IF YOU have ever walked down a tree-lined street on a hot day, you know the value of urban trees. They provide shade and respite from the concrete jungle. Some, like the , a few of which are centuries old, are simply magnificent to behold. City trees have a range of less obvious benefits, too, from flood control . It is no wonder, then, that around the world efforts to expand urban forests are growing. But there is a problem.
Take Boston, Massachusetts, where researchers have scrutinised the local trees for decades. There, around of planting, far too early to realise their benefits. It is the same elsewhere. In New York, more than a quarter of trees planted in 2009 were . Despite planting drives, urban tree cover across the US between 2009 and 2014. What is going on?
We know these trees face unique challenges, from excess to a surfeit of nutrients from dog urine. Poor, under-resourced management is also a factor. But now, emerging research suggests that all these may be the roots of another problem for urban trees: unhealthy microbiomes. Without the right kind of microbial communities, they may be more susceptible to the stresses they face in cities, and things look even worse for the most isolated or “lonely” trees.
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The good news is that improving trees’ microbiomes could help them live longer – to the benefit of all city dwellers.
Urban green spaces
The majority of the human population are now urbanites, and the urban environment is forecast to be three times the size it was at the turn of this century by 2030. The importance of urban forests is growing accordingly. For instance, city trees help reduce the urban heat island effect by limiting the amount of the sun’s radiation absorbed by the built environment, as well as . Trees by intercepting rain. They also clean pollution from the air and water, serve as habitat for a host of urban wildlife and store carbon.
While city authorities have long recognised these benefits, most haven’t invested enough to maintain their urban forests, or done so equitably. In the US, decades of discriminatory planning practices have left many neighbourhoods that are inhabited primarily by people of colour with fewer trees than whiter neighbourhoods. Low-income neighbourhoods in the US have than wealthier ones, and . Hotter than average temperatures in these neighbourhoods are .
Growing recognition of those inequities, along with rapid urbanisation and billions of dollars in climate change-related funding, has spurred urban authorities to redouble efforts to maintain and expand urban forests. Many local residents have also got involved (see “Help your street trees”, below). However, if a lot of these trees die within a few years of planting, much of this effort is wasted. “Trees need to get really big to benefit people,” says at Boston University. It takes some 10 years after a specimen is planted for it to become established and grow a cooling canopy, she adds. Depending on the city and the species, a tree must survive produced by planting and maintaining it. A city of saplings is no replacement for a long-lived urban forest. This is why early mortality among city trees is so concerning.
To tackle the problem, we must first understand why trees are dying. But making sense of the fate of millions of them in complex urban ecosystems isn’t straightforward. “It’s very hard to figure out what’s happening,” says Bhatnagar. Not only are there classic ecological factors like heat and drought; there are unpredictable human elements, too. Soil can be compacted or . City air is polluted with particulates, ozone and other gases. Humans may introduce invasive pests and pathogens. Management is also a big factor, with trees planted in poor soils, then underwatered, over-pruned, vandalised or felled to make way for developments. “In other areas of ecology, you don’t even think about people,” says Bhatnagar.
One clue to why city trees die is when they tend to die. There is a general pattern that can be seen clearly in the closely studied trees of Boston. When at Boston University and her colleagues , they found that both died at high rates when young. Urban trees were also more likely to die when they are bigger, whereas mortality rates fell in rural trees the longer they survived.
This suggests different drivers of tree death in urban and rural settings, says Bhatnagar. She thinks that older city trees regularly succumb to over-zealous pruning or removal because they are deemed to be in the way or to pose a hazard. For younger urban trees, she suspects their deaths are often due to a failure to establish roots and access water. (Conversely, in rural environments tree seeds germinate profusely, but the population of saplings is naturally culled as they compete for light and nutrients.) City trees’ failure to establish roots may be caused by stressors unique to cities, such as densely packed soils, excess nutrients and hotter temperatures.
New evidence, meanwhile, points to a problem with the microbes that live on and inside the roots of urban trees. Bhatnagar, a microbial ecologist, is a pioneer of this research. Until she learned about the plight of Boston’s trees, she hadn’t thought much about urban ecology. But in 2018, she started working with Hutyra’s group to explore whether problems with the microbiomes of city trees might be playing a role in driving high mortality rates. “It’s the most interesting science I’ve ever done,” she says.
Unhealthy root microbiomes
One finding that intrigued her was from a small study conducted in Ontario, Canada, in 2009. The researchers found that the roots of than comparable rural trees, and that there were fewer species present. Such mycorrhizal fungi are known to form mutualistic relationships with many species of plant. The fungi encase or penetrate tree roots with thin filaments to exchange nutrients such as nitrogen and phosphorus in return for sugars from the plant. They can also form underground mycorrhizal networks, which may help trees to share nutrients with each other through a “wood-wide web” of molecular exchange. Some researchers have argued – though not without controversy – that this is evidence of trees helping or communicating with one another.
Bhatnagar thought the finding in Ontario was striking. “It’s a very clear impact of a city on an organism,” she says. But no one had followed up to see if the effect was more widespread, or what the consequences might be for tree health. So, to get a more complete view, she and her colleagues looked at data on the soil microbiome of eight urban and rural forest plots in Massachusetts. They focused on ectomycorrhizal fungi or “ectos”, ones that sheathe the exterior of roots in some tree species.
