THAT TIME FORGOT
There are still plenty of them, but the great stands of cottonwood trees that form the backbone of Montana's rich habitat along streams and rivers are getting older. Landowners, recreationists and researchers all would like to know why new generations of cottonwoods seem to be absent in many places.
Some of the blame can be attributed to dams or diversions, which change the natural flow of rivers, and to bank stabilization to protect roads, bridges and homes, all of which interfere with the conditions cottonwoods need to get started. But research from UM's College of Forestry and Conservation, among others, shows that is not the complete story.
In the spring, fluffy cottonwood seeds drift in the breeze and pile up against rocks and grass along stream banks. It seems miraculous that such big, beautiful trees — often the only trees to be found out on the hot, dry prairie — can grow from these insubstantial beginnings. Cottonwoods require bare, moist mineral soil to grow, primarily gravel bars that form on the inside of river bends. Controlling a river's natural flow or movement reduces the gravel bars deposited by the stream. Buffeted by high water and chewed on by beavers and other animals, a cottonwood's early years are full of struggle. Once established, however, they anchor the stream edge, providing food and shelter for dozens of bird and mammal species.
Cottonwoods also have a well-understood relationship to stream dynamics, says Mike Merigliano, a UM research associate who studies river systems in Montana and surrounding states. He explains that cottonwoods are a natural clock for creating a timeline of river changes.
they establish and grow on new but stable gravel bars, you can age a
tree [determine how old it is] to find out when the bar was deposited," Merigliano
says. "Then if you map the aged-tree patches, some of which are
quite far from the present river course, you can see how the river has
changed over time."
Merigliano's work focuses on how plants, especially cottonwoods, relate to their environment and what will happen to them if the environment changes or doesn't change. He has stacks of data waiting to be published but has difficulty finding time to write, since he is so often called by citizen groups or government agencies to do field studies.
Last fall, for instance, McCone County ranchers and federal National Resource Conservation Service agents were concerned about the lack of young cottonwoods along the Redwater River, which flows to the Missouri from near Circle.
Not only do cottonwood trees provide sustenance for wild animals, they are the principal source of shade for livestock in that harsh environment.
An initial survey found many stands of old trees, but very few young ones. A graduate student will return to do a more detailed assessment, but Merigliano's guess is that a combination of ice jams, which scour the river bed and nearby vegetation, plus a general change in climate that is leading to a less active stream, are making the difference. New tree generations also may be impacted by naturally occurring salt concentrations — particularly along the upper portion of the stream — and the many small water developments that together may be reducing the incidence of big floods required to deposit new gravel bars.
Merigliano, UM forestry Assistant Professor Scott Woods and graduate student Millie Bowman are working on another project, one near Ovando on Kleinschmidt Flat — a glacial outwash plain extending from the Scapegoat Wilderness. The North Fork of the Blackfoot River runs along the edge of the flat, and a number of irrigation ditches were put in decades ago to divert water to farmers' fields. Now the U.S. Fish and Wildlife Service plans to retire one of the ditches to increase the river's summer flow helping bull trout migrate to their spawning grounds in wilderness-area creeks. Local landowners are supportive of the plan, but some are concerned about the impact on the large cottonwoods that have become established along the ditches.
To find out what the effects of draining the ditches will be, the researchers are studying the trees in the area to find out where they get their water and how much they need to survive and be healthy.
"How much drought stress can the plants take, and can they adapt to a system with less water?" Merigliano asks. "As the flow in the ditch changes, will that change the water tension in these trees?"
Water tension is a measure of how hard a tree has to work to pull water out of the ground. The higher the tension, the higher a tree's stress. At some point, the stress becomes too high and the plant will wilt or die.
their initial measurements, the ditch clogged up for a couple
of days and the researchers were able to observe an increase
in water tension in some of the trees. This indicates that
removing water from these ditches may have an impact on the
cottonwoods, Merigliano says. If so, the trees may have time
to adjust to the new conditions if the flow is reduced incrementally
rather than shut off all at once. Knowing where these trees
get their water at different times of
body of water has its own isotope characteristics, or the
ratio of "heavy" oxygen
and hydrogen atoms — those containing extra neutrons — to
their normal form. Different isotopic ratios are a function of
the conditions under which the water formed. Water precipitated
as a heavy rainfall has a different signature from water that
seeped into the ground from melting snow, which is different
from water that has been percolating through the ground for a
can look for a match between likely water sources and water
within the trees to find out where they are
This information helps landowners predict what will happen to their cottonwoods when the diversion ditch is retired. It also helps Merigliano piece together the larger cottonwood story.
"Considering studies of large cottonwood systems along several rivers, including the free-flowing Yellowstone River, we see that they are skewed toward oldstands," Merigliano says.
It also appears that up to about 150 years ago, coinciding with the end of the Little Ice Age, floodplains were larger and more active than today, he says.
To test his idea, Merigliano intends to travel north to study rivers and streams in the colder, wetter climates of Canada and Alaska to see if streams are more active there than here, and to see if that's where cottonwood youth can be found.
"I'm always looking for links between climate change, geomorphology and vegetation," he says. "Even if I'm just out having fun, I'm always looking at how things connect. But now I'd like to quantify my observations."