Tree Roots

Cross-section of primary root

Young roots begin as thin non-woody structures. These young roots are covered in a single layer of epidermal cells. This epidermis is specialized for absorption and usually bears root hairs which increase the surface area for absorption of water and mineral salts from the soil. In addition to this essential nutritive function, roots anchor and stabilize the plant and are important storage organs, storing surplus food manufactured by photosynthesis in the green shoots and leaves, largely in the form of starch.

The epidermis is covered by a thin layer of secretion, forming the cuticle. Secretions from the root may assist in mobilizing soil minerals, whilst the root tip may secrete mucilage to ease the passage of the growing root tip through the soil. The epidermis may persist for a long time in some herbaceous perennials, with the cuticle thickening with age.

exodermis is one or more layers of cells beneath the epidermis. In the older roots of some plants the epidermis may disintegrate and, if the cortex remains, then the exodermis forms the outermost layer.

cortex of the root consists of parenchyma cells and may develop sclerenchyma if it persists. Conspicuous air spaces occur between the cortical cells, allowing easy transport of oxygen from the aerial plant parts. In waterlogged soils, the root may develop aerenchyma, as in willow trees.

Primary growth of the root

Young growing root tips initially develop in a similar manner in herbaceous annuals, perennials and woody plants alike. This is primary growth (later the roots of perennials and woody plants also undergo secondary growth). Primary growth involves elongation of the root tip and development of the different tissue types that make up the root (differentiation). The growing root tip is protected by the root cap, behind which a meristematic region produces new cells by mitosis. Columns of cells are formed, extending back behind the root tip. Depending on the position of these files of cells within the root they develop into the various tissue types of the epidermis, cortex and vascular cylinder.

root tip meristem

Above: meristematic zones in an onion root tip (in longitudinal section). The intensity of shading indicates the rate of cell division (mitosis). Further from the root tip the cells begin to differentiate into epidermis and vascular tissues.

Secondary growth of the root

In perennial plants and woody plants, undifferentiated parenchyma cells between the xylem and the phloem become the vascular cambium. This is formed initially as a series of strips (four in the case of our root above) but eventually a complete cylinder of cambium forms. This cambium produces (secondary) xylem on the inside and (secondary) phloem on the outside. Radial rays of parenchyma cells may appear, dividing the secondary xylem into sectors as in woody stems, except root rays are often much thicker than stem rays. During secondary growth, the primary xylem and primary phloem may become more-or-less crushed and non-functional.

pericycle is the first cylinder of cells immediately inside the endodermis of the vascular bundle and as secondary xylem and phloem are produced, the pericycle increases in thickness, becoming several cell layers thick. The expansion of the vascular cylinder and pericycle pushes the cortex outwards until the cortex ruptures and is shed, along with the epidermis and endodermis. The outer part of the pericycle may produce a phellogen (cork cambium) which produces phellem (cork cells) towards the outside (and a phelloderm of parenchyma cells may be produced on the inside). As the root ages, further phellogens may arise deeper within the root, giving rise to a rhytidome (outer bark).

Gymnosperm secondary roots are similar to those of dicotyledons, just described, except they have tracheids instead of xylem vessels. In both, exposure to air and light may cause the root wood to develop more of the characteristics of stem wood.

exposed beech tree roots

Above: Beech Tree (Fagus sylvatica) roots

A river flows near to here, and when it overflows it cuts away the soil around the roots of these oak trees. These oaks are rooted on an embankment and lean forwards in arcs as they have righted themselves where the soil slumped in the past when the trees were younger. It always seems especially dark here, for as the trees curve forwards their canopies cover the sky above this transient river bed especially well. It is dark and damp and this part of the woods can be creepy! It is easy to imagine serpents living in the hollows amongst these exposed roots!

Some of the oaks here are centuries old; one has a trunk diameter of about 2 metres and looks as if it is 600-700 years old, though its crown is still youthful. Perhaps the trees grow fast on this rich flood soil, or perhaps the dampness has stopped them suffering the drought damage that contributes to the 'stag's-head' appearance of many ancient oaks. It is certainly 400 years old at the very least. The one above is much younger, probably about 300-400 years old.

exposed oak tree roots

There is a common misconception about the architecture of tree roots. Most trees have shallow roots, with almost all their roots within the top few centimeters of the soil surface and their deepest tap-root typically no more than a meter or two in depth. The roots are not structured so much like the crown, but more like a broad lamp-stand that extends out to about the crown perimeter. This is to trap rainwater and the nutrients in the rich decomposing organic layers. Tiny feeder roots extend up into the nitrogen-rich leaf litter.

This so-called 'root-disc' maybe shallow, but it's great diameter gives the tree great stability against being toppled by high winds, especially when one considers the total weight of soil on top. Only in arid places, where water comes from the water-table deep below rather than from frequent rainfall do trees have immensely deep tap-roots. Usually the bedrock will make it hard for roots too penetrate more than a metre or two. It looks as if where soil erosion has exposed the roots do the roots actively grow deeper down in these temperate oaks, anchoring the tree.

rhizosphere, or that part of the soil rich in plant roots, is an important social place for trees. The roots of neighbouring trees, especially trees of the same species or parentage, often fuse together and exchange sap, allowing the stronger trees to nourish the weaker trees. Roots also store food and many a tree-stump will put forth new branches, not only by consuming its own food reserves, but by drawing nutrient from neighbouring trees with whom its roots have naturally grafted. Most trees also form relationships with fungi, forming mycorhizae ('fungus-roots'). The fungi form vast networks of very fine white and hair-like filaments that are much better at absorbing nutrients from the soil than are the root hairs of the tree. The fungus gives some of these mineral nutrients to the tree, via its roots to which the fungus is joined, in exchange for sugars that the tree's green parts have made by photosynthesis, since fungi cannot photosynthesise and need a source of carbon.

