The life cycle of a leaf

A maple or oak leaf lives for a single season. It starts as a tiny folded bud, unfolds in spring, spends the summer making food for the tree, puts on a show of color in fall, and drops to the ground to become soil. Here’s what happens at each stage, and why.

1. The bud

A leaf’s life starts long before spring. On many trees, including maples and oaks, next year’s leaves form inside buds during the summer before, tiny and tightly folded. Tough bud scales wrap them through the winter and protect them from drying out and from the cold. Look closely at a bare twig in winter and you can see the buds lined up along it, each one holding a leaf or a whole new shoot.

When a leaf falls in autumn, the bud for next year is usually already sitting just above the scar where its stem was attached.

2. Bud break

In spring, after the buds have had enough winter cold, a run of warm days sets them growing. The bud scales open, and the new leaf unfolds and expands. Young leaves are thin, soft, and often a pale or yellowish green. On some trees, including red maple and many oaks, new leaves come out tinged with red from the same red pigments that show up again in fall.

Spring green-up runs in roughly the opposite direction from fall color. It starts early in the South and in the lowlands, then moves north and up the mountains, while fall color starts in the North and the high country and moves south and downhill.

3. The summer leaf

By early summer the leaf is full size, tougher, and deep green. Now it does its real job: photosynthesis. Chlorophyll in the leaf captures sunlight and uses its energy to turn carbon dioxide from the air and water from the roots into sugars that feed the whole tree, giving off oxygen as it goes.

Tiny pores called stomata, mostly on the underside of the leaf, let carbon dioxide in and let water vapor out. That water loss pulls more water up from the roots, which is how a tall tree lifts water to its top branches.

The leaf also holds yellow and orange pigments called carotenoids all summer long. They help gather light and protect the leaf from too much sun, but you can’t see them because there’s so much green chlorophyll covering them up. Chlorophyll wears out in bright light, so all summer the leaf keeps breaking it down and making more.

4. Fall color

As the nights get longer and cooler in late summer and fall, the tree begins shutting the leaf down. It stops making new chlorophyll and starts taking apart what’s there, pulling valuable nutrients such as nitrogen back into its twigs and roots to use next spring. As the green fades, the yellow and orange carotenoids that were there all along begin to show.

Some trees also make new red and purple pigments called anthocyanins in fall, from sugars still in the leaf. Sunny days and cool nights that stay above freezing make the most of them, which is why some years are redder than others. The full story is in why leaves change color.

Different trees take different paths. Sugar maples run from yellow through orange to red, aspens and birches turn gold, and oaks tend toward russet and deep red later in the season. Use the fall foliage calendar to see when color peaks across the country.

5. Leaf drop

Near the base of each leaf stem is a narrow band of cells called the abscission zone, which formed early in the leaf’s life. In fall, as the leaf winds down, the cells there change. A layer of cells weakens and separates, and on the tree’s side, a protective corky layer seals the spot where the stem was attached. That seal is the leaf scar you can see on a twig in winter.

Once the separation layer gives way, it takes very little to bring the leaf down: a breeze, a rain, or just the leaf’s own weight. Wind and heavy rain can strip color early, and a hard freeze can kill leaves before they finish, leaving them brown and hanging. More on the timing is in when do leaves fall.

A few trees don’t follow the usual pattern. Some oaks and American beech keep their dead, papery leaves all winter, a habit explained in why oaks and beeches keep their leaves. Ginkgo does the opposite and can drop nearly all its leaves within a few days, as covered in why ginkgo drops its leaves at once.

6. Back to the soil

On the ground, the leaf’s last stage begins. Fungi and bacteria start breaking it down, and earthworms, millipedes, mites, and insects chew it into smaller pieces. Over months to a few years, depending on the kind of leaf and how wet and warm it is, the leaf becomes part of the soil, returning the nutrients it didn’t send back to the tree. Soft leaves like maple break down faster than tough, tannin-rich oak leaves.

Until then, the layer of fallen leaves is a winter home. It holds moisture, shelters seeds, and protects many insects through the cold, including the fuzzy black-and-orange woolly bear caterpillar.

What about evergreens?

Pines, spruces, firs, and other evergreens follow a longer cycle. Each needle lives for several years, and the tree sheds its oldest ones a few at a time, so it’s never bare. Many pines drop their older, inner needles in fall, and those turn yellow first, which can look alarming but is normal. A few conifers, such as the larch and the bald cypress, turn color and drop all their needles every fall, just like a maple. See why evergreens stay green.

A leaf’s year at a glance