By Adrian Higgins
A pear seedling selection named Bradford was cloned by the gazillion to become the ubiquitous street tree of America’s postwar suburban expansion. Then it turned invasive.
Carole Bergmann pulls her small parks department SUV into an aging 1980s subdivision in Germantown, Md., and takes me to the edge of an expansive meadow. A dense screen of charcoal-gray trees stands between the open ground and the backyards of several houses. The trees are callery pears, the escaped offspring of landscape specimens and street trees from the neighborhood. With no gardener to guide them, the spindly wildlings form an impenetrable thicket of dark twigs with three-inch thorns.
Bergmann, a field botanist for the Montgomery County Parks Department, extricates herself from the thicket and in the meadow shows me that what I take to be blades of grass are actually shoots of trees, mowed to a few inches high. There are countless thousands, hiding in plain sight in Great Seneca Stream Valley Park. If it were not cut back once a year, the meadow would become like the adjacent screen, wall upon wall, acre upon acre of black-limbed, armored trees worthy of Sleeping Beauty’s castle.
“You can’t mow this once and walk away,” said Bergmann, who began her 25-year career in the department as a forest ecologist but has been consumed by an ever-pressing need to address the escape of the Bradford pear and other variants of callery pear, a species that originated in China, along with other invasive exotics.
By Maria Dolan
Austin, Texas, and King County, Washington, are testing carbon credits for planting and protecting urban trees.
The evidence is in: Urban trees improve air and water quality, reduce energy costs, and improve human health, even as they offer the benefit of storing carbon. And in cities across the country, they are disappearing.
A recent paper by two U.S. Forest Service scientists reported that metropolitan areas in the U.S. are losing about 36 million trees each year. The paper, by David Nowak and Eric Greenfield, was an expansion of the same researchers’ 2012 study that found significant tree loss in 17 out of the 20 U.S. cities studied.
This arboreal decline is happening even in some areas that promote “million-tree” campaigns, Arbor Day plantings, and street-tree giveaways. Cash-strapped municipalities just can’t find enough green to maintain the green. Additionally, many cities are adjusting to population booms, and to temperature increases and drought due to climate change—both conditions that can be hard on trees (while increasing their value as sources of cooling and cleaner air). There’s also a growing recognition of the inequity of tree-canopy distribution in many cities, with lush cover in wealthy neighborhoods and far fewer trees in disadvantaged areas.
To find more funding for urban trees, some local governments, including Austin, Texas and King County, Washington (where Seattle is located), are running pilot projects with a Seattle-based nonprofit called City Forest Credits (CFC). The nonprofit is developing a new approach: generating funding for city tree canopies from private companies (and individuals) that wish to offset their carbon emissions by buying credits for tree planting or preservation.
The vast majority of forest carbon credits worldwide have been issued for trees in tropical rainforests and other forests far from urban areas. A study released last year of the forest offsets in California’s cap-and-trade program found that they are effective at reducing emissions.
By Neil Shaw
JOHANNESBURG — One of the world’s largest urban forests is under threat from a tiny beetle.
The polyphagous shot hole borer is thought to have made its way to Johannesburg from Southeast Asia on packing crates or through the trade in plant materials.
Trudy Paap, a forest pathologist at the University of Pretoria, discovered the beetle in the Pietermaritzburg Botanical Gardens last year. She published her discovery in the journal Australasian Plant Pathology, calling it part of “the surge in the global spread of invasive forest pests” because of globalization.
The beetle has since moved to Johannesburg, 200 miles away, and spread across its urban forest, which according to the Massachusetts Institute of Technology initiative Treepedia has the world’s sixth-largest green canopy cover.
Today, many of Johannesburg’s estimated 6 to 10 million trees are dying, a crisis obscured only by the current winter season. Some of the infected trees have the telltale holes the 2-millimeter-long beetle makes in their bark.
“This beetle doesn’t actually eat the trees,” Paap said. Instead it carries a fungus that blocks the vessels that transport water and nutrients, “which ultimately leads to die-back and death of the tree.”
Though scientists don’t know just how many trees have died from the beetles’ invasion, the outlook for Johannesburg is grim: “The city is going to lose a lot of trees.”
The trees do not have an evolved resistance to the polyphagous shothole borer, unlike in Asia where the beetles naturally occur.
It is the older, more established trees that are at risk, said arborist Neil Hill. “So that’s going to leave a gap in the landscape. And if we don’t start to plant straight way with new trees that gap is going to become more and more of a concern as far as urban blight, pollution, aesthetic beauty.”
BY EILLIE ANZILOTTI
A new tower will have greenery lining the balconies and roofs to clean up the air and provide a new environment for pollinators and humans alike.
Toronto has long been serious about its urban canopy. The Ontario city is already home to around 10 million trees, which cover around 26% of the city. The current mayor, John Tory, wants to grow that to 40%.
Brisbin Brook Beynon, a local architecture firm, is already giving the city a leg up on that goal, albeit in an unconventional way: a 27-story residential building that will be covered with around 450 trees, growing on its balconies and roofs. This “vertical forest,” as BBB terms it, takes inspiration from the Bosco Verticale–residential towers in Milan that went up in 2014 with as many as 11,000 plants lining the sides. Since then, copycat buildings have been built in cities like Nanjing and in Taiwan–designed to combat pollution and prove that green space does not need to be limited to the ground. This latest iteration could open as early as later next year.
For Brian Brisbin, principal at BBB, bringing the vertical forest concept to Toronto aligned perfectly with the mayor’s goals for increasing tree coverage. And when he began researching the concept by studying the Bosco Verticale, he realized that all of the technology that enabled the Milanese building to function originated in Canada and North America. “That felt fairly profound,” Brisbin says.
