While the focus of so much environmental attention these days relates to how species and ecosystems are going to respond to all the changes we are throwing at them, certain species that thrive on change find themselves living in an artificially stabilized world, with equally problematic results.
River systems are a great example. There’s that old saying that “no man steps into the same river twice.” This is very true; rivers are naturally dynamic, and the streamside, or riparian, plant species that live along their banks have adapted to the ever-shifting landscape that results from cycles of flooding and summer drought.
These flood cycles can be described by their timing, magnitude, frequency, and duration, collectively referred to as the flood “disturbance regime.”
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Water has direct effects on plants, as well as indirect effects mediated by the stuff that it picks up and moves around (e.g., sand and gravel that gets eroded, transported, and deposited during floods). In rivers that meander across wide valley bottoms, like those found in California’s Central Valley, the path of a river naturally shifts course and wanders back and forth across its floodplain.
In some bends in the river, the river moves a little bit each year, eroding the outer bank and “growing” the inner one (called a point bar). Periodically, though, the path of the river shifts dramatically, and a bend of the river channel gets cut off and “abandoned” by the main flow of water (see time lapse images below). The result of both processes is the creation or renewal of patches of cleared land, upon which new vegetation can germinate and grow.
On this cleared land along both the main channel and the abandoned channels of a river, the new plants that begin to grow are typically good “pioneers” – the early arrivers that have strategies to deal with lots of environmental stresses. Later, the “resource competitors” will arrive – plants that can compete with established vegetation for resources such as nutrients, water, or light. Like the settlers that fanned across the plains during westward expansion, pioneer riparian trees like cottonwoods and willows are particularly hardy and well suited to the unpredictable life of a changing river. Due in part to their fast growth and large size, these pioneer tree species provide important structure and function, for example, in food web dynamics, nutrient cycling, erosion control, and as a habitat for fish (as trees and branches fall into the river) and wildlife.
However, pioneer riparian trees like cottonwoods and willows that depend on flood disturbances for regeneration opportunities have been declining. These declines are a direct result of human activity. Through floodplain land use changes and water flow regulation from dams built for water supply, flood control, and power generation, the dynamic rivers of much of the Western U.S. (and worldwide) have been stabilized. This stabilization is beneficial from a human perspective, but does also affect the trees: cottonwood seedlings that would normally germinate and grow along the main channel of a river with natural flood cycles typically don’t survive past the first summer growing season anymore. Without the underlying processes that allow for new seedlings to survive and mature, someday there may not be enough young trees to replace the aging ones, and eventually we may lose them from the system altogether.