Until recently forest ecosystems were believed to be stable, closed systems. They were thought to exist in a steady-state condition -- the climax or old growth forest -- that was their permanent condition unless perturbed by unnatural events. Scientists now recognize that forests are extremely complex and dynamic -- that they constantly change in structure and species composition as a consequence of plant growth, disturbances, species migrations, climatic changes, and other processes.
The systems approach was developed to grapple with complex and dynamic subjects -- from subatomic physics to astronomy, from medical research to industrial factory management. The systems approach manages complexity and dynamics by grouping things at different levels within a hierarchy and evaluating patterns and dynamics within and among different groups and levels. The ecosystem concept arose from the application of the systems approach to complexity in nature.
Forest ecosystems can be organized in a hierarchy with individual trees as the basic unit. Trees can be grouped into stands, stands into landscapes, landscapes into watersheds, and so on up to the scale of the whole earth.