Background The selection of tree species is critical for the soil and water conservation and vegetation restoration in arid-barren mountainous areas. There are 40% areas in Shandong province belonging to mountain area, where water is one of important environment factors limiting vegetation restoration. Thus, it is very important to choose tree species with developed roots. The examination on the anatomical characteristics of tree roots would be helpful for the tree species selection.

Methods The study site is located in Xueye county of Laiwu city, belonging to typical arid-barren mountainous areas of Shandong province. After afforestation, ten common tree species were selected and all the roots of trees for each species were sampled. According to the branching orders, the roots were grouped into five classes, and root traits (i.e., the root diameter, cortex thickness, stele diameter, ratio of vascular cylinder to root diameter, vessel diameter, and vessel density) were examined by paraffin sectioning, Nikon Eclipse E200 microscopy and Scope Image 9.0 software. The differences of root anatomical traits among tree species were analyzed and clustered by One-way ANOVA and SPSS 19.0 with Excel.

Results The fine root diameter of tree species increased with the root order. The first order roots showed larger diameter in conifer trees than in those deciduous trees. The diameters of deciduous shrub roots were smaller than those arbor and vines; however, the difference among species was not significant (P < 0.05).The stele diameters of fine roots demonstrated similar changes with the average diameter of fine roots. The cortex thickness of the first and second order roots in the deciduous trees was large. However, the root vessel diameter of all the tree species increased with the increase of root order, the vessel diameters in vines roots (i.e., Lonicera japonica and Euonymus fortunei) were smaller than those in arbors and shrubs roots. The vessel diameters in deciduous trees were larger than those of other tree species. The changes of vessel density were opposite to that of root diameter. Based on the anatomical characteristics of fine roots, the 10 species were clustered into three categories, the first one was the thin cortex and compacted transport tissue, including L. japonica, Lespedeza bicolor, Vitex negundo var. heterophylla, Amorpha fruticosa and Pinus thunbergii; the second one was the thin cortex and loose transport tissue, including E. fortunei, Cotinus coggygria, Robinia pseudoacacia and Platycladus orientalis; the third one was the thick cortex and loose transport tissue, including Gleditsia sinensis.

Conclusions The anatomical structure of tree root in arid habitat is related to its life type, which is the ecological adaptability in the process of long-term tree species evolution. Root's anatomical structure is a key index to explore the survival strategy of tree species in drought stress environment.