Not the 'native' label, but ecological compatibility basis of new Utrecht planting policy

ARTICLE

PLANT SELECTION

Hein van Iersel, Monday 17 March 2025

197 sec

Leiden List a starting point for Utrecht municipality

We need trees in the city. But which trees? In the Dutch city Utrecht, a new policy should provide guidance for citizens, managers, designers, and contractors. Trade magazine Boomzorg speaks with tree and greenery manager Jeroen van Gaalen and green advisor Geert-Willem Arentsen.

Geert Willem Arentsen (left) and Jeroen van Gaalen

You could say that the motto for planting trees in the city over the last few decades has been: the right tree in the right place. As of 2025, this no longer seems sufficient. It's not just about the tree itself, but about the tree in relation to various ecosystem services it can provide. Naturally, trees must feel at home in the city squares where they are placed, but this alone is not enough considering the disappearing biodiversity and the changing climate.

Leiden List

Therefore, since February 1, the municipality of Utrecht has been implementing a new planting and replanting policy, where ecological compatibility is central. The goal is to ensure that biodiversity in the city remains stable, as far as possible, of course. Policy officer Arentsen: 'The easiest solution would obviously be to say: we only plant native species. But that would overlook the fact that conditions in the city have changed significantly over the past decades. The climate we have in the Netherlands today strongly resembles that of central France fifty years ago. And in our cities, this effect is even more pronounced. Cities are drier and hotter than the surrounding countryside. Species like beech, but also the common rowan, which are indeed native, are actually no longer suitable for urban settings. Therefore, native is not the starting point, but ecological compatibility. This concept was borrowed by the municipality of Utrecht from the so-called 'Leiden List' (see box). This system assumes that native species have the highest biodiversity and best align with the local food web. However, because you cannot plant these species under all circumstances, you may switch to species that originate from nearby geographical areas. In our case, this could include species from the Mediterranean region.'

'From these species, you can expect that they are interesting for the insects that naturally occur here'

Policy

Greenery manager Van Gaalen: 'What we as a municipality need is a clear policy that you can explain to managers, but also to citizens and other stakeholders. Our principle is: native is wonderful, but given the above not realistic. Therefore, we also welcome an assortment that originates from France or Northern Italy. That's what we call ecological compatibility. From these species, you can expect that they are interesting for the insects that naturally occur here. Naturally, insects will also live on a tree with an American or Chinese origin that is placed in a Utrecht park. But we are especially looking for special insects. Insects that fly or crawl and that are attracted to every tree are usually not the most special species. We want to attract specific, unique insects.' And according to Van Gaalen, this means you have a broad, but carefully considered choice. To give an example: there are about 1,600 species on the list, of which more than 900 can be planted. We are talking about species with a rating of 4 points (representing the highest level of biodiversity) to minus 0.5 points.'

'The new policy is not an absolute law. If a walnut tree falls in a park due to a storm, it can perfectly be replaced by another walnut'

Zones

Arentsen: 'What we have done is divide Utrecht into two zones. First, the urban zones. Here, you can draw from the full Leiden List, so all the assortment with a rating from 4 to minus 0.5 points. The second zone includes the green areas. There, we primarily want trees with a high biodiversity value, so species with a rating of 3 or 4 on the list.'

Policy no absolute law

Van Gaalen: 'The new policy is that it is not an absolute law. If a walnut tree falls in a park due to a storm, it can perfectly be replaced by another walnut, because you can then demonstrate that it represents a certain cultural-historical value. This is only possible with solitary trees. If in an avenue with only plane trees one plane tree fails, another species must really be planted in between, because the plane tree scores moderately on the Leiden List.'

Other pillars

The new policy of Utrecht includes two other important pillars. The second pillar concerns dealing with the difference between large and small trees. A tree of the first size category must be replaced by another tree of the same size. It is not allowed to replace an oak with a cherry, for example. If it is not possible to replant a tree of the first size, it can be replaced by two trees of the second size category.

The third pillar of the replanting policy is that the municipality of Utrecht consciously wants to counteract the constant planting of large-size trees. Van Gaalen: 'We have the idea that if you plant a bit smaller - we are thinking of size 16-18 - such a tree will take root faster and after a few years will have overtaken its much larger neighbor.'

Leiden List

The basis for the new Utrecht policy is the so-called 'Leiden List'. This is a list compiled by ecologists Roelant Jonker and Wouter Moerland from the municipality of Leiden. It has subsequently been tested by various experts from other municipalities and institutions. The Leiden Register for ecological trees uses a rating system that ranges from -1 to 4. Where -1 stands for invasive exotics that should not be planted and the figure 4 for species with very high ecological value. These are, for example, species that serve both as host plants and provide shelter. The register includes 1600 species and focuses on promoting native species that contribute to local biodiversity and climate adaptation.

This article was previously published on 17 March 2025 on the website of Boomzorg.

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