Urban trees in Europe are under threat

Trees are a familiar sight in every European city. They line streets, provide shade in parks, and clean the air, offering cooling relief during hot summers. For the more than 70% of Europe’s population who live in urban areas, these trees are much more than just ornamental. They are an essential part of the infrastructure.

However, these silent guardians are in danger.

Climate change, urbanisation and global trade have created ideal conditions for invasive pathogens and insects to spread. European trees often have no natural defences against these alien species, and the consequences can be devastating. The damage affects ecosystems, city budgets and the cultural value of heritage trees and green spaces.
European response rooted in collaboration

To respond to this growing threat, researchers and stakeholders from across Europe joined forces under the COST Action Urban Tree Guard – Safeguarding European urban trees and forests through improved biosecurity (UB3Guard). With the support of the COST Programme, the Action created a collaborative network of scientists, city planners, arborists, and local activists across 31 countries, all working towards the same goal of strengthening the biosecurity of Europe’s urban forests.

Watch the introductory video of UB3Guard

Mapping Europe’s urban forests and nurturing the next generation of scientists

One of UB3Guard’s most remarkable achievements is the creation of the European Urban Tree Inventory (EUTI), a database containing records of over 9.5 million trees in 200 municipalities across 31 countries. The EUTI is the most comprehensive dataset of its kind in Europe, offering a powerful new tool for assessing and managing risks to urban forests.

What started as a scientific network quickly grew into a Teaching Tool Platform for practical solutions, shared learning and career development, particularly for young researchers.

“They performed impressively,” says Prof. Johanna Witzell, the Action Chair from Linnéuniversitetet in Sweden. “They took on active roles as working group or task leaders, using the network’s connections to gather data directly from municipalities.“

“We were able to build trust, mutual understanding, and lasting partnerships across sectors.” Prof. Johanna Witzell, Chair of UB3Guard

Benno Augustinus, a UB3Guard member from Switzerland, recalls the motivation behind the creation of the inventory: “Being an active Action member grew my network and opened doors outside of UB3Guard.” Dr Benno Augustinus, Action working group member

“When we started, we noticed that many urban tree inventories used and maintained by municipalities were not easily accessible to researchers. We saw an opportunity to create something unique by collecting inventories of trees from all over Europe. It was a huge task and a valuable learning experience, but it paid off with the creation of an excellent product.”

Working on the EUTI turned out to be a career-changing experience for Benno:

“To present our work on urban trees, I was invited to give a keynote at the Forest Invasive Species Network for Europe and Central Asia (REUFIS) conference in Skopje, North Macedonia. Being an active member, therefore, grew my network and opened doors outside of UB3Guard.”

Yasin Korkmaz, the young Action Social Media Coordinator from Germany, confides:

“I joined UB3Guard shortly before starting my PhD, and it has become one of the most meaningful milestones of my career. COST Action has given me the opportunity to co-author articles and book chapters, promote events and grants, and, most recently, produce a documentary film to introduce the Action. Being able to speak up, feeling welcomed and supported, and truly feeling like a valued member has given me more confidence.”

“UB3Guard has become one of the most meaningful milestones of my career.” Yasin Korkmaz, Social Media Coordinator in UB3Guard Action
The networking tools provided by the Action allowed young researchers to share ideas with many leading experts. For instance, the ‘Horizon Scanning’ meetings were designed to discuss the current state of urban tree biosecurity, as well as analyse emerging and future tools and strategies for protecting trees in urban areas more effectively. The makeathons (joint projects in which each participant contributes to reach a common outcome) began with the creation of an interactive map of pests and pathogens in Europe. These were organised alongside more familiar tools, such as training schools and workshops.

From workshops to real-world impact

The project also enabled young researchers to apply what they had learnt directly in their home countries.

For Jovana Blagojević, a young researcher from Serbia, participating in UB3Guard was a real turning point in her career.

“I see how much we can achieve when we work across disciplines and across borders. UB3Guard has changed the way I think about research and its role in society.” Dr Jovana Blagojević, Action working group member

“I couldn’t have found this kind of intense, hands-on learning experience anywhere else, combining theory and practice across biology, phytopathology, the social sciences and governance. Through meetings and training schools and by contributing to several papers, I developed skills in science communication, stakeholder engagement, interdisciplinary teamwork and innovative thinking in urban forest biosecurity.”

However, the true test came when she returned to Serbia, where the management of urban trees is often fragmented across different agencies and landowners.

“UB3Guard shaped my professional path and continues to influence my work every day, giving me the tools, confidence, and network to contribute to innovative, science-based solutions in urban tree biosecurity across Europe.”

The Action also actively involved government representatives, urban planners, and arborists in its events and training schools. This facilitated the immediate transfer of practical knowledge, providing direct benefits to local communities.

“These interactions helped us develop a shared language and identify effective solutions to common challenges,” explains the Action Chair. “We were also able to build trust, mutual understanding, and lasting partnerships across sectors.”

Although the UB3Guard project formally ended in October 2025, the network it created continues to grow. The team has already secured a COST Innovators Grant to launch a new project called One-Stop Shop for Resilient, Future-proofed Urban Treescapes (FREE3).

As Prof. Witzell explains: “The network is simply too valuable to dissolve, and there is still much to be done. FREE3 is a great opportunity for us to continue our collaboration and develop a decision-support platform aimed at helping small and medium-sized cities maintain healthy, green treescapes.”

The impact of UB3Guard will be visible for years to come. It will be evident in academic publications and on city streets lined with healthier trees. It will also be seen in municipal officials using better decision-making tools and in a new generation of empowered researchers.

As Jovana Blagojević puts it “This Action gave me the opportunity to meet amazing people from different backgrounds. It also gave me a sense of responsibility. I now see how much we can achieve when we work across disciplines and across borders. UB3Guard has changed the way I think about research and its role in society.”

Thanks to the COST Programme, Europe’s urban trees are now supported by a strong network of researchers who are better connected, better trained, and better prepared to face future challenges.

UB3 Action website

FREE3 website

Network website

UB3Guard flyers