Characters of root biomass spatial distribution of Robinia pseudoacacia in Weibei loess areas

s: In this research, soil auger is used to investigate root distribution characters of the main planted species, R. pseudoacacia, in loess area. Based on the spatial analysis of the root biomass distribution characters, results indicated that there were obvious dif-ferences in root spatial distribution characters on different sites. On northern hillsides, root biomass of R. pseudoacacia is distributed relatively evenly in the soil of different depth, which favored to extend the absorption space of the tree, and promoted the growth of the upper part of the tree. On southern slope, its root is concentrated in upper soil of 40~80 cm, which limited the absorption space of the root system, and consequently the growth of the upper part of the tree. Judging from its horizontal distribution characters, on southern hillsides, root biomass decreased rapidly with the increase of distance to the trunk; while on the northern hillsides, root sys-tem has maximum biomass on the distance of 1.0 m, 1.5 m and 2.0 m to the trunk, which helped to indicated that the root system of R. pseudoacacia on northern hillside has larger absorption space than that on southern hillsides. Analysis of the distribution characters of root biomass with different diameter class showed that the differences in root distribution patterns were mainly caused by root with larger root diameters (d3 mm). Further analysis indicated that on the two sites, root biomass with diameter between 1 mm and 3 mm has similar distribution patterns; but there was obvious differences in the root biomass distribution patterns with its diameter less than 1 mm, which was in accordance with the site differences, and can be treated as physiological effective root to reflect the effect of site on tree growth. Combined with the root distinction coefficient, root biomass distribution characters were analyzed quan-titatively. Results indicated that on southern slope, root distribution coefficient of total root biomass, root biomass with diameter less than 3 mm, 1 mm was 0.984, 0.980 and 0.975 respectively, which indicated that relative small root, especially fine root, was distrib-uted in deeper soil. While on northern slope, root distribution coefficient of total root biomass, root biomass with diameter less than 3 mm, 1 mm was 0.988, 0.990 and 0.990 respectively, which indicated that there was relative lager root, especially fine root was dis-tributed in deeper soil. Comparatively, the root biomass distribution pattern on northern slope extended the absorption space of the tree, favored the absorption of soil moisture and nutrient in deeper soil, and promoted the growth and development of the upper part.