Vegetation is used at two different levels in the classification.
Pioneer species that briefly invade wetlands when conditions are favorable are treated at the Subclass level because they are transient and often not true wetland species. Subclasses are named on the basis of the predominant life form. The nature of the substrate reflects regional and local variations in geology and the influence of wind, waves, and currents on erosion and deposition of substrate materials. Bottoms, Shores, and Streambeds are separated on the basis of duration of inundation. In the Riverine, Lacustrine, and Palustrine Systems, Bottoms are submerged all or most of the time, whereas Streambeds and Shores are exposed all or most of the time.