Trees are well-known shade providers and pollution fighters, but cities could see big returns on their investments in planting more trees, especially as they look for ways to adapt to climate change.
That's the conclusion of a new analysis by the nonprofit Nature Conservancy, which analyzed the tree canopies of 245 cities around the world and tried to measure the benefits of planting trees as a return on investment.
Dhaka, Bangladesh, and Karachi, Pakistan, were among the global cities that showed the greatest benefit. Minneapolis was among 16 North American cities where the study found a return on investment for trees providing both a cooling effect and reducing air pollution.
The report noted that adding trees would have the most impact on denser, dirtier cities. It did not quantify other potential benefits from trees, such as reducing storm water runoff and providing aesthetic or even psychological benefits.
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Past studies have provided plenty of evidence showing that trees benefit the urban environment, but the Nature Conservancy wanted to address the question it often gets, "How much of the overall problem in my city can trees solve?" said Rob MacDonald, an urban ecologist with the group.
Despite their importance and the payoffs seen in many cities, the report acknowledged most of the urban areas studied by the Nature Conservancy were losing trees, not gaining, MacDonald added. "The trend globally is a slow decline."
The trend between 2000 and 2010 shows Minneapolis and many other North American cities have lost forest cover.
It's not hard to find evidence of that in Minneapolis. Fall is prime time for city crews to remove diseased and storm damaged trees. The Minneapolis Park Board removes between 3,000 and 4,000 trees each year from public parks and boulevards. The advance of the emerald ash borer has added 5,000 trees to that total and is the biggest threat right now to the city's canopy.
The Minneapolis Park Board is two years into an eight-year plan to remove all of the 40,000 public ash trees. Many are being removed before they are infested. Crews plant about 10,000 trees each spring to maintain the city's tree canopy, and they're being a lot more strategic about it, said Justin Long, assistant superintendent for environmental stewardship at the park board.
"What we're seeing is we're becoming more susceptible to outside influences," he added. "It is better to actually have up to nine different species on a street so that should something else come, we're able to not be impacted as greatly."
The Nature Conservancy's work in Minnesota has focused more on northeastern Minnesota's vast forests rather than the urban landscape. The group has planted about 2 million trees in the last 10 years or so, said Jim Manolis, forest conservation program director for the Nature Conservancy in Minnesota and the Dakotas.
The plantings are aimed at diversifying the forests to make them more resilient to climate change. But Manolis said the organization is evaluating how it spends its resources and expects "urban tree planting is an area we'll be looking at more."