Trees which become hollow with age, largely due to decomposition of the dead heartwood, will often put down new roots inside the tree to absorb and recycle nutrients from the decomposing deadwood.

Fallen oak still growing upright!

The root system can fail in trees, especially in young immature trees, but even in old veterans. It may fail in high winds, especially if the soil is water-logged and fluid, or it may fail when it is eroded away. This oak seems to have fallen a long time ago, perhaps when waters undercut its roots during a flood. What looks like the original stem is now growing prostate, but the oak has accommodated by investing in one of its upright side-branches which is now the main stem. Trees that are young enough can pull or push, forcing their trunks and branches to bend and resume an upright posture, but if the tree is too heavy it has to reconfigure itself as this one seems to have done - no problem so long as some of the roots remain rooted.

Notice the smaller vertical sinker roots growing off the main lateral (horizontal / sideways) roots seen here.

It is surprising how weather can affect not only our own personal mood as we perceive it, but also what we perceive to be the atmosphere of the woodland. This set of pictures of the oak with the exposed roots were taken on a dryer sunny day, compared to the dark, damp day of the photos shown at the top of the page. The latter appeared far less sinister and the damp rank smell of this area had gone and was replaced with a fresh open feel.

It is important to realize that roots don't only function in anchorage and absorption of water, but also in absorption of soil nutrients and in the storage of food reserves. However, the main woody roots here are not the absorptive roots, absorption of water and minerals is the function of the finest end branched of the roots and their root hairs.

The main roots of a tree are the framework roots, consisting of lateral roots, growing out sideways just beneath the soil surface, and sometimes a vertical taproot. The more-or-less central taproot which penetrates straight downwards (to about half a meter in an oak tree) is more important in the young root systems of oaks, pines and walnut tress; whilst spruces, limes, willows, poplars and birches have small taproots, relying instead on more fibrous root systems. The main lateral roots grow out sideways from the top of the taproot and are very close to the surface in spruce and fir trees, but initially angle down to a depth of about 10 cm in an oak tree before growing out horizontally. Where they join the trunk (the root collar) they may be 30 cm or more in diameter and rapidly taper over the first meter of their length and become less rigid and more rope-like further from the trunk. The lateral roots fork, branch and criss-cross one-another, fusing together when in contact, forming the rigid root plate, which is about as wide as the canopy. The lateral roots may drop down a number of deeper sinker roots within 2 metres or so of the trunk. Sinkers may be as thick as the tap root an, like the taproot, will grow downwards until they meet an obstruction or the water table or reach a point where the oxygen content of the soil is too low to support further growth, and then stop or put out lateral branches. Roots of trees, such as willow, have been known to considerable distances along storm sewers.

Yew Tree roots

Yew Tree (Taxus baccata) roots

Framework roots are typically quite shallow, typically only 1-2 meters deep, but up to 5 meters or so in well-drained soil. They may extend outwards from the trunk for up to 2 to 4 times the canopy radius (or up to twice the height of the tree). In dry habitats, however, roots can grow much deeper to reach the water table, up to about 120 m in fig trees, for example. In these cases, the taproot and sinkers dominate over the laterals. The European Beech (Fagus sylvatica) often grows in shallow soils on slopes and has short, slow-growing roots with many branches to absorb more moisture from a smaller soil volume.

The woody framework roots have little, if any, role in absorption. This is carried out by much smaller and finer
absorptive roots (which have more permeable outer coverings and a much higher surface-area to volume ratio). The lateral roots branch 5 times or so, with the smallest branches ending in fans of fine non-woody roots. The finest roots may taper to less than one-tenth of a millimetre in diameter at their tips, and may be 1 mm long or less, and just behind their tips is the zone of very fine root hairs where most absorption occurs. Even this gives the roots limited absorptive capacity, however, especially for certain minerals, and for this the roots rely on the vast fine networks of fungal hyphae of those fungi that exchange materials with the tree's roots in a mutually beneficial symbiosis (the mycorhizal fungi). The fungus supplies the tree with water, phosphorus and nitrogen, and in return the tree supplies the fungus with organic carbon which the tree fixed from carbon dioxide in the air during photosynthesis. A mature oak tree has an estimated 500 million live root tips which may be involved in absorption.

Feeder roots are fine absorptive roots which grow from the laterals into the surface soil, sometimes upright into the leaf litter for absorption of minerals from the decaying leaves as well as rainwater. Most trees utilize mainly rainwater, with access to the water table only being essential in extremely arid or salty areas. Deep roots may transport water during the night, ready for use by the tree during the day (during transpiration: see transport in plants).

Canopy roots grow out from the trunk and branches (they are a type of adventitious root, that is a root growing where it is not usually expected to). In rainforests, these roots grow into the mats of epiphytes which cover the trees, especially in branch forks and hollows. Here the tree can absorb valuable nutrients from decomposing dead plant matter. Roots may grow from the trunk down into its hollow middle, in old trees, to absorb the minerals released as fungi and, to a lesser extent, bacteria rot away the dead wood. Red mangroves (Rhizophora mangle) have a number of species of symbiotic sponges growing attached to their submerged roots, and   fibrous roots may grow into the sponge itself. The sponge supplies the tree with nitrogen in return for fixed organic carbon, such as sugars.

Article updated:

30 Dec 2016
12 May 2021