Study review: Growth of street trees in structural cells and structural soil

If you live in a city, you may have noticed that trees planted along the streets don't always seem to thrive as well as those in parks or forests. The combination of compacted soils, limited space, and exposure to urban stresses such as pollution, heat, and traffic can make it challenging for street trees to grow and survive. However, innovations in urban soil technology, such as structural cells and structural soil, are changing the game for urban forestry. These solutions provide a way to create a healthy growing environment for street trees, allowing them to flourish and provide the many benefits that trees offer to urban communities. In this blog post, we'll explore the study "Growth of street trees in urban ecosystems: structural cells and structural soil" and its findings on the benefits of these technologies for street tree growth.

The original study can be found here.

The study investigates the growth of street trees in urban environments, specifically looking at the role of two factors: structural cells and structural soil. Structural cells are a type of modular system that can be used to support pavement and soil around tree roots, while structural soil is a special type of soil that allows for better drainage and aeration.

The researchers studied the growth of trees in three different types of environments. Using structural cells, using structural soil, and one using an open space setting:

1. Open space

The study used an open landscape area without physical rooting confinement to evaluate the advantages of using designed soil and cell systems for tree growth. Two species of trees were planted in soil with a specific proportion of sand, silt, and clay to a depth of 1.5m. The trees were equally spaced at 3.5m across a 100m experimental plot, which was located adjacent to the areas where structural soil and cells were used. 

2. Structural soil

The structural soil used in the study consisted of 80% granular crushed stone (2-4 cm in diameter) and 20% soil, similar to the CU-soil described in Grabosky et al. (1998). The pH was 5.5, and the organic matter averaged at 5%. The materials were compacted to 95%. 

3. Structural cells

The structural cells used in the study were made from recycled polypropylene and measured 1.2m (L) by 0.6m (W) by 0.4m (D) of fixed shape and dimensions. Two units were stacked on top of each other to achieve a final depth of 0.8m. The cells were installed according to the manufacturer's specifications, and each cell was filled with the same loamy soil made up of sand, silt, and clay.

Key findings:

The study found that trees in the structural cells had significantly better growth than those in the other two environments. They also found that the structural soil had a similar growth to that of the open space setting. The growth was measured by comparing the diameter at breast height (DBH) and the tree’s height. 

Structural cells;

• Provide improved aeration of soil.

• Create more space for tree roots to grow.

• Allows for better drainage and root health.

• Reduces soil compaction.

• Improves tree longevity.

Structural Soil;

• Is a viable option for trees in urban locations which would otherwise be planted with high compaction.

• Allows trees to perform similar to that in a large open lawn setting, with improved water permeability and increased room for root growth.

• Is a cost effective solution which allows for good tree growth in an urban environment.

Open Space;

• Trees without any soil improvement through structural soil or cells are still successful if given adequate soil area and permeability.

• Issues with compaction and poor root growth can still occur, likely due to reduced excavation and soil cultivation.

• When translated to a highly urban setting with reduced soil area or permeability, tree health is reduced.

You may also find my previous blog about the pros and cons of different urban tree planting methods useful. This article covers the various benefits and issues faced by developers when looking to improve tree health. It also includes a breakdown of common structural soil profiles. Click here to read more.