There is a commonly held, but largely unfounded fear, that “the tree is too tall”.
BS3998:2010 Tree work: recommendations states:
Trees are dynamic, continually self-optimizing organisms, i.e. each year, by producing new shoots, roots and radial increments of wood and bark, they maintain both their physiological functions and their structural integrity. Thus, the often massive structure of a mature tree above ground, consisting of the stem, branches, twigs and the attached foliage, is highly efficient in intercepting, using and storing solar energy, while also bearing its own weight and dissipating the potentially damaging forces of the wind. Below ground, although far less obvious, the extensive root system is equally efficient both in providing anchorage and in pervading the soil in order to absorb the water and mineral nutrients that are essential for survival, growth, flowering and fruiting.
So, trees will grow as tall and as wide as the local conditions will allow (and not always as tall as suggested in the guide books), the size of the tree’s canopy should not be a cause for concern on its own.
Limiting factors to tree growth include:
- the volume of soil available,
- the quality of the available soil – including its chemical composition, the availability of water and oxygen in the soil that may be exploited by the tree’s roots, the security of the tree’s “anchor” in the ground,
- the local climate – not too hot or too cold, the right amount of rainfall at the right time of year,
- the available sunlight – the right amount at the right time of day and of year,
- exposure – which may be quite extreme in an urban setting,
- local pollution – which may be quite extreme in an urban setting.
The practice of reducing the size of a tree by cutting a tree’s branches down to a lower height, known as topping, has long been widely regarded as an unacceptable response to the concern that the tree is too tall for a number of reasons:
- the practice often removes a large proportion of a tree’s crown and leaf area in one operation and so will seriously weaken the tree by reducing its ability to produce chemical defences, for example by hindering the production of energy through photosynthesis,
- the pruning wounds created by topping seldom heal properly, and so the tree’s inner tissues are exposed to the risk of disease and insect pests,
- post-topping, to compensate for the loss of leaf area, trees will often respond by putting out a profusion of dense, upright shoots from the cut wound surface; a tree with insufficient stored energy reserves may die as a consequence,
- additionally, this new growth sprouts from latent buds located just below the bark and concentrated around the cut wounds; these shoots are only weakly attached to the wood from which they have emerged and very prone to breaking off, particularly in high winds. Topping may therefore create a hazardous situation at a significant height that cannot be easily inspected or managed, and serious injury or damage to property may occur as a consequence of branch union failure; rather than reducing the perceived danger of a tree that is too tall, topping is likely to make a tree a greater risk,
- topping destroys the natural form and grace of a tree forever; the tree will never recover its natural habit and so will appear disfigured and mutilated, especially when it is without leaves during the winter,
- a topped tree, should it survive, will often quickly grow back to its original height and with a denser crown than before it was pruned. In other words, topping in the long term, is unlikely to have the desired effect of reducing the size of a tree. Moreover, the tree is likely to need pruning again when, within a few years, it regains or surpasses its old size, or its new branches break or become a hazard.
The unfounded fear that the tree is too tall should not result in any pruning work because such action will create long-term problems, will leave the tree looking unsightly and anyway is unlikely to have the desired effect. The only realistic arboricultural solution if a tree has genuinely become too tall for its location is to remove the tree altogether.