Fine Root Morphology, Anatomy And Tissue Nitrogen And Carbon Of The First Five Order Roots In Twenty-Seven Chinese Tropical Hardwood Tree Species

Trees fine roots play a key role in carbon (C) flow and nutrients cycling in forest ecosystems. In the long term organic evolution, during their development tree roots branch into different hierarchies, i.e. branch orders, in ordering to adapt to heterogeneous soil environments. However, how root morphology, anatomy and tissue chemistry vary with branch orders, as well as what the relationships exits among these root traits are poorly understood. In this study, we investigated root morphology, anatomy and tissue chemistry of the first five orders in twenty seven tropical tree species, aimed to reveal the variation of root traits and underlying mechanism across tree species. Our results showed that:There were general pattern of the fine root morphology with ascending branch orders in 27 Chinese tropical hardwood species. With the increase of the branch order, root diameter and length increased across 27 species. Specific root length (SRL) inversely correlated with root branch orders. However,13 out 27 tree species showed that root tissue density increased with branch order increasing.Vascular bundle diameter and V/R ratio (vascular diameter and root diameter ratio) increased as root order increasing in the first five orders. The lower order roots (the distal two or three branch orders) consistently had primary development with an intact cortex, may mainly provide most of the root surface for resource uptake, whereas higher order mother roots showed secondary development and may serve mainly transport and storage functions. Correlation analysis showed that the most important influencing factor of root diameter is the cortex thickness.With root order increasing, tissue C content increased, tissue N content decreased, and the C/N ratio increased across tree species. Although some tree species had differed mean root diameter, tissue N concentration, tissue C concentration and C/N ratio were relatively similar. The results showed that the same order roots across tree species may be formed with similar anatomical structure, and play similar functions in the root systems.Our results suggested that there were systematic variation in morphology anatomical structure and tissue chemistry among the first five order roots, and there were close linkages between these root traits across 27 tree species. These findings may further advance our understanding in the relationship between root morphology, anatomical structure and tissue chemistry, and provide mechanistic explanation of fine root turnover in woody plants.