Within four years of first becoming infested, the ash trees died -- over 100 million since the plague began twenty years ago. In some cases, their death has an immediate impact; no more cooling shade, loss of birds and the buffering effect from a noisy street. In the long term, their disappearance means parks and neighborhoods, once tree-lined, are now bare.
Something else, less readily apparent, may have happened as well. When the U.S. Forest Service looked at mortality rates in counties affected by the emerald ash borer, they found increased mortality rates. Specifically, more people were dying of cardiovascular and lower respiratory tract illness -- the first and third most common causes of death in the U.S. As the infestation took over in each of these places, the connection to poor health strengthened.
In an analysis of 18 years of data from 1,296 counties in 15 states, researchers found that Americans living in areas infested by the emerald ash borer, a beetle that kills ash trees, suffered from an additional 15,000 deaths from cardiovascular disease and 6,000 more deaths from lower respiratory disease when compared to uninfected areas. When emerald ash borer comes into a community, city streets lined with ash trees become treeless.
The researchers analyzed demographic, human mortality, and forest health data at the county level between 1990 and 2007. The data came from counties in states with at least one confirmed case of the emerald ash borer in 2010. The findings—which hold true after accounting for the influence of demographic differences, like income, race, and education—are published in the current issue of the American Journal of Preventive Medicine.
"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 author of the sturdy. "But we saw the same pattern repeated over and over in counties with very different demographic makeups."
Although the study shows the association between loss of trees and human mortality from cardiovascular and lower respiratory disease, it did not prove a causal link. The reason for the association is yet to be determined. The presence of trees can make communities healthier. Their absence? This study goes to show we are all part of the interconnectedness of nature; when one dies, we all suffer.
The emerald ash borer was first discovered near Detroit, Michigan, in 2002. The United States has about 7.5 billion ash trees. In some forests, more than half the trees are ash. The borer attacks all 22 species of North American ash and kills virtually all of the trees it infests, unless treated.
