The plan launched to great fanfare in 2015. Copenhagen would plant 100,000 new trees in the Danish capital by 2025. Like many similar tree-planting pledges, the commitment was pitched as a key prong of its plan to reduce carbon emissions.

But six years later, many of the saplings have already withered and died. 

“People bash into them with bikes and cars,” says Sandra Hoj, an urban tree campaigner in the city, “and they often don’t have proper protection.” The bark gets slashed, or they get poisoned in the winter because the municipality has put salt on the roads instead of some alternative that’s safe for the trees. 

“Meanwhile, the city is still replacing some beautiful mature trees with tiny little twigs, which die, are replanted, and then die again,” says Hoj. 

Copenhagen’s record doesn’t necessarily stand out as especially bad: The city points out that its tree-planting schemes have increased Copenhagen’s canopy cover by 1.5% since 2015. 

The problem is, too often, tree-planting is perceived as a feel-good cure-all for global warming. In the U.S., conservative leaders like former President Donald Trump have touted tree-planting while working to eliminate emissions regulations.

And such large-scale initiatives are anything but simple. The missteps that Copenhagen encountered are common, revealing the little-discussed dark side of these tree-planting sprees: Popular campaigns to plant 1 million trees are announced to much fanfare in cities from London to Los Angeles to Phoenix, Arizona, but often fall short of their goals. 

Many trees don’t survive, or thrive, or deliver their promised benefits.

Some never make it into the ground. In both L.A. and Denver, million-tree campaigns started under one mayor failed to reach their target before their successors changed strategies. L.A. had planted 407,000, and Denver, between 250,000 and 500,000.

“I think that urban tree planting initiatives can do a huge amount of good, but I think a lot of them are done a bit haphazardly,” says Lucy Hutyra, an ecologist at Boston University. 

The Cost-Benefit Balance of New Trees

There is good reason for the enthusiasm around planting trees. Their ability to store carbon makes them potentially powerful tools against climate change. Ample studies have shown they also alleviate air pollution, reduce the effects of urban heat islands and make streets more walkable. At the same time, ongoing development and increasingly extreme weather conditions have led to a rapid decline of existing urban tree canopies in most places.

Yet planting a massive number of trees is not necessarily a positive investment if not enough of them survive to become mature plants. “It’s not just about planting a million trees. It’s about planting and taking care of a million, and in the right places,” says Lara Roman, a U.S. Forest Service researcher who studies tree mortality.

There’s also a carbon cost to tree-planting, meaning that trees have to survive years before they offset that cost. The largest environmental gain comes when trees mature, sometimes decades after they’re planted.

Keeping new trees alive in the city is tricky. And it’s not cheap to plant trees right. Too often, when cities set their eyes on planting an impressive number of trees, Hutyra says, they underestimate the investment — natural resources, labor and funding — needed to keep them alive long enough to see those gains. 

“Depending on where exactly it’s located, the soil may be rather poor quality, or people may be dumping coffee or salt — from road salt applications — and the growing space the tree has may not be very much,” says Hutyra.

In Copenhagen, the city started to cut corners as it faced the reality of its aspiration’s high costs. It switched its planting priorities from streets — where trees were most needed — to parks and peripheral sites, where trees require less care and face fewer threats and competitors for space. Instead of planting saplings along Copenhagen streets, the city focused its efforts on a peripheral forest site around a trash facility on the island of Amager, where the city planted tens of thousands of acorns in lieu of young trees. According to Hoj, many of these have already died.

The city acknowledges this to an extent. It says that 61,600 trees have been planted so far, 58,000 of which are in an urban forest in Amager. In a statement, a city spokesperson says the new trees growing on the city’s edge will ultimately provide a welcome new forest for Copenhageners and that the city does have a careful watering system for new trees. Above all, it highlights that, while so far reduced in scope, the city’s scheme still shows ambition and commitment, and has increased the city’s overall tree canopy. 

“It is our experience that approximately 50% of trees planted in open ground, such as the ones on Amager, survive,” said the city spokesperson. “The trees on Amager are still small but they are alive, and they will grow big,”

Los Angeles: A Case Study

In the U.S., mass tree-planting gained steam during the early 2000s as Denver, L.A. and New York City all kicked off their own efforts. The goal was particularly ambitious in arid L.A., where former Mayor Antonio Villaraigosa set out to plant 1 million trees by the end of his first term in 2009 — in just four years. 

There was a steep learning curve. As many as 700,000 of the million trees needed to be planted on private property because space elsewhere was tight, according to the city, which meant that the maintenance and watering largely fell on residents.

The city hosted public giveaways. But some residents took fragile seedlings, only to then leave them sitting on their patios or balconies, the Los Angeles Times reported. The city also partnered with local nonprofits to plant trees along median strips, in school yards and in low-income communities where tree cover was sorely lacking. But planting street trees on private property — which required landowners’ participation and, in many cases, a commitment to water them for the first five years — proved difficult, according to a study on the program.

“There was a kind of throw-everything-in-the-kitchen-sink approach to get trees into the ground all over Los Angeles, and most of it was done in this cobbled-together way using nonprofits as a kind of tip of the spear,” says Stephanie Pincetl, the author of that study and a professor at UCLA’s Institute of the Environment and Sustainability. Behind the scenes, organizers scrambled to raise private funds to support the planting effort, but “there was no money really available to water the trees.”

