BISMARCK — This year’s historic drought could signal hard times ahead for one of North Dakota’s oldest and most recognizable residents: the cottonwood tree.
Predominant across the American West, cottonwoods have become an unofficial symbol of the Dakotas, where they cover the banks of the Missouri River and its many tributaries.
But with the construction of large-scale dams decades ago, humans have already crippled the future of cottonwoods along the country’s longest river. Now, scientists believe increasing weather volatility under the planet’s warming climate will exacerbate the hard times ahead for these iconic trees, especially on the Missouri.
“I’d say there’s about another 50 years and you will have reached the lifespan of 90% of the trees,” said Carter Johnson, an emeritus professor at South Dakota State University and a leading expert in the ecology of the Dakotas.
This projected near-eradication of cottonwoods in the Missouri River Valley is not new. Johnson and colleagues first studied the potentially grave consequences of the federal damming effort more than 50 years ago, predicting at the time that the tight management of the formerly natural river system would deplete its cottonwood population.
While the longtime Dakotas ecologist said he has not yet logged the effects of climate change on these trees, he said an increasing prevalence of warming-induced droughts would spell an even grimmer future for cottonwoods.
Stewart Rood, a professor of environmental science at the University of Lethbridge in Canada and a specialist in cottonwoods, similarly predicted that climate change will pose a “double whammy” to the floodplain zones these trees occupy. Higher temperatures and less snowpack feeding the Missouri River at its origins in Montana, Rood said, will lead to both lower flows and heightened water demand from irrigating farmers.
And though Rood said we may not observe the consequences of these water reductions immediately, he urged action today to head off the not-so-distant ramifications of warming for cottonwoods and the surrounding life that depends on them.
“The good news is, we’re still gonna have cottonwood forests, we’re still gonna have birds and fish and wildlife by 2050,” Rood said. “But by 2100, things might, in fact, be more troubling.”
Before the massive damming projects of the 1950s, the Missouri River was dynamic and volatile, prone to floods that could dramatically shift its pathways and bank lines. That kind of movement was ideal for cottonwoods, which depend on the river as a consistent source of water, as it allowed for new trees to take root outside the shadows of their established parents.
Now, the Missouri River’s cottonwoods “are just aging in place,” said Jonathan Friedman, a researcher of rivers and riparian vegetation with the U.S. Geological Survey. Friedman said there are hardly any young cottonwoods on the Missouri anymore, and reduced flows in the river would mean fewer and fewer new trees to fill in when the old ones die off.
Much of Friedman’s research has focused on the Little Missouri River of western North Dakota, a tributary that feeds into the larger river. While the big Missouri was fundamentally changed by dams — which Johnson noted injected so much noise into the system that it can be hard to discern the kinds of incremental differences spurred by climate change there — the Little Missouri offers a clearer picture of the effects of climate change.
Friedman said an overall warming trend in western North Dakota since the 1970s has resulted in lower water and less migration of the Little Missouri. As a result, the area of new cottonwood forests produced each year has declined.
The ramifications of warming aren't limited to cottonwoods, but ecologists noted they may be some of the first species to respond to these changing conditions. Rood noted that cottonwood trees, even more so than their neighbors, are specialists: They reproduce best in the narrow period after the recession of a flood, making them especially vulnerable to the fluctuations in river flows brought on by climate change.
Their long lifespans and keystone position in floodplain ecosystems also make cottonwoods a "sentinel species," Rood explained, providing scientists with historical markers of the dry periods, the flood years and the gradual evolution of the weather.
“In terms of climate changes, they’re excellent records,” Rood said.
Because of the importance of these trees to their ecosystems — offering cooler temperatures, stabilizing riverbanks and anchoring the region's most vibrant habitats — a dramatic decrease in their number would have costly ripple effects. Rood said that when cottonwoods drop off, as has already happened along some North American rivers like the St. Mary of Alberta, Canada, “the whole floodplain unravels.”
“It is sort of catastrophic, and I don’t think we’re being hyperbolic about it,” Johnson said, noting the parallel declines of several other species in the Missouri River floodplain. Even without climate change, the South Dakotan said, there would be few remaining old cottonwoods to speak of in the Missouri River floodplain in half a century. “With climate change, they’ll even be fewer.”
When Johnson and his colleagues first began studying the floodplains of the Missouri River, most of the cottonwoods were young, having taken root in the wake of the 1952 flood and just before the dams went in. Today, those trees are in their 60s, over half the lifespan that Johnson would expect under the conditions provided by the dammed Missouri River.
Johnson said cottonwoods will always line the Missouri's edges, but once the old trees of the valley die, there will be few new ones to replace them.
“Someday, 100 years from now, they’ll blow the dams out, and we’ll go back to something,” he said. “But it won’t be what we had before.”
Readers can reach reporter Adam Willis, a Report for America corps member, at firstname.lastname@example.org.