Biologically, the mature plant community is characterized by diversity. It has diversity of species, diversity of ages, diversity of niches and habitats, and a diversity of life forms.

Life forms can generally be associated with the structure of the plant at the time of its maturity. For instance, herbs and grasses have a typical life form; shrubs have another that is typical for their growth habit; trees have an arboreal life form; and vines constitute yet another. Many herbs and flowering plants of the prairies differ significantly from grasses of the prairie in that prairie grasses have a great deal of underground structure in the form of roots and rhizomes, which make them particularly well adapted to fire and drought. Each life form has a different requirement for growth and maturation. Similarly, among life forms of a different type, some are very tolerant of low light intensity, while others require high light intensity. Based on two characteristics then, size and shape at maturity and ability to grow in light of varying intensities, plant species segregate themselves. Those that are capable of living in the habitats and environments provided by others tend to congregate together to form communities.

The mature plant community is dynamic in the sense that it is self-regenerating and is able to compete for and hold a site against all competition. Moreover, mature plant communities usually occupy the best sites and are able to maintain and/or extend their boundaries. The mature plant community develops as the result of the growth and development of many individuals, and is replaced, renewed, and regenerated by the growth, development, and replacement of these same individuals.

The mature plant community is clearly demonstrated by the deciduous temperate forest. In its most fully developed form, the deciduous temperate forest may include several species of large trees which, because of the nature of their growth habits, protrude above the general canopy level to form an overstory. The main structural formation of the deciduous temperate forest, however, is determined primarily by the canopy trees, and these are the dominant species which grow to 40-80 feet. They include the familiar forest trees such as oaks, maples, elms, etc. Their interlocking and overlapping leafed branches produce a continuous cover over the landscape, and they reproduce themselves at the site. Dominant canopy species are very versatile; they thrive in the shade of the canopy when they are young and ultimately prosper in full sunlight when full-grown.

The creation of shade or screening of light by the canopy produces a habitat which normally would not occur without it, and in the shade of the canopy grow trees of lesser structure; that is to say, trees of the same species as the canopy but which have not grown into the canopy. Their growth rate is perhaps reduced by low light intensity or inhibited by competition. In addition, other species of deciduous trees which never attain the stature or the height of the canopy trees are a part of this second layer. These are species such as ironwood, bluebeech, dogwood, and redbud which, even under optimal conditions, would not attain the size of the dominant canopy trees.

Lower still on the deciduous totem pole is the shrub layer, usually consisting of such things as mountain laurel, rhododendron, and similar types of plants. Finally, there is an herbaceous layer of grasses and flowering plants on the forest floor, and perhaps some vines and vine-like materials such as trailing arbutus or winterberry. In considering this remarkable combination of life forms—arboreal, shrub, herb, grass, and vine—each with its different requirement for light intensity, it is easy to see that there is a measure of interdependency between and among species. Due to the incredibly efficient fashion in which a fully developed plant community utilizes energy, little is wasted. Sunlight is first intercepted at the canopy level and photosynthesis, growth, and development occur there. Next, sunlight of lesser intensity filters through to the secondary layer, and more photosynthesis, growth, and development occur there. And the process continues through the stages until the reduced sunlight energy reaches the herbs of the forest floor.

Not only do plants have differing life forms, but in a sense they have differing life styles. Plants can be classified as annuals, i.e., those plants which complete their life cycle in one year; biennials, i.e., those plants which complete their life cycle in two years; or perennials, i.e., those plants with a very long life span. Not only are there differences in the life forms of plants with respect to the completion and development of their life spans, but there are great differences in the reaction of plants to climatic conditions. The factor of adaptation to climate is as important as any other factor in determining which plants will grow in any given place. Plants native to a tropical rainforest will not grow in a temperate deciduous forest because they cannot survive the winter. Conversely, the trees of the temperate deciduous forest have adapted to the cold, and such plants will not flourish in hot, humid, nontemperate climates. Winter dormancy, which is essential to survival in a temperate winter, does not allow for the continuous growth that warmer climates permit. Life forms and the ability to survive the prevailing climatic conditions are the compelling forces in community structures, and must be fully understood when the city is considered ecologically.