The Wisdom of Trees by Max Adams

Author:Max Adams [Adams, Max]
Language: eng
Format: epub
ISBN: 9781781855454
Publisher: Head of Zeus

Hydraulics

Shifting water and sugar around an organism as large as a tree requires some serious engineering. Four separate forces are at work: water adhesion, transpiration and surface tension all working one way, with gravity acting against them. Water is sticky, more sticky than any other non-metallic liquid. Its hydrogen and oxygen atoms bind together very effectively. But if the forces working on water appear simple, they operate together in ways that are not. First, water in leaves is drawn out by evaporation; since water sticks to itself (surface tension, or cohesion), the evaporating water pulls other water molecules along in its wake. So long as there is no gap in the column, water in the roots, connected by surface tension with water at the leaves, is drawn up continuously. As all children know, even if there is a gap the effect of sucking (negative pressure) caused by transpiration at the top of the column ought to be able to keep that column rising.

Well, no actually. Engineers tell us that there is a limit to the power of suction, a rather precise limit: 33 feet 10 inches, to be exact; or about 10 metres. No pump can work beyond that limit with suction alone because the effect of gravity on a column of water eventually overcomes the forces of suction and surface tension. The column breaks irrevocably: water snaps. In plants, the breaking of the water column is called ‘cavitation’, which is like an air embolism (often fatal) in animals. In summer, when it is very dry, you can sometimes hear the cracking sound from a tree trunk in which the column suddenly breaks. New research seems to show that beech trees are peculiarly vulnerable to long dry spells. Yet trees can grow much taller than 33 feet 10 inches, and in any case cavitation in trees is generally caused not by suction but by drought or frost. So how does a tree get water up so high?

There are two possible answers: one is that the engineering of the cells that transport water and sugars around the tree brings a third force into play: water adhesion. Water coheres to itself, but it also adheres to other materials. Its adherence to the walls of the woody cells, combined with surface tension in the water column, seems to allow the tree to supplement the pull of transpiration and keep that water column from cavitating. There is also another force at work, one which we do not yet understand properly: the ability of many trees to accumulate positive pressure in their roots and force the water column upwards. This is partly a response to cavitation, and it happens in spring. Think birch wine and maple syrup: that’s positive root pressure.

This is all very well and it reminds one just how brilliant trees are. Inconveniently for us, scientists can’t agree with one another on how trees draw water to so great a height. There is no current plausible explanation for their ability to create positive root pressure.

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