Where are the roots?

Given the impressive height of many of the UK’s native tree species, it’s perhaps not surprising that a good number of people think their root systems must burrow to equally impressive depths.

In fact, in the majority of tree species, the depth to which they extend below the ground is only a small proportion of their height. For instance, when you look at an average-sized mature oak tree extending 20 metres or so above the ground, if you stripped away the earth below the tree, you would be unlikely to see significant roots below 3 metres and the majority of roots would most likely
be in the first metre of soil.

But where are these roots?

While the depth of tree root systems may not cause great excitement, they can spread extensively, and in some species, like willow, as much as up to 40 metres. To use a somewhat hackneyed comparison, that’s the length of three and a half London buses.

So, what are they doing down there?

The roots that are closer to the base of the tree are providing stability and stopping the tree from falling over. This ‘root plate’ comprises of thick roots and a mass of interwoven smaller roots that bind soil to them and provide a heavy anchor for the tree sufficient to support its weight and the forces of most winds.  In mature trees they will typically extend to between 1 m and 2 m radius around the base of the tree.

As an example, a root plate that has a 3.5 m diameter and is 0.8 m deep would have a volume of around 7.7m³ and a weight of around 13 tonnes which is equivalent to the bell that is Big Ben.

Then there are the fine roots extending out through the soil to draw in the water, minerals and other nutrients that the tree depends on to thrive. Typically, the top 600 mm is where most of the action is going on when it comes to tree health.  This is where the soil is most alive with microorganisms (there are over a billion microorganisms in a gram of healthy soil) and tree roots depend on this area of the soil and the symbiotic interactions with fungi, bacteria and other microorganisms, for the nutrients that the tree needs to grow, to create healthy leaves, flowers and fruit.

Tree roots form close networks with different microorganisms in the soil. Mycorrhizal fungi is considered one of the most important associations

When you look at a tree, you might think that the roots spread out equally in all directions creating a circle that the tree depends on. While this can be true, it is not always the case and so to be certain, we need to look more closely.

What we see above the ground often gives us some important clues as to what’s happening in the root system – allowing us to develop better strategies for managing specific trees, or groups of trees.

Knowing what the roots are doing, how far they spread and how deep they go and what the impacts are likely to be from certain activities within the root area, requires an advanced level of expertise and experience. This is true whether advising on the health of ancient trees, how to ensure the success of new planting or how to design and construct buildings near to mature trees.

There are some very simple key points to bear in mind when thinking about these matters.  Trees can only thrive if they have a healthy root system and tree roots depend on healthy soil. Tree roots and soil can be degraded in all sorts of ways.

Here are a few to be aware of:

• Soil compaction – healthy tree roots require good soil conditions to thrive. Soil should ideally be crumbly and structured in a way to aid aeration as well as water percolation and absorption. Compacted soil can negatively impact on the overall tree health over time and so the additional pressure on soil during construction work or in areas with a heavy footfall, grazing or traffic, require careful management.
• Excavation/physical damage – hands up if you’ve ever put a spade through a root by accident? Probably most of us at some time or another. This is also a common danger in earthworks and major landscaping projects.
• Level changes – where earthworks or landscaping materially affects the level of surrounding land and soil, this can have a highly detrimental effect on tree health as it can fundamentally alter the delivery of water, nutrients and oxygen to the root system.
• Constant soil structure damage – it’s very easy to think that all the brown stuff beneath our feet is hardy and resilient, whereas in fact the ecosystem contained in soil is actually very fragile. Where soil is being constantly mined, damaged or stressed, the delicate relationship it has with the tree can be crucially changed for the worse.
• Contamination – soil pollution is more common than you might think and there are a wide range of causes – from direct chemical contamination, to the slow but sure poisoning effect of vehicle exhausts.

By understanding how all these factors can impact on tree health and being able to use the experience that comes from years of working with, managing and recording trees and their health, we can make informed judgments that deliver successful strategies for works involving trees.

Our Continuing Professional Development training, Tree Root and Soil Systems “What is going on beneath our feet?” is very popular among people and organisations who need to know about the impacts of their designs and decisions on the roots of trees.

Expertise and knowledge in observing and interpreting above ground signs can indicate where tree roots are likely to be as well as their condition.  When greater accuracy and detail are needed, we use Ground Penetrating Radar to map roots below the ground and Static Load Tests to measure and produce detailed information, based on engineering principles, of root plate stability. Get in touch with us to find out more.