Root Identification And Varitation In Architecture Of Fine Roots Of Subtropical Tree Species In Southern China

Fine root of the trees is vital organs which are important in the acquisition of water and essential nutrients and maintaining normal physiological activities, they make a significant contribution to material circulation and energy flow at the ecosystem level, especially to the carbon cycling.Therefore, the fine roots more and more attentions of plant physiological ecologist.Fine root is not a simple homogeneous system with a complex branching structure, the traditional standards to define fine roots by the size of diameter (<0.5mm,<1mm, <2mm or <5mm) are not uniform, the roots in the same diameter size may include a number of branching fine root orders, which have different morphological and functional heterogeneity. So far there is still no uniform standard to define the fine roots. Therefore, researching the functional heterogeneity from the perspective of root orders is necessary, it benefit to definition of fine root.In this study, we studied root architecture across the first five or six root branch orders of 6 typical tree species (Liquidanbar formosana, Choerospondias axillaris, Cyclobalanopsis glauca, Lithocarpus glaber, Cunninghamia lanceolata and Pinus massoniana) in Dashanchong State Forestry Farm in Changsha County, Hunan Province, China. All fine root samples were collected by using excavation method and the roots were dissected according to the branching described by Pregitzer. Our objectives were:(1) to examine architecture among six subtropical tree species; (2) to determine nutrient concentrations across different branch of fine roots; (3) and to have knowledge of the relationship between root architecture and nutrient exploration strategy. The results are presented as follows:(1) The diameter, length, specific length, specific surface area and tissue density among six different species are of great heterogeneity. The average value of root diameter (0.23-0.48 mm), length (0.07-0.24 cm), tissue density (0.17-0.21g·cm-3) of first root order presented minimum, the specific root length (31.8-118.09m·g-1), specific surface area (480.65-1157.82cm2·g-1) of first order were maximum. The average value of diameter (0.6-1.56 mm), length (0.55-0.97cm) and tissue density (0.27-0.55 g·cm-3) of sixth order achieved maximum, the specific root length (2.11-12.39 m·g-1) and specific surface area (84.54-198.48 cm2·g-1) of sixth order were minimum. With the increase of the root orders, the average diameter、root length and tissue density of fine roots increased (p<0.05), while the specific root length and surface area were significantly decreased (p<0.05).(2) The C content of fine root increased at first and then decreased with root order increase. Among them, the C content of 1-4 roots of C. glauca, L. glaber and C. axillaris increase, with the increase of the root orders increases, however, the C content of fifth root order less than forth order. The C content of 1-3 roots of P. massoniana, C. lanceolata and L. formosana increase, with the increase of the root orders increases. Apart from C. axillaris, N content in fine root decrease as root order increase, but C:N ratio was increase from low to high root order. C content in fine root of C. glauca was the highest one and that of L. glaber was the lowest. N content in fine root of C. axillaris was the highest while that of P. massoniana was the lowest. The C:N ratio in fine root of P. massoniana was the highest and that of C. axillaris was the lowest.(3) The diameter of stele increased significantly in six species,with root order increase (p<0.05), the percentage of stele in total diameter also increase; while the cortex thickness’s increase trend is not significant with the increase of root orders; the thickness of fine roots were mainly due to the growth of the stele. In addition, the cortex of the forth and higher orders in all species came off or became incomplete, and the third orders of C. glauca and L. glaber came off and became incomplete. The results showed that the first two orders were the main module with absorptive function, and the third order has absorptive function to some extent.