Wood is secondary xylem, and the vascular cambium is a thin layer of cells that makes new xylem and phloem, while xylem cells account for most of the diameter growth in a tree each year. 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, the central, supporting pillar of the tree. Conifer tracheids are much smaller than hardwood vessels, seldom exceeding five millimeters in length and 30 microns in diameter. 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.

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. Some living bristlecone pines are more than 4,000 years old. For height, 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. There is an anchored tree-ring chronology for approximately the last 10,000 years, 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. In 2004 Nature reported that Koch's team suspected the difficulty of getting water so far up into the sky is what ultimately constrains tree growth, and in April 2004 Nature gave the height of the world's tallest tree as 112.7 metres.

Now look below the ground, where a tree gets its water and food. 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, 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. Red anthocyanin pigment appears later in the growing season as a result of a combination of factors. Evergreen trees stay green all year, most have needle-shaped leaves, and the narrow needles of a Douglas fir can expose as much as three acres of chlorophyll surface to the sun. Plants lose most of the water their roots take in to the air through their leaves, a process called transpiration, and across plant species an average of 400 water molecules are lost for each CO2 molecule gained. Redwood leaves near the ground are broad and well-spaced, while those at the top are small and scrunched.

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.

Words to know

• tracheids: conifer water cells
• dendrochronology: tree-ring dating
• transpiration: leaf water loss
• mycorrhizae: root-fungus partnerships
• suberin: waxy cork substance