Wood is secondary xylem, and the word xylem comes from the Greek word xylon, meaning wood. A tree grows in two ways. Primary growth happens at the tips of roots and stems and makes them taller or longer. The vascular cambium is a thin layer of cells that makes new xylem and phloem, and xylem cells account for most of the diameter growth in a tree each year. In places with cold winters, a tree's cambium rests in winter and grows in summer. Annual growth rings result from yearly changes in the activity of the cambium.
In spring, when a tree is growing fast, its xylem cells are large with thin walls. The width of a tree's ring changes with the weather and other conditions of that year, and drought makes narrow rings while plenty of rain makes wider rings. Sometimes trees produce extra or false rings in a year. Trees growing in the same area tend to develop the same pattern of tree rings as long as they experience the same conditions. Scientists learn about past weather patterns and forest fires by studying the annual rings of trees, a science called dendrochronology. Prometheus, a bristlecone pine in Nevada, was found by counting its rings to have 4,862 growth rings, and it was estimated at being 4,900 years old, the oldest known tree of its time.
Unlike animals, plants have no heart-like pump to move fluid through their vascular system. Xylem carries water and minerals up from the roots to the leaves, and most of the water absorbed and moved through plants is moved by negative pressure. Water molecules stick to each other through forces generated by hydrogen bonding, and water's stickiness helps explain how water can be moved to tree canopies 100 meters (328 feet) above the soil. Water-carrying xylem cells are dead, so they cannot pump water. Plants lose most of the water their roots take in to the air through their leaves, a process called transpiration, and plants keep less than 5% of the water their roots absorb.
Roots take in water and food from the soil for the tree. Most tree roots are in the top three feet of soil, and a tree's roots often spread out much wider than its crown. Coast redwoods have root systems only 6 to 12 feet deep, and giant sequoia roots are shallow but spread wide. Tree roots give fungi food the tree has made, and the fungi give back water and minerals. Mycorrhizae are partnerships between fungi and roots that help the tree take up water and nutrients.
Leaves make food for the tree, and they get their green color from a pigment called chlorophyll. As nights get longer in autumn, trees get a signal that the growing season is ending, and the green chlorophyll fades. Shortening day length is the most dependable seasonal signal for autumn leaf color, more than chilly mornings. Yellow and orange colors in leaves come from carotenoids, which are in the leaf all summer, while red anthocyanin pigment appears later in the growing season as a result of a combination of factors.
Coast redwoods are the tallest trees in the world, as of November 2026, and Hyperion, the tallest living tree, is estimated to be between 600 and 800 years old, as of November 2026. Redwood forests grow naturally in a narrow band along the Pacific Coast, from central California to southern Oregon. Mature coast redwoods are resistant to fire because of bark 12 inches thick and a lack of flammable pitch or resin. The tallest known coast redwood is about 380 feet tall, as of November 2026.
The General Sherman Tree is the largest tree in the world by trunk volume, at 52,508 cubic feet, as of November 2026, and it stands 275 feet tall and is more than 100 feet around, as of November 2026. Giant sequoias are known to reach ages of up to 3,400 years, as of November 2026, though they are not the world's oldest trees. Rapid growth over a long life accounts for the size of giant sequoias, but they are not as old as once thought, as of November 2026. Fire is part of the life cycle of giant sequoias: fire's heat opens giant sequoia cones and releases their seeds. Giant sequoia cones keep their seeds in closed cones for perhaps 20 years, unlike other trees in their forests.
Unlike animals and people, trees do not replace damaged tissues. As new rings of sapwood form, the inner cells of older sapwood turn into heartwood, which is the central, supporting pillar of the tree. A heartwood post only 12 inches long and 1 by 2 inches across can hold up twenty tons. The lumber in the General Sherman Tree would be enough to build 120 average-sized houses. In a paper mill, the cellulose fibers from wood are trapped on a wire screen as the water drains away, and they stick to each other to form the paper web.
Words to know
- xylem: water pipeline
- cambium: growing layer
- dendrochronology: tree-ring dating
- transpiration: leaf water loss
- mycorrhizae: root-fungus partnerships
- anthocyanin: red pigment
