What do leaves do for a tree?

A leaf looks simple, but it does most of a tree’s work. It makes the food, lifts water from the roots to the top of the crown, and trades gases with the air around it. Here’s how each of those jobs works, and why a maple or an oak gives it all up every fall.

Leaves make the tree’s food

Almost everything a tree builds starts in its leaves. Using the energy in sunlight, a leaf joins carbon dioxide from the air with water from the roots to make simple sugars, and it gives off oxygen as a by-product. This is photosynthesis, and the light is caught by chlorophyll, the green pigment packed into tiny bodies called chloroplasts inside the leaf’s cells (NC State Extension).

Most of that work happens just under the leaf’s upper surface, in a layer of tall, tightly packed cells called the palisade layer. Its cells hold several times as many chloroplasts as the looser, spongy layer beneath, which is full of air spaces that connect to pores on the leaf’s underside (University of Hamburg). See the parts of a leaf for the outside view.

The sugar doesn’t stay put. Some is burned right away for energy. Some becomes cellulose, the stuff of cell walls and, eventually, wood; dry wood is about half carbon by weight, and nearly all of that carbon ultimately came from carbon dioxide captured through photosynthesis (USDA Forest Service). The rest is stored as starch, mostly in the roots, and drawn on again in spring. The twigs of many trees hold enough stored energy to grow a whole new set of leaves (University of Illinois Extension). There’s more on that in what trees do in winter.

Why leaves are green

Sunlight carries every color. Chlorophyll soaks up red and blue light and uses that energy, but it reflects green, so green is what reaches your eye (NASA Earth Observatory). Healthy leaves also reflect a lot of near-infrared light, which we can’t see. Satellites can, and scientists use that glow to map how green and healthy forests and crops are from space.

Leaves also hold yellow and orange pigments all summer, hidden under the green. When the tree stops making chlorophyll in fall, they show through. That story is in why leaves change color.

Leaves breathe through tiny pores

To make sugar, a leaf needs carbon dioxide, and it gets it through stomata, microscopic pores that are usually most plentiful on the underside. Each pore sits between two curved guard cells that swell to open it and relax to close it. Stomata generally close in the dark, and they also close in heat or very dry air to save water (NC State Extension).

Open

Carbon dioxide in, water vapor out

Closed

Water saved, but no carbon dioxide

There are a lot of them. A worked estimate for an English oak leaf puts about 500 stomata in every square millimeter of its underside and about 100 on top, or roughly 900,000 on one leaf (The Open University).

Every open pore comes with a cost. The same opening that lets carbon dioxide in lets water vapor out, and the trade is lopsided: by one common plant-physiology estimate, a plant gives up roughly 200 pounds of water for every pound of dry plant matter it builds (College of St. Benedict and St. John’s University). When a tree runs short of water and closes its stomata, photosynthesis slows too, because the carbon dioxide can’t get in.

Leaves breathe the other way as well. Like animals, a leaf uses oxygen and gives off carbon dioxide around the clock as it burns sugar. In daylight, photosynthesis outpaces that, so a forest takes in carbon dioxide and gives off oxygen. At night only the breathing continues, and carbon dioxide over the forest rises until morning (University College London).

Leaves pump water to the top of the tree

A tree has no heart to push water upward. The leaves pull it. As water evaporates from inside the leaf and escapes through the stomata, it tugs on the water behind it, and because water molecules cling to one another, the pull runs as an unbroken thread down through the wood to the roots. The sun supplies the energy. This evaporation is called transpiration, and about 90 percent of the water a plant takes up leaves this way. Along the way, the stream carries minerals up from the soil, keeps the plant’s cells firm, and cools the leaves on hot days (NC State Extension).

The volumes are large. A big oak can give off about 40,000 gallons of water in a year, and heat, wind, and dry air all speed the rate up (U.S. Geological Survey).

Built for where they grow

Even on one tree, not all leaves are alike. Leaves in full sun near the top and outside of the crown tend to be smaller and thicker, with extra layers of food-making cells and more wax and hair, and they often angle more steeply. Shade leaves deeper inside are larger, thinner, and held flatter to catch what light gets through (Northern Woodlands). The waxy coating on the surface, the cuticle, is generally thicker on leaves that get more sun, and it slows drying (NC State Extension).

Broad, thin leaves are superb at catching light but poorly suited to cold winters. Their tissues can be damaged by freezing, and they continue to lose water when replacement water may be difficult to obtain, so most deciduous broadleaf trees in cold climates drop them. Conifer needles are built tougher, with a heavy wax coat and cell fluids that resist freezing, and they can last several years (USDA Forest Service). See are pine needles leaves? and why evergreens stay green.

How leaves know fall is coming

Temperature and rain vary from year to year, but the length of the night doesn’t, so many deciduous trees use it as one of their most dependable seasonal signals. As nights lengthen in late summer, leaves slow and then stop making chlorophyll (USDA Forest Service). Research on hybrid aspen points to a light-sensing pigment called phytochrome as part of how trees read day length when they wind down growth for the year (Frontiers in Plant Science).

Frost isn’t required to trigger fall color. In fact, a hard frost or freeze can spoil the show by damaging or killing leaf cells (USDA).

When summer runs dry

Because leaves lose so much water, they are often among the first places you can see signs of drought stress. When a leaf loses water faster than the roots can replace it, its edges and tips turn brown first, a condition called leaf scorch. It usually shows up in July and August and is often worse on the sunny or windy side of the tree (The Morton Arboretum).

Dry weather can also change fall. Drought stress can make leaves drop early, which leaves fewer to turn (University of Illinois), and a very dry, hot fall in Pennsylvania in 2019 brought muted color and early leaf drop (Penn State). The Forest Service notes that a severe summer drought can instead delay fall color by a few weeks (USDA Forest Service). Which way it goes depends on the timing and the trees.

Why the job ends in fall

In a cold winter, a broad leaf stops paying its way. Short days give it little light, freezing nights damage it, and water moves poorly through cold soil and frozen wood while thin leaves keep losing it. So the tree shuts its leaves down and salvages what it can first.

Before a leaf falls, the tree pulls much of its most valuable material back into the twigs and roots. Averaged across the world’s plants, leaves give back about 62 percent of their nitrogen, 65 percent of their phosphorus, and 70 percent of their potassium before they drop, but only about 11 percent of their calcium (Vergutz and others, Stanford). Then a layer of cells at the base of the leaf stem separates, the veins close off, and the leaf lets go (USDA Forest Service). The timing is covered in when do leaves fall?, and what happens next in what trees do in winter and when trees leaf out again.

A leaf’s jobs at a glance

And the leaf’s work isn’t over when it falls. On the ground it shelters insects and other small animals through the winter, then breaks down and returns what’s left to the soil. Whether to leave it there is its own debate: see should you rake your leaves?