When a forest service researcher realized that the loss of 100 million trees infested by the emerald ash borer could present a unique opportunity to learn more about the value of trees, he probably didn't expect to find such dramatic effects of treeless streets.

But in the neighborhoods hit by the beetle that kills ash trees, researchers noticed a stark rise in human mortality from cardiovascular and lower respiratory disease: there were 15,000 more deaths from cardiovascular disease, or 16.7 additional deaths per year per 100,000 adults, and 6,000 more deaths from lower respiratory disease than in unaffected areas, or 6.8 additional deaths per year per 100,000 adults.

The deaths were reported in 18 years of data from 1,296 counties in 15 states, between 1990 and 2007, and are published in the American Journal of Preventive Medicine.

The researchers controlled for demographic differences, such as income, race, and education.

"There's a natural tendency to see our findings and conclude that, surely, the higher mortality rates are because of some confounding variable, like income or education, and not the loss of trees," said Geoffrey Donovan, a research forester at the Forest Service's Pacific Northwest Research Station, said in a press release. "But we saw the same pattern repeated over and over in counties with very different demographic makeups."

The results are significant enough to make researchers suspect that there could be more than an association between the increased risk of death and the lack of trees. Future research may focus on a causal relationship, and could include how trees may improve anything from air quality to human stress and physical activity.