| By using soil-drilling method for sampling by means of choosing artificial forest of slope of different direction and setting temporary plots, this research investigated the root distribution features of Robinia pseudoacacia and Pinus tabulaeformis at Yongshou and Changwu counties in Weibei areas. The results indicate that there were remarkable differences in both the vertical and horizontal root distribution patterns of Robinia pseudoacacia and Pinus tabulaeformis on sites of different direction. Soil water is a main factor of affecting develop of fine roots. The fine roots in the north-facing site has more wider absorption space of water and other nutrients. Root length and surface areas of unit volume in the north-facing site are much greater and more equable than in the south-facing site's in vertical and horizontal directions within the 50 cm soil layer.In the north-facing site, the root distribution depth is deeper than that in the south-facing site's on the both spot of 0.5 m and 1.5 m distance to the trunk. As of relative abundance soil water in the north-facing site , The growing potentiality of Pinus tabulaeformis and Robinia pseudoacacia is active, and the difference of the root distribution features is not clear. On the contrary, the difference is very clear in the south-facing. The distribution depth of Pinus tabulaeformis fine root length and surface areas of unit volume is between 10 and 70 cm soil layer greatly, while is between 20 and 120 cm to much extent for Robinia pseudoacacia in the south-facing site . The opposite relative of between the bulk density and root distribution is available to Pinus tabulaeformis, but is unavailable to Robinia pseudoacacia. The result indicates that Robinia pseudoacacia is much better than Pinus tabulaeformis in absorbing more deeper soil water and in adapting and in producing in Loess Plateau. At the same time, this result indicates that anti-effect of Robinia pseudoacacia to soil water ecology environment is heavier than that of Pinus tabulaeformis in Weibei Loess Plateau which has half-humid climate.The perlite and sand of have great effect on the physical property of different combination soil by using pot experiment. The bulk density decreased with the increasing of perlite, but the total porosity increased in the same time. When the ratio of sand increased , the bulk density increased and the total porosity decreased .On the basis of root distribution investigation, by using experimental ecology method, a study wasmade on the relative of plantlet root distribution patterns of the four tree species and the physical property of soil. The results indicate that the growing potentiality and weight of plantlet fine roots and broughs and leaves are most eligiblest in the proper bulk density and porosity. Too high bulk density and total porosity and too low bulk density and total porosity have adverse effect on growing of roots to some extent. The effects on the bulk density to plantlet roots of Robinia pseudoacacia and Pinus tabulaeformis are more clearer than that of Platycladus orientalis and Prunus armeniaca var.ansu Maxim. The root distribution character of Platycladus orientalis was mainly controlled by its genetic features, little by site conditions, while is showed that the roots are well developed in spite of relative high the bulk density. The main root of Prunus armeniaca is remarkable taproot type, and root vigor is strong. So it is controlled little by the bulk density. The effects on the bulk density to plantlet roots of Pinus tabulaeformis are more stronger than that of Robinia pseudoacacia. The result shows further that although root growing could be hindered by high bulk density to some extent, the roots can keep well growing potentiality in the site of keeping ideal water and other nutrients in spite of relative high bulk density.
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