Trees that pollute

The unintended consequences of growing poplar plantations

Because well-fed hybrid poplars grow an astounding 5 to 8 feet a year they’re flourishing in plantations all over the temperate world. After they reach maturity at the age of 20 years the 60-foot trees are harvested and confected into furniture, plywood, guitars, matchsticks, chop sticks, paper, and jet fuel. They’re used as windbreaks and ground cover and are effective in treating contaminated soil and processing effluent from sewage treatment plants.

That’s the good news. The bad news? They pollute. That is, they release airborne chemicals that react with the gases spewed by internal combustion engines to produce ground-level ozone (0₃).

Breeding hybrid poplars is sort of like breeding horses, without the monkey business of teasing a stallion in a barn with a mare in heat, training him to mount a dummy female and ejaculating into a fake vagina. In the case of poplars you take cuttings from a male Eastern Cottonwood (Populus deltoides), for example, and a female Black Poplar (Populus nigra) and grow them next to one another in a greenhouse. After they produce catkins—those drooping clusters of tiny flowers—you use a small paintbrush to remove the yellow pollen from the cottonwood’s catkins and paint the pollen onto the poplar’s stigma, the sticky female reproductive organ at the center of each flower. Plant the resulting seeds and the result will be trees that grow fast and tall like a cottonwood without the mess of cottonwood’s white seed fluff.

On hot days hybrid poplar foliage emits chemicals called Volatile Organic Compounds (VOCs). These include isoprene, methanol, and terpenes, vapors that surround the trees and are thought to protect their cell membranes from the stress of overheating. They’re called “volatile” because they evaporate in sunlight and readily react with other compounds. When they come in contact with the nitrogen oxides in vehicle exhaust the result is ozone, which damages lung tissue. Isoprene is used to make rubber. Ethanol is wood alcohol. Terpenes are responsible for the smell and taste of many plants and are thought to give different strains of cannabis what is perceived as their unique buzz.

Poplars are not the only trees that exude VOCs. Eucalyptus, willows and conifers are especially effusive. The blue haze that gives the Great Smokey Mountains their name is created by terpene molecules from fir and spruce that attract water vapor, which reflects the color blue. Although Ronald Reagan was savaged after stating that “Trees cause more pollution than automobiles do” he was right in the sense that VOCs reacting with exhaust produce smog, a major trigger of respiratory diseases such as asthma and COPD.

Todd Rosenstiel, a researcher at Portland State University, advises the growing biofuel industry to think carefully about where to site tree plantations that emit high volumes of VOCs. Eastern Oregon, for example is a good candidate for temperate species because of the low concentration of nitrogen oxides.

If the City of Missoula had taken this advice it never would have planted 90,000 hybrid poplars on a 130-acre plantation located next to a crowded Walmart and an asphalt plant and a few hundred feet from the busiest intersection in Montana. Every day the trees drink more than a million gallons of what’s called grey water—sanitized sewage effluent produced by the treatment plant next to the plantation. The nitrogen and phosphorous in this effluent that would have been poured into the Clark Fork are processed by the poplars instead. The trees apparently like their supercharged diet. On hot days they fill the air with VOCs. Meanwhile, 40,000 vehicles spew exhaust daily into this smelly, frenetic neighborhood.

The Missoula health department measures ozone levels with an air quality monitor and posts live-time reports online. But this device is located three miles from the plantation and reports 0₃ levels only as city-wide averages. On hot days the reading might rise as high as 47 parts per billion—50 ppb is considered the threshold to unhealthy air. But the numbers drop dramatically if the wind starts blowing, suggesting that a monitor placed at the intersection would return much higher ozone readings than the one miles away.

To address the hyper-local nature of ozone and other airborne pollutants, California has deployed monitors installed in a fleet of vehicles that roam around specific neighborhoods in 64 communities, including smoggy Los Angeles and the most polluted city in America—Bakersfield. They collect block-by-block data that will be used to identify pollution hot spots.

Missoula officials once planned to harvest what are now mature and dying trees in 2026 and sell the wood for ceiling molding and painted furniture. Enthusiastic documents claimed the project would cost $1.375 million but would recoup its expenses when the lumber is sold. Whatever local market for hybrid poplar sawlogs might have existed when the plantation scheme was hatched in the early 2000s there is now no demand. Admitting its mistake, the city is considering grinding the trees into compost. A more profitable use would be to cut them down, run them through a chipper and convert this biomass into fuel for vehicles and airplanes. But the nearest biofuel plant is 500 miles away in Seattle.

Now the city is thinking of planting alfalfa after the trees are harvested. But because of poplar’s vast and complex lateral root system this will require bulldozing or poisoning the entire 130 acres. To appreciate what planners are up against in trying to plant any kind of crop, consider the Pando clone. This is a 106-acre patch in Utah’s Fish Lake National Forest dominated by a mass of quaking aspen—a close relative of hybrid poplars in the Populus genus. Due to that fact that these 47,000 trees are all connected by a single root system, Pando is regarded by botanists as one single tree. In terms of weight and landmass it is the largest tree in America and one of the largest organisms in the world.

In Latin Pando means “I spread.”