Emerging invasion risks of non-native urban trees in continental Europe under a changing climate

This study utilizes the TPS-ISK screening tool to assess 34 non-native urban tree species in continental Europe, revealing that while nearly 30% currently pose a high invasion risk, climate change projections significantly elevate these threats, necessitating urgent proactive management and monitoring strategies.

The Gist

Imagine your city's parks and gardens as a grand, international buffet. For decades, we've been inviting exotic trees from all over the world to join the feast because they look beautiful, grow fast, or provide shade. Most of these guests behave themselves, staying politely within the flowerbeds where we planted them.

But a few of these guests have a secret: they are "escape artists." If they slip out of the garden and into the wild forests or riversides, they can take over, crowding out the local native trees and changing the ecosystem forever. These are the invasive species.

This paper is like a security check for 34 of these popular "guest trees" that are currently planted in cities across continental Europe. The researchers wanted to answer two big questions:

1. Right now: Which of these trees are likely to escape and cause trouble?
2. In the future: How will a warming climate change the rules of the game?

Here is the breakdown of their findings, using some everyday analogies:

1. The "Background Check" (The Screening Tool)

The researchers used a digital checklist called TPS-ISK. Think of this like a security scanner at an airport.

• Instead of checking for weapons, it checks for "invasion traits": Can the tree make a million seeds? Does it grow fast? Can it survive in dry soil or poor conditions?
• They gave each tree a "risk score."
  • Low Risk: A polite guest who will likely stay in the garden.
  • Medium Risk: A guest who might wander a bit but probably won't cause a riot.
  • High Risk: A guest with a history of causing trouble who is likely to run wild.

2. The Current Situation: "The Quiet Before the Storm"

Under today's climate, they found:

• 10 trees were flagged as High Risk. These are the troublemakers we need to watch closely right now.
• 23 trees were Medium Risk. They aren't causing chaos yet, but they have the potential to.
• 1 tree was Low Risk. It's safe.

The "False Alarms" and "Missed Detections":

• The scanner was pretty good. It correctly identified famous troublemakers like the Tree of Heaven (Ailanthus) and the Northern Red Oak (Quercus rubra) as high-risk.
• However, it missed one known troublemaker (the Japanese Cherry), labeling it "Low Risk." This is a "false negative"—like a security scanner missing a small knife.
• It also flagged a couple of trees as "High Risk" that haven't caused much trouble yet (like the Amur Maple). This is a "false positive"—like a scanner beeping because you have a belt buckle, not a weapon. But in this case, it's better to be safe than sorry!

3. The Climate Change Twist: "Turning Up the Heat"

This is the most critical part of the study. The researchers asked: "What happens if the weather gets hotter and drier?"

Imagine the climate as a thermostat. Right now, the thermostat is set to "Cool," which keeps some of these exotic trees from growing too wild in the northern parts of Europe. But as we turn up the heat (climate change), the thermostat is set to "Tropical."

• The Result: The "High Risk" list grew.
• New Danger: 7 trees jumped from "Medium" or "High" risk to "Very High Risk."
• Why? These trees are like survivalists. They are tough, reproduce quickly, and can handle stress. When the climate gets warmer, the "cold barriers" that used to stop them from spreading north disappear. Suddenly, a tree that was stuck in a warm southern city can now thrive in a forest in Poland or Germany.

The "Super-Invaders":
The study highlighted a few "Super-Invaders" that are now at the top of the danger list:

• Tree of Heaven (Ailanthus altissima): The ultimate escape artist. It grows like a weed, chokes out other plants, and is nearly impossible to kill.
• Northern Red Oak (Quercus rubra): A fast-growing giant that is already taking over forests in some places.
• American Persimmon (Diospyros virginiana): Currently mostly in gardens, but the warming climate is opening the door for it to spread.

4. The "Urban Heat Island" Effect

The paper also mentions that cities are like heat islands. They are often 5–8°C warmer than the surrounding countryside.

• The Analogy: Think of cities as "training camps" for these invasive trees. The warm city environment teaches them how to survive heat. Once they get strong in the city, they are ready to march out into the wild forests, especially as the global climate warms up to match the city's temperature.

The Bottom Line: What Should We Do?

The authors are sounding an alarm bell, but not to panic. They are saying: "Don't wait until the invasion happens to start fighting it."

• Early Detection: We need to spot these trees escaping the garden before they take over the forest.
• Proactive Management: We need to be careful about which trees we plant in the future. Just because a tree looks pretty doesn't mean it's safe.
• Policy Change: Some of these trees are currently legal to sell and plant, but this study suggests they should be on a "watch list" or even banned in certain areas, similar to how we ban certain dangerous animals.

In short: Climate change is removing the "fences" that keep these exotic trees contained. If we don't manage them now, we might wake up in 20 years to find our local forests replaced by a few aggressive, non-native species that we planted for their beauty.

Technical Summary

1. Problem Statement

Urban green areas in Europe frequently contain numerous non-native tree species introduced through ornamental horticulture. While many of these species are currently managed within gardens, they possess functional traits (e.g., high reproductive output, stress tolerance) that predispose them to escape cultivation and become invasive.

• The Gap: There is a lack of comprehensive risk screening for non-native urban trees specifically within continental Europe.
• The Threat: Climate change is expected to expand the climatic niches of these species, allowing them to establish self-sustaining populations in regions previously too cold or climatically unsuitable. This creates an "invasion debt" where species currently confined to urban heat islands may spread to natural ecosystems as regional climates warm.
• Objective: To assess the invasion risk of 34 non-native urban tree species in continental Europe under both current and projected future climate scenarios to inform proactive management and policy.

