Getting to the root of tree survival

SESL Australia

The urban landscape setting can be a hostile environment for tree growth. There are many soil issues that trees face during establishment and growth in heavily urbanised spaces. Therefore, when SESL investigates streetscapes for potential tree planting, we need to make sure that the oxygen levels, carbon dioxide levels, light, water, nutrients, space and temperature are within the desired range for optimum tree growth. If any of these factors are out of balance, the tree is highly likely to show signs of stress resulting in poor growth or worse, decline and death. 

This article aims to cover the importance of getting these plant requirements in balance, choosing the right tree for the environment and whether to choose structural support soils to support the soil. 

Streetscapes often resemble the harshest natural environments high-temperature variations and extreme evaporative stress combining with limited root volumes to hinder and stunt growth. Engineering requirements for road construction are a major limitation on root expansion due to the level of compaction of adjacent road base and often “accidental” compaction of the soils surrounding them. The road base is comprised of a range of fraction sizes (well-graded) that is compacted heavily leaving very little pore space as smaller fractions can fit within the larger pore spaces. The level of compaction required for constructing pavements is >1.7g/cc of which trees roots cannot penetrate. Sealing of the surface with concrete and/or bitumen further prevents effective soil conditions. While it is necessary to exclude roots for road construction, we often find that the soil in median strips and verges is also highly compacted due to vehicle traffic and the engineer’s propensity to compact everything! Such compaction leads to insufficient air entry and oxygenation and hindrance of root penetration. Air is 21% oxygen. For a root to survive, it requires 2.5% oxygen and 5% oxygen to grow. To absorb nutrients, it needs at least15% oxygen. Oxygen levels in compacted soils are often so low a streetscape tree root will typically not grow into the hostile soil surrounding roadworks, therefore, it is imperative that the allocated soil space is of a sufficient volume and quality for a tree to grow to its intended size. 

The smaller the rooting space available, the more limiting it is for a tree to meet its growth requirements as its roots will be limited to exploiting the small volume of soil suitable immediately adjacent to its rootball.  The tree will subsequently take on a squat bonsai appearance, or it may even perish. 

Rooting space is an essential requirement for tree growth. When space is lacking, a tree will resort to following pathways where there is sufficient oxygen, and the roots will change direction, stop growing or grow close to the surface, often into the space under concrete pathways or in the porous granular materials used to backfill services. The result can be heaved pavements and damaged services.  How many times do we see street trees planted in spaces as small as 1m3 and then we curse the tree for lifting the pavement? 

So how much root volume does a tree need? This is not an easy question to answer. Tree size, useful life expectancy and the harshness of the environment all come into such decision making, and it is not an exact science but ultimately, a judgemental one. 

A useful resource for assessing the desired soil volume is the soil volume simulator by Leake and Haege (2014) which accesses the key factors influencing suitable minimum soil rooting volume for new trees in limited spaces. Instead of looking at the tree at maturity it instead looks at design size which is a more accurate representation for urban trees. Other factors such as climate, soil, maintenance and whether the tree will be sharing space with other trees are also considered. 

The soil volume simulator can be found using the following web link: https://elkeh.com.au/

In this work we came to the following general conclusions-

  • Street trees mostly get much less available rooting volume than tree in natural soils
  • The kinds of trees that are most often chosen for urban environments, tested by experience, can tolerate such reduced volumes
  • They often respond to this by stunting growth
  • A stunted tree can still provide an acceptable design solution.

In our estimation method we consider all these factors to arrive at sensible volumes where a tree has a good chance of developing without severe stunting. The above web-site takes you through a step by step approach or “ready-reckoner” to give a suggested minimum volume. It also considers discounts for shared rooting volumes where two or more trees are planted in the same bed.

Another technique to help overcome some of the issues that occur with streetscapes such as over-compaction, concrete cracking and heaving is either structural support soils or “Strata Cells”. Both these techniques involve engineered structures to support pavement and allow roots to grown underground without heaving the above hard surfaces. The primary difference between using structural soil and strata cells is the amount of effective soil volume available to the plants. If using Strata Cells there is 90% effective soil volume whereas this is reduced to 20-30% functional soil in structural soil. Therefore, to achieve the equivalent effective soil volume you would need 3-4 times greater total soil volume for SSS than for a vault system. Table 1 gives an example of soil volume requirements.

 Table 1. Required soil volume and structural soil required.

Tree Size
Ideal soil volume for optimum growth and development
Extent of Structural Soil
Small Trees
>9m3
>27m3
Medium Trees
>12m3
>36m3
Large Trees
>18m3
>54m3

As a consequence of street trees not being able to tolerate their streetscape surroundings they are likely to show ill-health, perish or cause damage to roads, pavers and infrastructure. SESL has 30 years in experience with providing professional advice on helping you choose the right tree for the right soil for the right place to ensure that your streetscape has an optimum outcome.