Vertical gradient of light is often important. Light often a limiting factor for plant growth, all other things being equal. The vertical gradient of light changes through the seasons in a deciduous forest. There is reduced red and blue light in the understory. Heavy seeds and vegetation leads to clumping near the parent tree or plant.
Herbaceous plants are common where sunlight reaches the floor of the forest.
Changes in pH, nitrogen, herbivores, pathogens, shading, adding organic matter to the soil, etc. Sometimes species phase themselves out. Frequent in chaparral, grasslands, etc. Prairies in Illinois were stabilized by fire. In the last 500,000 years, coastal forests in California have undergone retrogressive succession.