Leaves cheap enough to keep
Why Don't Pine Trees Lose Their Leaves?
A needle is built to survive a winter. A broad leaf is not, so the tree throws it away instead.
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Step 1 of 5
Winter is dangerous for a leaf mostly because of water.
When the ground is frozen the roots can draw nothing up, but a wide leaf keeps losing water to the air. It is thirst, not cold, that does the damage.
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Step 2 of 5
A needle hardly loses any.
It is narrow, so there is little surface. It is coated in wax, and its breathing holes are sunk into pits where the wind cannot reach them.
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Step 3 of 5
So it survives, and the tree keeps it for years.
A pine needle stays on the tree for two to seven years. The tree drops a few all the time, so it never stands bare.
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Step 4 of 5
Which means it can work on any mild day of the winter.
A bare tree has to wait until spring, then build a whole set of leaves before it earns anything. An evergreen is ready the moment the light is.
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Step 5 of 5
That bargain wins where the growing season is short or the soil is poor.
Far north, high up a mountain, on thin sandy ground: nowhere with enough spare to build a new set of leaves every single year.
Trees that drop their leaves each autumn are called deciduous, and a few conifers — the larch among them — do exactly that.
The short version
Evergreens keep their needles because a waxy, narrow needle survives a winter of frozen ground — so the tree is already working when the light returns.
Try it yourself
Roll a pine needle between your fingers and feel how waxy and stiff it is compared with a broad leaf. That is the winter-proofing.