It's the successives steps for an area to go from the bare ground to the "stable" ecosystem, a beech and maple forest in this exemple.
Each ecosystem has its own sequence of species, depending on all the factors involved (ground, climate, light, water supply ...) The first ones are pioneer and transitional, preparing the area and changing the details to match the imperatives of the final ones. Then the ecosystem reaches a state somewhat stable with very few changes in the cover, plants and animals (until the next change or accident). The ground composition and its layering is one of these details.