Columbia University researchers discovered 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.
US rates of childhood asthma soared by 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, trees are also a source of pollen, which may potentially exacerbate asthma symptoms in vulnerable children. 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.”