What is the value of a tree’s shadow?

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Many cities around the world have pledged to increase urban tree cover because trees deliver diverse benefits for the climate, the environment and society. Urban trees purify the air, provide refuge for urban biodiversity and protect the passers-by from the sun, for example, which positively impacts urban public spaces. Tree care influences tree health and vitality, which, in turn, determine the level of the benefits provided. However, tree planting and care costs are significant and increasing due to climate change and opportunity costs associated with alternative use of public space such as commerce or parking.

Does it pay to plant trees in the cities at all? In order to answer this question, we need to know both the costs and benefits associated with urban tree cover. While estimating the costs associated with tree planting in cities is relatively easy, the calculation of trees´ benefits is not so straightforward. This is because many of these benefits generate public value rather than private benefits and are difficult to enumerate. The value of a tree´s benefits is a sum of values of all its benefits, e.g. value of rainwater retention, carbon stored or noise reduction. Microclimate regulation is among the most important benefits that can help cities adapt to climate change, particularly in light of the increasing frequency, duration and severity of heatwaves. Yet, valuation methods for the different aspects of tree microclimate regulation function are lacking. Microclimate regulation has mainly been assessed by the changes in energy consumption of buildings resulting from tree shade and climate effects, and by productivity loss at different outdoor temperatures. However, this leaves out the many other aspects of climate regulation – particularly those for the people actually walking in the streets.

In a study recently published in the Urban Forestry & Urban Greening journal, we proposed a new approach for estimating the value of the trees’ ability to cool the outdoor environment. The replacement cost method was applied to estimate the value of the shade benefit of urban trees. The replacement cost method estimates the economic value of an ecosystem service by the cost of replacing the service with a human-made substitute. In this case, we assume that parasols, similarly to trees, provide shade, which cools the local temperature by reflecting solar radiation. Hence, the calculation of the shading benefit of a tree was approximated by the cost of providing the same benefit through a parasol (that is by its price). We developed the method for a generic urban tree in Prague, Czech Republic, and then applied the method to calculate the value of shade from trees growing on embankments in the city centre.

The results showed that the costs of tree planting and maintenance were higher than the estimated shading benefits in the short term (20–30 years), but the situation reversed when the tree life expectancy increased (> 40 years). Street trees are hence a long-term investment in terms of microclimate regulation. Nevertheless, other benefits trees provide could make up for the higher costs in the short term. These might be potentially quantified using other existing methodologies and added up to the values calculated by the proposed method.

Since the assessment of all relevant costs and benefits is essential for efficient decision making, the method can assist city planners in estimating the value of urban tree benefits. The proposed method represents a quick, easily applicable and easily understandable approach for the assessment of tree microclimate benefits. Enumerating these benefits can help to make a stronger case for investments in urban tree cover against other alternative uses of public space.

However, care needs to be taken in the application of our approach to ensure that the method satisfies the validity conditions, i.e. the lowest costs of parasols, in areas that are frequented in summer and need shade from the sun. Our future work will aim to more clearly specify these conditions and improve the method itself.

Reference of the cited article:

Horváthová, Eva, Tomas Badura, and Helena Duchková. 2021. “The Value of the Shading Function of Urban Trees: A Replacement Cost Approach.” Urban Forestry and Urban Greening 62. https://doi.org/10.1016/j.ufug.2021.127166.

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