This study is a series to analyse the succession of the slope vegetation of expressways. In this paper, the mechanism and function of tree root of the model stands were studied in relation to the process of succession.
The mechanism of succession of the slope vegetation has been represented only by the structure of the above-ground parts of communities. But the growth and property of distribution of root, the underground parts, are very significant to analyse the succession. The author tried to make clear the distribution and form of roots for the purpose of examining both the characters of tree root and the responses of them to site factors.
He made a case study on Meishin expressway. 13 or 14 years elapse after seeding on slopes, and recently dominant tree species made several types of communities.
The investigated stands were chosen taking an interest in tree species, slope types, and soil conditions. Cutting and fill slopes were desired to be the same soil material to compare the growth of the same tree root on both slope types. 6 stands of Pinus densiflora, 2 stands of Robinia pseudo-acacia, and 2 stands of Alnus sieboldiana were investigated according with the purpose of this study.
The horizontal and vertical distribution of root biomass and the classification of root types were studied in relation to the site conditions of slopes such as the slope types and soil conditions.
The root size was classified into 5 ; fine root (diameter is under 2mm), small root (2 to 5mm), medium-1 root (5 to 10mm), medium-2 root (10 to 20mm), and large root (over 20mm).
The results of survey are as follows.
1. The significant correlations were clarified between the rcot bicmass and the above-ground biomass.
2. The root type of Pinus densiflora and Alnus sieboldiana were different according to the slope types. Both species were vertically of flat-root type on cutting slopes and deep-root type on fill slopes. From the soil hardness test, this difference was considerd to depend on soil hardness of the two types of slopes. These phenomena are very interesting.
3. The root system played an important role for the process of succession, because of the competition for a living space under the ground.
4. The distribution of root system of woody species is much wider horizontally and vertically than herbaceous species, therfore, the progression of succession is desirable for the conservation of the slopes from erosion and landslide.

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