How do tall trees stay hydrated?

07 May 2019

ASH-TREE

Two ash trees in a park

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Question

How do tall trees suck water right to the top of them? How are some trees capable of supporting themselves so so high into the forest canopy?

Answer

This tall order came into our Facebook inbox. Izzie Clarke asked Howard Griffiths, plant ecologist from the University of Cambridge, to dig up an answer.

Howard -  Well that's a really great question. And it's one that's puzzled physiologists for many years and we think we have an answer. Basically, we recognise that trees can be up to 120 m high, that's taller than the St Pauls Cathedral dome, for instance. And the reason that water can move up from the soil right through that trunk of the tree and then be lost through evaporation through the leaves is because of the amazing powers of water. It's got this most amazing tensile strength which... well it's remarkable, have you noticed how water remains a liquid exactly between nought and a hundred degrees? But within that range that's basically why water supports life on earth but it's also that hydrogen bonding which holds the water molecules together which gives it an amazing strength. Then the water rises up capillaries, due to two properties known as cohesion and adhesion.

Izzie -  Are they just almost like little tubes that run up and down a tree, essentially?

Howard - They're tiny tubes. Most of them are around the diameter of a human hair, okay, but they act like little capillaries. They're only maybe less than a centimetre, often only a couple of millimetres in length and so water moves through those tubules. And you can visualise those tubules; they're in the tissues that we call the xylem, which forms the heart wood trees. And if you take a cross-section through a tree, if you look through a fence post, you can often see the annual rings which make up the new increments of growth of the xylem that we get every year.

So the water is drawn up through that xylem, held by that tensile strength, and then what actually causes it to come right to the top of the tree is the dryness of the air at the surface at the top. Because there's such a change in energy between the free energy of water vapour and the liquid water that's being drawn up through the moist tissues,  that when the liquid evaporates into the moist cavities of the leaf and then leaves the leaves to the atmosphere, that draws up more water through those micro capillaries to that site of evaporation within those cells of the leaves.

Izzie - And even as a tree gets older, does that process diminish? Trees obviously can keep going for so long, how can they repair that system if something were to happen to it?

Howard - Ah well, that's another good question. Partly because they grow new xylem every year. New annual rings represent the new growth and clearly some very very old trees don't make very much growth. But what's been intriguing scientists over the last 20 years or so is what happens if those water columns snap? And you can actually hear them clicking sometimes. Now it's not often you see plant physiologists sticking their ear to the tree but you can, with a microphone, pick up the sound of those water columns snapping. And we think there are some mechanisms which may help trees, and particularly shrubs and so on repair those water columns overnight.

Izzie - How much water does a tree actually need? If you've got your plants in the garden, you’re like ‘I've got to make sure I look after them and they don't die’, but how does it work for trees?

Howard - Well, for a big tree like a big beech tree that you might see in a park, they might use something like let's say 400 litres of water. So if you imagine 200 large bottles of fizzy drink, that's the amount of water they need every day.

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