The Decline of Cottonwoods

courtesy of Montana Audubon

 

To continue to have healthy cottonwood communities, we need to have a 1:1 ratio of replacement trees (seedling, sapling and pole stages) to mature trees. What researchers are finding is that at best the replacement rate is 75%. At that rate, we will lose one-fourth of all cottonwood sites in Montana in the next 50 years. And that is assuming that all saplings somehow will survive to become mature trees. The reasons for the decline of cottonwood forests are many and varied.

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Dams

Dams affect riparian zones in many ways. The velocity of water decreases as streams approach reservoirs, which means that sediment precipitates and drops to the floor of the reservoir instead of flowing down river. Reservoir levels are usually lower in spring in anticipation of spring runoffs. Therefore much of the water received from spring snow melt is contained behind the dams. The reductions of water flow caused by damming also impedes the ability of the river channel to change course, thereby reducing the meandering nature of a river. And dam management activities usually prohibit natural spring floods and a gradual reduction of flows through out the summer – just the cycle that cottonwoods need.

Canyon Ferry damA study on the Missouri River determined that before the installation of dams it was a natural occurrence to have spring floods with flows in excess of 50,000 cubic feet per second (cfs) on an average of once every three years. The same study also concluded that cottonwoods on the Missouri need a runoff of at least 50,000 cfs once every nine years in order to continually reproduce successfully. In the high water spring of 1998, following one of the wettest winters on record, the highest recorded level was only 42,000 cfs.

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Development

Because of our need for and attraction to water we continue to alter our landscape in ways that threaten the existence of these important cottonwood forests. More and more subdivisions are blotting the rural areas of Montana, directly impacting riparian habitat. In addition, many landowners are faced with “ripraping” and shoring up of river banks. These actions forever change the natural processes that will occur downstream–processes necessary for the survival of cottonwoods.

We need to educate landowners about the need to protect riparian areas. Quite often people build homes on our rivers without realizing that flood plain areas are a natural part of the river’s yearly cycle and that cottonwoods are uniquely adapted to these areas–and in fact cannot survive elsewhere.

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The Life of Cottonwoods

To understand cottonwoods, we must understand the dynamics of western rivers. When streams are unfettered by dams and other unnatural obstructions, each spring begins a process of rising and receding water that is paramount to the survival of cottonwoods. The high water of spring scours the riverbanks and moves material downstream where it is deposited as a layer of fine silty soil on sand and gravel bars. As the waters begin to recede, the cottonwoods release their small seeds (there are approximately 500,000 seeds to a pound) and these seeds are dispersed widely by the water and breezes and deposited on the sand and gravel bars which, thanks to the natural process of the spring flood, are adorned with that layer of fine, silty soil that is ideal for cottonwood germination.

Cottonwood seeds require bare soil, moisture and sunlight to germinate and begin to grow. However, they are short-lived (less than two weeks) and if conditions aren’t right, a full year must pass before the cottonwoods will release the seeds under positive growing conditions.Even if the seeds do germinate, the survival rate is quite low and after eight weeks they have thinned considerably. Cottonwoods must be located low enough in the river channel to receive adequate moisture, yet high enough to be protected from scouring floods or ice floes. Their root systems must grow quite fast when they are young to keep up with the decline in water level as the stream recedes. Cottonwoods use water inefficiently, which is a disadvantage on sites prone to drought, but an advantage on infertile, continually moist sites. Cottonwoods are therefore closely adapted to the dynamics of meandering streams.