The new IPCC report, the Fifth Assessment tells us once again that climate change is not a future threat but a present peril. We have added sufficient greenhouse gases to the atmosphere that we are committed to substantial warming even if we dramatically reduce our fossil fuel emissions within a few decades.
Most of us now live in cities. In 2014, 54% of us live in cities, and that will rise to 66% by 2050 (United Nations). Increasing urbanization is generally a good thing for the planet – city dwellers are more energy efficient, for example. However, cities tend to amplify the effects of global warming because of the urban heat island effect and greater air pollution than in rural areas. A healthy urban forest can mitigate the urban heat island and reduce the impacts of global warming.
You may not think of your city as an urban forest, but it probably is. Most urban areas try to plant trees along streets, in parks and many property owners plant trees. The total of all these trees is the urban forest, and it is a lot more than a collection of trees. Urban forests create many of the same environmental services as wild forests. The urban forest decreases erosion, reduces stormwater runoff, improves water quality, and improves the physical and spiritual well-being of urban dwellers.
One important function of a healthy urban forest is to reduce urban heat. Trees cool cities two ways: 1) shade reduces the heat load on urban surfaces, especially on dark pavement; and 2) the evaporation of water from leaves has a cooling effect. The cooling effect of trees extends beyond the shaded area, as this figure shows (scroll down to continue reading; click image for larger version):
If a modest number of trees can mitigate the urban heat island effect, a dense canopy cover can do even more. American Forests recommends a canopy cover target of 40% – meaning that for every acre of urban forest land, 0.4 acres are shaded by trees. While some American cities are doing even better than that – Nashville, for example, has 47% canopy cover – the urban canopy cover of most cities is declining.
In 2012, David Nowak and Eric Greenfield of the USDA Forest Service reported that urban forest canopy cover is declining across the US, with a loss rate of about 4 million trees per year (summary of the article; original article, pdf). In spite of the huge benefits of a healthy urban forest, and substantial amounts of money invested in urban forestry efforts, cities are falling behind in maintaining an adequate canopy cover.
What is replacing those trees? The short answer is impervious cover – pavement, mostly. Impervious cover has exactly the opposite effects of a tree canopy – the dark surface heats the city, and water immediately runs off into drainage basins and creeks without soaking into the ground. We’ve all had the experience of stepping out of a car into a parking lot on a hot day – the parking lot is roasting hot, and we flee for the shelter of the air-conditioned mall or office building.
To counter this problem, many cities have made commitments to increasing their urban forest canopy. New York launched an ambitious plan to plant 1 million trees to increase its canopy cover, but has fallen behind because so many of the newly-planted trees have died, and because their is no budget for maintenance of the new trees or older trees.
Louisville, Kentucky, has the dubious honor of the fastest growing urban heat problem in the nation, according to research by Brian Stone and his Urban Climate Lab (see Stone’s book, or visit the Climate Lab web site). Between 1961 and 2000, the temperature difference between urban Louisville and surrounding rural areas increased by 1.67F per decade. Louisville is nestled in a valley next to the Ohio River, and therefore suffers from the low elevation lack of wind, and the mugginess of the nearby river. Little can be done about that. Louisville also has the lowest urban tree canopy cover of any city in Stone’s study. Although the city as a whole had 30% cover, downtown was less than 10%. Louisville’s canopy cover is declining steeply. The preliminary results of a recent canopy cover study showed a 9% decline in canopy cover in one area of town in only 12 years. The combination of weather, including ice and wind storms, and the lack of any real urban forest policy caused the loss, but it is now getting much worse because of the depredations of the emerald ash borer.
Planting more trees is not necessarily the solution to making cities more resilient in the face of climate change. It is the fate of those trees that is important. There is little point in planting more urban trees if they die after a few years. The cost of replacing trees every five or ten years is far greater than the benefit of those trees. On the other hand, the benefits of a tree that lives 100 years or more are many times greater than the cost of their planting and maintenance.
There is a huge difference between an urban forestry program and planting a bunch of trees. Urban forestry is a management approach to maintaining a healthy, diverse forest canopy in our cities. Tree planting programs tend to be short-sighted and lack a comprehensive plan.
The next articles in this series will examine best practices in urban forestry, worst practices in urban forestry, and the challenge of tropical urban forestry.