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The best trees to reduce air pollution
City trees can help reduce air pollution from New York to Beijing, but which trees do the best job? Future Planet weighs the options.
Cities usually come at the price of green spaces. Sinceprehistoric timespeople have been busy clearing forests to make way for settlements. But increasinglyGreen has found its way back into modern urban landscapes, And for a good reason. Vegetation helps cities become better habitats for animals and people, and helps make city air safer.
Trees have a remarkable range of properties that can help reduce urban air pollution, and cities around the world are trying to take advantage of them. In January 2019 theThe Mayor of London announced that 7,000 trees would be planted by the end of the yearnext year. Meanwhile, China's Hebei province, home to Beijing, has been working on a "green necklace" made from plants that could help reduce pollution from factories around the capital. andParis is planning an urban forest that will include its most famous landmarksin an effort to adapt to climate change and also improve the city's air quality.
While trees are generally effective at reducing air pollution, it's not as simple as saying that the more trees you have in an urban space, the better the air. Some trees filter pollutants out of the air much better than others. In order to optimally influence the air quality in a street or city, it has to be the right tree.
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And of course, trees are just one way to filter pollution; It's better to reduce pollutant emissions in the first place, notes David Nowak, a senior scientist at the US Forest Service who has been studying the contribution of plants to air quality for 30 years. “But trees can be a big help,” he says.
Trees can directly and indirectly improve air quality. Indirect,They can help by shading surfaces and reducing temperatures. When buildings are shaded by trees, this is the casereduces the need for traditional air conditioning, and associated greenhouse gas emissions. In addition, lower temperatures reduce the risk of harmful pollutants such as ground-level ozonePeak values are common on hot days in urban areas.
Air pollution in Beijing regularly exceeds 10 times the WHO recommended levels, but tree planting programs are being deployed to reduce pollution (Image credit: Getty Images)
But trees also play an important role in removing pollutants directly from the air. Plants are often considered the "lungs" of an ecosystem because they take in carbon dioxide and give off oxygen, says Rita Baraldi, a plant physiologist at the Institute of Bioeconomy of the Italian National Research Council. But they also act as the “liver” of the ecosystem, filtering air pollutants like sulfur dioxide and nitrogen dioxide through their leaves.
Trees are particularly effective at removing particulate matter (PM), Nowak adds. PM comes in the form of tiny particles of organic chemicals, acids, metals and dust emitted by fossil fuel vehicles, factories and construction sites. The largest of these particles are up to 10 microns in diameter (known as PM10).about a fifth the width of a human hair. Then there are PM2.5s with a diameter of 2.5 microns andeven smaller pollution by nanoparticles.
Particulate matter can easily enter human respiratory tract and cause pulmonary and cardiovascular diseases orWorsening of a respiratory disease. It was tooassociated with inflammationandheart disease. After an estimate8.9 million deaths per yearworldwide could be due to outdoor exposure to particulate matter.
Clean the air
From an urban perspective, plants act as a readily available collection of PM purifiers. "Trees can help reduce particulate matter in two main ways," says Prashant Kumar, founding director of the Global Center for Clean Air Research at the University of Surrey.
Conifers offer the best PM reduction as they are an evergreen species. But that doesn't automatically make them fit for every context - David Nowak
The first is dispersion – by impacting trees and plants, concentrated clouds of tiny particles are dispersed and thus diluted by the air, reducing the risk of inhalation by humans. The second is deposition. PM can easily get caught in the waxy, hairy leaves of trees and shrubs. When it rains, most of these particles are carried away by the water into the sewers.
"The extent to which each species performs such filtering activity depends primarily on canopy size, leaf size and leaf structure," says Baraldi. Larger canopies can trap more particles than smaller ones, and larger leaves can trap more pollutants than small ones. When it comes to leaf species, it is those with rough, rugged, and hairy surfaces that act as the "best filters" for PM.
London plane trees lining Victoria Embankment emit high levels of volatile organic compounds that can affect urban air pollution (Image credit: Getty Images)
Recent research suggests that tiny hairs on plant leaves in particular can play a major role in trapping the pestssolid and liquid particlesthey form PM. In a recentto learn, Barbara Maher and colleagues from the University of Lancaster tested the ability of nine tree species to capture particulate matter in wind tunnel experiments.Silver Birch, Yew, and Elder were the most effective at trapping particles, and it was the hairs on their leaves that contributed to reduction rates of 79%, 71%, and 70%, respectively. In contrast, stinging nettles turned out to be the least beneficial of the species studied, although they still made up a respectable 32%.
