Branching order of lateral roots is a fundamental characteristic of form in root system, roots of different orders display markedly differences in morphology, anatomy, physiology, and longevity. However, the position of an individual root on the complex lateral branching network has often been ignored, and the knowledge of relationships between the formation of the branching roots and function is limited. More recently, studies on root order have found that fine roots of the first-order at the distal end of a root system, which primary function is nutrient uptake rather than storage and transport, are thinner in root diameter with higher tissue nitrogen (N) concentrations, and lower total nonstructural carbohydrates (TNC) concentrations. Thus, they have a shorter longevity in contrast with higher order roots. The questions are what changes in morphology with increasing root orders and how many fine roots will die? To answer these questions is critical for understanding the function of branching order and the linkage between fine root mortality and branching system. The objectives of this study are: 1) to examine variations of fine root diameter, length, specific root length (SRL) and biomass with increasing orders; and 2) to determinate the proportion between mother roots and daughter roots among five orders through two temperature species of Fraxinus mandshurica and Larix gmelinii.
The study site is located at Maoershan Forest Research Station (45°21′～45°25′N, 127°30′～127°34′ E) owned by Northeast Forestry University, in Harbin, China. Both forests were planted in 1986. In each plantation, we established three plots (20 m×30 m) on the site of 480～500 m in altitude. On May 15, July 15 and September 15 of 2003, three/four small intact segments of the fine root system were excavated carefully at a random location in each plot. Soil blocks (20 cm×20 cm×10 cm) were excavated from the sties at 0～10 cm and 11～20 cm in depth and all the intact root segments were collect. Once excavated, the intact segments were put into plastic bags with ice, and were severed at a set temperature of 0～2℃. In the laboratory, each individual root was dissected by order beginning with the distal end of a root system, and increasing sequentially with each branch from the first-order to higher order roots. After the dissection, diameter, length, and dry weight of a given order were determined. 
The results showed that mean diameter and length of fine root in both trees increased significantly (p<0.05) from the first- to fifth-order roots, and SRL decreased significantly (p<0.05). The first-order roots at the distal end of a root system had smaller diameter, shorter length, and larger SRL in contrast with higher order roots. Root biomass varied significantly with root order, the fifth-order roots had the highest biomass, biomass of the first-order roots was higher than that of the second- to fourth orders in Fraxinus mandshurica and higher than that of the second-orders in Larix gmelinii, respectively. In both trees, root numbers deceased significantly (p<0.05) from the first- to fifth-order roots, the first-order roots accounted for 80～90% of total root numbers among the five orders, therefore, the vast majority of roots was the first-order roots. Within the same order, root number varied with different seasons, suggesting that only a subset of the fine root population undergoes the transition from the first-order to higher-order root. It is noteworthy that root number in surface soil (0～10cm) was double higher than that in subsurface soil (11～20cm). In contrast to root number, proportions between mother roots and daughter roots form the fifth- to second-order was 3, and had similar branch patterns in both species and soil depth. However, the proportion between the first-order to second-order was about 10～12 (for Fraxinus mandshurica) and 8 (for Larix gmelinii), which exhibited a branch pattern of herringbone or cluster in the first-order roots. If the branch pattern of roots in higher order was similar between the second- and first-order roots, 65%～75% of the first-order roots in total root number will be dead, these dead roots accounted for 40%～50% of total root length, and 20%～30% of total biomass, respectively.