In this paper, the relative dominance of Populus tremula var. davidiana, Betula platyphylla and Picea wilsonii has been regarded as the measure of the niche overlap. Their final result of competition has been discussed with Lotka-Volterra equation of competition. The results are as follows:

1. The progressing succession of coniferous forest is a process of internal, directional and linear succession. The important power is the competition among dominant plant populations.

2. The result of competition among dominant plant populations has no relation to the inherent growing ability (r) of every population and the first state, and has relation to the competitive coefficient (α)between populations and the accommodeaed capacity (K) of every population.

3. The competition among dominant plant populations. may be regarded as a mathematical theory and a method studying succession of plant community.

In this paper, the characteristics of the vertical distribution of the forest vegetation on Emei Mountain was discussed. Using the methods of quantitative classification and the analysis combining the ecological physiognomy and floristic composition, according to the altitudinal distribution of plant communities, four vertical zones of forest vegetation may be discerned:

1. The evergreen broad-leaf forest zone below 1900m;

2. The evergreen and deciduous broad-leaf mixed forest zone 1500—2000m;

3. The needle-leaf and broad-leaf mixed forest zone 2000—2500m;

4. The cold-temperate coniferous forest zone 2500—3099m.

In this text the layer-cutting method in crop field (M. Monsi and T. Saeki, 1953) is adopted. The produce structure of communities, the distribution of light and the variation of extinction coefficient within communities under different density condition of the millet are studied. The aim is to findout a rational density of millet and a simple method for determinating daily average extinction coefficient.

The experiment results are shown as follows:

1．Aerial biomass, ear weight and LAI etc. of the community of medium density are 724.5g/m2, 289.81g/m2, 2.95 respectively, they are the highest in the communities with different density. It is shown the density between 45000--50000/mu are suitable.

2. It assumes positive correlation between density of community and extinction coefficient. The extinction coefficient increases with density. It shows negative correlation, between the extinction coefficient of communities and the average value of light intensity within communities.

3. The tendency of diurnal variation of extinction coefficient of different millet communities is presented as a depression shape curve i. c. higher at the morning and evening, lower at noon. Through practical measure, we can see that the instantaneous k value at 9. a. m. or 3. P. m. approximates to diurnal average value of the community during the day. Thus each of them instantaneous k value may be considered as approximate value of daily average k value.