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The Garden Scoop

Benefits of Urban Trees

Urban trees provide beauty and definition to our cities and most of us have a love and appreciation of these denizens of the boulevard and backyard.  In recent years, a body of research has emerged showing even greater benefits than previously understood to both the environment and our existence as humans. 

Taking a global perspective and looking toward the future, trees are going to be increasingly important as urbanization expands and human populations stake a larger claim on space.  By 2050, it is predicted that 70% of the world’s population will live in urban areas.  That is almost a 20% increase from the slightly greater than 50% of the population that currently resides in urban areas.  And the trend throughout recent history shows a dramatic surge in urban populations considering the fact that a mere 16% of the world’s population lived in cities in 1900.  This sharply increasing trend, combined with the many challenges of a larger human foot print, such as climate change, emphasizes the future importance of our urban forests at the global scale.

The canopies of urban forests provide extremely valuable shade in cities and can work to counteract the urban “heat island” effect.  Since our cities are filled with concrete, asphalt, bricks and other materials that absorb heat and slowly release it, urban areas are known to be “heat islands” that are warmer than surrounding rural areas. This effect is most evident if you observe spring flowering trees and shrubs.  The plants in town tend bloom about a week earlier than plants in the countryside. 

A recent analysis by researchers at the Morton Arboretum in Lisle, IL looked at the effect of trees on temperature around the globe and showed that trees greatly contribute to urban cooling, with to a 16⁰F reduction in temperature.  Urban trees provide heat relief from shading, but also thought evapotranspiration by actively cooling the air in cities.  Although this benefit makes our beautiful tree-lined city streets more comfortable to walk down, it also helps to lower energy costs for cooling, which further reduces our carbon footprint.

On an annual basis, it is estimated that US urban trees provide $18 million in economic benefits by reducing air pollution, energy use, and pollutant emissions while increasing carbo sequestration. A more diverse and mature urban forest has been shown to increase both environmental and economic benefits of trees.   While other infrastructure depreciates over time, trees appreciate as they mature.  Compared to planting or maintenance costs, the annual benefit from trees far outweighs their costs.

Beyond environmental or economic benefits, trees provide a great number of social and health benefits to humankind.  Residents of tree-filled communities are happier and have fewer cardiovascular health aliments. Research has also shown that having more trees and plants in our communities is linked to reduced negative thoughts, reduced symptoms of depression, better moods, and increased life satisfaction.  Overall, trees makes us happier people.

School children that simply have views of trees and greenery out their classroom windows have shown greater academic performance and a quicker recovery from stressful events.  Hospital patients with views of trees have quicker recovery times.  Several studies have found reduced blood pressure and stress when research participants had a view of plants and trees.  The simple presence of trees can improve health of patients with neurodegenerative diseases.

On the negative side, one 2013 study showed an increase in cardiovascular and respiratory deaths in communities across 15 states with a significant number of dead or ailing ash trees infested with the emerald ash borer.  Loss of ash trees over time in Cincinnati was found to be positively related to increases in crime.  So beyond the negative aesthetics, the emerald ash borer is having other, more serious effects on our communities, further emphasizing the need to replant trees. 

Earlier this week I was teaching an Urban Tree Health and Identification Workshop in the Peoria area and some of the attendees raised the question of “how many trees are too many,” with one lady noting her front yard was full but she still planned to add more.  Although I could not find an answer to “how many trees are too many,” there is substantial amount of evidence supporting the need for trees in our communities. 

As I discussed in last week’s ‘In the Garden’, fall is an excellent time to plant trees.  If trees are planted in the right location for their size and needs, I don’t see an upper limit to the number of trees we can add to our yards, parks and communities.