How Stockholm Is Sprouting Healthy Trees From Concrete

When Stockholm’s Traffic Office conducted a general assessment of street traffic in the Swedish capital in 2001, it came to the shocking conclusion that two-thirds of all trees in the city center were dead or dying.

City authorities agreed that an urgent response was needed to nurse these leafy urban ecosystem pillars back to health.

Enter Björn Embrén, Stockholm’s first “tree officer.”

Under his leadership, various technologies and materials were tested in an attempt to create a more suitable living space for trees in the urban environment.

“I knew that they could grow if the circumstances were right,” says Embrén, a former professional gardener.

The root of the problem was the fact that many trees in Stockholm were planted in the 19th century. Back then, the streets were comparatively underdeveloped, so the trees could grow relatively freely in the space underground. But as Stockholm has developed over the decades, like many cities across the world, its streets have been increasingly paved over with concrete, and more pipes — for plumbing, electricity, internet and beyond — have been laid underneath them. As a consequence, the roots of the trees have often been damaged through disruptive building works and, due to the lack of space and impermeable concrete, they have also been starved of oxygen and water.

A 2001 assessment found that a shocking two-thirds of all trees in the city center were dead or dying. Credit: Per-Boge / Shutterstock “It’s like putting a plastic bag over your head and tightening it over your neck,” says Embrén. “That’s what happened to the trees in Stockholm. They were dying.”

Eventually, inspired by railroad embankments — which require only a small amount of organic matter for healthy trees to grow — Embrén concocted what was to become known as the “Stockholm tree pit” model.

By 2002, Embrén had drawn up designs. And by 2003, they were already building.

The design involves digging a pit and constructing a frame underground around the tree’s roots, and then filling said pit with a mixture of soil and stone, sometimes including biochar, to both aerate and fertilize the soil. These permeable layers are very strong and physically adaptable but also allow stormwater to flow in, meaning the trees are provided with sufficient air and water naturally. They also allow rainwater to be soaked up — a necessity amid more extreme weather brought on by climate change.

“We found that the more breathable the materials we used, the happier the trees were,” adds Embrén.

Proponents say the method has a number of benefits, including the fact that pits can be installed around existing trees, they can bear the weight of heavy-vehicle traffic, they require little topsoil — a resource that is becoming scarce — and they need less watering than traditionally-planted trees.

This approach, which allows tree roots to thrive beneath hard surfacing, ergo allowing healthy trees to grow within the modern built environment, is particularly relevant as cities attempt to re-green and reforest in the face of climate change.

OP and further reading https://reasonstobecheerful.world/stockholm-tree-pit-method/