2. Methodology

The study employed a standardized, evidence-based risk assessment framework using the Terrestrial Plant Species Invasiveness Screening Kit (TPS-ISK v2.4.1).

• Species Selection:

  • Sample Size: 34 taxa (33 species, 1 subspecies) of non-native urban trees.
  • Criteria: Species were selected based on (1) occurrence in urban park inventories (Zagreb, Croatia), (2) documented use in European urban horticulture (RHS Encyclopedia), and (3) verified non-native status in continental Europe (via Euro+Med Plantbase and GBIF).
  • A Priori Categorization: Species were pre-classified as "invasive" or "non-invasive" based on global databases (GISD, EASIN, GBIF) and literature to validate the screening tool's accuracy.

• Risk Assessment Protocol:

  • Tool: TPS-ISK, which consists of 55 questions divided into:
    • Basic Risk Assessment (BRA): 49 questions covering biogeography, invasion history, and biology/ecology.
    • Climate Change Assessment (CCA): 6 questions predicting how future climate conditions will alter the BRA scores regarding introduction, establishment, dispersal, and impact.
  • Execution: Eight expert assessors screened the species individually. Quality control was performed by two senior authors.
  • Scoring & Thresholds:
    • Scores were calculated for BRA and BRA+CCA.
    • Risk Categories: Low (<1), Medium (1–41), High (>41).
    • Calibration: Receiver Operating Characteristic (ROC) curve analysis was used to determine the optimal threshold score (AUC = 0.6947) to distinguish between medium and high risk. A threshold of 41 was established.
    • Very High Risk: An ad hoc threshold of ≥50 was applied to identify species of extreme concern.

• Statistical Analysis:

  • Permutational ANOVA was used to test differences in confidence factors between BRA and BRA+CCA.
  • False positive/negative rates were calculated against the a priori categorization.

3. Key Contributions

• First Continental-Scale Screening: This is the first study to provide a detailed, systematic risk screening of non-native urban trees specifically for continental Europe, addressing a critical knowledge gap in invasion biology.
• Integration of Climate Change: The study explicitly quantifies how climate change projections alter invasion risk, moving beyond static risk assessments to dynamic, future-oriented scenarios.
• Validation of TPS-ISK: The study demonstrates the applicability and robustness of the TPS-ISK tool for terrestrial woody plants in a European context, validating its use for policy support under EU Regulation No. 1143/2014.
• Identification of "Hidden" Risks: The research highlights species that are currently not listed on the EU Invasive Alien Species list but possess high future invasion potential due to climate shifts.

4. Key Results

Risk Classification (Current vs. Future):

• Current Conditions (BRA):
  • High Risk: 10 species (29.4%).
  • Medium Risk: 23 species (67.6%).
  • Low Risk: 1 species (2.9%).
• Future Conditions (BRA+CCA):
  • High Risk: Increased to 11 species.
  • Very High Risk (Score ≥ 50): 7 species identified (up from 2 under current conditions).
  • Impact of CCA: The inclusion of climate change increased risk scores for 44.1% of species, decreased scores for 17.6%, and left 38.2% unchanged.

Specific High-Risk Species:

• Very High Risk (BRA+CCA):
  • Diospyros virginiana (American persimmon)
  • Quercus rubra (Northern red oak)
  • Ailanthus altissima (Tree of heaven)
  • Acer tataricum ssp. ginnala (Amur maple)
  • Koelreuteria paniculata (Goldenrain tree)
  • Magnolia kobus (Kobus magnolia)
  • Phellodendron amurense (Amur cork tree)
  • Pseudotsuga menziesii (Douglas fir)
  • Robinia pseudoacacia (Black locust)
• Notable Findings:
  • Ailanthus altissima, Diospyros virginiana, and Quercus rubra were consistently ranked as the highest immediate risks.
  • Magnolia kobus shifted from Medium to Very High risk under future climate scenarios.
  • False Positives/Negatives: The model correctly identified 8 of 15 known invasive species as High Risk (True Positives). It flagged 2 known non-invasives as High Risk (False Positives: Acer tataricum ssp. ginnala, Phellodendron amurense) and missed 1 known invasive (Prunus serrulata, classified as Low Risk).

Drivers of Risk:
The high-risk species share traits such as ecological plasticity, high reproductive performance, broad environmental tolerance, and the ability to thrive in disturbed urban environments. Climate change is projected to reduce frost limitations, allowing these species to expand their ranges northward and to higher altitudes.

5. Significance and Implications

• Policy and Management: The study provides a scientific basis for updating the EU List of Invasive Alien Species of Union Concern. It argues for the inclusion of species like Quercus rubra and Acer tataricum ssp. ginnala, which are currently excluded despite high risk scores.
• Preventive Action: The results emphasize the need for early detection and continuous monitoring of species that are currently widely used in horticulture but are not yet invasive. Urban areas act as "pre-adaptation zones" where species acclimate to warmer conditions before spreading to natural habitats.
• Strategic Shift: The findings suggest that management strategies must shift from reactive control of established invaders to proactive prevention of high-risk non-natives, particularly those with high climate-change-driven expansion potential.
• Economic and Ecological Protection: By identifying these risks early, stakeholders can mitigate future economic losses and ecological disruptions caused by the displacement of native flora and alteration of ecosystem services.

In conclusion, this manuscript establishes that climate change will significantly exacerbate the invasion risk of non-native urban trees in Europe, necessitating immediate updates to regulatory frameworks and management practices to safeguard biodiversity.