Urban Landscapes

When Björn Embrén took charge of managing the trees planted in the city of Stockholm, a 2001 survey found that two thirds of them were dead or dying.¹ The situation will be similar in any city where tree roots are situated under roads, sidewalks and parking lots, often in compacted construction waste and soil. Trees need water and mineral nutrients to thrive, as well as a free exchange of gases in their root systems, particularly so that carbon dioxide does not accumulate, but also so that tree root systems and the microbiota that support them have access to oxygen.

City infrastructure construction projects often put a signficant strain on existing trees. Roots are often exposed and cut by excavators to make way for pipes, cables, sewer and stormwater drainage systems. Often mixed gravel suited for road substructures is used to backfill the excavation, and it is compacted with machines to create a stable surface for the parking lot, street or sidewalk that it must support, as per contractual requirements designed . Then an impermeable, hard surface of concrete or asphalt is applied as a top layer, and the storm water is diverted to a stormwater drainage system that channels the water to an expensive central processing facility to prevent the flooding that would otherwise easily occur.

Dying trees — A group of dying trees at Tegelbacken junction in central Stockholm, in the summer of 2010

Tree roots will often penetrate into stormwater drainage pipes to find water and air, and block them. The flooding that ensues forces the city to dig up the street or sidewalk or parking lot, and then the drainage pipe, to find the blockage or blockages. More tree roots are cut, the drainage pipe and street drainage grates are replaced, the excavation backfilled and compacted. And then it all gets covered again with asphalt or concrete. This, of course, is costly, ongoing maintenence that a city would rather avoid.

Tree roots in drain pipe — City tree roots commonly infiltrate stormwater drainage pipes in search of water and air.

Knowing these details, it is easy to understand why the trees in any city do not survive well. It is also easy to understand why some city managers might think that the best solution is to cut them all down.

But as you might suspect, research² has shown that urban trees and landscapes provide a wide variety of socioeconomic benefits to the people living and working in cities. Neighborhoods with trees are better off economically. Businesses thrive, residents earn more money. People are both mentally and physically healthier when trees are nearby. There is less crime in regions with trees. There is more social cohesion and interaction - people are more friendly and cooperative in the presence of trees than they are in an urban desert. ³ ⁴

The City of Stockholm has embarked on an innovative project, developed and led by Björn Embrén, that has not only saved the trees of Stockholm, but has caused them to thrive in a remarkable way. The approach he developed not only ensures the well being of trees and other city landscapes, including grassy areas, but also provides a distributed system for the drainage and filtration of stormwater. The tree and landscape planting pits are an investment that makes sense not only for the social benefits, but also because the city saves money in stormwater drainage system maintenence and overall capacity provisioning and maintenance, tree replacement and maintenance, and grassy area maintenance such as a park.

The key innovation that Björn developed is a so-called "structural soil" that is comprised of layers of similar sized stones, called "macadam", that are compacted to provide a very stable foundation for the road, sidewalk or parking lot that will be installed above a tree planting pit. A mixture of biochar and compost is flushed down between the large stones (~15 cm / 6" diameter) of the bottom layer (60 cm / 2 feet deep), which tree roots can easily infiltrated because of the large gaps between the stones. The gaps in this layer are filled with a mixture of compost and biochar, or a nutrient enriched biochar.

Above this foundational layer is an aerated bearing layer comprised of medium sized stones (~5 cm / 2" diameter, 15 cm / 6" thick). The gaps between the stones are preserved, and this layer is connected to the street, sidewalk or surface drain to allow water into the planting pit and gas exchange with the root zone, so carbon dioxide can escape and oxygen can enter. Sand and silt is prevented from entering the aerated bearing layer with a layer of geotextile above it, so the gaps between the stones are preserved.

Stockholm Solution tree planting pit — Diagram of a structural soil tree planting bed as implemented in Stockholm.
Source: Planting beds in Stockholm city  - a handbook 2017 ¹

Tree plant bed with structural soil

Pavement with superstructure
Rainwater gutter
Aeration well: infiltration of stormwater and gas exchange of oxygen and carbon dioxide
Surface grate
Cover material, macadam 4/8 mm
Root collar at the same level as in nursery
Macadam 2/6 mm + 25% by volume mixture of nutritionally enriched biochar and compost (50/50)
Tree pit foundations in concrete
Geotextile
Leveling stones, macadam 8 / 11mm
Aerated bearing layer, macadam 32 / 63mm
Stones 90/150 mm with nutrient-enriched biochar flushed down into the cavities
Biochar layer
Gas exchange (carbon dioxide, oxygen)

In addition to planting pits for trees, biochar macadam is used in Stockholm as a soil substrate for perennial plant beds and grass. Grassy areas in cities are often damaged by either human traffic, particularly the short cut paths that get worn to mud largely because of soil compaction, or utility and construction vehicles that drive over them or use them as parking areas, often leaving muddy ruts behind that have to be continually repaired and reseeded. Grass planted in a biochar-compost macadam is highly resilient to such damage, leading to much lower maintenence costs, and is also remarkably lush and green.

Stockholm grass — This grass was seeded in a mixture of small pebbles, biochar and compost. It is very resilient to compaction damage that is often caused by construction and utility vehicles or people taking shortcuts across it.

Björn Embrén has given a number of informative presentations on his techniques. Videos of two of these talks entitled "Porosity With Stone Biochar and Compost" ⁵ and "The Stockolm Solution - an integrated approach to trees and infrastructure in the urban realm" ⁶ are linked below. There are also slide decks available from presentations titled "More Porosity with Stone, Biochar and Compost" ⁷, "The Stockholm Solution - Combined Urban Tree and Storm Water Management" ⁸ and "Trees and Stormwater Management: The Stockholm Solution" ⁹.

As referenced above, the City of Stockholm has also developed an instruction manual called "Planting beds in Stockholm city - a handbook 2017" ¹ which we have translated into English. There are also drawings referenced in this document, the first to be translated is titled "Trees in Paved Areas with Structural Soil" ¹⁰. The rest of the drawings will be translated soon and be available on this webpage.

Urban Landscapes

Tree plant bed with structural soil

References