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HOW COME? Evergreen trees hold on to needles, even in winter

How come trees like pines stay green all year, while other trees grow new leaves each spring? asks Lucas Schmotzer, a reader in Huntington, NY.

The color of spring is a vibrant green, the color of tiny new leaves unfurling on formerly bare branches. But pine trees -- along with redwoods and spruce trees -- seem exempt from the seasonal cycle.

Maple, oak, and birch tree leaves turn from green to red or gold, falling to the ground as autumn turns to winter. Pines hold onto their deep green needles, even if they're covered in ice.

How do pines (and other evergreens) do it? Botanists think pines probably first evolved on the sides of mountains in cold climates. Under such conditions, the water that trees need to live may be trapped in ice for part of the year. Like a cactus in the desert, a pine tree adapts to harsh conditions by conserving all the energy it can.

One way plants conserve energy is by growing very slowly. A tree uses up precious energy growing new leaves, just as humans burn more calories as growing teenagers than as adults. So pine trees evolved leaves that last a long time. But there is more to a pine needle than its long life. Its shape and its structure also enable a pine tree to survive in cold and dry places.

Compare a pine needle to a maple leaf. The maple leaf is flat, thin and delicate. Cold, dry air can easily whisk moisture away from the leaf's large, open surface. But a pine needle is compact, with very little surface area. Each needle is encased in a hardened layer called the cuticle, made partly of wax. Underneath, inside the needle, thick-walled cells are packed tightly together.

The combination of limited surface area, a tough covering, and compact cells helps a needle hold onto its water, even through a long winter.

And while their leaves don't drop off in the fall, pines do lose needles. A needle on your backyard pine tree may live for two to four years. It's a lot like the hairs on our head. Older needles are always falling out; new needles are always growing in to replace them. The evidence is under the tree: that springy layer of brown pine needles, carpeting the ground.

But a bristlecone pine needle may hang on for 40 years before it finally falls to the ground. Bristlecone pine trees are among the oldest individual organisms on our planet. Growing at a glacial pace, a bristlecone may take 3,000 years to reach its full height of 15 to 50 feet. The oldest known bristlecone in North America, in California's White Mountains, was a baby pine more than 5,000 years ago.

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