New York is currently planting more trees
Children who live in tree-lined streets have lower rates of asthma, a New York-based study suggests.
Columbia University researchers found that asthma rates among children aged four and five fell by 25% for every extra 343 trees per square kilometre.
They believe more trees may aid air quality or simply encourage children to play outside, although they say the true reason for the finding is unclear.
The study appears in the Journal of Epidemiology and Community Health.
US rates of childhood asthma soared 50% between 1980 and 2000, with particularly high rates in poor, urban communities.
In New York City, asthma is the leading cause of admission to hospital among children under 15.
The researchers found the city had an average of 613 street trees per square kilometre, and 9% of young children had asthma.
The link between numbers of trees and asthma cases held true even after taking into account sources of pollution, levels of affluence and population density, the researchers said.
However, once these factors were taken into account, the number of trees in a street did not appear to have any impact on the number of children whose asthma was so severe that they required hospital treatment.
Some experts believe that children who are exposed to few microbes in early life are at an increased risk of asthma because their immune systems do not get the practice they need at fighting infection.
Therefore, if a tree-lined street encourages outside play, it might help reduce the risk of asthma by maximising the odds that children will be exposed to microbes.
However, trees are also a source of pollen, which may potentially exacerbate asthma symptoms in vulnerable children.
Lead researcher Dr Gina Lovasi admitted the effect, if any, of trees was far from clear.
She said: "There may be something else healthful about the areas that had more trees.
"For example, trees could be more abundant in areas that are well maintained in other ways."
Leanne Male, assistant director of research at the charity Asthma UK, said: "Previous research looking at the influence of the environment on levels of asthma has focused on negative aspects, such as pollution and chemical exposure.
"This innovative report is the first to look specifically at the potentially beneficial effects of trees in urban areas and raises some interesting issues.
"However, there are a number of other factors that have not been considered, for example whether the families involved have pets.
"Despite the need for further work, this is a positive first step into a new area of research linking the environment and asthma."
New York City is planning to plant 1 million extra trees by 2017.