Why trees also promote biodiversity in the city

• The trees and woods present in the city are home to a great diversity of organisms representative of a dynamic biodiversity, according to our partner The Conversation.

• Maintaining the presence of trees in our cities can therefore “buffer” the effects of urbanization on biodiversity.

• This analysis was conducted by Bastien Castagneyrol and Elena Valdés-Correcher, researchers in ecology.

Trees and cities are not antagonistic;

at least, they are no longer quite so.

In recent years, many cities have in fact set up ambitious tree planting programs, demonstrating a real enthusiasm for the subject.

However, there is a paradox, since the extension of cities continues to take place to the detriment of natural spaces, leading to the fragmentation of habitats and the disappearance of wooded areas, which are replaced by impermeable surfaces that store and radiate heat.

The tree in the city is often presented as a "nature-based solution" to cool cities.

But trees also play a more discreet role, and yet just as fundamental: that of sustainably supporting biodiversity.

For a long time, the latter was only considered from the angle of the number of species present in a given environment.

But there is another dimension whose importance is increasingly recognised: that of the diversity of interactions between species.

They are indeed at the origin of the evolution of living organisms and their adaptation to environmental constraints.

And it is also on them that “nature-based solutions” could be created.

However, there is still a long way to go in this direction.

An urban and dynamic biodiversity

In cities, trees and woods are home to a wide variety of organisms.

The city reader will perhaps think of the aphids whose honeydew drips onto the roofs of cars and bicycles parked under the lime trees, of the processionary caterpillars which are so worrying at the start of spring, or of the starlings which take shelter at night in the foliage. (so many examples exist!).

All of these organisms are in constant interaction with the tree and with each other.

Raise your head under an oak tree, you will see traces of the passage of herbivorous insects: caterpillars have eaten the edge of the leaves, aphids and bedbugs have drilled small holes there to suck the sap.

But if there are leaves left to observe, it is also because predators, notably birds, have attacked these insects, thus ensuring the protection of the tree.

Trees, herbivorous insects and insectivorous birds represent a dimension of urban biodiversity, and what is more, of a dynamic biodiversity.

​Interactions still poorly known

Numerous research studies have also shown that urbanization homogenizes biodiversity: biodiversity is more similar between two towns in the same region than between a town and its neighboring countryside.

With consequences on the interactions between organisms, and by extension on the dynamics of biodiversity and the services provided to city dwellers, which are still poorly understood.

It must be said that the town planner does not facilitate the task of the ecologist!

Just as the city is not a biological desert, it is not a homogeneous territory either.

The amount of trees can vary considerably from one area to another, which can have a very significant effect on the biodiversity it supports.

Researchers at North Carolina State University, for example, have shown that less treed areas provide fewer resources for herbivores.

This decreases their abundance and therefore the damage caused to trees.

On the contrary, researchers from the University of Queensland in Australia have shown that the preservation of adult trees in the streets makes it possible to maintain the diversity of birds.

​The more trees there are, the more insects attack them

We studied the heterogeneity of biodiversity linked to urban trees within the framework of a participatory science project carried out at the European level.

52 schools and 41 scientists sampled oak leaves in 17 countries.

Some were in town, others in the countryside.

By examining the 18,060 leaves sent by all the project partners, we were able to study the impact of urbanization on insects associated with the pedunculate oak (

) in most of its range in Europe.

We found contrasting effects of impermeable surfaces (typical of urbanized areas) and tree density on herbivorous insects linked to oaks: oaks surrounded by many trees were on average more attacked (suggesting greater abundance and diversity herbivores), while oaks growing in a highly urbanized environment were less attacked.

These results were expected.

What was less so was that the density of trees could modulate the effect of urbanization.

In particular, a higher density of trees reinforced its negative impact on insects responsible for the formation of galls on oak leaves, and on the contrary attenuated the effect of urbanization on insects digging galleries in the leaves.

Our study confirms the importance of maintaining the presence of trees in our cities, obviously for our own immediate benefit, but also because their density can buffer the effects of urbanization on biodiversity.

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At the very least, given our still limited understanding of the complex webs of interactions between trees and overall urban biodiversity, one should be vigilant about the cascading consequences of any action taken on trees in the city. .

We will continue the work next year with the participation of new schools in the project.

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This analysis was written by Bastien Castagneyrol and Elena Valdés-Correcher, ecology researchers at the National Research Institute for Agriculture, Food and the Environment (INRAE).

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