In northern floodplain forests, willows are usually the species closest to water's edge, followed by cottonwoods, elms, boxelder, and ashes. Northern floodplain forests originate with stream channel movement and after floods. Willows, particularly sandbar willow, have abundant fibrous roots which help stabilize soils. Their roots and stems also help catch silts and clays carried by floodwaters, further adding to soil development. Cottonwoods exhibit very rapid growth and usually overtop willows, except at the water's edge where the more flood-tolerant willows outlive cottonwoods.
Older forests contain more perennial grasses and shrubs than do young stands.
Very few cottonwood saplings and seedlings are found under cottonwood canopies. In western North Dakota, replacement of cottonwood trees occurs from about 30 to 60 years.