And it also, Brisbin says, made bringing the concept to Toronto feel much more feasible. “We have a lot of depth of specialty in this area in Toronto, with horticultural and agricultural universities and research facilities,” he says, “and we’ve brought a lot of together to take a very science-based approach to developing this project.”
By Theodore Endreny
Megacities are on the rise. There are currently 47 such areas around the globe, each housing more than 10 million residents.
More than half the global population now lives in urban areas, comprising about 3 percent of the Earth. The ecological footprint of this growth is vast and there’s far more that can be done to improve life for urban residents around the world.
When it comes to natural spaces, trees are keystone species in the urban ecosystem, providing a number of services that benefit people. My research team has calculated just how much a tree matters for many urban areas, particularly megacities. Trees clean the air and water, reduce stormwater floods, improve building energy use and mitigate climate change, among other things.
For every dollar invested in planting, cities see an average of US$2.25 return on their investment each year.
By Morgan Erickson-Davis
Nations are hurrying to reduce greenhouse gas emissions and slow global warming, and one way they’re going about this is by encouraging the protection of forests. Trees trap carbon in their biomass and in the soil, and it’s hoped that keeping them in the ground will keep their carbon out of the atmosphere.
Climate-focused forest conservation policies and programs tend to be focused on rainforests. Covering vast areas, rainforests have earned the moniker “lungs of the planet” for their ability to sequester carbon dioxide while producing oxygen.
But pound for pound, other types of forest give rainforests a run for their money. A hectare of mangrove, for instance, can store four times more carbon than can a hectare of rainforest. And now, new research shows that even temperate forests in cities may be able to sequester nearly as much carbon as a similarly sized area of rainforest.
The study was conducted by a team of scientists from University College London, who mapped the carbon stores of areas of tree cover in the London Borough of Camden. Their results were published recently in the journal Carbon Balance and Management.
The team used remotely sensed LiDAR (which stands for “Light Detection and Ranging”) data that provided high-resolution information about tree structure. Armed with specific numbers on the dimensions and extent of Camden’s aboveground biomass (i.e., the parts of trees that aren’t underground), the researchers were able to estimate how much carbon is contained in each pocket of urban forest.
“Urban trees are a vital resource for our cities that people walk past every day,” said lead author Phil Wilkes. “We were able to map the size and shape of every tree in Camden, from forests in large parks to individual trees in back gardens. This not only allows us to measure how much carbon is stored in these trees but also assess other important services they provide such as habitat for birds and insects.”
Their results indicate Camden’s trees contain more carbon than estimated by previous studies. And while, as a whole, the borough’s median carbon density is on the low side when compared to many natural ecosystems – roughly equivalent to subtropical steppe – its urban forests are carbon storage powerhouses. The maximum value they uncovered was in a large, 320-hectare park called Hampstead Heath. There, carbon density approaches that of tropical rainforest.
By James Barron
The city has 10,542 acres of forests. The Natural Areas Conservancy, which says they are at a tipping point, is thinking about how to care for them.
Sarah Charlop-Powers was comparing New York City’s forests to its subways.
The city has more than 840 miles of tracks for one. It has 10,542 acres of the other, about half as much as the Congaree Swamp in South Carolina, a small national park with an old-growth forest and, according to the website I Love National Parks, “more bugs than you can imagine.”
No doubt there are many bugs in New York’s forests, even cockroaches that have yet to find their way to somebody’s basement or bathtub. But Ms. Charlop-Powers, the executive director of a Manhattan-based nonprofit group called the Natural Areas Conservancy, is not focused on them. She sees the trees — and the forest. Most of the city’s forest is deep in parks, and on the worry spectrum, she is “concerned” about them.
“The situation is not dissimilar to the subways, in that we’re at a crucial moment,” she said.
Fortunately, urban forests appear to be at the point the subways reached decades ago, before transit policymakers decided that maintenance could be deferred.
BY TEE CLARKSON Special correspondent
As a city of considerable age, Richmond is fortunate to have a significant number of older trees in varying densities across the landscape.
The importance of these trees, particularly in urban areas, is substantial.
“We are way past the hippie, tree-hugger mentality when it comes to the benefits of urban trees,” said Joel Koci, an associate with the Urban Forestry Extension at Virginia State University.
“We have real science to back up the benefits of urban forest.”
Koci pointed specifically to the benefits of trees in sequestering carbon in leaves, trunk and root systems, the absorbing of particulate matter such as harmful chemicals and heavy metals, the absorbing of solar radiation, as well as the reduction of storm water runoff and erosion.
By Dean Fosdick | AP
Landscape designers in cities are creating quieter living spaces by using trees to mute loud noises like sirens and air brakes. It’s called “soundscaping,” and it aims to restore peaceful, natural sounds like wind whispering through leaves, birds chirping or rain dripping from branches.
“Massive walls are often installed to quiet freeway noise in major cities, but there are more aesthetic ways to handle it,” said Tim Moloney, who teaches landscape design at the University of Missouri. “Use vegetation for minimizing the background clatter.”The denser a tree’s lower branches, the better it masks or deflects bothersome noise, Moloney said. Evergreens are the preferred vegetative sound barriers because they are densely branched and are attractive year-round. Ideally, shrubs would be a major component of any green muting mix.
Planting trees is a cost-effective way to tackle urban air pollution, which is a growing problem for many cities.
A study by US-based The Nature Conservancy (TNC) reported than the average reduction of particulate matter near a tree was between 7% and 24%.
Particulate matter (PM) is microscopic particles that become trapped in the lungs of people breathing polluted air.
PM pollution could claim an estimated 6.2 million lives each year by 2050, the study suggests.
Lead author Rob McDonald said that city trees were already providing a lot of benefits to people living in urban areas.