Unexpectedly, the study, published in August, found that in the soil than rural forests did. However, as in Ontario, there were fewer fungi colonising the tree roots. The researchers interpret this as evidence of a “breakdown” in the relationship between the fungi and their tree hosts. “In cities, the rules are broken for how microbes interact with trees,” says Bhatnagar. She thinks this could be due to excess nitrogen in urban soils, both from pollution and from hotter temperatures favouring bacteria that fix nitrogen in soil. “If there’s tons of free nitrogen in the soil, the plants will kick off the ectos,” she says.
As-yet-unpublished research by at the University of Wisconsin-Madison and her colleagues seems to back up this idea. In the very different setting of Bogotá, Colombia, Pringle says they are finding that trees growing in parks have fewer symbiotic fungi – both ectomycorrhizal and endomycorrhizal ones, which penetrate tree roots – compared with trees growing in tropical forests outside the city. However, the microbiomes of street trees growing in pits surrounded by concrete are comparable with those of rural trees. Unlike soil in parks, which may more often be subjected to nitrogen-rich dog urine, “the hard surfaces don’t get peed on”, says Pringle.
The consequences of lower fungal colonisation rates on tree health, if any, aren’t yet clear. at the University of British Columbia in Canada says it would be premature to draw a link between changes in microbiomes and higher mortality of trees in urban forests. “We don’t know what these conditions are doing to [large organisms], let alone what they’re doing to micros,” she says. But, she adds, these findings hint at yet another way in which urban environments can undermine complex ecological interactions.
Pringle is more concerned. “It is a sign of trouble,” she says. The loss of the fungal sheaths around roots could expose trees to more microbial pathogens, for instance. Indeed, Bhatnagar’s group has found that hotter, drier soils at the edges of fragmented forests have a higher abundance of pathogenic microbes and a lower abundance of symbiotic fungi. An unhealthy microbiome could also affect the immune systems of city trees and make them less resilient to heat and drought. But if missing fungi and other microbes are part of the problem with urban trees, they could also be part of the solution.
Rebooting the wood wide web
One researcher exploring this possibility is Bhatnagar’s colleague at Harvard University. After reading about ecologist Suzanne Simard’s groundbreaking work on mycorrhizal networks of old-growth forests, she wondered if she could facilitate such connections among isolated city trees. In a greenhouse at the Harvard Forest research station in western Massachusetts, she and her team have set up simulated sidewalks in the form of long wooden boxes, with soil conditions mimicking those in Boston. First, Harden plants white oak saplings at either end of each box, then, wearing latex gloves to avoid contamination, she adds a handful of extra soil collected from a stand of oaks at the Harvard Forest to one of the trees. This inoculates its roots with an intact community of fungi. Between the trees, she makes a “soil conduit” – a strip of loosened soil that should make it easier for the roots of both trees to spread.
Harden hopes to demonstrate that the mycorrhizae will colonise the first tree and spread through the conduit to the second, with health benefits for both. If the treatment works, city trees separated from others might benefit from soil conduits cut through roads or sidewalks. By her estimate, New York City alone has around 300,000 such “lonely” trees. “We have people plopping down trees without understanding what they need,” she says.
Alongside Harden’s team, Bhatnagar and a handful of other researchers have planted saplings in pots for a parallel experiment. The idea is to test how inoculating the plants with different types of urban and rural soils, each with distinct microbiomes, affects the growth and health of the tree. The results could inform Boston’s tree-planting efforts and help correct what Bhatnagar sees as the “Home Depot-isation” of tree planting: spraying fertiliser and fungicide on any old soil and plonking in a tree.
Neither experiment will have clear results until 2025. However, research suggests that inoculating trees with native fungi and other microbes can improve growth and health, at least in non-urban settings. One Texas-based company called Funga is now using the microbes that produce the best results to make trees on a pine plantation grow bigger and faster. In Bogotá, Pringle is also experimenting with inoculation and says her group has found that trees treated with native soils and fertiliser grow best. However, she adds that one major challenge with this method is access to native soil, which can’t yet be cultivated and instead must be collected from intact forest.
Hart thinks inoculation could help urban trees, but is sceptical that it would ever be enough to address the larger-scale issues that disrupt urban soil microbiomes. “We have to fix these problems if we want the microbes to survive,” she says. “We need more plants and less concrete, and fewer cars.”
Nevertheless, Bhatnagar believes these approaches are worth a try, given the desire to expand urban forests. “Folks are desperate,” she says. “They don’t want to go through the process of greening a space just to then see it fail.”
Help your street trees
Many cities are planting trees to expand their urban green spaces, but there often aren't enough resources to ensure these trees are properly maintained. "It's really the people in the community who are going to keep the trees alive," says arborist Claire Corcoran at Speak for the Trees, a non-profit organisation working to expand Boston's green canopy. It is likely that your town or city has "adopt-a-tree" programmes or local volunteer groups with specific ideas and opportunities to care for urban trees. But Corcoran suggests some general ways you can help:
Trees need watering for up to five years to become established, although be .
Densely-packed soil can make it harder for tree roots to expand and for water to reach them. Creating a small barrier so that people don't walk or cycle over roots can help.
Excess nutrients from dog urine and faeces, or from trash, can pollute the soil. Avoid contaminating soil around trees and keep tree pits free of rubbish.
Covering soil in mulch can help it retain water and slows nutrient inputs.
Roots also need oxygen, so gently rake soil around street trees to increase aeration.
People treat trees better when they understand the benefits they provide, so spread the word.
James Dinneen is a New York‑based environment reporter at New Scientist
