Rainforest giants defy physics to create clouds, but what happens if they fail?


Have you ever wondered how liquid moves up a drinking straw? It’s all to do with pressure gradients.


Your suction creates lower air pressure at the top of the straw than at the bottom, which draws the water up to your lips. But if you stood on the balcony of


a tall building and tried to suck liquid up an incredibly long straw, it would get to around 10 metres (three stories high) and then stop. Even if you created a perfect vacuum at the top (which no one can do with their mouth), the liquid wouldn’t go any higher. Gravity combined with low pressure would


prevent it from travelling any higher – instead, it would start to boil inside the straw. Yet trees are sucking water out of


the soil into their roots and transporting it up their trunks to the leaves all the time. It’s part of a process called transpiration, and many trees grow to over 10 metres tall. Some, the redwood, the Himalayan cypress and the dipterocarp trees of Southeast Asia, grow up to 100 metres tall – that’s as high as a 30-storey building! So how do they defy the laws of physics and move water to such great heights without boiling to death? Trees have evolved incredible


biological ‘plumbing’ systems. Inside their trunks, they have a network of thin, hollow vessels full of water called the sapwood or xylem. Unlike drinking straws, these vessels have evolved


intricate structural adaptations that maintain the water in liquid form, even under the incredibly low pressures required to move it above 10 metres. Each vessel forms a continuous column of water from the roots to the leaves. As Richard Powers put it in his novel


The Overstory, ‘A tree is a passage between earth and sky.’ A 60-metre- tall rainforest tree can take up to 1,200 litres of water out of the ground in one day, but it only needs a small fraction of this. The rest passes up the trunk and branches and evaporates through tiny holes in the leaves into the sky. The loss of water through these holes (called stomata) creates lower pressure in the leaf (like at the top of the straw being sucked on), and water is drawn up from below, starting at the roots, where the pressure is higher. Providing there is enough water in the soil, it will


Photo: Richard Whitcombe/Shutterstock


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