The hidden pathogen threatening our urban forests

Walk down almost any tree-lined street in a European city and you’ll likely see the common lime tree, Tilia × europaea. With its broad green canopy, sweet-scented flowers, and generous shade, it’s one of the unsung heroes of urban life. These trees cool our neighborhoods during summer heatwaves, filter out pollution, and offer a sense of calm amid the bustle of city life. But like many quiet heroes, lime trees have a hidden vulnerability. A soil-dwelling pathogen called Phytophthora plurivora is slowly undermining their health, with consequences that ripple far beyond the trees themselves.

That’s the story behind a study published in [Plant–Environment Interactions] by a research team at the University of Sheffield. The work of researchers Eleanor Absalom, Anthony Turner, Matthew Clements, Holly Croft, and Jill Edmondson explores what happens when these beloved street trees fall ill, and why the results matter for the future of our cities.

City trees are important natural infrastructure serving multiple functions. Trees act like living air conditioners. Through a process called transpiration, essentially, sweating water through their leaves, they cool the surrounding air. In heatwaves, this effect can mean the difference between life and death for vulnerable city residents. Also, studies consistently show that access to trees lowers stress, boosts mood, and even improves recovery rates in hospitals. Finally, trees absorb carbon, filter pollutants, and reduce stormwater flooding by soaking up excess rainfall.

But these benefits depend on healthy, mature trees. Big trees with leafy crowns and strong root systems do far more for cities than young saplings. So when a disease cuts into their strength, the whole urban ecosystem feels it.

Phytophthora (literally “plant destroyer”) is a notorious genus of pathogens. These microscopic organisms aren’t fungi, though they behave like them; they’re water molds, thriving in damp soils. The Irish potato famine of the 1840s? That was caused by a Phytophthora species. Phytophthora plurivora is especially dangerous because it’s a generalist, it doesn’t stick to just one kind of tree. It has been linked to the decline of beech, oak, and alder in European forests, and it’s spreading through urban areas too. Infected lime trees often show bleeding bark, root rot, leaf yellowing, and canopy dieback.

The pathogen has been present in Sheffield for years, with local lime trees first showing symptoms in the late 2000s. But until now, no one had looked closely at how infection affects not just the trees’ survival but also the ecosystem services they provide.

The research team took a novel approach: they wired up ten mature lime trees, five healthy-looking, five infected, with Internet-of-Things (IoT) sap flow sensors and tiny devices called dendrometers. Sap flow sensors measured how much water was moving through the trees, minute by minute. Since water movement is tied to cooling and carbon uptake, this gave a direct measure of how well each tree was functioning. Additionally, dendrometers tracked subtle changes in trunk diameter, allowing the team to see whether trees were growing, shrinking, or stagnating. They also collected data on leaf density, chlorophyll levels, and local weather conditions to paint a full picture.

This high-tech monitoring ran through the blistering summer of 2022, when Sheffield saw record-breaking heatwaves and drought. If there was ever a stress test for urban trees, this was it.

The results showed a quiet but steep decline. Infected trees used 87% less water than their healthy neighbors. On average, a healthy tree cycled through about 198 liters of water per day, while an infected one managed only 25 liters. Because less water flowed, infected trees lost much of their natural air-conditioning capacity. Their “urban cooling” effect was cut by a similar margin, dropping from nearly 486 kilowatts of energy loss per day in healthy trees to just 61 kW in the sick ones. Healthy trees grew thicker over the summer, adding about 0.35% to their trunk diameter. Infected trees, by contrast, actually shrank slightly (–0.22%). However, not all infected trees were equally impaired. Some still managed water flow and growth similar to healthy ones, possibly because their disease was less advanced.

Put simply: when these trees got sick, they look worse and stop performing their ecological role.

The study highlights how diseased trees can no longer protect cities in the ways we depend on them to. This is especially worrying for several reasons. First, climate change is making it worse. Warmer, wetter winters help pathogens spread, while hotter, drier summers make stressed trees more vulnerable. Second, urban forests are genetically uniform. As many city streets are lined with cloned cultivars of just a few species, the lack of diversity creates a buffet for pathogens. Third, management decisions are tough. A tree that looks outwardly healthy but is harboring infection might still be cooling the street and cleaning the air. Should it be cut down to stop the spread, or kept to preserve its benefits?

The research team that conducted the study emphasizes this trade-off. Some infected trees were still functioning nearly as well as healthy ones. Removing them too soon could rob communities of cooling shade during critical heatwaves. Waiting too long, however, risks sudden branch failures and wider spread of disease.

So, what can we take away from this research? Early detection matters. To this end, smart sensors could give city managers a way to spot infected trees before they collapse. That’s safer for the public and helps in making nuanced decisions about removal. Also, diversity is key. Relying on cloned cultivars or a narrow palette of species sets up urban forests for disaster. A more diverse planting strategy spreads the risk. Moreover, trees need allies. Protecting urban trees from disease also means tackling climate change and reducing stresses like soil compaction and pollution. Healthy trees resist infection better. Finally, policy must balance trade-offs. A one-size-fits-all removal policy might backfire. Sometimes, letting a partially infected but still-functional tree stand could provide vital ecosystem services in the short term.

Street trees often fade into the background of daily life, but they’re silent partners in making cities livable. This study reminds us that those partners are vulnerable. A microscopic invader in the soil can ripple up to affect city temperatures, public health, and even climate resilience. Because in the end, protecting urban trees isn’t just about trees, it’s about protecting ourselves.

If you want to learn more, read the original article titled "Impact of Phytophthora disease on the growth, physiology and ecosystem services of Common Lime (Tilia x europaea) street trees" on Plant-Environment Interactions at http://dx.doi.org/10.1002/pei3.70054.