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Urban Fogged -- Countryside Dogged

Global urbanization is accelerating and making large impacts on the vegetation in cities, but what about the countryside? Extensive monitoring, warning, and effects research has focused on city centres with the pervasive perception that rural environments are safe havens from urban pollution. However, this premise may be a myth. According to a study published in the scientific journal Nature, cottonwood tree saplings grew twice as large amidst the pollutants of New York City as in the surrounding countryside. Let us take this space to summarize these eye-opening findings (Gregg et al. 2003).

Urban air contains high concentrations of many pollutants including numerous gases, particulates, and photochemicals. Urban soils are loaded in heavy metals and are usually more hydrophobic (that is, they repel water molecules) and acidic than rural and forested areas. Taken separately or together these factors are often detrimental to plant growth. On the other hand, urban environments also have higher rates of nutrient and base-cation deposition (Ca²⁺ and Mg²⁺ - important for soil fertility), warmer temperatures (due to the black-top effect), and increased CO₂ concentrations-factors that often enhance plant growth. The net effect of these factors on plant growth has largely remained unknown, especially comparing urban and rural ecosystems.

In a unique study, a team of scientists used a clone of Eastern cottonwood (Populus deltoids) as a living "phytometer" to analyze how multiple anthropogenic environmental changes affect plant growth in New York City compared to surrounding rural environments. Cottonwoods were chosen because of their rapid growth rates, continuous growth throughout the season, and responsiveness to the above environmental factors.

Contrary to what any nature-loving, clean-air-breathing hominid might expect, the cottonwood saplings grew twice as large amid the pollutants of New York City as they did in the rural countryside. Greater urban biomass was found in all urban-rural site comparisons and determined to be independent of soil type. Thus the greater urban plant biomass had to be due to atmospheric conditions rather than soil alterations.

The beneficial effects of increased nutrient and base-cation deposition in urban areas was ruled out when urban atmospheric deposition was neutralized, yet the increased urban growth trend persisted despite the equal nutrient playing fields (literally of course). Higher temperatures and CO₂ concentrations were ruled out after chamber experiments simulating urban and rural thermal and CO₂ environments failed to reveal individual or combined effects on biomass.

Environmental comparisons provided no scientific evidence that greater urban cottonwood biomass was due to enhanced growth in the urban atmosphere. Therefore, the pattern must have arisen from detrimental effects reducing growth in the country. If soil, nutrient budgets, temperatures, and CO₂ concentrations could not account for increased growth in the city, then these factors could not account for reduced growth in the country. In addition, most atmospheric gases, suspended particulates, and wet deposition components that could reduce plant growth were either higher in New York City or did not differ between urban and rural sites.

However, there was one exception: ozone (O₃). Ambient ozone concentrations were significantly higher at rural sites and with little doubt this was the smoking gun that reduced rural growth. To understand this one needs to know that primary O₃ precursors are emitted in cities (the NOx gases we hear so much about in the news), but react in sunlight to form O₃ as air masses move to rural environments. Ozone exposures are therefore consistently higher in rural sites. A chamber experiment showed a 50 percent reduction in cottonwood biomass at the greater O₃ exposures of rural vs. urban sites-a comparable effect to that found in the field trials.

Analysis of the EPA's Alliance of Information and Referral Systems (AIRS) database by the scientists showed that the O₃ exposures of the rural study sites were representative of the mean nonurban agricultural and forested exposures throughout the northeastern U.S. Meaning, the detrimental effect of higher rural to urban O₃ was not due to higher exposures downwind of huge urban centres like New York City, but instead, was consistent with a pattern of lower urban exposures to nonurban sites throughout the entire northeast.

In this particular study the greatest effect of the multiple anthropogenic environmental changes was the secondary reactions that produced the higher rural O₃ exposures thus reducing plant growth in the country. It always seems that the greatest negative effects come from the secondary conditions caused by the primary factors. Extensive global change research has studied temperature change, nitrogen loading, increased CO² concentrations, but this is the first time O₃ has been proven to be an overriding impact amid these more studied and publicized factors.

Cottonwood is in the middle range of O₃ sensitivity. Many species show greater responses to ambient O₃ exposures, so this effect is not likely to be limited to cottonwood clones. Remember, the cottonwoods did not grow better in New York City, but instead grew significantly poorer in the countryside. As a global society we need to reconsider pollution impacts on both rural and urban environments as all air sheds merge throughout the world. There are no safe havens. This is one more reason, among thousands, to use renewable energy sources.

Reference: Gregg, Jillian W. et al. Urbanization effects on tree growth in the vicinity of New York City. Nature. 424, 183-187 (2003).