimension.One end represented higher tissue density with long life-span and conservative strategies,while another end represented higher fine root nitrogen content with fast turnover and acquisitive strategies.Unlike aboveground leaf economic spectrum,belowground fine root economic space is more complex.(5)Phylogenetic signals among tree species were determined by using the established phylogenetic tree for investigating the phylogenetic effects on fine root functional traits.The results showed that higher Blomberg’s K values for fine root diameter,specific root length and specific surface area,indicating that the morphological characteristics of fine roots in the cooperative dimension were influenced by stronger phylogenetic signals and higher Blomberg’s K values for fine root diameter,specific root length and specific surface area.This indicates a stronger phylogenetic signal,while.Chemical properties on the fast-slow ladder dimension were less influenced by phylogenetic signals.This study demonstrated that there are significant differences in the functional traits of fine roots of common subtropical tree species,which contribute to the separation of underground fine root ecotone and promote the coexistence of tree species to obtain sufficient nutrients from the soil,The economic space of fine roots of subtropical tree species is two-dimensional and closely related to phylogeny,and species in different phylogenetic branches have different functional traits of fine roots to promote species coexistence. |
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