Trees 'prefer cities to the countryside'

Trees now fare much better in the city than in the countryside, according to a study that reveals how the rural environment is not so safe for plants.

Pollutants tend to negate each other's effects in urban environments. But over time they increase ozone levels affecting nearby countryside, making rural life tougher than in the city.

This effect outweighs the impact on plants of polluted soils, rising temperatures, carbon dioxide and nitrogen levels linked with climate change.

Compared with Brooklyn, identical cloned cottonwood trees planted downwind of New York City were found to grow only half as well - a surprising finding that the ecologists from Cornell University, Ithaca, and the Institute of Ecosystem Studies, Millbrook, report today in the journal Nature.

Differences in soil composition, temperature, carbon dioxide concentrations, nutrient deposition, urban air pollutants and microclimatic changes could not account for the greater "biomass" of urban plants.

The culprit is ozone pollution. While individual one-hour peak ozone concentrations are often higher in urban environments, in rural environments ozone remains in the atmosphere for a longer period. Hence the cumulative ozone exposures are much greater.

10 July 2003