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  • Fossil records tell us that about 250 million years ago, there wasn't a tree on Earth designed

  • to survive in temperatures below freezing. They were all built for life in the tropics,

  • where it's always warm, water is always liquid, and leaves are safe year round without fear of frost.

  • If you took a tropical tree (whether ancient or modern) and transported it to Siberia

  • or Patagonia in winter, its water would freeze into sharp-edged ice crystals,

  • fatally puncturing the living cells in all of its leaves. The same thing that happens to lettuce and spinach when it freezes.

  • Cold weather also means that water in the tree's plumbing system freezes, and dangerous

  • bubbles form in the ice from air that was previously dissolved in the liquid water.

  • The ice itself doesn't cause much harm, but when it thaws, the bubbles remain, which is

  • a problem because the whole plumbing system relies on the inter-molecular attraction of

  • water molecules pulling each other upward against gravity. Air bubbles break the chain

  • of molecules, essentially shutting off the flow of water.

  • So to survive in cold weather, trees need to avoid two things: bubbles in their pipes

  • and direct damage to living cells due to ice crystals.

  • They solved the first problem before ever leaving hot climates, because air bubbles

  • are also a problem during drought. When plants work harder to get water from the soil, their

  • water-conducting pipes can accidentally suck in tiny pockets of air from the surrounding

  • tissue. To combat this, trees in the dry tropics developed skinnier pipes, which, thanks to

  • the counterintuitive physics of bubbles and water, develop fewer bubble blockages than the high-capacity

  • pipes of their wet-tropical cousins. So the plumbing of ancestral drought-adapted trees

  • was accidentally pre-adapted to the cold as well, well before they began to spread beyond

  • the tropics.

  • Upon arriving in cold places, trees then evolved two techniques they still use to avoid frozen

  • leaves: one is to fill living leaf cells with concentrated sugary sap, a biological version

  • of anti-freeze. Some trees -- mostly evergreen conifers like pines or spruce -- use solely

  • this technique and keep their needle-like leaves unfrozen year-round with a super strong

  • anti-freeze. But other species, like maples and birches and larches, combine a less extreme

  • level of sugar in their leaves with the practice of going leafless during the winter to avoid

  • foliar frostbite.

  • These techniques are uniquely cold-beating adaptations - well, they were, until descendants

  • of leaf-dropping trees made it back to the dry tropics, where their strategy helps them

  • deal with the extended seasonal droughts that occur in monsoonal climates.

  • As far as we know, sugary sap remains the only adaptation that's only useful in cold

  • places, which is why you have to come to the north if you want to tap into the sweet, sweet,

  • anti-freeze that might just help you survive the winter - we call it maple syrup.

Fossil records tell us that about 250 million years ago, there wasn't a tree on Earth designed

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B1 water freeze plumbing winter survive sap

How do Trees Survive Winter?

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    Mark Lin posted on 2014/02/10
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