Fine roots are the core of the belowground ecological processes and play an important role in tree root systems. Fine roots are the major organ trees use to absorb water and nutrients. They play an important and significant role in the processes of carbon allocation and nutrient cycling of forested ecosystems. Therefore, an understanding of the structure and function of root systems allows us to enhance forest ecosystem productivity and to understand the way available resources are used belowground.
No single, widely accepted definition of what makes up a fine root exists, although most researchers refer to roots ≤ 2 mm in diameter as fine roots. We studied eight forest types in a hilly red soil area in South China during the 2010 growing season. We measured important root characteristics at a variety of soil depths such as the distribution of RLD (root length density) in several size (diameter) classes, fractal dimensions and the relationships between RLD and soil bulk density, and between soil organic carbon and total nitrogen. The results indicate Pinus massoniana low-benefit forest (PMLF) has the lowest RLD, but larger densities were found in Chinese fir low efficiency forest (CLF) and Pinus elliottii forest. The RLD in topsoil was significantly correlated to the shrub-herb layer coverage (r=0.793, P<0.05). RLD of all forest types decreased with the increasing of soil depth. The effects of plant species and growth conditions on RLD declined with increasing soil depth. Also, the differences of RLD among forest types gradually decreased as soil depth increased. In the different habitats, no obvious changes were found between RLD and diameter class in the same soil layer. The RLD in the 0 to ≤ 0.2 mm diameter class was the highest in Pinus massoniana Low-benefit Forest (PLF), and the RLD usually peaked in the 0.2 to ≤ 0.5 mm diameter class in all forest types except Pinus massoniana Low-benefit Forest (PLF). RLD in all diameter classes all declined as soil depth increased. As root diameter increased, the differences of RLD increased significantly in the shallowest soil layer and in the deepest soil sample. Correlation analysis indicated RLD was significantly correlated with soil organic carbon and total-N, while there was a negative correlation between RLD and soil bulk density. RLD in the 1-2 mm diameter class had a significant impact on soil structure, soil organic carbon and total-N accumulation. The fractal dimensions of root length ranged from 2.0973 to 2.6063 in all forest types. Samples with fractal dimensions had higher concentrations of roots in the 0-0.2 mm diameter length class. This suggests the succession process of the root system not only affected by plant genetic characteristics and climate factors, but it also influences soil quality; and its various characteristics were also closely related. The function of roots in improving soil structure or increasing nutrient availability would be weakened if the diameter class was too large or too small with the same root length. Therefore, ecological management and forest development should include the consideration of root characteristics. Forest stand structure, which consists of trees, shrubs and herbs, should be managed in a way which enhances ecological system functions and increases and maintains soil productivity.