Source: Katherine J. Willis, Gillian Petrokofsky, “The natural capital of city trees,” Science
Surrey, UK (April 28, 2017) – The term “natural capital” refers to elements of nature that, directly or indirectly, produce value for people. Determining the location and quality of natural capital assets, and the ecosystem services that they provide for human well-being, is now underway in many countries, not just in the countryside but across cities.
One example of such natural capital is provided by city trees, which can take up substantial amounts of carbon dioxide and also cause local cooling, thereby ameliorating the urban heat island effect. City vegetation can also reduce pollution and improve human health. However, understanding the characteristics of particular species is critical, and planting the wrong species in the wrong places can cause unintended problems.
Some tree species are more pollution-resistant than others. For example, the London plane (Platanus x hispanica) has thrived alongside city streets for many years, partly because it has an inbuilt pollutant-cleansing process through its ability to shed its bark. Vegetation can also act as a natural filter, removing particulate matter (PM) from the air either through the interception of airborne particles or the uptake of gaseous air pollution via stomata on the leaf surface. The amount of PM removed, however, largely depends on the species. For example, Chen et al. recently showed that the effectiveness of different plant species in removing PM from Beijing’s air (and thus their potential for helping to reduce the severe smog that plagues the city every winter) varied by up to 14 times between species.
The main reasons for this variation were differences in leaf surface area, quantity of foliage cover, and physical shape of the species. Trees were most effective at removing PM, although tree height was important (shorter trees performed better). Good PM-removing trees included species of elm, magnolia, ash, and holly. Certain species of climbing vines performed better than some trees for PM capture, an important consideration when creating green walls and green roofing on city structures. Those not so effective at PM capture included some common street species, such as ginkgo (Ginkgo biloba) and honeysuckle (Lonicera maackii).
Deciduousness of the trees is also important, as are the size, shape, and waxiness of the leaves. Sæbø et al. have reported that among 27 species of trees and shrubs commonly planted in Norwegian and Polish cities, coniferous species—in particular the Scots pine (Pinus sylvestris)—are the most efficient at capturing PM. By contrast, broad-leaved deciduous species such as lime (Tilia cordata, the iconic “linden” tree of Berlin) were less efficient. An online tool developed to capture these data to assist with urban tree planting, i-Tree, developed by the U.S. Forest Service, is revealing some remarkable amounts of PM capture by different city trees. For example, a recent study using i-Tree estimated that the trees in public spaces in Strasbourg, France, removed 88.23 metric tons of pollutants between July 2012 and June 2013.
In addition to pollution control, there is limited, but persuasive, evidence for positive effects of city trees on physical and mental health, which complements psychological research that has substantiated the benefits of parks and green spaces as health resources for urban populations.
For example, when Kardan et al. compared neighborhoods with different densities of street trees in Toronto, Canada, with high-quality data sets on public health and demographics, they found that higher tree density (maple, locust, spruce, ash, linden, oak, cherry, and birch) was correlated with higher perception of health and lower incidence of heart and metabolic disease. The authors estimate that planting just 10 or more trees per city block is equivalent to saving more than $10,000 Canadian dollars per household in health-related costs—a figure that far exceeds the estimated cost of planting and maintaining those additional 10 trees.
Read the full study and see notes and references: “The natural capital of city trees“