A, due to the influence of environmental and human factors, the population density of the same section is different, and it is often mosaic distribution, which is the horizontal structure of the community, and A is correct;

From B, B to D, it is inferred that the succession process is: herb community → shrub community → tree community; In the process of succession, the dominant species are constantly changing, but herbs and shrubs exist but do not disappear, B error;

C, in the whole succession process, because the energy assimilated by producers is more and more, it eventually tends to be stable, and the population of each group generally increases in an S-shape, and C is wrong;

D, the community type at the time of succession to a relatively stable state is determined by environmental factors such as climate, soil, moisture and temperature, and D is wrong.

So choose: a.