by Maria DeLaundreau
This year I am serving the Mississippi River Fund through MN GreenCorps, an AmeriCorps program coordinated by the Minnesota Pollution Control Agency. The goal of GreenCorps is to “preserve and protect Minnesota’s environment while training a new generation of environmental professionals.” GreenCorps places AmeriCorps members at host sites around the state where they will carry out their term of service.
As part of my MN GreenCorps year with the Mississippi River Fund, I am researching the best way to restore cottonwoods to the Mississippi National River and Recreation Area’s floodplain. Cottonwood trees are an important species of floodplain forests. They are extremely flood tolerant, and often grow very close to the river channel. Their location and sheer size make them easy to notice, but it’s what you can’t see that gives us a clue about the future viability of their population. I was surprised to learn that these trees, which each produce thousands of seeds each year, are not regenerating in many areas of the Mississippi River’s floodplain. Old cottonwoods tower above all other species, but in the understory there are very few cottonwood seedlings or saplings taking hold.
Standing in front of a cottonwood tree in the Mississippi River floodplain.
Young cottonwoods are having trouble growing because the flow of the Mississippi River has changed. Flood waters pick up soil upstream and deposit it downstream when the river loses enough energy that it can no longer hold the soil in suspension. Dams now control floods, and prevent spring floods from scouring the banks and picking up soil as they once did. This is good for landowners because it prevents the river from rapidly moving its channels and changing where the river sits on their land. However, this change in river flow is hard on cottonwood seedlings. Cottonwoods are great at growing on the soils that flooding leaves behind, in fact they require disturbed, open soils.
The timing of flooding has changed as well. Historically, major spring floods were in April/May. Since then the climate has changed so that now the river floods during the June/July growing season. The later flooding may mean that recently germinated cottonwood seeds have been scoured away or flooded out.
Part of why cottonwood trees are so critical to the floodplain is that they are a foundation species, meaning that it is a species that creates stable conditions for other species. In other words, the ecosystem along the river would be very different if there were no cottonwoods present. This species is one of the first to grow in recently flooded areas, which has the effect of stabilizing dynamic river channels by reducing erosion and making this habitat more suitable for other vegetation. Cottonwoods continue to facilitate the growth of other plants as they grow. As cottonwoods grow larger they shield the area around them from harsh levels of sunlight, thus enabling the growth of trees that require shadier conditions to establish. Over time, cottonwood trees turn sandbars into mature floodplain forest habitat that can support a larger variety of species.
The majestic bald eagle.
Many birds and other land animals rely on cottonwood trees and the habitat they create. For example, they are the preferred nesting trees for bald eagles because they require large branches to support their hefty nest size, and these birds require perches with a high vantage point so they can scan larger portions of the river for prey.
Cottonwood trees also improve aquatic habitat. In time, cottonwood trees will provide shelter for fish. When cottonwoods grow near a river channel, the river erodes the banks around them until the tree is left on unstable footing. When it falls into the river, the branches, which previously would have been home to terrestrial birds and insects, will shelter aquatic invertebrates and fish. The branches in the river also help to improve water clarity by acting as a filter to remove soil and dirt sediment in the water.
I’m still working with experts to determine which approaches we should investigate further. The future goal is that by experimenting with the best way to establish cottonwoods we will be able to produce a cottonwood regeneration plan and add younger cottonwoods to the riparian ecosystem on National Park Service islands along the Mississippi River. This action will allow bald eagles, other wildlife, and the river to continue benefiting from cottonwood trees while we learn more about what needs to be done for cottonwoods to regenerate naturally once again.