Trees and people suffer together

Tree dying leads to health problems in the population

22/01/2013

Tree dying has a massive impact on human health. On behalf of the US Department of Forestry, scientists have investigated possible relationships between the deaths of millions of ash trees and deaths in nearly 1,300 counties in 15 US states. The study shows a significant correlation between the condition of the trees and the deaths due to respiratory and cardiovascular disease, said lead author Geoffrey Donovan from Forest Service, Pacific Northwest Research Station. The study was published.

From previous studies it was known that "exposure to the natural environment can improve human health", but previous findings were always controversial due to structural weaknesses in the study, argue the US scientists their research approach. Considering the biological invasion by the Asian ash beetle (Agrilus planipennis) and the related tree extinction have provided a unique opportunity to assess impacts on human health, Donovan and colleagues continue.

100 million trees died due to introduced beetles
The introduced European ash beetle "was first discovered near Detroit, Michigan, in 2002," the researchers report. The beetle attacks all 22 species of North American ash and kills almost all the trees it infests, write Donovan and colleagues. In other adjacent US states and in the southeast of Canada, the tree pests are now widespread. Overall, the invasion of the beetles has so far led to the death of around 100 million trees, according to experts in the affected North American regions. In their current study, the scientists have now compared this tree dying with the medical population data of the last 18 years from 1,296 counties in 15 states. In order to avoid distortions due to other factors as far as possible, the researchers also included the demographic data from the years 1990 to 2007 as part of their study. The influence of variables such as income, race or educational status on health could thus be largely excluded.

21,000 additional deaths from tree deaths
In both regression models used for the calculation, according to the US researchers, there was a clear statistical correlation between the death of trees by the European ash beetles and the deaths due to cardiovascular and respiratory diseases. Between 2002 and 2007, there were "15,000 additional deaths from cardiovascular disease and 6,000 additional deaths from suffered respiratory illnesses" in affected regions. The presence of Asian ashbird beetle in the states is associated with an increase of close to seven deaths per 100,000 population a year in cardiovascular disease and an increase in annual respiratory deaths by about 17 per 100,000, Donovan and colleagues write.

Causal relationship between nature and human health
"Of course, there is a tendency to see, when looking at the results, that the higher mortality is due to some confounding factors, such as income or education, variable rather than the loss of trees," explained Donovan. However, the pattern had to be proven "in districts with very different demographic conditions". The statistical correlation between the dying of the tree and the increased number of deaths was confirmed to be significant even considering the demographic factors. The current investigation provides clear evidence of a causal relationship, but further studies are needed to prove it clearly. The "results give no direct insight into how trees reduce mortality related to cardiovascular and respiratory disease," but researchers argue that there are several plausible explanations. For example, improving air quality, reducing stress levels, increasing physical activity, or even the impact of trees on the microclimate or temperature could play a role, Donovan and colleagues report. It is clear that the presence of trees has a positive effect on the health of the population, and that the dying of trees also has a lasting negative impact on human health. (Fp)

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Picture: Luise