excerpts from Cottonwoods: Pondering Our Dominant Riparian Trees
by Jenifer Morrissey
Turkey Trot Springs, Apishapa River Watershed, Rocky Mountain Region, North America
Cottonwood has kept us warm this winter.
In summer, cottonwood provides us with shade.
Cottonwood’s gold in fall is delightful,
And spring is when cotton’s made.
I’ve been wonderstruck of late by cottonwoods. They dominate our canyon bottom. They are the forest of the valley floor. Predominantly they are along the creek, though they also mark seeps, springs, and old streambeds. The cottonwoods are important here. They provide refuge from the summer sun for all manner of birds, bugs, and beasts. They shelter the creek and reduce evaporative losses. They slow the flow of water and stabilize streambanks. They provide diversity, a break from the valley uplands and the coniferous forests above that. I appreciate the oasis they represent here and on the nearby, relatively tree-less plains. I am filled with amazement at their existence. I took them for granted once; now I marvel at their presence here.
Cottonwoods are trees that are easy to take for granted. They are large and seemingly commonplace along the water courses of our bioregion. They sprout in pastures and can form dense stands in riparian areas. Their wood is of relatively low economic value, so they are often deemed of little worth. However, unlike in the eastern part of our continent, here they are the dominant and sometimes exclusive tree of our riparian forests. Their importance increases by their singularity.
I knew to ponder cottonwoods. I’d been told years ago that we really shouldn’t take them for granted because these riparian forests aren’t regenerating in Colorado. That’s certainly a sweeping statement, but in many watersheds human-induced changes have made it difficult for new stands to be established. Perhaps it took living intimately with one, though, to make the academic observation have real meaning.
I am awed by cottonwoods because they so perfectly fit this place. They have learned over the eons to take advantage of the nuances of their locale to ensure perpetuity, to ensure survival. It seems a pretty risky venture, actually. They put out vast quantities of cotton as spring floods are receding in the hopes that a seed will find an ideal bare spot; that it will then germinate; following that it will grow roots that will pursue the water table down as it recedes with the season. The risks continue with the threat of summer drought and thunderstorm-induced floods, scouring winter ice flows, voracious beaver and hungry deer and elk, and the return of the spring flood flows. Will the young sapling withstand these threats to be part of a new stand that will reign dominant for a century? I am awed by cottonwoods.
To appreciate cottonwoods, it seems necessary to appreciate how streams and their beds change over time. One dynamic of the water cycle that is important to cottonwoods is channel narrowing. Channel narrowing typically occurs after one to several years when peak flows are lower than normal, so the stream concentrates in one part of its normal bed. Without higher flows, the stream deepens the narrower bed, and vegetation is able to grow on the adjacent bench, further stabilizing the channel at the narrower width. Cottonwood stands often establish on the benches after narrowing. The stands have irregular shapes, though they tend to run parallel to the channel.
It is common, given all the conditions necessary for a seedling’s survival, for cottonwoods to grow in groves where all the trees are the same age, where seeds all found favorable conditions together to begin their journey in life. I marvel at the intricate web of wind and water, soil and sun, heat and cool, freeze and thaw, that lead to a young cottonwood tree. After that first year, cottonwoods employ diverse strategies to counter the myriad threats to their survival. If shoots are destroyed by fire or aggressive grazing by beaver or other animals, cottonwoods can regrow from the stump (coppicing.) If drought becomes an issue, cottonwoods can sacrifice selected branches to ensure that the remaining ones can survive on the available water. It’s as if the trees realize how rare was their success at germinating, so they employ the many means available to them to make good on their start.
In ballroom dancing, it is customary for partners to hold each other in a relatively close position. This close position requires the two people to then coordinate their movements as they move to the music. The woman’s foot, for instance, must go back an instant before her partner’s foot comes forward as they travel around the dance floor. The frame that the partners create with their arms and upper bodies facilitates the necessary communication about direction of travel, speed, and approaching obstacles. What I find marvelous about ballroom dancing is that the partners aren’t moving identically; they are coordinating their movements to enable them both to flow about the dance floor in time to the music.
Perhaps it is my experience with ballroom dancing that makes me see dances between non-human partners. For instance, I can imagine streams dancing with the land, moving together slowly over time to a dance band only they can hear. The water is the dominant partner leading changes in direction and speed, and the land gets out of the way or moves into an open space so the dance can continue. The trees are a third partner of sorts, dancing in between the other two. There is nothing in my mind quite so beautiful as a well-executed waltz between two partners accustomed to dancing together. It seems to me that cottonwoods dance with the environment they call home, and I find the execution of that dance to have rare beauty, too.
Cottonwoods have learned to dance with the water and the wind and the land over millennia. In just the past two centuries, a minuscule bit of time by comparison, humans have been altering the flow of water and the lay of the land in ways detrimental to cottonwoods.
One of the ways that people have altered the flow of water is with dams and diversions. Dams are installed to regulate the flow of rivers and streams. Often their intent is to capture the peak flow of spring run-off so it can be used later in the season for irrigation or power generation. The effect of dams is to change the water cycle. From the cottonwood’s perspective, the cycle is changed in many ways that make it harder to regenerate. (Irrigation diversions may have similar adverse affects.) For instance, the reduced peak spring run-off flows mean that suitable seedbeds may not be created by the scouring and cleansing effects of flooding. Even if a seed is able to germinate, high flows later in the season for irrigation may inundate the seedbed site, drowning the germinating seed or nascent tree. Third, fall flows may be moderated so that seedlings may not get the moisture they need to survive the winter. Dams and other management activities also reduce the ability of rivers to change course. These human-induced changes to the dynamics of the water cycle impact the ability of cottonwoods to regenerate.
