by Matt Rosenberg January 16th, 2013
Trees aren’t just a nice-to-have, their widespread removal due to pest infestations can actually be associated with higher rates of mortality, according to a new study published in the American Journal of Preventive Medicine by Geoffrey Donovan and a team of researchers. He’s an economist of forestry for the Portland-based Pacific Northwest Research Station of the U.S. Department of Agriculture. Donovan probed changes in mortality rates from cardiovascular disease and lower respiratory tract diseases such as pneumonia and bronchitis in 15 U.S. states where the spread of a beetle called the Emerald Ash Borer wiped out roughly 100 million ash trees since its appearances in Detroit in 2002. He found that controlling for a range of other factors, and compared to uninfected areas, the pest’s presence and resulting loss of tree canopy was associated with increased mortality of an additional 6.8 deaths per year per 100,000 adults (or 6,113 total) due to lower respiratory system illnesses; and another 16.7 deaths per year per 100,000 adults (or 15,080 total) connected to cardiovascular malfunction. The infected and uninfected areas were within 1,296 counties in the 15 states.
However, Donovan warned against jumping to conclusions based on the study. In an accompanying commentary. The Dean of the University of Washington’s School of Public Health Howard Frumkin also urged caution but said the study was important as part of a growing wholistic approach in the public health field, to try to quantify the benefits of nature.
Data were collected from 1990 to 2007 across 1,296 U.S. counties in the 15 states that had one or more confirmed borer infestation in 2010. Some areas within those counties had no infestation, others did, and they were compared through a detailed analysis accounting for race, ethnicity, income and education. Data analysis occurred over 2011 and 2012. The borer “kills all 22 species of North American ash and virtually all infested trees, so it is a good proxy for ash tree death,” writes Donovan. And when the borer is detected in an area, many healthy ash trees are often cut down to prevent its spread.
The mortality rate effects grew as the infestation progressed, and was more pronounced in counties with median household incomes that were above-average. That may be because there are more trees, including ash trees, in wealthier areas, Donovan says.
Donovan cautioned in a statement that association doesn’t equate with causation and he notes in the study that more research is needed to firm up plausible hypotheses that human health does benefit from abundant greenery – through lowered stress, better air quality and more physical activity.
Frumkin, a widely published public health researcher and dean of the School of Public Health at UW, in an accompanying commentary in The Journal of Preventive Medicine, stressed that although Donovan’s study is not scientifically conclusive it does underscore the continuing need to apply “full benefit accounting” to societal assets such as trees, something he says “biomedical research rarely delivers.”
Donovan has published a series of scientific articles on the possible associations between extent of urban tree cover and crime, property values and birth outcomes. His work has attracted substantial mainstream media coverage.