Organizers eventually scaled back their expectations, and by the end of Villaraigosa’s second term in 2013, the program had resulted in 407,000 new trees. In contrast, New York City, which launched its campaign in 2007 under then-Mayor Michael Bloomberg, planted its millionth tree in 2015, two years ahead of schedule. (Michael Bloomberg is the founder and majority owner of Bloomberg CityLab’s parent company, Bloomberg LP.). 

“The program [in L.A.] didn’t reach its goal, so some would say it failed,” says Greg McPherson, a now-retired forester for the National Forestry Service. “Others would say a lot of trees were planted that would not have been planted otherwise if [Villaraigosa] hadn’t taken a risk.”

But basing success on numbers alone doesn’t tell the full story. Roman lauded NYC’s effort not because they planted 1 million trees, but because they had a robust strategy that included a mix of private and public funding for both planting and stewardship, which was shared by the city and its partners. 

That’s not to say New York’s program was perfect, or that Los Angeles’ wasn’t without its successes. McPherson’s 2014 study of the nearly 92,000 trees planted in L.A. between 2006 and 2010 found the annual mortality rate to be 4.4% for street trees. That’s in line with rates elsewhere that range between 3% and 5%, according to a 2011 literature review of 16 street tree studies. He also concluded that the 79.8% survivorship rate for street trees was relatively high for a large and dry city. New York City’s program, meanwhile, has been criticized for disparities between neighborhoods in tree mortality rates.

“??It’s about improving the overall governance around trees and making sure there’s a sustainable system of partners and funding dollars to take care of the trees for the long haul,” Roman says.

Not All Trees Are Equal

It’s not just maturation that increases the environmental benefit of trees; it’s also the kind of tree.

“Every benefit that trees can deliver is delivered to a far, far greater extent by large trees,” says Mary Gagen, professor of geography at Swansea University. The bigger the tree, the more carbon it stores. Larger trees can offer shade and large volumes of temperature-moderating vapor that, together, reduce the need for summer air conditioning and winter heating. They help to mitigate flood events. And large tree canopies provide a connective grid for wildlife across a city in a way that saplings simply cannot, Gagen points out. “They’ve got a big leafy canopy and cavities in their trunks that give wildlife a number of different niches to live in.” 

But bigger trees aren’t always better, especially in dense urban environments. 

“Smaller trees are never going to provide the same benefit as, say, a London plane, which not only offers more cover, but will live for over 100 years,” says Paul Wood, an urban tree expert and author of London’s Street Trees: A Field Guide to the Urban Forest. “But smaller trees, like the cherries or rowans that are popular in London streets, might still be more appropriate for some areas with dense buildings.”

That’s why if cities really want to measure the success of their tree programs, they need to factor in the maturity and types of trees. One good way to do this is canopy cover, which allows cities to better assess the relative benefits of different species.

A recent study from the nonprofit American Forests that measured canopy cover across the U.S. identified stark inequalities. Neighborhoods that are majority people of color have, on average, 33% less tree canopy than majority-white communities, the research found. And the poorest neighborhoods have 41% less canopy than the wealthiest ones. These kinds of disparities are another factor in assessing the success of tree-planting efforts.

Be Ready for Climate Change

Many cities may have close association with a particular tree — lindens in Berlin, plane trees in London, palm trees in Los Angeles — but that does not mean those species will always be the most resilient choice for the future. Though iconic, L.A.’s palm trees are slowly dying from both disease and old age, and most will not be replaced. Instead, the city will swap them out for trees that offer more shade and require less water.

Heat island effects already make cities considerably warmer than their rural hinterlands — as Wood notes, nighttime temperatures in London can be as much as 10 degrees Celsius (18 Fahrenheit) higher than in the surrounding countryside. That means that non-native species can be the best choices in cities, where intense human activity has to an extent estranged climates from their broader surroundings. 

As climate change makes epidemics increasingly likely for plants as well as people, it’s also important to make sure new tree plantings are not too homogenous. With 500 species of trees, London already performs well for this, meaning it is more likely to avoid the fate of cities that have seen most of their trees wiped out at once. 

“Diversity is key to resilience,” says Wood. “Milwaukee was once a monoculture of elms, which meant that when Dutch elm disease arrived in the 1970s, more or less the entire city had to be felled. Going back further, New York used to be filled with American chestnuts, all of which died in the early 20th century.” 

Climate considerations are particularly complex in drought-prone hot cities, where larger trees that cool the environment may also require more water. For L.A., that’s meant a balancing act — and a tree-planting strategy that’s less focused simply on tree volume.

In 2019, L.A. Mayor Eric Garcetti kicked off a new tree-planting initiative with a more modest goal — 90,000 trees by 2021, or 30,000 trees each year — and more focus on tree maintenance. He hired Rachel Malarich to be the city’s first forest manager, and one of her first jobs is to develop a plan to respond to the drought. The city is also building out its first tree inventory since the ’90s, which will help officials monitor trees that are stressed.

As for the new trees, the city has planted a little more than half its goal — after a slowdown due to the pandemic and related budget cuts — and she says they’ll continue even after the original target year of 2021. They’ll give more consideration to species that are more climate resilient, choosing from a list put together by McPherson and other Forest Service researchers behind the Climate Ready Trees research project.

“L.A. has a long history of importing from other Mediterranean climates,” says Malarich, “and the main addition of the trees that we may be testing over time are ones from the Pacific Southwest area, that do well in hot dry climates.”

Originally published by bloomberg.com