Coniferous trees, like pine and cypress, are also good natural cleaners. In 2015, Jun Yang, an urban ecologist at the Center for Earth System Science at Tsinghua University in Beijing, ranked the most abundant species in cities based on their PM 2.5 absorption capacity. The ranking also took into account the species' ability to survive in urban contexts and any negative impacts on air quality, such as the production of allergens and volatile organic compounds (VOCs) - a range of substances that can interact with gases emitted by vehicles , such as nitrogen dioxide. In the presence of sunlight, these reactions can contributeto ground-level ozone, which is harmful to human health. The impact can be significant; When Berlin was hit by a heat wave in 2006, ozone was created by the interaction of VOCs from plants and pollutants from vehiclesled to a sudden deterioration in air quality.
To Yang's surprise, his ranking system showed that the most common tree species were not the best pollutant filters. "Of the 10 most common species, only the London plane tree,Silberahornandhoney grasshopperrated above average,” he says. These were coniferous trees such asPine treesandcypresses, those were the best dirt filters. Planting conifers, Yang concluded, would make the most sense in polluted cities like Beijing to reduce PM2.5. The Chinese capital routinelyreports PM valuesover 125 micrograms per cubic meter, more than 10 times greater thanWorld Health Organization recommended limit of 10 micrograms per cubic metre.
There's a lot of conflicting advice and wishful thinking about the benefits of urban trees for reducing air pollution - Stephanie Carlisle
Part of the reason for the success of conifers in reducing particulate matter is their canopy structure – the dense canopy of needle-like leaves typical of conifersvery effectivein capturing pollutants. And their seasonal biology helps, too. "Conifers offer the best PM reduction because they're an evergreen species," says Nowak. Unlike deciduous trees, which lose their leaves in winter, evergreen species act as year-round filters. "But that doesn't automatically make them fit for every context."
Evergreen trees can be excellent scavengers of pollutants and, unlike their deciduous cousins, work year-round (Image credit: Getty Images)
The problem with conifers, Nowak says, is that many species can be very differentsensitive to soil salinity, which tend to be high in urban areas, particularly where salt is used to de-ice roads. To make matters worse, the year-round canopy of conifers can block sunlight from melting snow and ice, which can cause traffic problems in cities with cold temperatures, Nowak notes. These two disadvantages of conifers were also cited by Yang as caveats to consider in his recommendations.
Some broadleaf tree species can also have side effects, Nowak says. For example, trees commonly found in northern hemisphere cities — such as poplars or black rubber trees — can emithigh VOC levels.
"Ideally, you want to be able to identify species that can maximize PM absorption but minimize ozone precursor production," says Margarita Préndez, an organic chemist at the University of Chile who has studied how different species affect air quality impact in Santiago. Nowak cites conifers such as hemlock, juniper and deciduous trees such as elm, horse chestnut and linden as examples of low-VOC plants.
"Based on data from Santiago and other Chilean cities, native trees emit less VOCs than non-native trees," adds Préndez. In Santiago, non-native species such as the Prunus and London plane tree can produce up to 30 times more VOCs than native species.
But that rule may not apply everywhere, and Yang says one can't generalize when it comes to endemic or introduced trees. "Some of the best species for reducing air pollution are alien," he says. "We shouldn't exclude them for ideological reasons."
Yew hedges are good roadside additions to reduce pollution, but they also have poisonous leaves and berries, meaning they're not suitable everywhere (Image credit: Getty Images)
Finding the right trees for a city is a delicately balanced matter. But that's just the beginning, says Nowak. The next question is where to plant them.
Many well-meaning plans have suffered from poorly planned planting. "Some cities, like Beijing and Mexico City, have planted trees quite far from the city centers," says Rob McDonald, senior scientist at The Nature Conservancy. "That might not be so advantageous."
McDonald, who works with local governments to manage urban forests, says the rule of thumb is that trees must be planted close to people — and sources of pollution.
And since wind direction and landscape structure can affect the way pollution moves, trees need to be planted accordingly, Nowak adds. In narrow streets surrounded by tall buildings like those in downtown Manhattan,Air flow can trap pollutants near the ground.Planting tall trees with large canopies can make things worse in this situation by preventing pollution from spreading. A recentlyTree planning scheme in Beijingpartly for this reason the pollution in certain areas is trapped.
