Objective To analyze the size distribution and determine the effects of stand age on root biomass in Pinus kesiya var. langbianensis plantation based on the data of fine root, coarse root and dead root biomass among different species groups.

Method This study was conducted in 5-year-old, 8-year-old, 15-year-old, 25-year-old and 36-year-old P. kesiya" var.langbianensis plantations located in Pu'er city, Yunnan province, China. The soil cores with diameter of 8.5 cm were collected in October, 2014 at different stand age. Each soil core was separated into three sections at the depths of 010, 1020, and 2030 cm.

Result The fine root biomass of P. kesiya var. langbianensis decreased with the stand age, while the fine root biomass of other species groups increased with the stand age. The maximum of fine root biomass was found in the 36-year-old P. kesiya var. langbianensis plantation. There were no significant difference in the coarse root of P. kesiya var. langbianensis and dead root of different stand age in P. kesiya var. langbianensis plantations. On the contrary, the coarse root biomass of other species group and stand, and root biomass increased with the stand age. The fine root biomass mainly distributed in the soil with the depth of 010 cm. The fine root biomass of P. kesiya var. langbianensis, the other species group and all stand as well as root biomass decreased with the increase of soil depth. The stand age and soil depth had significant effect on the fine root biomass of P. kesiya var. langbianensis and other species group, respectively. The cross effect of stand age and soil depth had no significant influences on the fine root biomass of P. kesiya var. langbianensis. The stand age had a significant effect on the dead root biomass. The stand age, soil depth and the cross of stand age and soil depth had significant effect on the coarse root and all root biomass.

Conclusion With the growth of P. kesiya var. langbianensis plantations, the community structures and species composition will change and has a significant effect on the root biomass.