Abstract: In this study, we analyzed the vertical distribution characteristics of root biomass density, root length density, root surface area density in monoculture and mixture of Pinus tabuliformis and Robinia pseudoacacia plantations in Caijiachuan small watershed of Jixian County, Shanxi. We examined their relationships with soil physical and chemical properties in different stand types. The results showed that the total root biomass density of P. tabuliformis and R. pseudoacacia in mixture was more than 75% higher than that in monoculture. Root system of P. tabuli-formis mainly distributed in shallow layer (0-40 cm), while that of R. pseudoacacia was deeper (40-80 cm). Fine roots were predominant in different diameter classes. Length density and surface area density of fine roots were in the order of R. pseudoacacia in mixture > P. tabuliformis in mixture > R. pseudoacacia stand > P. tabuliformis stand. Root biomass density of fine roots was in the order of P. tabuliformis in mixture ＞ R. pseudoacacia in mixture ＞ P. tabuliformis stand ＞ R. pseudoacacia stand. In vertical profile, the total root and fine root biomass, root length and root surface area density of P. tabuliformis stand, R. pseudoacacia stand, P. tabuliformis in mixture and R. pseudoacacia in mixture showed a rule of decreasing with the increases of soil depth. Under different stand types, fine root length density, root biomass density and total root length density were positively correlated with soil total nitrogen, soil organic carbon, and soil water contents. Total root surface area density was significantly positively correlated with soil organic carbon, soil water content, and soil total nitrogen. The distribution of roots in pure and mixed stands of P. tabuliformis and R. pseudoacacia showed different patterns. Compared with the pure stand, the mixed stand had higher root biomass, soil nutrient contents, and soil water content.

Key words: root distribution, mixed forest, pure forest, Pinus tabuliformis, Robinia pseudoacacia