Beresford-Kroeger, 63, is a native of Ireland who has a bachelor’s degree in medical biochemistry and botany, and has worked as a PhD-level researcher at the University of Ottawa School of medicine, where she published several papers on the chemistry of artificial blood. She calls herself a renegade scientist, however, because she tries to bring together aboriginal healing, Western medicine and botany to advocate an unusual role for trees.

The tree is a chemical factory, she explained, and its products are part of a sophisticated survival strategy. The flowers contain trepan oils, which repel mammals that might feed on them. But the ash needs to attract pollinators, and so it has a powerful lactones fragrance that appeals to large butterflies and honeybees. The chemicals in the wafer ash, in turn, she said, provide chemical protection for the butterflies from birds, making them taste bitter.

Wafer ash, for example, could be used in organic farming, she said, planted in hedgerows to attract butterflies away from crops. Black walnut and honey locusts could be planted along roads to absorb pollutants, she said.

But some of Beresford-Kroeger’s claims for the health effects of trees reach far outside the mainstream. Although some compounds found in trees do have medicinal properties and are the subject of research and treatment, she jumps beyond the evidence to say they also affect human health in their natural forms. The black walnut, for example, contains limonene, which is found in citrus fruit and elsewhere and has been shown to have anticancer effects in some studies of laboratory animals. Beresford-Kroeger has suggested, without evidence, that limonene inhaled in aerosol form by humans will help prevent cancer.

Trees also absorb pollutants from the ground, comb particulates from the air and house beneficial insects.

Some studies support a role for trees in human health. A recent study by researchers at Columbia University found that children in neighborhoods that are tree-lined have asthma rates a quarter less than in neighborhoods without trees. The Center for Urban Forest Research estimates that each tree removes 1.5 pounds of pollutants from the air. Trees are also used to remove mercury and other pollutants from the ground, something called phytoremediation. And, of course, trees store carbon dioxide, which mitigates global warming.