Wood is secondary xylem, and the word xylem comes from the Greek word xylon, meaning wood. 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, although one or two months after their birth, differentiating xylem cells reach their final mature state; cork cambium adds to the girth of a tree, and cork contains a waxy substance called suberin. Ray cells grow through the annual growth rings and hold the rings together like staples or nails; sapwood is usually about the five youngest, outer, annual growth rings, depending on species and vigor, and as new rings of sapwood form, the inner cells of older sapwood turn into heartwood. Conifer tracheids are much smaller than hardwood vessels, seldom exceeding five millimeters in length and 30 microns in diameter, and in winter the dormant cambium is only a few layers of cells, three to six, with thickened primary cell walls.
In places with cold winters, a tree's cambium rests in winter and grows in summer, and 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, but trees growing in the same area tend to develop the same pattern of tree rings as long as they experience the same conditions; tree-ring data from many individual trees are collected and standardized to remove variation that comes from isolated factors, and anchored dendrochronologies are sets of average tree-ring widths that have a known start and end date. There is an anchored tree-ring chronology for approximately the last 10,000 years, as of November 2026.
Scientists learn about past weather patterns and forest fires by studying the annual rings of trees, and the science of studying the past by looking at tree rings is 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; some living bristlecone pines are more than 4,000 years old. Coast redwoods are the tallest trees in the world, as of November 2026, and in April 2004 Nature gave the height of the world's tallest tree as 112.7 metres; in 2004 Nature also reported that Koch's team suspected the difficulty of getting water so far up into the sky is what ultimately constrains tree growth. Hyperion, the tallest living tree, is estimated to be between 600 and 800 years old, as of November 2026.
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; water moves from one cell to the next when there is a pressure difference between the two, and scientists call the continuous water transport pathway the Soil Plant Atmosphere Continuum, or SPAC. Theory estimates the cohesion of water at as much as 15,000 atmospheres, but in experiments the cohesion of water appears to be much less, at only 25 to 30 atmospheres; plants lose most of the water their roots take in to the air through their leaves, a process called transpiration, and redwood leaves near the ground are broad and well-spaced, while those at the top are small and scrunched.
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. 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, and arbuscular mycorrhizal fungal species are generalists and can associate with hundreds of host species. 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, and yellow and orange colors in leaves come from carotenoids, which are in the leaf all summer; across plant species an average of 400 water molecules are lost for each CO2 molecule gained.
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, and windthrow is the main cause of natural death in mature coast redwoods; the General Sherman Tree is the largest tree in the world by trunk volume, at 52,508 cubic feet, as of November 2026, and volume figures for the biggest sequoias count the trunk only, not the branches, leaves or roots. Giant sequoias are known to reach ages of up to 3,400 years, as of November 2026, but rapid growth over a long life accounts for their size, and they are not as old as once thought; fire is part of the life cycle of giant sequoias: fire's heat opens giant sequoia cones and releases their seeds, and the cones keep their seeds closed for perhaps 20 years, unlike other trees in their forests.
Bark is a tree's protection from the outside world, and it keeps a tree warm in cold and cool in heat, and it keeps insects away, while coast redwood bark is thick and protects the tree from fire. Unlike animals and people, trees do not replace damaged tissues, and the area of chemical change around a tree wound is called the reaction zone; heartwood is the central, supporting pillar of the tree, and 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. Wood composite technologies have been used for decades to create building and home furnishing products.
Words to know
- xylem: water-carrying wood
- dendrochronology: tree-ring dating
- cohesion: water's stickiness
- mycorrhizae: root-fungus partnerships
- tracheids: conifer water cells
- suberin: waxy cork substance
