Wednesday, June 16, 2010

Cottonwoods: Western tree of life

Forests of Populus trichocarpa are a magnet for many animal species

Express Staff Writer

Soaring cottonwood trees—some more than 100 feet tall—provide sanctuary for humans and critters alike in the Wood River Land Trust’s 84-acre Draper Wood River Preserve on the west side of Hailey. Throughout the year, native wildlife such as moose, deer and numerous species of songbirds use these close-to-town forests for cover and foraging. Photo by Jason Kauffman

Stepping foot into the Wood River Land Trust's heavily forested Draper Wood River Preserve in western Hailey can be a transformational experience.

The hiker's ear is immediately deluged with a cacophony of birdsong. Once one is cloistered within the preserve's 84 acres of protected forest, the hubbub of the nearby city is drowned out and replaced with the pleasant chorus of birds and subtle rustling of overhead leaves.

On an especially wet and misty day last week, a moist fragrance permeated the woods. If the day had been a hot June day instead, hikers would likely have noticed another remarkable fact of these rare forests: the substantial cooling effect that's inherent in this dense riparian greenway.

Wildlife know these facts well. More species—especially birds—use these forests for foraging and cover than any other Western habitat. But more on that later.

Often unseen by Western residents as they go about their day-to-day lives is the tree that more than any other defines the narrow ribbons of green that hug most rivers and streams draining the Rocky Mountains. The Big Wood River is no exception. The tree is Populus trichocarpa—the black cottonwood.

The water-loving tree is the West's tallest native broad-leafed tree.

"It's uniquely suited to our environment," said Scott Boettger, executive director of Hailey-based Wood River Land Trust. "It evolved in this environment to be the predominant tree in the riparian areas."

The tree's local significance is recognized in the names of the Big Wood River and Wood River Valley. The first wave of settlers who arrived here in the late 1800s found impenetrable tangles of heavy cottonwood logs and giant root wads backed up against the river's shores, bends and islands.

Though most people would look askance at the practice today, valley residents went to great lengths in the 1960s and 1970s to remove these logjams from the river. Heavy bulldozers were commonly driven into the river to root out these tangled piles of debris.

Even today, some look at these trees with a critical eye and consider them no more than a nuisance. One need look no further than the 150 or so tall cottonwoods that were clear-cut south of Bellevue on Gannett Road in March.

The trees stood along a half-mile stretch of the Wood River Valley Irrigation District 45 canal, which delivers water to about 8,000 acres of farmland. Officials with the water district cut the trees because they absorb considerable amounts of irrigation water.

The impression many have of this common tree isn't helped by the coughing and sneezing fits cottonwood seeds induce in especially asthmatic individuals. It's best not to inhale while passing through these white clouds of "cotton" that are blown about the valley every July.

Municipal foresters are also challenged by cottonwoods. Not only do the trees drop copious numbers of limbs each year, they can also be a safety hazard because of their relatively short lifespans and tendency to easily topple. In Idaho, cottonwoods generally live no more than 100 years.

Jen Smith, Ketchum parks superintendent and city arborist, recently learned of an interesting statistic from the National Arbor Day Foundation: The city of Aspen, Colo., spends about $40 on tree care per year for each citizen of the city. By comparison, Ketchum spends just $20 per person and the city of Boise only $4 per person.

Smith said one explanation for Aspen's exorbitant tree-care costs became clear to her when she drove into the Colorado town recently.

"Aspen's main street leading into town is lined with cottonwoods," she said.

That doesn't mean people shouldn't plant a cottonwood on their in-town lot, Smith said. It just requires more attention.

Despite their challenges, downed cottonwoods in the river and those standing tall along the banks are now recognized by many as necessary components of a healthy riparian ecosystem.

It's a story witnessed anywhere that moving water exists in the West. Depending on the location—from the desert rivers of southern Arizona to the rainforests of coastal Alaska—one of several varieties of cottonwood is likely present. Here in Idaho, the two primary species are the black cottonwood—whose range stretches from northern California through the northern Rockies all the way to Alaska—and the narrowleaf cottonwood, which in Idaho grows farther south along such rivers as the South Fork of the Snake.

The Southwest's native variety is the Fremont cottonwood. Along streams like the San Pedro River on the Arizona-Mexico border, bright green cottonwoods provide a surprising strip of valuable shade in an otherwise blazing desert ecosystem.

Cottonwoods play an important role in shaping the rivers from which they draw water. It's a role that often puts them at odds with human ambition.

Take the single cottonwood tree Boettger pointed out last week while strolling through the Draper Wood River Preserve. The tree's barren limbs and few leaves signal it's nearing the end of its life.

Someday soon, the tree will lose its struggle against the strong river currents that tug at its increasingly exposed roots. With a heavy crash, it will finally topple over into the water.

The benefits of this single occurrence will be many, Boettger said. Not only will it provide cover and a slower downstream eddy for fish, but it will also direct the river's flows toward the opposite bank. This will in turn lead to more scouring and other trees falling into the river.

Thousands of years of this back-and-forth toppling of massive cottonwoods have played a significant role in shaping the sinuous, S-shaped turns of rivers like the Big Wood. In a sense, cottonwoods played a big role in directing the erosion that created this valley here in the central Idaho mountains.

"If it wasn't for the cottonwood, we wouldn't have the valley we have today," Boettger said.

But for all its ecological importance, the actual extent of the cottonwood in the arid West is actually quite small. According to Vicki Saab, a research biologist at the U.S. Forest Service Rocky Mountain Research Station in Bozeman, Mont., cottonwood-dominated riparian areas make up less than 10 percent of the arid Western landscape.

However, more than half the region's bird species rely on these cottonwood stands, often called "gallery forests," during some portion of their life cycle, said Saab, who has studied how songbirds use cottonwood forests along the South Fork of the Snake River in eastern Idaho. Moose, elk, deer and other wildlife also rely heavily on these stands during significant portions of their lives.

"They're so minimal on the landscape and yet they're so important for wildlife," Saab said. "I can safely say that it's the most rich habitat in the arid West, for both aquatic and terrestrial wildlife and insects."

For all their hardiness, cottonwoods are easily impacted by human activities. Hydroelectric dams can prevent cottonwoods from reseeding themselves because of the disruption of the high spring flows that scour riverbanks and allow new seedlings to germinate. Without these flows, some cottonwood stands are nearing the end of their life and are not reproducing. Along some dam-controlled rivers, officials have started to artificially raise river levels during the spring to mimic the floods these trees require.

People like to build their homes in riverside cottonwood forests. This has the effect of requiring artificial bank stabilization or riprapping to protect their investments, which disrupts the natural flows of rivers and can lead to channelization. None of this is good for cottonwoods or the wildlife that depends on them, biologists say.

Finding a way to preserve enough open space along the Big Wood to allow the river to function as naturally as possible is one of the key aims of the Wood River Land Trust, Boettger said. Up and down the Big Wood, the organization has protected hundreds of private riparian acres. These open lands—full of healthy cottonwood stands—allow the river to continue functioning as a natural river should.

"It's got to have the ability to meander up and down the valley," Boettger said.

Jason Kauffman:

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