Kumar and his team recentlyspecific recommendations madefor city planners on this point. Hedges or green walls are generally preferable to trees in narrow streets flanked by tall buildings. While wide streets surrounded by low-rise buildings typical of American suburbs allow air to flow more freely, there is less risk of trapping pollutants, making both trees and hedgerows viable options. Examples of street hedges that work well include viburnum, red-tipped photinia, privet, and laurel, Baraldi adds.
Securing biodiversity is also essential, even if a tree species is an outstanding winner in terms of its pollutant binding. Kumar recommends that no more than 5-10% of an urban forest should be of the same species or family. And a final factor Nowak notes is that one should be realistic about maintenance and longevity—plants that require little attention and last for several decades are to be preferred.
Through the maze
With such a range of variables to consider, knowing which species work best in a given location can be a challenge. "There's a lot of conflicting advice and wishful thinking about the benefits of urban trees for reducing air pollution," says Stephanie Carlisle, an urban ecologist at the University of Pennsylvania who studies the interaction of natural and built environments. "Plus, it's really difficult to measure." In fact, some studies report that plants can reduce fine dust entering indoor spacesonly 1%, while others call for reduction rates of 60%.
However, scientists are building tools designed to help urban planners identify the most suitable species for a given location. For example, free software provided by the US Forest Service,iTree species, ranks species based on a number of variables, including air pollution removal abilities, carbon storage, and VOC emissions.
Choosing the right tree involves a realistic assessment of its life expectancy and care needs (Image credit: Getty Images)
The city of Oakville, Ontario, Canada was one of the early adopters of iTree and is a good example of how difficult it is to find the right balance. The community found that Norway maple provides more air quality benefits than any other tree, according to iTree. However, since Norway maple already made up more than 10% of the city's tree canopy, the city decided against planting more to preserve biodiversity. However, the community also stopped planting hawthorn after iTree revealed it was of little help for air quality. Hawthorn was gradually replaced by American elm - but then it turned out that it was susceptible to Dutch elm disease.
As Nowak explains, the species ranking in the iTree tool gives a general indication of the best species without considering much local context. "Species that become established may be invasive or not well suited to the ecosystem in question," he says. "As such, it's always best to check with local experts to determine which species are doing well based on local conditions." Next, Nowak and his team plan to further investigate how local ecosystems affect how well a tree grows plants and how best to use its natural properties.
The importance of local context is a point reiterated by Kumar. Earlier this year he co-authored aGuide to help urban planners select species that can help reduce roadside pollution. He listed 61 species that have 12 characteristics suitable for reducing air pollution, such as: B. Ragged leaves. The guide also takes into account “undesirable” variables such as pollen, VOCs and high-maintenance species. "Evergreen oaks, pine species and common yew were chosen as potentially the most effective," says Kumar. "Because they are relatively tolerant of pollution, are evergreen, and contain a number of beneficial leaf traits."
Such advice is not just for city planners. Citizens can now rely on a growing selection of toolkits to help them with their planting choices. Jennifer Gabrys, a sociologist at the University of Cambridge, has created a digital tool that allows anyone to play a part in reducing air pollution. CalledPhyto-Sensor, and developed while Gabrys was previously at Goldsmiths University, the tool lists plants that have been shown to be effective in reducing PM, such as wallflowers and ivy, and offers suggestions for planting locations.
More green space in general can help with urban air pollution, including plants other than trees in parks and gardens (Credit: Getty Images)
But ultimately, context determines whether a species is beneficial or detrimental. "Even the 'best performing trees' may not work in some cases," says Kumar. "For example, we would not recommend planting yew near schoolyards because it is poisonous."
If all of this proves anything, it's that clinging to a tree as a cure-all won't get you very far. "Some designers tend to think in terms of objects rather than complex ecological systems," says Carlisle. "But without a holistic understanding of urban ecosystems, there is a risk of doing more harm than good."
In this sense, planting trees to combat pollution is like many other aspects of urban design – the key to success is understanding local and ecological nuances. It decides whether city trees are a breath of fresh air or a big headache.
The emissions from the trips required to cover this story were 0 kg CO2: the author interviewed sources far from the safety of the lockdown. The digital emissions of this story are estimated to be 1.2 to 3.6 g CO2 per page view.Learn more about how we calculated this number here.
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