Friday 15 December 2017

Strong heartwood of a great work of nature

THE severe storm of recent days brought down many a tree, usually old, sick trees that had been weakened. But still the losses are relatively few, and it is nothing short of amazing how trees can withstand the brutal force of such strong winds without more of them falling or breaking at least.

A tree is a great work of nature. Its structure confers an enormous advantage because it lifts up the leaf canopy where it can capture vital sunlight and rain before these reach lower plants. Trees try to outgrow their competition and, in a forest, they force each other up to great heights.

To reach such heights, a tree must make a large volume of wood to support its canopy and the branch structure on which it is carried. To make the volume of wood of the trunk and main limbs takes a lot of sunlight energy, energy which is released when the wood is burned or rots.

A tree must make its structure strong enough to withstand severe winds, adding more strength to carry the heavy extra loading generated by gusts. To stand -- at a safe distance -- near a big oak or beech tree as it tosses and bends in a roaring gale is awe-inspiring.

Twenty or 30 metres high, these great old trees flex and sway their large branches, some of them weighing several tonnes, ducking under the load generated by the gusts. The youngest, most flexible wood is on the outside and the rings of strong heartwood absorb and dissipate the force of the wind.

The structure of trees has evolved so efficiently that they rarely break or fall without good reason. Wood-rotting fungi cause the tree's heartwood to decay, creating a weak spot that threatens the structure. Trees do not get old, as such, but fungi inevitably shorten the life of trees.

Water-logged soil or a change of water-table can cause anchor roots to die and to rot and, deprived of its great anchor roots, a tree will eventually come down. In some tree species, notably conifers, a circling root, not straightened at planting, can cause poor anchorage and bring a tree down.

Trees often break or lose a big limb because of poor branch structure. For insta-nce, if the fork between two branches of equal size is narrow, bark will be included as they expand and this prevents the union of the wood of the two limbs. It is practically inevitable that one branch, or both, will be ripped away in a gale.

Sunday Independent

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