Terrestrial Plants and Plant Communities

A terrestrial plant community is best described by its composition and dynamics. It is most important to know for each plant speciesits numbers, the area covered by it, its rate of reproduction, and its longevity. It is also important to know how soon adult plants are replaced by their young or other species and the frequency with which one species occurs relative to others in the community. All these characteristics used to describe the normal composition and dynamics of the community (sometimes called the baseline condition) are influenced by the qualities of the environment humidity, temperature, sunlight, soil composition, and the presence or absence of toxicants. Normally, a plant community is relatively stable and displays an equilibrium condition that changes only slowly. It also tends to be resilient in the sense that short-term perturbations to the system do not persist. But prolonged periods of stress, e.g. due to extremes of climate, may reduce the growth of sensitivespecies; the more tolerant species then become predominant. When a toxic agent is introduced into a plant community (often in the air), some species will be affected more adversely than others. These sensitivespeciesmay be no longer able to compete successfullyfor their place in the system, and disappear. High levels of toxic agent or prolonged exposure may destroy large numbers of ~pecie~with con~equent disruption to the plant community. Ultimately the system