In the Presence of Trees

tree-philly-2Spring has arrived in earnest in Philadelphia – the forsythia and azaleas have come and gone, rhododendrons are starting to bloom, and the large deciduous trees have now fully leafed out.  It’s a big difference from the stark landscape I moved into last December.  Trees mean a lot to me.  In the presence of trees, I feel happy – or it’s actually more basic than that – I feel soothed and calm.  We made trees a priority in choosing our new neighborhood when we decided that living in northwest Philadelphia, on tree-lined streets and near wooded parkland, is worth the added commute to work in Center City.  I know that trees make a difference in my personal happiness and comfort, but do they affect human health in a more tangible way?

Quantification of the relationship between trees and human health is provided in two recently published studies from US Forest Service researchers.  In a fascinating paper published January 2013 in the American Journal of Preventive Medicine, Dr. Geoffrey Donovan and colleagues linked tree loss from the emerald ash borer infestation to human deaths from cardiovascular disease and lower respiratory tract illness.  The emerald ash borer was first discovered in the US in 2002, and has since devastated the ash tree population, resulting in loss of approximately 100 million trees.  As explained by Dr. Donovan, the potential loss is still far greater, as there are upwards of 7.5 billion ash trees in the US.

For each US county, the researchers used documented infestation by the emerald ash borer (yes/no) and the number of years since first infestation as proxies for tree loss.  They then studied the correlation of county-level infestation with county-level death rates over time, from 1990 to 2007.  They also adjusted for other variables that could conceivably be related to the emerald ash borer infestation across time and space (such as income), although the researchers considered this type of confounding to be unlikely.

Emerald ash borer infestation was significantly associated with higher county-level mortality from both cardiovascular disease and lower respiratory tract illness, and these associations were stronger with greater number of years since first infestation.  To illustrate, infestation in a county was associated with an additional 16.7 deaths per year from cardiovascular disease, and the excess deaths ranged from 9.7 in counties with 1 year since infestation, to 38.6 with 6 years since infestation.  Interestingly, the relationships were strongest in higher-income counties, meaning that loss of ash trees from the borer had a greater effect on health in wealthier areas.

There are several reasons why these results provide such strong evidence for an effect of trees on health.  This is the first of studies on this topic that has taken advantage of a ‘natural experiment’ to look at how changes in green space result in changes in health, within the same geographic region.  This type of design, in which health within the same area is compared before and after an intervening event (in this case, emerald ash borer infestation) is much less prone to bias than studies that compare health in neighborhoods with green landscape to health in other, different neighborhoods with less green landscape, since neighborhoods can differ on many measured and unmeasured factors that are also related to health and may confuse the question of interest.  Another convincing aspect of their study is that the researchers also looked at death rates from accidents – a cause of death not conceivably affected by loss of ash trees – and found no effect from the ash borer.

Trees may affect health in several ways – by relieving stress and depression, reducing ambient temperatures, increasing neighborhood physical activity, and by reducing air pollution.  Trees act as pollution sinks, capturing particles on their leaves before re-releasing the particles to the atmosphere or depositing the pollutants on the ground during a precipitation event.  Broadleaf, deciduous trees (such as ash) are more effective pollution traps than most evergreen species.  In another recent study focusing on trees and human health (published April 2013 in Environmental Pollution), Dr. David Nowak from the US Forest Service estimated improvements in air quality from trees in several US cities – in terms of the amount of fine particulate matter pollution (PM2.5) sequestered, the number of prevented adverse health events, and the monetary value tied to those health events.  For example, they figured that Philadelphia tree cover, estimated as 20.8% of the city’s land area, is responsible for removal of about 12.3 tons of PM2.5 per year, translating to $9.9 million savings in health-related costs per year.  In a comparison of modeled effects for major US cities, the largest health-related bonuses from tree cover were found for the most populous and polluted areas, like New York City, in which their 20% tree cover results in about $60 million health-related cost savings per year.  Los Angeles, another highly polluted city with similar tree cover, has less reduction in PM2.5 air pollution from trees because of lower rainfall than other areas, and thus relatively lower expected health benefits.  These projections are a useful quantification of the benefits of trees, but it’s important to note that they surely represent only a small part of the expected health benefits, as PM2.5 is only one of myriad air pollutants that trees circumvent, and since there is evidence that trees may positively affect human health in ways that don’t depend on pollution control (such as by improving mood or by encouraging outdoor physical activity).

Cities are starting to recognize trees and green space as a local asset to community health.  In Philadelphia, several governmental and non-profit groups have declared tree planting as their mission, such as the Philadelphia Parks & Recreation’s TreePhilly effort, which aims to increase the city’s tree canopy from 20% to 30% by 2015 through a combination of community education, strategic tree planting, and free tree giveaways, and the Philadelphia Horticultural Society’s multistate Plant One Million campaign, which trains local citizens as community Tree Tenders.  Community engagement is particularly important in this effort because much of the land available for tree planting exists on private property – in people’s yards.  I look forward to doing my part in my own yard and perhaps by educating others, and it will exciting to see how collective efforts improve the local landscape in years to come.  Based on emerging research discussed here and elsewhere, the benefits could be far greater than anticipated.

by Anneclaire De Roos

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