Car exhaust, heavy metals in the soil, concrete, excessive wind, heat, and cold all sound like stressors for urban trees. Common sense would say that urban trees are more stressed than their rural counterparts and grow more slowly. However, recent research in New York City is questioning this widespread belief.
Cornell University researchers planted clones of the Eastern cottonwood (Populus deltoides) at four sites in the cities' boroughs and at three other upstate New York and Long Island locations. Light levels, rainfall, and insect damage were similar at each site. Rural and urban soils were interchanged to reduce growth variation due to nutrients and heavy metal content.
After three years, the city trees were twice as tall as their rural cousins. Researchers couldn't find out what was accelerating the city trees' growth, so they started looking at what was depressing the rural trees.
Surprisingly, they found the ozone levels in rural areas were almost double those in the city. Ground level ozone has been proven to depress tree growth. Even though ozone levels may spike during certain periods of the day in the city, cumulative ozone levels were higher in rural areas because of urban pollution that blows in from elsewhere. They theorize that the presence of other pollutants in cities, such as nitric oxide, may react with the ozone and actually lower the levels.
For more on this research visit Cornell News.