Tree Anatomy: Parts of a Tree and the Layers of a Trunk in 3D

In tree anatomy, a tree has three main parts: the roots, the trunk and the crown of branches and leaves. The roots anchor the tree and take up water and minerals; the trunk holds the crown up and carries water up and sugar down; the leaves make the tree's food by photosynthesis. Wood scientists group the trunk, branches and leaves as the shoot (Wood Handbook, ch. 3).

Table after the Arbor Day Foundation (Arbor Day Foundation). Inside a leaf, see the leaf cross section and stomata diagram; for the tissues all plants share, see plant anatomy.

What counts as a tree? Usually a woody plant with a single stem at least 2 m tall, or several stems with at least one 5 cm across at breast height. On that definition there are 60,065 tree species, about 20% of all seed plants (Beech et al. 2017); the Royal Forestry Society says more than 80,000 (Royal Forestry Society).

A tree trunk has five main layers. From the outside in they are the outer bark, the inner bark (phloem), the vascular cambium, the sapwood and the heartwood, with a small, soft pith at the very centre (Wood Handbook, ch. 3). Everything outside the vascular cambium is bark; everything inside it is wood.

Table sources: the USDA Forest Service tree guide (USDA Forest Service), OpenStax (OpenStax 30.2) and the Wood Handbook.

Sapwood vs heartwood

Sapwood is the outer wood that still conducts water and has living parenchyma cells; heartwood is the darker wood inside it, which no longer conducts. Living cells at the boundary make the extractives that darken heartwood and protect it from decay fungi (Wood Handbook, ch. 3). In white oak the sapwood is nearly white and roughly 2 to 5 cm (1 to 2 in) wide, and the heartwood greyish brown (Wood Handbook, ch. 2), or light to medium brown in The Wood Database's description (The Wood Database).

This is the text version of the 3D model: every labeled structure, grouped as in the viewer, with what it does.

A tree grows longer only at its tips and thicker along its whole length. Growth in length (primary growth) comes from apical meristems at the tips of shoots and roots. Growth in thickness (secondary growth) comes from two lateral meristems: the vascular cambium and the cork cambium (OpenStax 30.2).

The vascular cambium is a thin layer of cells between the inner bark and the wood. By repeated cell divisions it makes wood (secondary xylem) to the inside and secondary phloem to the outside. Its long fusiform initials make the vertical cells of the wood and its ray initials make the rays, and it divides sideways too, so that it keeps up with the growing girth of the trunk (Wood Handbook, ch. 3). Each spring it starts again in response to auxin, a hormone made by the opening buds at the branch tips that travels down through the phloem (Arbor Day Foundation).

The cork cambium makes cork cells whose walls contain suberin, a waxy, water-repelling substance. Cork, the cork cambium and a thin layer called the phelloderm together form the periderm, and openings in it called lenticels let the living cells inside exchange gases with the air. Herbaceous plants mostly show only primary growth (OpenStax 30.2).

A tree ring is the wood made in one growing season. Early in the season the cambium makes earlywood, with large, thin-walled cells; later it makes latewood, which is denser, with thicker walls (OpenStax 30.2). The change from one year's latewood to the next year's earlywood is the line you count. The old names springwood and summerwood are no longer recommended, and many tropical trees form no visible rings at all (Wood Handbook, ch. 3).

All examples are from the Wood Handbook (Wood Handbook, ch. 3). The same genus can differ by climate: temperate oaks, ashes and hackberries are ring-porous, while their tropical relatives are diffuse-porous. Where the reference tree on this page sits: white oak is ring-porous, with very large earlywood pores in two to four rows and small latewood pores in radial groups (The Wood Database).

Dendrochronology is the dating and study of annual rings in trees. Its basic technique, crossdating, matches patterns of wide and narrow rings across many samples from one area, so that every ring is assigned a single calendar year (Laboratory of Tree-Ring Research). Ring widths vary with the conditions in the year they formed, so a dated series of rings is also a record of past climate.

The astronomer A. E. Douglass collected his first tree-ring specimens in 1904 (LTRR history) and founded the Laboratory of Tree-Ring Research at the University of Arizona in 1937 (LTRR). One roof beam, HH-39, linked living trees to old timbers and dated many Puebloan ruins to the exact year.

Overlapping oak and pine timbers from central Europe now form a continuous record more than 12,400 years long. Its authors called it the 12,460-year Hohenheim chronology, absolutely dated back to 10,461 BC (Friedrich et al. 2004); a later popular account gives 12,468 years (wissenschaft.de).

The oldest known individual trees are Great Basin bristlecone pines. The tree called Prometheus, cut down in 1964, had 4,862 growth rings and was later put at almost 4,900 years old (NPS, Great Basin) (NPS, Bryce Canyon). Sources disagree on the oldest living one: the Bryce Canyon page gives the tree called Methuselah as 4,765 years old, while the Great Basin page gives 4,847 years for the oldest known living tree after Prometheus and adds that in 2012 another bristlecone from the same area proved to be 5,065 years old.

How to tell the age of a tree

Count the rings on a stump, or on a pencil-thin core taken with an increment borer at breast height, 4½ feet (1.4 m) up; the core must reach the centre (Northern Woodlands). Then add the years the tree took to reach that height: about 5 for aspen and softwoods and 10 for hardwoods (MSU Extension).

