In 1980, when Ronald Reagan famously said that most air pollution was caused by plants, most people guffawed. Yet he may have been onto something. Scientists have since learned that some flora do produce volatile organic compounds (VOCs) that contribute to smog, and now a study has found that widespread land-management and forest-growing practices are making the problem worse by allowing certain VOC-emitting species to thrive.
The chief culprit is isoprene, a VOC that some young, fast-growing trees churn out like chimneys. Over the past few years, isoprene emissions in the southeastern half of the U.S. have increased, largely as a result of the natural invasion of sweet gum trees in Southern pine plantations. Other contributors are the natural oak stands that have overtaken huge swaths of abandoned farmland throughout the East.
The short-lived isoprene leaked from leaves packs a chemical wallop in the lower atmosphere. It reacts with man-made nitrous oxide (NOx), emitted mainly by cars and factories, to create ground-level ozone, otherwise known as smog [see illustration]. While there’s thus some truth to Reagan’s assertion, scientists say fossil-fuel burning humans are really to blame. “In the eastern U.S., we have enough NOx around that the VOCs end up forming ozone,” says Drew Purves, a postdoctoral fellow in the department of ecology and evolutionary biology at Princeton University and
the lead author of the study. “We control every aspect of U.S. forests.”
Tree Pollution in the Eastern U.S.
To gauge the recent increase in tree-borne VOCs east of the Mississippi, researchers used forest data from the U.S. Department of Agriculture, which surveyed 2.7 million trees on 250,000 square plots. The data, coupled with lab measurements of species-specific isoprene emissions and nitrous oxide emissions, were used to model isoprene output and smog.
The Northeast New forests are already maturing, with old-growth, low-emitting species such as sugar maple and beech overtaking the high-emitting oaks. Because the region is choked with NOx, even scarce
biogenic VOCs will generate more ozone.
The Midwest Biofuel facilities proposed in this region would raise huge plantations of willow and hybrid poplar (the top isoprene maker per acre) in areas with little existing forest, sending up plumes of VOCs that would react with NOx from Midwestern industries to make ozone.
The South Fire-suppression practices have subverted the regrowth cycle of trees and promoted VOC production across the South, where isoprene-happy oaks and sweet gum trees are settling in instead of periodically burning up. Yet with the region’s low NOx emissions, the VOCs may actually reduce ozone instead of increasing it.
Biogenic VOCs can either reduce ozone pollution or create it, depending on what else is brewing in the atmosphere. Bright, balmy conditions are best for VOC production, which trees may perform to protect against thermal stress. On a 95-degree day, a forest of 10,000 poplar trees can crank out more than three kilograms of isoprene an hour, the equivalent vapor emission of spilling three gallons of gasoline. Isoprene forms ozone when it mingles with NOx in the atmosphere. But if NOx levels are low, it destroys ozone.
These Three Species Emit the Bulk of Nature’s Volatile Organic Compounds
Sweet Gum A fast grower, this tree is spreading across pine plantations and forests in the Deep South, where the hot, humid weather exacerbates its isoprene output.
Poplar Good candidates for biofuel plantations, various species are grown rapidly in the northern U.S., where their isoprene output is tempered only by the cold.
Oak The many species of this genus grow at different rates in various climates; in Missouri’s oak-heavy Ozarks, the trees may emit up to 300 tons of isoprene a day.