Forest Groups. Rethinking how trees grow in cities

Francesco Procacci
Mar 13
6 min read

The recent post about forest groups in cities generated an unexpectedly intense discussion.

Many readers immediately recognized the ecological intuition behind the idea: trees rarely grow alone in natural environments. They grow in groups where soil, shade, water and biological interactions create a shared living system.

Others, however, reacted with skepticism.

Some questioned whether clustered vegetation could reduce visibility and affect safety in public spaces. Others raised practical concerns about infrastructure conflicts, maintenance complexity and the technical feasibility of such systems in dense urban environments.

These reactions are not surprising.

For more than a century, urban design has developed a very specific image of what trees should look like in cities. We expect them to appear as orderly rows aligned along streets and boulevards, evenly spaced and visually predictable.

This image has become so familiar that it often feels natural.

Yet it reflects a particular design logic—one that prioritizes visual order over ecological relationships.

The discussion about forest groups therefore reveals a deeper question: how should trees actually function within the structure of the city?

The Tradition of the Urban Tree Line

Tree-lined streets have long been one of the most recognizable elements of urban design.

From the boulevards of Paris to contemporary masterplans, aligned rows of trees structure the rhythm of the street. They frame perspectives, provide shade and reinforce the geometry of the urban grid.

In drawings and renderings, this arrangement appears both elegant and rational.

But this visual order also reflects the way trees have historically been integrated into urban form.

The street is designed first: lanes, sidewalks, parking strips and underground infrastructure define the section. Once this structure is established, trees are inserted into the remaining fragments of soil.

Each tree occupies a small, isolated space.

Its position follows the geometry of the street rather than the ecological relationships that allow vegetation to develop as a system.

Trees Do Not Grow Alone

In natural environments, trees rarely grow as isolated individuals.

They develop in communities where root systems interact, moisture is shared and layered vegetation creates complex microclimates.

A forest is not simply a collection of trees.

It is a network of relationships.

Within this network, soil organisms, roots, shade and water cycles interact continuously. These interactions allow vegetation to regulate temperature, support biodiversity and maintain resilient ecological conditions.

When trees are planted as isolated elements along streets, most of these relationships disappear.

Each tree must survive within a limited soil volume, often surrounded by compacted ground, asphalt surfaces and underground infrastructure.

Under such conditions, growth is constrained.

The canopy remains smaller than it could be.

Root systems struggle to expand.

The ecological capacity of the tree remains limited.

The city may appear greener, yet its vegetation often performs far below its potential.

From Lines to Groups

The idea of forest groups emerges from this ecological observation.

Instead of distributing trees as isolated individuals along a line, urban design can organize vegetation in clusters where multiple trees share soil volumes and interact within a common ecological space.

Different species can coexist within the same planting area.

Roots can expand within continuous soil systems.

Shade can regulate temperature and moisture conditions.

Within these groups, vegetation begins to function less like decoration and more like a small ecosystem embedded within the urban fabric.

Such arrangements do not attempt to replicate natural forests literally.

Cities remain dense environments shaped by infrastructure, mobility systems and public space requirements.

Yet even within these constraints, the spatial organization of vegetation can move closer to ecological principles.

What a Forest Group Actually Means

The concept of a forest group is sometimes misunderstood as simply planting many trees close together.

In reality, it is less about density and more about how vegetation is organized as a system.

A forest group is typically composed of several trees that share a continuous soil volume rather than isolated planting pits. Instead of being separated by pavement every few meters, the trees grow within a common ecological ground where roots, water and microorganisms can interact.

Within this shared soil structure, different species can coexist. Some trees develop a large canopy, others remain smaller and occupy intermediate layers. Shrubs, grasses and ground vegetation may also be integrated, creating a more complex vertical structure.

The spatial arrangement does not need to follow a rigid alignment. Trees can be positioned in small clusters or loose compositions that allow both ecological interaction and clear circulation space for pedestrians and urban life.

In practice, this often means replacing many small planting pits with fewer but larger soil areas capable of supporting multiple trees.

These areas function as ecological pockets within streets, plazas or public spaces, where vegetation operates collectively rather than as isolated individuals.

Such groups do not transform the city into a forest.

They simply allow trees to behave more like the living systems they are.

A Possible Example in a European Street

To imagine how a forest group might function in practice, consider a typical street in a temperate European city.

The climate is similar to that of cities such as Milan, Lyon or Vienna: warm summers, cool winters and moderate rainfall distributed throughout the year.

In a conventional design, the street might be planted with evenly spaced trees—often a single species such as plane trees or lindens—each placed within its own small planting pit between pavement and asphalt.

A forest group approach would reorganize this structure.

Instead of ten isolated trees along the street, the design might create three or four larger planting areas where several trees share a continuous soil volume.

Within each group, different species could interact while remaining compatible with the urban environment.

A possible composition might include:

• London Plane Tree (Platanus × acerifolia) forming the upper canopy

• Small-leaved Lime (Tilia cordata) providing intermediate shade

• Field Maple (Acer campestre) adding seasonal variation and ecological diversity

These trees would not be planted randomly. They would be arranged within a shared soil area large enough to allow roots to expand and interact.

Beneath them, permeable surfaces and understory vegetation could support rainwater infiltration and soil life. Low shrubs or perennial plants might occupy the edges of the group, creating a gradual transition between the ecological pocket and the surrounding street.

Pedestrian circulation would remain clear. Visibility could be preserved through careful spacing and pruning strategies.

The street would still function as a public space for movement and everyday urban life.

Yet the vegetation would begin to operate differently.

Instead of isolated trees struggling within narrow pits, the planting would form a small urban ecosystem capable of supporting larger canopies, richer biodiversity and more stable microclimates.

The difference may appear subtle in drawings.

But over time, it changes how vegetation actually lives within the city.

The Question of Safety and Visibility

One of the most common concerns raised in the discussion around forest groups relates to visibility and safety.

Open sightlines have long been considered an important component of public space design. Dense vegetation can sometimes create spaces that feel less controlled or less visible.

These concerns are legitimate.

But they do not necessarily contradict the ecological logic of clustered vegetation.

Urban forest groups do not imply uncontrolled density. Their spatial configuration can be carefully designed to maintain clear circulation paths, visual permeability and accessible public space.

In many successful urban parks and landscapes, vegetation already operates through layered compositions that balance ecological complexity with spatial clarity.

The challenge is not whether such environments are possible.

It is whether urban design chooses to pursue them.

Designing Living Systems

The discussion around forest groups ultimately reveals a broader shift in urban thinking.

For a long time, vegetation has been treated primarily as a compositional element within the urban landscape. Trees structure avenues, soften plazas and provide shade along sidewalks.

But when trees are understood as living systems, their role within the city begins to change.

They require soil continuity, water infiltration and spatial conditions that allow ecological processes to emerge.

Designing for these systems inevitably affects the structure of the urban section itself.

Mobility priorities, surface materials and underground infrastructure all become part of the ecological equation.

Urban nature, in other words, cannot simply be inserted into the city after the fact.

It must be supported by the way the city is designed from the beginning.

Conclusion

The debate around forest groups is therefore not merely about planting techniques.

It reflects a deeper question about how cities imagine nature within their own structure.

Rows of trees will likely remain an important element of many streets. They provide shade, order and a familiar urban identity.

But alongside these traditional forms, new ecological approaches are beginning to emerge.

Clusters of vegetation, shared soil systems and more complex planting strategies allow trees to function as interconnected systems rather than isolated objects.

When this happens, urban nature stops being a decorative layer applied to the city.

It becomes part of the living infrastructure that shapes the urban environment itself.