Ring counts are not always exact. A year that starts dry and turns very wet can make what looks like two rings, and in very dry years a tree can skip a ring altogether (NIST); bristlecones high in the mountains sometimes add no ring in a year (NPS, Great Basin). Crossdating many trees is how dendrochronologists catch these false and missing rings.

Water is pulled up a tree through the xylem of the sapwood. Water evaporating from the leaves (transpiration) creates a tension that draws a continuous column of water up from the roots; cohesion between water molecules and adhesion to the xylem walls hold the column together. This is the cohesion–tension theory (OpenStax 30.5).

Water vapour leaves through the stomata, pores in the leaf surface that also let in carbon dioxide, so a tree trades water for carbon. Most of the water a tree takes up is lost this way, but sources give different shares: OpenStax says up to 90% of the water taken up by roots may be lost by transpiration (OpenStax 30.5), while South Dakota State University Extension says almost all the water absorbed on a summer day is lost, and only 1 to 2% is used for photosynthesis and other needs (SDSU Extension).

A large oak can transpire 40,000 gallons (151,000 litres) a year (USGS), and a mature elm more than 100 gallons (380 litres) on one hot summer day (SDSU Extension). The same pull lifts water to the top of a 116 m tree (OpenStax 30.5); the tallest known tree, the redwood Hyperion, is 115.61 m (Royal Forestry Society).

Sugar travels the other way, and to anywhere it is needed. Phloem sap flows by pressure from sources such as leaves to sinks such as roots, young shoots and developing seeds (OpenStax 30.5). That is why girdling, removing a complete band of bark, kills a tree: the phloem is cut, the roots starve and eventually stop sending water to the leaves. Removing a vertical strip less than a quarter of the way round harms but does not kill the tree (NMSU Extension).

Hardwood and softwood are botanical groups, not measures of hardness. Hardwoods come from flowering trees (angiosperms) such as oak, maple and birch; softwoods come from gymnosperms, mostly conifers such as pine and spruce. The single most important difference in the wood is that hardwoods have vessels (pores) and softwoods do not (Wood Handbook, ch. 3).

Sources: Wood Handbook chapter 3 for all rows, with the 90% tracheid figure matched by The Wood Database (Softwood anatomy), which also notes that resin canals are unique to conifers. The names mislead: balsa, one of the lightest woods, is a hardwood (Wood Handbook, ch. 3). Reaction wood forms whenever a trunk or branch leans more than a degree or two from the vertical, so every sloping branch contains a lot of it.

Under a hand lens, white oak end grain shows large earlywood pores at the start of each ring, dense latewood with small pores in radial groups, and rays of two sizes (The Wood Database), as in the magnified block of the 3D model.

• Vessels are stacks of short cells joined end to end through holes called perforation plates; on the end grain they show as open pores (Wood Handbook, ch. 3).
• Rays run from pith to bark and continue into the inner bark. They carry substances sideways and do much of the wood's storage. Oak's broad rays are more than eight cells wide and several centimetres tall (Wood Handbook, ch. 3), which is why quartersawn oak shows broad, conspicuous ray flecks (Wood Handbook, ch. 2).
• Tyloses are bubble-like growths that fill the pores (The Wood Database). They plug most heartwood vessels of white oak, which makes it suitable for tight barrels, while red oak usually lacks them and leaks (Wood Handbook, ch. 2).

Most tree roots are shallow and wide. University of Illinois Extension says over 90% of tree root biomass lies in the top 18 inches (46 cm) of soil, almost half of it in the top 6 inches (15 cm), and that roots spread up to three times the width of the crown, up to five times in loose sandy soil and as little as half in heavy clay (Illinois Extension). The Arbor Day Foundation gives a deeper and wider picture: roots mostly in the top three feet (90 cm), spreading two to four times the width of the crown (Arbor Day Foundation). Both agree that roots reach well beyond the dripline, the edge of the crown.

Large perennial roots and small, short-lived feeder roots together absorb water and minerals, anchor the tree and store food for winter (Arbor Day Foundation); root hairs add surface for absorption (OpenStax 30.3). Oaks and hickories keep a taproot later in life (Illinois Extension).

Tree roots work with fungi. In a mycorrhiza, the fungus supplies the plant mainly with phosphate and other minerals, such as zinc and copper, and takes sugars from the root. Ectomycorrhizal fungi wrap the root in a dense sheath called a mantle; endomycorrhizal (arbuscular) fungi grow inside the root tissue and are found in more than 80% of land plants (OpenStax 31.3), or, in another count, about 80% of plant species and two-thirds of all land plants (Moore, World of Fungi). Oaks depend on ectomycorrhizal fungi for growth and survival: root tips from just five white oaks in Arkansas yielded at least 32 fungal taxa, most often species of Russula (Ben Hassine Ben Ali et al. 2018).

A knot is the part of a branch that has become incorporated in the trunk. A knot cut across, as in a flatsawn board, is round; cut lengthwise, as on the radial face of the 3D model, it is a long "spike" knot. While the branch lives, the trunk's growth is continuous with it and the knot is intergrown; after it dies, the trunk grows around it and the knot is encased. Knots weaken wood because the fibres change direction around them (Wood Handbook, ch. 5).

Outside, the base of a branch has a branch collar, a slightly swollen ring of stem tissue, and in the angle between branch and trunk a branch bark ridge, a line of bark pushed up as branch and stem grew. A correct pruning cut starts just outside the branch bark ridge and avoids the collar; flush cuts injure the tree far more (University of Nebraska–Lincoln) (Iowa State Extension).