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Half the rain that falls in the Amazon comes from water that trees release through their leaves.
Every plant and tree raises water 100 meters into the air with nothing but the power of sunlight, silently and sustainably, with no limit on how much. That silent, solar-powered water lift is the trick behind the title Nature's Magicians, and one chapter of the book is called A Spell of Rain.
The water leaves through evaporation from the leaf. That makes holding leaves a big cost in water for a plant, and even dead leaves can be a drain for the same reason.
How does water climb 100 meters without a pump?
Evaporation from the leaf pulls on a continuous column of water in the wood, and the column holds together because water molecules cling to each other. The pull passes down through narrow tubes called xylem vessels to the roots. Botanists Henry Dixon and John Joly proposed this cohesion-tension idea in 1894. The water in a tall tree sits under negative pressure, stretched like a rope.
How do trees end up making rain?
Water vapor released by leaves rises, cools, and condenses into clouds that can drop rain farther downwind. In the 1970s, Eneas Salati and colleagues used water isotope measurements in the Amazon basin and found that roughly half the rain there had been recycled from the forest itself. Rain that falls far inland therefore partly comes from trees nearer the coast. Modeling studies of large-scale Amazon clearing have projected drier conditions downwind.
Why does a leaf lose so much water?
A leaf takes in carbon dioxide through tiny pores called stomata, and water vapor escapes through the same openings. The air holds a low concentration of carbon dioxide and a far higher concentration of water inside the leaf, so each open pore lets out much more than it takes in. For plants like wheat and rice, several hundred water molecules are lost for every carbon dioxide molecule fixed. Plants with a different carbon-fixing system, such as maize, lose fewer.
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