Ecological systems are categories which describe general ecological functions and processes. Upland (terrestrial), wetland (palustrine), and estuarine systems are included in this classification. The wetland definitions mainly follow the classification of Cowardin et al. (1979). True aquatic systems such as lakes, oceans, and rivers (called lacustrine, marine, and riverine systems, respectively) are not included. The upland, wetland, and estuarine systems are further subdivided into more specific classes. The classes are defined by the dominant life form of the vegetation which reflects the basic physiognomy or appearance of the habitat. Forest, woodland–barrens, shrub, and herbaceous are the classes used in this classification. The systems and classes are combined in the following definitions. These definitions are necessary because they help us distinguish between a wetland and an upland—or a forest and a woodland. The differences between systems or classes are often technical and difficult to recognize in the field. In order to analyze a site and determine the appropriate ecological community, it is necessary to be familiar with the systems and classes. The species lists for ecological communities are organized by regions and according to the following systems and classes.