The word "ecology" comes from the Greek "oikos, meaning "household" or "place to live". These ecosystems or communities are usually self sustaining. I say "usually" because some of these communities can become unbalanced very quickly when detrimental factors occur. Some ecosystems, like tundra, coral reefs, wetlands and grasslands are very fragile and very small changes can effect their health. Larger ecosystems with wide diversity are much more stable and somewhat resistant to harmful changes.
A forest ecosystem community is directly related to species diversity. Generally, you can assume that the more complex the structure, the greater is its species diversity. You should remember that a forest community is much more than just the sum of its trees. A forest is a system that supports interacting units including trees, soil, insects, animals, and man.
Forest ecosystems tend to always be moving toward maturity or into what foresters call a climax forest. This maturing, also called forest succession, of the ecosystem increases diversity up to the point of old age where the system slowly collapses. One forestry example of this is growth of trees and the entire system toward an old growth forest. When the ecosystem is exploited and exploitation is maintained or when components of the forest begins to naturally die, then the maturity of the forest ecosystem declines.
Management of forests for sustainability is desirable when forest diversity is threatened by overuse, resource exploitation, old age and poor management. Forest ecosystems can be disrupted and harmed when not properly sustained. A sustained forest that is certified by a qualified certification program gives some assurance that the forest is managed to allow maximum diversity while satisfying the manager's environmental and economic demands.
Scientists and foresters have dedicated their entire careers trying to understand even a small part of forest ecosystems. Complex forest ecosystems are extremely diverse, ranging from dry desert shrub land to large temperate rain forests. These natural resource professionals have categorized forest ecosystems in North America by placing them into forest biomes. Forest biomes are broad categories of natural tree/plant communities.