When planting a large number of trees -- for example, in a park, green belt, or along roadways -- the species landscape planners choose could have a significant effect on future air quality, according to University of California Cooperative Extension environmental horticulture farm advisor John Karlik.

Some trees and shrubs emit high rates of certain volatile organic compounds (VOC), which react with nitrogen oxides (NOx) and sunlight to form ozone, a ground-level air pollutant. Other plants emit very little VOC.

The Kern County farm advisor studied VOC emissions from trees and shrubs with researchers at UCLA and UC Riverside as part of a project funded by the California Air Resources Board. Karlik's team measured isoprene emission rates for more than 60 California trees and shrubs by enclosing branches with Teflon sheeting supported by a frame. Clean air was blown through the enclosure. Samples of the exiting air stream were taken and analyzed for isoprene. Although isoprene is not the only compound released by trees, it is often the dominant compound and was therefore selected to be measured for this study.

Of the species studied, Karlik found that the highest isoprene emitters were certain oak, poplar and eucalyptus species. Plants that emitted very low levels of isoprene included Modesto ash, Chinese pistache, blackwood acacia and euryops daisy.

"For effective air quality attainment strategies, it's important to understand nature's contributions," Karlik said. "In California, we've made a great effort to capture VOCs from sources such as gasoline nozzles to prevent vapors from escaping into the atmosphere. We should also consider volatile emissions from tree species when large-scale planting programs are contemplated. One or two trees in a backyard landscape don't have a significant impact on emission of VOCs. But 10,000 trees could."

Emissions from a large-scale tree planting might be compared to vapors from gasoline spilled at the pump. For example, a medium-sized liquidambar tree on a warm summer day emits about one gram of isoprene per hour. For 10,000 trees, the isoprene emission would be about 10 kilograms per hour, an amount comparable to the evaporative emissions caused by spilling about 12 gallons of gasoline per hour. Since many tree species emit more isoprene than liquidambar and grow substantially larger, their effect on air quality would be still greater.

Karlik notes that all trees should be credited with cleaning the air. They absorb carbon dioxide, release oxygen and provide surfaces for deposition of airborne particles and unhealthful gases such as ozone. Water evaporating from tree leaves, which cools air, and tree shade can cut energy consumption, reducing the need for air-polluting energy generation.

"Certainly there are many benefits to having trees in our environment," Karlik said, "but we must also recognize the VOC emissions contribution."

Landscape planners consider a wide variety of characteristics in selecting trees for projects, such as how well the plant is adapted to the climate, its size, shape, color, flowers and fruit, growth rate, maintenance requirements, availability, disease and insect resistance.

"I'm suggesting that the VOC emission rates of trees should also be considered," Karlik said. "It may not be the most important characteristic, but in a huge tree planting it becomes increasingly significant."

Jeannette Warnert
Public Information Representative
Ag and Natural Resources
UC Center
550 E. Shaw Avenue
Fresno, CA 93710