Tree Root Systems

Tree roots extend radially in every direction to a distance equal to at least the height of the tree (assuming no physical barriers) and grow predominantly near the soil surface.

 

Typically 90% of all roots and virtually all the large structural supporting roots are in the upper 60cm of the soil.

Soil disturbance within the rooting area should be avoided, whenever and wherever possible as this can significantly adversely affect tree health and tree stability.

 

Introduction

Tree roots absorb water and nutrients from the soil, serve as a store for carbohydrates and form a structural system which supports the trunk and crown.

 

Root System Development

Initially, a germinating seed has a single root, the radicle, or taproot, which grows vertically downwards, provided soil conditions are suitable.

Elongation is most rapid during the first two or three years but decreases with tree age and increasing soil depth.

Horizontally growing side roots (laterals) form at an early stage and soon become largely responsible for structural support.

Development of the taproot then declines with the result that only a small proportion of trees have a sizeable taproot at maturity.

Lateral roots near the soil surface thicken over successive years, eventually becoming the large woody roots of the framework root system of a mature tree - there are usually between four and eleven such roots which may become 30cm or more in diameter close to the stem.

They taper rapidly until at 2-3m distance they are usually only 2-5cm in diameter, by which stage they have lost much of their rigidity and physical strength.

Beyond the 'zone of rapid taper', lateral roots extend outwards in a broad zone for many meters, without appreciable further decrease in size, typically maintaining a diameter of 1-2cm.

In some species, e.g. ash, cherry, thorn and some pines these roots tend to grow within the upper 10cm of soil.

In other species, e.g. birch, lime and oak, the lateral roots descend diagonally to a depth of 20-50cm at a distance of about 2m from the trunk and then continue growing outwards horizontally.

Associated with roots are much finer, thread-like, mycorrhizae. Mycorrhizae are symbiotic fungi which grow on or in roots, an association which is mutually beneficial to both the tree and the fungus. They are extremely efficient at nutrient absorption, especially phosphorus, and many trees cannot survive without them.

Fine roots and their mycorrhizae are jointly responsible for moisture and nutrient uptake, whilst the perennial woody roots primarily act as conducting vessels to and from the trunk.

 

Root Distribution

The variability of soil conditions and the presence of barriers to growth, result in a variable and unpredictable distribution of roots.

This is because root growth is opportunistic, occurring only where the soil environment can sustain it.

Roots proliferate wherever they encounter favourable conditions, which is why the greatest root concentration is found close to the soil surface i.e. where the soil is loosest and water, oxygen and nutrients are most readily available.

Soil bulk density increases and aeration decreases with increasing soil depth and consequently root numbers and size decline sharply with depth thus below 1m it is rare-to-find many roots which are larger than a few mm in diameter.

a) Root Depth

The deepest roots are usually found directly below, or near to, the trunk as tap, oblique lateral or sinker roots.

Maximum root depth varies greatly, from only 10-20cm in waterlogged peaty soils to, exceptionally, tens of metres in loose, well-aerated soils or fissured rock. However, there is no such thing as an intrinsically 'deep-rooted' or 'shallow-rooted' tree species.

All trees can develop a deep root system (2-3m deep) if soil conditions allow. Apparent differences in rooting ability depend on the genetically determined capacity of roots to tolerate difficult soil conditions such as poor aeration and compaction. It is this propensity which results in the root systems of some trees being deeper than others under the same conditions.

Whilst genetic characteristics of a tree play some part in the rooting pattern, soil conditions are of overriding importance.

A survey of the root plates of windblown trees in southern England after the storms of 1987 and 1990 revealed that 44% of root plates were shallower than 1m, 95% were shallower than 2m and the deepest root plate was only 3m.

Average root depths are typically in the range 1 to 2m.

b) Root Spread

Root spread is not confined to the area delineated by the spread of the branches or 'dripline'.

Excavation has revealed that roots can grow for a considerable distance beyond the branch spread; typically extending outwards for a distance equivalent to at least the tree's height, and in some cases (particularly in infertile or compacted soils) up to three times the height of the tree.

Roots distant from the trunk are usually very close to the soil surface. Obstacles in the soil such as rocks, kerbs and building foundations provide a physical barrier to root extension.

Roots meeting such obstacles are typically deflected by them and once clear of the obstruction, they often resume their original direction of growth.

 

Factors Affecting Root Distribution

 

a) Bulk Density

Root growth declines rapidly with increasing the bulk density of soil.

Compaction can be a natural feature on a site or it can be induced, e.g. by the repeated passage of pedestrians, livestock or vehicles over the soil surface.

Where soils have been compacted it is often difficult to establish new trees.

Established trees that experience sudden compaction of the soil frequently suffer root death and crown dieback often occurs because of the inability of the tree to adapt quickly to the rapid change in soil conditions.

b) Aeration

In order for roots to survive, oxygen must be available in the immediately surrounding soil. Oxygen supply to roots is governed by soil structure and texture. In loose or coarse textured soils the air gaps between the soil particles are relatively large and atmospheric oxygen diffuses readily into the soil, while the waste product of respiration, carbon dioxide, can diffuse away.

Poor soil aeration inhibits the growth of new roots and can result in the death and decay of a large proportion of the existing root system.

The roots of dormant trees tolerate periods of poor aeration better than those of actively growing trees because their respiration rate is reduced and they need less oxygen.

c) Fertility

The roots of established trees proliferate in areas of moist soil that are rich in nutrients, especially nitrogen and phosphorus.

In general, soils with low fertility produce trees with poorly developed root systems and sites with higher fertility produce trees with well-developed root systems.

 

Tree Roots & The Water Table

In most parts of the UK, the water table is situated deep within the soil and well beyond the reach of tree roots and contributes nothing to meeting the water demand of trees.

Trees are typically wholly dependent on recent rainfall and the water stored in the soil.

 

Practical implications

Tree roots extend radially to a distance equivalent to at least the height of the tree and are located primarily in the upper 60cm of soil.

The main structural roots are usually found in the upper 30cm and taper substantially within about 3m of the trunk.

The vast majority of fine absorbing roots are even closer to the soil surface. Thus, any soil disturbance within the rooting zone will damage tree roots and should be avoided.

Within the rooting area the following should especially be avoided:

  • Soil stripping and site grading
  • Trenching
  • Soil compaction
  • Deposit of toxic or impermeable materials

The nearer to the trunk the above operations take place the greater the damage and loss of roots.

 

Reference

ARIN 130/95 Tree Root Systems

FCIN 11/05 The Influence of Soils and Species on Tree Root Depth