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Soils and tree roots: key requirements for tree health

The soil gives a tree’s root system five critical requirements: oxygen, water, nutrients, space to explore and anchorage. Soil compaction through human and livestock activities is detrimental to all of these, and thus to tree health.

Oxygen and gaseous exchange

Gases diffuse through pores and voids in the soil. Compaction by traffic (both vehicular and foot) reduces the amount of pore space and thus the rate of gaseous exchange. This limits the depth at which roots can effectively function. Excessive use of mulch can have a similar effect. So too can burial of undecomposed organic matter, because bacteria use up the available oxygen to break it down. Unlike leaves, roots take in oxygen and give out carbon dioxide. So an uninterrupted supply of oxygen is vital.

Water

Water displaces air in soil (think of what happens during rain), reducing the amount of pore space and the effective diffusion of gases. If your soils are permanently waterlogged, roots must grow very shallow to obtain enough oxygen. This places them at greater risk of soil compaction. And in compacted soil, the smaller pore space is more quickly filled with water. A compacted soil not only reduces infiltration (lowering the effective rainfall at the site), but also dries out more quickly. Thus, soil compaction reduces the effective rainfall.

Nutrients

Disturbance to the natural cycling of organic matter on the forest floor disrupts a tree’s access to the nutrients it needs. A truncated rootzone obviously leads to a lower exploitable root volume and thus less available nutrients. The addition of pollutants and even ill-conceived fertiliser or mulch programs can further limit the tree’s ability to get the nutrients it needs; for example, a very woody mulch can cause nitrogen depletion in the soil by encouraging bacterial and fungal growth.

The topsoil layer contains most of the available nutrients in an ecosystem. Consequently, scalping and erosion can have severely debilitating effects on tree health by removing these nutrients.

Trace elements are rarely a problem in urban soils, and usually deficiency is not the greatest problem. An exception is iron, which can become chronically deficient in soils affected by excessive lime or phosphorus.

Space to explore

Roots need to grow continuously in order to find new sources of nutrients, water and oxygen. But their growth can be blocked by compacted soil, concrete, paving and other barriers. Compacted soil is strong and resists penetration by feeder rootlets, which need to grow near the surface but can be prevented from doing so by the strength of the compacted soil surface.

Anchorage

A tree that has been unable to establish a deep root system as a result of compaction and drainage problems will be poorly anchored. It was obvious during the aftermath of the severe wind storms in 1991 on Sydney’s North Shore that most trees felled had grown in drainage lines where waterlogging mandated a shallow root system. Epicormic growth (flushing of new growth from trunks) and poor canopy health are outward symptoms of root decline.

Improving conditions for trees

In a future article, we will cover techniques we use that attempt to undo the effects of:

• soil compaction
• changes in soil pH
• raised phosphorus levels
• loss of topsoil
• burial by fill and topdressing
• loss of available soil volume by buried services.

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