The Impact of Urban Trees on Allergies and Asthma

The Changing Urban Tree Landscape

In the past, cities were lined with perfect-flowered, insect-pollinated trees like the American elm. However, today's urban forestry favors wind-pollinated species that are often unisexual—either dioecious with separate male and female plants or monoecious with both sexes on one plant. The trend towards "seedless" or "litter-free" trees has inadvertently led to an overabundance of male trees, which release large quantities of pollen, exacerbating allergies and asthma.

The Botany Behind Allergenic Trees

Dioecious trees, such as ash, willow, and poplars, are easily propagated to be male-only, while monoecious trees can also be bred to favor male flowers, resulting in all-male cultivars. These engineered trees, which are not found in nature, contribute significantly to urban pollen counts. The first mention of this selective breeding in urban forestry was noted in a USDA booklet from 1982 titled "Genetic Improvement of Urban Street Trees."

The Escalation of Allergies and Asthma

Allergy rates in the U.S. have skyrocketed from 2-5% in 1959 to 38% in 1999, with asthma now the leading chronic childhood disease. Contrary to what one might expect, allergies and asthma are more prevalent in cities than in the countryside. This is due to several factors:

1. Pavement in cities does not trap pollen effectively, allowing it to become airborne again.
2. Higher levels of air pollution in urban areas weaken immune and lung function.
3. City dwellers often experience more stress, which can trigger asthma and allergies.
4. Increased carbon dioxide levels in cities stimulate more frequent and intense pollen production.
5. The imbalance of male plants in urban areas leads to higher pollen loads.
6. The scarcity of female plants, which naturally trap pollen, leaves more allergens in the air.

Enhancing Health with Allergy-Friendly Tree Selection

Landscaping with allergy considerations in mind is gaining traction. The Ogren Plant-Allergy Scale (OPALS™) is a tool that helps arborists design allergy-free plantings by ranking plants on a 1 to 10 scale based on their allergenic potential. Female cultivars typically rank the best, while those with abundant, highly allergenic pollen rank the worst.

Criteria for Allergy Ranking of Plants

OPALS™ is based on over a hundred criteria that distinguish allergy-causing plants from those that are not. Negative criteria include tiny flowers and small pollen grains, while positive criteria include complete flowers with sticky, heavy pollen. This scale is used by landscapers, city arborists, the USDA, and the American Lung Association to make informed planting decisions.

Identifying Male and Female Trees

Determining the sex of a tree can be challenging, but many city trees are now asexually produced with predetermined sexes. The book "Allergy-Free Gardening" by Thomas Leo Ogren provides extensive research on tree sex and allergy rankings. Some cities, like Albuquerque, New Mexico, have enacted pollen control ordinances to limit the planting of male cultivars.

As awareness grows, urban landscape professionals are expected to prioritize the planting of low-allergy trees to foster healthier environments.

References:

1. Lewis, Walter H., "Airborne and Allergenic Pollen of North America," John Hopkins University Press, Baltimore, Maryland, 1976.
2. Jacobson, Arthur Lee, "North American Landscape Trees," Ten Speed Press, Berkeley, California, 1996.
3. Koch, Christopher Von, "Allergy," Die Woche, pg. 27, July 7, 2000, Hamburg, Germany.
4. Dworschak, Manfred, "Der Spiegel," Feind am Straßenrand, Pp. 174, 175, Nr. 29, 2000.
5. Ogren, Thomas Leo, "Turn Back the Pollen Clock," New Scientist, London, Pp. 46, 47, June 3rd, 2000.

Learn more about the Ogren Plant-Allergy Scale (OPALS™) Explore the American Lung Association's resources on asthma and allergies Read about urban forestry and tree selection from the USDA