Natural Capital of Street Trees
April 28, 2017
In a new Perspectives paper published in Science on 28 Apr 2017, Kathy Willis and Gill Petrokfosky highlight the considerable advantages of street trees as a natural capital asset. Urban trees can take up large amounts of carbon dioxide while also providing local cooling, which is enormously valued in increasingly hot cities, and natural filters for air pollution, which benefits human health. The paper draws attention to the importance of understanding the characteristics of tree species because planting the wrong species in the wrong places can cause unintended problems. Selecting species which have characteristics that maximise benefits and minimise problems is a strategy better rooted in the scientific evidence base. Interestingly, some climbing vines can be even better than trees as natural filters, which could be an important consideration when greening city structures such as walls and roofs.
There is a growing body of evidence of the positive effects green spaces for physical and mental health and this includes some persuasive data on the health benefits of city trees. A comparison of neighborhoods with different densities of street trees in Toronto, Canada found that higher tree densities were correlated with higher perception of health and lower incidence of heart and metabolic disease: planting just 10 or more trees per city block was estimated to be the equivalent of saving more than $10,000 Canadian dollars per household in health-related costs. This is much higher than the cost of planting and maintaining the additional trees.
It is important to take account of potential problems associated with street trees, such as the release of airborne pollen that causes human allergic reactions and the emission of biogenic volatile organic compounds (BVOC) associated with ozone formation. Many popular street trees belong to plant orders that are known to produce high amounts of allergenic pollen, notably birch (Betula spp.), ash (Fraxinus spp.), mesquite (Prosopis juliflora), plane (Platanus spp.), and cypress (Cupressus spp.). Other popular street trees include black gum (Nyssa sylvatica), poplar (Populus spp.), oak (Quercus spp.), false acacia (Robinia pseudoacacia), plane (Platanus spp.), and sycamore (Acer spp.) which are all high BVOC emitters. Trees in city planting schemes are mostly chosen to reflect what is considered attractive, and although some advice is provided about issues such as poisonous fruit, there are almost no warnings about pollen allergy potential and other less visible health hazards.
Future urban planting strategies should take account of these potential problems and aim for a diversity of species to help provide some resilience against future pest and pathogen outbreaks, and also consider adding rare, threatened, and endangered species to the lists of trees that line our streets and provide valuable ecosystem services to the people who live in cities.
Reprint of the article available by kind permission of AAAS/Science
Summary available here
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