Attributes other than growth requirements can influence the choice of a tree. Aesthetic bonuses of trees include fall colors, flowers and fragrance, fruit that is edible or attractive to birds. Adverse features that might be considered are poisonous seeds, pollen that activates allergies, and the emission of volatile organic compounds (VOCs) that are precursors of ground-level ozone.
As they grow, trees interact with their environment in several ways. Trees can alter the microclimate in their vicinity by reducing air temperature through transpiration and the shade from their canopies, and by slowing wind speed. The cooling effect can reduce emissions of air pollutants and slow the formation of ozone. In urban areas, trees, properly placed, can reduce energy use by shading buildings in summer and blocking winds in winter. Lower energy use reduces air pollution from energy production. On the other hand, emissions from vehicles and equipment used in the maintenance of urban trees can indirectly result in a net increase in pollution due to trees.
The removal of carbon dioxide from the air through photosynthesis is generally known. Trees also remove pollutants from the air. The leaves absorb gaseous pollutants—ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide. Particulate matter may also be absorbed, but is most often deposited on the leaf surface where it may be blown off by winds, washed off by rain, or dropped to the ground when the leaves fall.
Trees emit varying amounts of the biogenic VOCs (BVOCs)—isoprene, monoterpene, and other VOCs. Whether the trees in an urban area have a low or high rate of emission of VOCs can determine whether the trees cause a net decrease or increase in air polluting emissions. High emitters include eucalyptus, liquidambar, sycamore, poplar, oak, black locust, and willow trees. Low emitters include ash, pine, maple, palo verde, elm, and fruit trees. However, data is not available for many trees.
Although the emission of VOCs by vegetation has been known for decades, the extent of its importance to urban air quality has only recently been realized. The first BVOC emissions inventory by the California Air Resources Board (ARB) was done in the South Coast air basin in the 1980's. Dominant plant species were identified, and the BVOC rates determined for about one-third of these. Leaf-mass constants were also developed so that a range of BVOC emissions could be estimated for the survey area. Since then, survey techniques have been increasingly refined, and restudies of this area have been conducted. Other surveys continue to be conducted in Southern California.
A similar BVOC emissions inventory was completed for the San Joaquin Valley Air Quality Study and, most recently, for the Central California Ozone Study.
The databases and other information developed from these surveys are used to improve the accuracy of the Urban Airshed Model used in preparing the emissions inventories that are included in plans to attain federal and state air quality standards.
The importance of accurate emissions inventories in the development of air pollution control strategies prompted the passage of SB 2174 (Polanco), which requires the ARB to approve and validate emissions inventories for stationary, mobile, areawide, and biogenic sources.
Adelia Sabiston
For more information:
http://selectree.cagr.calpoly.edu
This resource allows a search based on one or more desirable or undesirable attributes for trees.
Biogenic emissions is one attribute that can be used.
http://www.lgc.org/bookstore/energy/downloads/socal_tree_guidelines.pdf
http://www.lgc.org/bookstore/energy/downloads/sjv_treeguidelines.pdf
These free publications from the Local Government Commission were developed for communities to use in
selecting trees, especially those planted in public areas. Sidebar list is from San Joaquin Valley guide.
http://www.pcl.org/bonds/urbantrees.html
Text of two papers, from 1994 and 1999, by David J. Nowak, a leading researcher in the field of biogenic
emissions.