MTPR

Cottonwoods: Born From The Floods

Jun 24, 2019

I’ve just started noticing cottonwood seeds in the air again — the bits of white fluff that bounce around on the breeze. It seems impossible that big, beautiful cottonwood trees can grow from such insubstantial beginnings.

Cottonwoods get their start in the floodplains of rivers. In wide, flat valleys, most streams and rivers meander or snake across the valley floor. At the inside bend, or point of each turn, there is usually a gravel or sand bar. These are called point bars and are part of the floodplain along with the land next to the stream channel. This surface floods on a regular basis, typically every one or two years. The floodplain soaks up and retains this water like a sponge, releasing it slowly through the summer.

This is where you will find cottonwood seedlings and saplings, the young trees. The seeds of the catkins, or cottonwood flowers, are blown by wind and land on these point bars. The timing of the seed dispersal usually coincides with the tail end of the flood peak during the early summer. As the water recedes, the point bar stays wet and the seeds germinate. This wet, bare, mineral substrate is necessary for cottonwood regeneration. If there is not a large flood the following year, the seedlings will sink roots and anchor themselves to this substrate. As young trees, they bend with the current and can survive the force of spring runoff. Although cottonwoods are related to aspen, and they do send out suckers and clone like aspen trees, this is not always successful. Future generations of cottonwoods must start from scratch on gravel stream bars.

Cottonwood "cotton" piled up near a curb.
Credit Josh Burnham

As you drive along a river, notice its pattern. Where the river is straight, for example, when it’s been channelized to accommodate the road, you probably won’t see stream bars and probably won’t see any young cottonwoods, either. They will be along stretches where the river meanders, along with willows, dogwood, and various herbaceous, or non-woody, plants. If you walk out to the river to where the huge cottonwoods are, you’ll see the mature trees are well above the stream on a terrace, or abandoned floodplain. But where did they germinate? On the dry terrace? No, they started as seedlings on the stream bars. This shows how dynamic streams are, and why it is so important to maintain the meandering stream pattern that facilitates the floodplain/stream interaction. It is through this process that cottonwood communities will perpetuate themselves over time.

Since these plant communities provide food, hiding cover, and nesting habitat to many species, including raptors, cavity nesters, and songbirds, not to mention reptiles and amphibians or amphibians, and small mammals and large mammals like deer and bear, it is important to keep these ribbons of green along our rivers in the West.

"Field Notes" is produced by the Montana Natural History Center.