A tree is built from growing tips
Most animals grow until they reach adult size and then stop. Trees behave differently. They keep adding new tissue for as long as they live, and nearly all of that growth happens in a few specific places. Everything else in the tree is older material that was laid down earlier and no longer grows in the same way.
Those growing places are called meristems. A tree has meristems at the tips of its shoots and roots, which make the tree longer, and a thin cylinder of dividing cells under the bark, which makes the tree thicker. Understanding these two kinds of growth, lengthening and thickening, explains most of what you can see in a tree.
The three main parts
Roots
Roots anchor the tree and take up water and dissolved minerals. The thick, woody roots near the trunk mostly provide support and act as pipelines. The real work of absorption is done by very fine roots near the tips, which are thin, short-lived and often renewed. Many of them partner with fungi, a relationship known as mycorrhizal fungi.
A common surprise is how shallow most root systems are. In many forests the majority of the roots that absorb water lie in the upper layers of soil, spreading sideways well beyond the edge of the crown. Depth and spread vary a great deal with species, soil and water supply, so it is safer to say that roots follow the resources than to give a fixed rule.
Trunk and branches
The trunk holds the crown up toward the light and carries materials between roots and leaves. Inside a mature trunk, the inner wood (often called heartwood in many species) is no longer actively moving water. It provides structure, and in some species it holds chemicals that make it resistant to decay. The outer wood, called sapwood, carries water upward. Sugars made in the leaves travel down through the thin inner bark.
Crown and leaves
The crown is the whole spread of branches and leaves. Leaves capture light and make sugar, a process usually called tree photosynthesis. The shape of a crown reflects both the species and its history: how much light it had, how crowded its neighbors were, and whether storms or browsing damaged it along the way.
How a tree gets taller
Height comes from buds. A bud is a tightly packed, protected package of unopened tissue: tiny stems, leaves or flowers waiting for the right season. In temperate climates, most trees form their buds in late summer or fall and keep them closed through winter, protected by scales. When spring warms the tree, the buds open and the shoot extends.
The bud at the very tip of a stem is the terminal bud. When it grows, the stem gets longer. Buds along the sides, called lateral buds, produce branches. In many trees the terminal bud suppresses the buds below it to some extent, which is one reason young trees often have a dominant central stem. If the terminal bud is damaged, side buds often take over.
Height growth is also seasonal. In many cool-climate species, most of the stretching happens in a short window in spring and early summer. Some species can continue to extend shoots for longer if conditions stay favorable. Trees in tropical regions may grow in pulses tied to rainy and dry periods or may grow nearly year-round, depending on the species.
How a tree gets thicker
A trunk widens through a thin layer of dividing cells called the vascular cambium. It sits between the wood and the bark and wraps around the whole trunk, branches and large roots. Each year it adds new wood cells toward the inside and new bark cells toward the outside, though far more wood than bark.
Because the cambium lays down wood on the outside of the existing wood, the oldest wood sits at the center and the newest just under the bark. That layering is why trees in climates with a distinct growing season show visible rings, and these tree rings can record past growing conditions.
This also explains a few folk puzzles. A nail driven into a trunk stays at the same height for decades, because the trunk lengthens only at the tips. A carving made on a young tree does not rise as the tree grows, though it may stretch sideways and become distorted as the trunk widens.
What controls growth rates
Growth is the result of how much sugar the tree makes and how it spends it. A tree must divide its sugar among several needs: new leaves and shoots, new roots, wood, defense chemicals, seed production and simply staying alive. When conditions are poor, a tree typically cuts back on growth before it cuts back on survival.
Several factors matter most:
- Light. Trees in shade grow more slowly than those in full sun, though many shade-tolerant species are adapted to wait for years under a canopy and then respond to a gap.
- Water. Dry spells reduce growth, and long or severe drought stress can damage the water-carrying tissues.
- Soil nutrients. Nitrogen and other minerals limit growth in many forests.
- Temperature and season length. The length of the growing season shapes how much wood is added each year.
- Competition. Neighbors compete for light, water and nutrients, which is why foresters practice stand thinning to give chosen trees more room.
- Species and age. Fast-growing pioneer species behave very differently from slow-growing, long-lived ones, and growth rates tend to change as a tree ages.
Evergreen and deciduous habits
Trees have two broad strategies for holding leaves. Deciduous trees drop their leaves in the cold or dry season and regrow them later, avoiding the cost of maintaining leaves when conditions are poor. Evergreen trees keep leaves through the year, often with tough or needle-shaped leaves that resist freezing and drying. The seasonal rhythm of deciduous forests is a direct result of this habit, while conifer forests are dominated by trees that hold needles for several years.
Note that “hardwood” and “softwood” are trade terms based on botanical group, not on how hard the wood is, a distinction that confuses many people.
Measuring growth
Foresters usually track growth by measuring how tree diameter at a standard height increases over time, along with total height. These simple measurements feed into estimates of volume and tree biomass, and they help guide decisions in silviculture, the practice of tending forests on purpose.
A slow process with fast consequences
A tree adds only a thin layer of wood each year, so the work of decades can be undone by a single severe event. But the same pattern of continuous growth gives trees a surprising ability to recover: new buds, new wood and new roots can form where tissue was lost. How well a particular tree recovers depends on its species, its age and how much of its living tissue remains.