How many trees should be in a sustainable city?

The theme for International Day of Forests 2018, “Forests and Sustainable Cities,” is a good opportunity to discuss cities’ forest cover goals. Tree planting campaigns implemented recently in many cities have certainly invigorated the urban forestry movement and engaged the public in pursuing sustainable development goals. But what was the goal of these campaigns? Were they to increase tree cover, or were they to counterbalance tree cover losses from 2000 to 2012?

The theme for International Day of Forests 2018, “Forests and Sustainable Cities,” is a good opportunity to discuss cities’ forest cover goals. Tree planting campaigns implemented recently in many cities have certainly invigorated the urban forestry movement and engaged the public in pursuing sustainable development goals. But what was the goal of these campaigns? Were they to increase tree cover, or were they to counterbalance tree cover losses from 2000 to 2012?

Forest cover changes during this period can be calculated for a given region using the publicly available satellite data on global tree cover from Google Earth Engine, with a spatial resolution of 30m. According to these data, the region situated between 40.6 and 40.8 degrees north and between 73.75 and 74.0 degrees west has lost tree cover over an area of 29 hectares, whereas tree cover gains have only been seen over 2 hectares.

This region covers a large part of New York City (Brooklyn, Bronx and Manhattan), where the Million Trees program was launched in 2007. The Million Trees program was responsible for the planting of 500,000 trees in New York City from 2007 to 2011. Despite this, the visible effect on tree cover was rather low: gains did not counterbalance losses.

Of course, a full picture of the scenario cannot be built from satellite images alone. More accurate results can now be obtained due to recent advances in high resolution remote sensing technology and the improvement of algorithms for individual tree detection. These algorithms make it possible to count the number of urban trees and from this derive an estimate of the number of trees that should be planted every year to replace fallen trees.

Keeping 2 million trees in a city requires 1 million trees to be planted every ten years.

This algorithm was recently applied in Berlin as part of a study by Tigges and Lakes published in Carbon Balance and Management. The algorithm in this case detected 1.4 million trees with a mean height of 15m. Annual tree mortality ranges from 1 to 3%, so approximately 14,000-45,000 of these trees will fall every year. About 50,000 trees should be planted every year in the study area to ensure that the city’s tree cover does not fall below its current level. The mortality of planted trees may actually be double this number, hence this case study concludes that keeping 2 million trees in a city requires 1 million trees to be planted every ten years.

These small calculations illustrate the cost and effort of avoiding urban deforestation and the importance of long-term support and cooperation from the general public. Engaging citizens is crucial for reducing the cost of benefits received from trees growing in the city’s parks streets and forests.

Among the many benefits of increasing urban tree cover is the removal of carbon dioxide from the atmosphere. Although it is not possible to balance CO2 emissions by net removals from urban forestry alone, the increase in urban tree cover may count towards countries’ Nationally Determined Contributions, which will go some way to achieving ‘net-zero’ emissions of greenhouse gases, as set out by Joachim Krug’s recent review, which was published in Carbon Balance and Management in January 2018 and covered on the SpringerOpen blog.

Obviously, a sustainable city should have as many trees as it can afford with the help of its citizens, and no fewer trees than in 2000. It may cost, but it’s worth it.

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