Roots play an important role in individual plant carbon budgets and ecosystem-level carbon and nutrient cycling. Considerable research has concentrated on the vertical distribution and seasonal dynamics of fine root biomass. However, the spatial distribution of root biomass has been poorly studied. Our objectives are to determine how fine root biomass changes with horizontal distance, and to examine the spatial distribution of fine root biomass in stands of different ages.
The horizontal and vertical distribution of root (0-10mm diameter) biomass for three Pinus massoniana plantations of different ages, 20, 30 and 46 year-old, in the Three Gorges Reservoir area, Hubei Province, China, was studied. Ten cm diameter soil cores were collected in July, 2010 at different horizontal distances (0.5, 1.0, 1.5 and 2.0m) from the stem. Each soil core was separated into five sections at depths of 0-10, 10-20, 20-30, 30-40 and 40-60 cm. Roots (0-10mm) were first separated into living and dead roots. Then both categories were classified into two diameter classes, that was, two groups for fine roots (0-1mm and 1-2mm), and two groups for coarse roots (2-5mm and 5-10mm). Roots were dried at 80℃ to a constant mass and weighed.
Our results indicated the average fine root biomass of P. massoniana in the Three Gorges Reservoir area was 0.56 t/hm², which is within the range of worldwide fine root biomass (<2-5mm diameter), but lower than the average fine root biomass of subtropical evergreen needleleaf forest. The total root biomass (≤ 10mm diameter) for P. massoniana forest for the 20, 30 and 46 year-old stands was 2.40, 4.72 t/hm², and 2.94 t/hm², respectively, and differences between them were statistically significant (P<0.05). However, the fine root biomass (0-2mm diameter) declined insignificantly with an increase in forest age (P>0.05). In all three stands, living root biomass was much higher than dead root biomass. Fine root biomass in all the stands peaked at 1.0 m from the stem, but fine root biomass in stands of different ages changed differently as the horizontal distance increased. Fine root biomass was mainly concentrated in the upper soil layer and decreased as soil depth increased. For all ages of stands, 47.53%-71.73% of living roots were observed in the 0-20cm soil layer. The majority of coarse roots (2-10mm diameter) were at a depth of 20-60cm. We concluded smaller roots were more sensitive to environmental change than larger roots, based on an analysis of the interactions between the spatial distribution of fine root biomass and soil depth, distance from stem and stand age. Spatial distribution of fine root biomass was very significantly influenced by soil depth (P<0.01), but effects of stand age and horizontal distance from the tree stem were not significant (P>0.05). Also, all these factors had no significant influence on the spatial distribution of coarse root biomass (P>0.05). The results indicate more consideration should be given to research on the spatial distribution of roots and the effects of these integrated factors on root distribution.