A forest is a community of trees, shrubs, herbs, microorganisms, and animals, the trees being the most obvious living structures. Trees can survive under a wide range of climatic conditions, but forests generally occupy the moister, less frigid parts of the terrestrial biosphere. To different human cultures at different times, forests have been regarded as places of danger, security, economic opportunity, recreation, and aesthetic pleasure. They take part in natural processes of nutrient cycling and water purification, and otherwise help maintain a clean environment. Forests are important sources of many products. Forestry is the science, art, and technology of managing these forest resources.

FORESTS

The large size and slow growth of trees make forests appear stable and permanent, but in fact they are dynamic sites of ongoing processes such as tree growth and death and soil formation. The tree species in a particular area are also constantly changing as species migrate and new trees invade disturbed areas. Climates themselves change, but this generally occurs so slowlyÑover tens or hundreds of yearsÑthat a given forest area appears to contain a constant group of species.

Ecology

The inhabitants of forest communities interact in complex ways. Trees compete with each other for sunlight, moisture, and mineral nutrients. These materials are necessary for photosynthesis, the process by which green plants produce organic compounds for energy to live and grow. As trees photosynthesize, they absorb carbon dioxide from the air and extract moisture from the soil. Trees help to retain water; heavy rains do not run rapidly off forest land. Natural or human activities that destroy forests result in increased runoff and in temporarily higher levels of carbon dioxide in the atmosphere. After this the growing forest increases the oxygen content of the atmosphere. A mature forest adds less oxygen to the atmosphere. A global research project designed to measure the overall influ