An Introduction to Tree Biology: How Trees Grow # 1

Understanding how trees grow enables us to better care for them throughout their lives.

Trees do not grow from the bottom up, continuing to push from the bottom of their stems, rather they elongate from their shoot tips and expand around their circumference by generating new cells annually.

The end of every shoot contains an area termed a meristem which develops new growth each year. This new growth is called extension growth. New layers of cells are developed at the end of an existing shoot.

The length of the extension growth in any one year is a result of the trees available energy and therefore can give an indication of the trees health. Extension growth may vary from year to year dependant on available energy however, in a healthy tree would appear uniform from year to year.

This picture (Picture 1 - Extension Growth) shows the extension growth on a Rowan tree (Sorbus commixta 'Embley'). The rings around the shoot at the bottom of the picture are the terminal or apical bud scale scar.

In other words this is where the tree grew to last year and marks the start of this years extension growth. (The terminal bud scale scar is not so obvious on all tree species however on rowan and cherry trees it is particularly easy to observe.)

As this is occurring another meristematic area beneath the bark of existing shoots, branches, and the main stem, develops new growth to provide an annual increase in the width of a tree, and its branches.

Therefore at the same time that the tree is growing taller by production of annual extension growth it is growing wider by production of annual tree rings. These are the rings that can be counted in the cross section of a felled tree from which people calculate tree ages.

When observing a young tree predictions can be made regarding its form in maturity from it’s existing branch structure.

This picture (Picture 2 - Beech Union) of a beech tree shows a co-dominant branch union, where two branches have developed almost at the same time and are therefore of a similar size.

The branches would have started growing with an acute angle between them indicating that in maturity, through the development of annual ring growth around the circumference of each branch, the branches would eventually come into contact with each other.

Tight unions can be structural defects occurring in stems, branches and shoots, and can be susceptible to failure. In the instance of the beech tree, in Picture 2, as the branches continue to develop growth around their circumference they push against each other and the result may be failure of one of the branches or splitting of the union.

The union in this Goat Willow (Picture 3 - Willow Union) has split and although the tree has managed to remain intact the split has progressed down the main stem and the tree is predisposed to failure. (There is another defect associated with tight unions termed included bark which will be discussed at a later date. Not all tight unions develop included bark.)

In a young tree identification of such defects means they can easily be remedied. In the instance of the beech tree (Picture 2) one of the branches could have been removed when the tree was young by means of secatuers resulting in a small wound that would have quickly callused over promoting the remaining branch as the lead stem of the tree.

As you can see it would be extremely difficult to remove one of these branches now and the resultant wound would be much larger and significant in terms of wounding.

Identifying growth and structural defects in young trees and affecting the necessary pruning to remedy them is termed formative pruning.

Formative pruning should be undertaken at the nursery stage. It is something to be aware of when purchasing trees. If formative pruning is undertaken it can negate the requirement for significant and costly tree pruning at a later date or prevent tree or branch failure.

It is important that if you have any concerns regarding the form or condition of your tree/s you seek professional advice from a qualified arboricultural consultant. They usually offer advice for free and will be able to advise you whether further investigation is required and what if any remedial actions you should or can undertake in order to address your concerns.

They will also be able to recommend a good arboricultural contractor to undertake any proposed tree works. Contractors are not all consultants and tree works and tree consultancy should be treated as separate areas of expertise. N.B. Many arboricultural contractors have a good consultancy knowledge however this should not be assumed.