If we look at the development of the forest community, of the grassland community, or of any community, we see that it proceeds through stages. There are many plants which are not capable of living except in high light intensity, yet whose life forms do not enable them to persist into the mature community. Consequently, as communities develop these species are eliminated. For example, the grasses that are found in a city lawn are plants that require relatively high light intensity. These grasses could not form the herb layer of a mature forest community because, when the forest community is fully matured, there is not enough light at the forest floor to support their growth. These plants must live in an area which, for whatever reason, has few trees. Many of the weeds, flowers, and herbs of open fields similarly require high light intensity, and are not capable of sustaining themselves in a mature forest community.

The structure of a city lawn is quite simple. The full intensity of the sun strikes a single photosynthetic layer, passes through it, and strikes the ground. A tremendous amount of energy, the same amount that can support the full growth of the forest, is absorbed by these grassy areas. It is this enormous amount of sunlight energy, capable of completely sustaining the growth of the mature forest, that must be overcome when grasses, lawns, meadows, and other nonforest community structures are to be maintained in a city that was built in a climate capable of supporting a deciduous forest.

As a community develops through pioneer stages, through herb, shrub, and secondary layer stages, and finally into a mature community with a closed canopy, many factors change. In an open community, where a great deal of light reaches the floor of the community, high evaporation rates and higher soil temperatures are likely to occur. Conversely, as a cover is formed over the soil, the temperature of the soil surface is likely to be reduced, and the rate of evaporation is likely to be less. Moreover, the climate at the center of a mature plant community is noticeably different from that at its fringes. Consequently, the structure of the mature plant community deep in the deciduous forest differs from the more open, pioneer community that lies at its edges.

The most complicated community structure to be found in terrestrial vegetation is that of the tropical rainforest. In addition to all of the layers described for the deciduous temperate forest, a tropical rainforest has great numbers of epiphytes which grow not only in the canopy species, but in the secondary species. Spanish moss is the epiphyte most common in the United States, particularly in Louisiana, Mississippi, Florida, and other southern States. Spanish moss is quite capable of sustaining its entire life cycle merely from the nutrients and water that it gets from the air. In recent years, acres of forest in southeastern Georgia, which normally abound in Spanish moss, have suffered a loss of this plant and air pollution is suspected as the culprit.

The changing factors in community development, then, proceed as the community progresses from its pioneer stage to its mature stage. As the community increases in complexity, with numbers of different kinds of species, representing different life forms, occupying different niches in the community, the photosynthetic efficiency of life processes for a given parcel of ground also increases. This increase in the complexity of the community provides a greater number of habitats for plants, as well as a greater number of habitats for animals.

The ecological niche which the mature community provides for animals is significant not only because of the great variety of habitats and niches but also because the greater increase and efficiency of the photosynthetic process make the community more likely to be able to support an animal population. Of course, a great abundance of wildlife is found at the perimeter of the forest. Here plants which are good for forage for ungulates such as deer and game birds such as pheasant, quail, and grouse are more plentiful than they are closer to the center of the forest.

The photosynthetic apparatus of green plants is particularly well adapted to harness the energy of the sun, and very little can stop, hinder, or slow down the process of capturing sunlight energy and its utilization in the living system.