Photosynthesis is the process by which trees absorb CO2 and produce wood, leaves and oxygen. The process reverses at night. Trees release carbon dioxide when the majority of the urban population is asleep. A question has always intrigued me: Do urban trees make a measurable difference in the neighbourhood environment?
The question points in several directions at once. For example, deciduous tree canopies grow in the spring just in time to provide ‘The Luxury of Shade” in the hot summer months. Just as predictably leaves fall in autum to permit the weaker rays of the winter sun to penetrate to the ground. The tree canopy growing season delivers regionally tailored cycles of shading and passive solar heating. When street tree leaves fall to the ground and are carted away, they take with them any particulate matter that may have stuck to the leaf over the growing season.
There are also aesthetic issues to consider. The view of green trees is thought to have beneficial effects whether the vantage point is from inside the dwelling or out walking in the neighbourhood. These benefits accrue whether the tree is in a private yard, the street or a park. In addition, the presence (or absence) of large-scale tree planting can assist with way finding and enhancing the sense of place.
Finally, there is the question of whether urban trees make a measurable contribution to the air quality in the neighbourhood. A recent study reports these findings (2007 Nowak – NYC urban tree forest):
An analysis of trees in New York City reveals that
- NYC has about 5.2 million trees with canopies covering 20.9 percent of the area.
- Most common species: tree of heaven, black cherry, and sweetgum.
- The urban forest currently stores about 1.35 million tons of carbon valued at $24.9 million.
- Trees remove about 42,300 tons of carbon per year ($779,000 per year) and
- About 2,202 tons of air pollution per year ($10.6 million per year).
- The structural, or compensatory, value is estimated at $5.2 billion.
Information on the structure and functions of the urban forest can be used to improve and augment support for urban forest management programs and to integrate urban forests within plans to improve environmental quality in the New York City area.