“One of the most extensive human-caused influences on riparian zones in the western United States has been livestock grazing.” [Note 1] Cattle naturally seek the shade of riparian areas, especially in summer, where there is also water and often abundant forage. Their presence in these areas impacts the cottonwoods that live there. If grass is scarce, seedlings and saplings are preferred forage. “Cattle preference for woody species often increases significantly in late summer and fall.” [Note 2] Seedlings can also be trampled by cattle grazing in riparian areas. While riparian areas can be a real asset for ranchers who graze cattle, the trees that help make them an asset can be put at risk by the presence of cattle that are not managed properly.
The Arapaho Indians believe that cottonwoods are fundamental to the creation of the star-studded night skies. When the night spirit needs stars, it asks the wind spirit to blow, and blow hard it does, until cottonwoods shed some of their branches. At the broken places is left a star-shaped pattern in the wood, where a new star was born into the sky. [Note 3] I have been picking up cottonwood branches and looking for star sticks ever since I heard this story. I am filled with delight when I find one.
I have benefited from dams and diversions, from cattle grazing, from human modifications to riparian areas. Drinking water, irrigation water, hamburgers, hiking trails, wood and paper products are all examples of things I’ve used whose creation and delivery had potentially detrimental effects on cottonwoods. And personally we have altered the landscape here. If we had allowed cottonwoods and their places to work their magic on us, would we have done things differently? Would we have worked so hard to change those places? Would we have needed those products of the modifications so badly? Would we have changed the tempo of the music to one the cottonwoods couldn’t dance to?
I’m saddened to think these changes we’ve made — to the landscape, the water cycle, the animal population — may threaten these trees’ chances for survival. I’m saddened to think what might happen to the oases of which these trees are a fundamental part. In our arid region, I’m saddened to think of the water we will lose out of our streams and rivers to evaporation because these trees aren’t there to provide cover. I’m saddened to think that an opportunity for delight might disappear from my life. I’m saddened to think of this place without cottonwoods.
Not all human-induced changes to the landscape have made cottonwood regeneration more difficult. Some have made it easier. For instance, people have created new cottonwood habitat by building irrigation ditches. These ditches often have high flows in the spring when penalty water is running, then regular flows all summer to nourish the seedling. In the winter there are no ice flows to threaten the young tree. And the ditches tend to capture snow blowing across the ground, so there is a source of winter moisture. Hence cottonwoods are prevalent along irrigation ditches here.
Human-induced changes along the South Platte and the Arkansas are further examples of changes to the landscape that have at least temporarily improved cottonwood survival. Dams were installed to prevent meandering. The result has been channel narrowing which has left benches suitable for cottonwood stand establishment. The challenge of these stands however is that the change was a one-time event. These forests are considered transient features of our bioregion. [Note 4]
So these giants outside my window
And the logs upon my fire
And the skeletons standing over the creek
These trees whose foliage I admire,
They have all beat the survival odds
That began in a puff of white.
They have used the wisdom of ages
To dance with water and land and light.
I am in awe of cottonwoods.
Notes:
- Successful Strategies for Grazing Cattle in Riparian Zones, Montana BLM Riparian Technical Bulletin No. 4, January 1998, p. 1. [Return]
- Montana BLM, p. 14. [Return]
- Pyle, Robert Michael. The Thunder Tree. New York: Houghton Mifflin Company, 1993, p. 187-8. [Return]
- Scott, M.L. et al. “Fluvial process and the establishment of bottomland trees,” Geomorphology 14 (1996), p. 336. [Return]
Additional Sources, with thanks to Bob Ehrhart, formerly of the Riparian and Wetland Research Program, University of Montana:
Baker, William L. “Climatic and hydrologic effects on the regeneration of Populus angustifolia James along the Animas River, Colorado,” Journal of Biogeography (1990) 17, 59-73.
Bradley, Cheryl E. and Derald G. Smith. “Plains cottonwood recruitment and survival on a prairie meandering river floodplain, Milk River, southern Alberta and northern Montana,” Canadian Journal of Botany, Vol 64, 1986, 1433-1442.
Fenner, Pattie, Ward W. Brady, and David R. Patton. “Effects of Regulated Water Flows on Regeneration of Fremont Cottonwood,” Journal of Range Management, 38(2), March 1985, 135-138.
Hansen, Paul L., Robert D. Pfister, Keith Boggs, Bradley J. Cook, John Joy, and Dan K. Hinckley. Classification and Management of Montana’s Riparian and Wetland Sites. Montana Forest and Conservation Experiment Station, School of Forestry, The University of Montana, Missoula, Montana, Miscellaneous Publication No. 54, May 1995.
Rood, Stewart B. and John M. Mahoney. “Collapse of Riparian Poplar Forests Downstream from Dams in Western Prairies: Probable Causes and Prospects for Mitigation,” Environmental Management, Vol. 14, No. 4, p. 451-464.