Why Do Evergreen Trees Keep Their Needles in Winter?

Bare branches and green ones

Walk through an Ontario park in November. The maples and oaks are bare, and their leaves lie in brown piles on the ground. A few steps away, a pine or a spruce is still covered in green. Both kinds of trees face the same snow and the same freezing nights, so why does one kind drop its leaves while the other keeps them?

Trees like maples are called deciduous: they drop their broad, flat leaves every fall. Trees like pines and spruces are called evergreen because they keep their leaves all winter. An evergreen's leaves are narrow needles. To see why needles can stay, we need to compare them with broad leaves, feature by feature.^[1]

Winter is a dry season for trees

Snow and ice are made of water, so it may seem strange to call winter dry. But the Open University explains that water that has turned into ice is not available to plants. When the ground freezes, roots can take up only a little water, and only very slowly.

Leaves lose water all the time. Water escapes from a leaf's surface and through its pores into the air. In winter the tree cannot easily replace that water, so a leaf that loses a lot of water can dry out and die. A NASA page about spruce trees describes the same problem: in very cold winters, the water in the soil is frozen and the trees cannot use it.^[1]

Three features that save water

First, look at the shape. A needle is narrow and thick, so it has a small surface compared with the amount of leaf inside it. A broad leaf is thin and flat, with a large surface. Less surface means less water can escape.

Second, a needle has a thick, waxy outer layer called a cuticle. Wax keeps water in, the way a waxed paper bag keeps a sandwich from drying out. NASA says the waxy coating on spruce needles helps the trees save water in very cold winters.

Third, a needle's pores are tucked away. Leaves have tiny pores called stomata that let gases move in and out. On a needle, the stomata sit in sunken pits. The Open University explains that all three features reduce the water a needle loses.^[1]

Antifreeze inside and snow outside

Cold is a second danger. An article in The Conversation explains that if ice forms inside the cells of a needle, it would be lethal. To protect themselves, conifer needles build up substances that lower the temperature at which water freezes and help guard the parts of each cell. You can think of them as a kind of natural antifreeze.

Snow is a third problem. The same article says thin, often drooping needles catch less snow than broad leaves would. Conifer branches spread outward and downward, and they are flexible. When heavy snow piles up, the branch bends until the snow slides off.^[2]

Every choice has a cost

If needles are so well protected, why do maples bother with broad leaves? Because the features that help needles survive winter have a cost. The Open University says they make needles less efficient at making food in spring and summer. A broad leaf does not pay that cost, so it works well in summer. But its large, thin surface would lose too much water in winter, so the tree drops it in fall and grows new leaves in spring.

An evergreen does not have to build a whole new set of leaves each spring. The Conversation explains that its needles can keep working for more than one growing season. Needles still do not last forever. White pine needles last about two seasons, so a pine does drop old needles; it just never drops them all at once.^[1]

The conifer that turns gold

Not every tree with needles is evergreen. The Canadian Encyclopedia says larches are Canada's only deciduous conifers. A conifer is a tree that makes cones. Larch needles turn golden yellow in fall and then drop, just like maple leaves.

Canada has three kinds of larch. One of them, the tamarack, grows in every province and territory. Its bluish-green needles grow in bundles of 10 to 30. So if you see a tree with bare, bumpy twigs in winter and cones still hanging on, it might be a tamarack, not a dead pine.^[4]