The cambium is a thin sheet of living, dividing cells wrapped around a stem or root just under the bark, and it is what lets a plant grow thicker over the years. Height comes from the tips of shoots, but every gain in trunk width comes from this one hidden layer. Because it sits so close to the surface, it is also the part of a tree most easily hurt by a mower, a stripped ring of bark, or a fire.
Where the Cambium Sits and What It Makes
Botanists call it the vascular cambium: a cylinder of dividing cells that forms among the stem’s first tissues.1PubMed Central. Vascular Cambium Development On its inner side it makes xylem, the wood that carries water up from the roots.2Biology Semester II. Plant Organs, Tissues, and Cells: Part Eight Secondary Growth On its outer side it makes phloem, the tissue that carries sugars from the leaves.3Biology LibreTexts. What Is the Cambium in a Plant and What Does It Do? The wood in a cut log was all laid down by the cambium, layer after layer.4Secondary Xylem Biology: Origins, Functions, and Applications. Secondary Xylem Biology That means the oldest wood is at the center and the newest sits right against the cambium. Because new wood is added around the outside rather than by stretching the trunk upward, a nail or swing hook stays at the same height for life. It just gets slowly buried as new layers grow around it.
You can find the area on a real twig. Scratch a young twig with a fingernail and look for the thin green layer just under the brown skin. Green, moist tissue means that part is alive; brown and dry means it is dead. Gardeners use this scratch test to check whether a branch survived winter. The cambium itself is too thin to see, so the green you notice is the living layer and tissue around it.
The cambium is made of two kinds of stem cells: long, narrow ones and small, roughly cube-shaped ones.5Southern Illinois University Carbondale. Vascular Cambium The long cells are the most common kind, and they divide to make the lengthwise water and food pipes of wood and inner bark.6Botanical Online Instruction. What Is the Cambium in a Plant and What Does It Do? The small cells make rays, thin bands of living cells that run outward from the center like spokes.7PubMed Central. Regulating the Vascular Cambium: Do Not Forget the Vascular Ray Initials and Their Derivatives Rays move nutrients sideways across the wood and inner bark,8PubMed Central. Rays, intrusive growth, and storied cambium in the inflorescence stems of Arabidopsis thaliana Heynh so the faint lines radiating across a cut stump are the cambium’s ray work.
In spring, when the cambium is actively dividing, bark often peels away easily because the freshly made, fragile cells in this zone tear apart. In winter the zone is firmer and the bark clings.
Why Damage to the Cambium Can Kill a Tree
When the cambium is killed all the way around so wounds that stay clean and protected have a far better chance.9CEC Research. A comparison of bark thickness and fire resistance of three Californian oak species, the stem usually dies even if the outer bark still looks intact.10CEC Research. A comparison of bark thickness and fire resistance of three Californian oak species In canker diseases, fungi kill the inner bark and cambium and block wound healing, leading to branch dieback.11Europe PMC / Elsevier (Physiological and Molecular Plant Pathology). Fungal pathogens of canker disease trigger canopy dieback in poplar saplings by inducing functional failure of the phloem and cambium and carbon starvation in the xylem So dieback above a sunken bark lesion often traces to dead cambium beneath it.
Yard trees pick up this kind of damage in a few common ways. String trimmers and mowers nick the base of the trunk. Tight ties, wires, or tree guards get left on as the trunk widens. Deer, rabbits, or voles chew bark in winter. Each wound removes cambium where it hits, which stops new growth in that spot.
A ring of damage is far worse than a patch. Food made in the leaves has to flow down the inner bark to reach the roots. A patch wound lets that food detour around the edges, but a complete ring blocks the path, and the roots slowly starve.
The trouble often hides for a while. A tree with a killed cambium can leaf out in spring on stored water and sugars, so it may look fine for weeks or months after the injury. Decline typically shows later as small, pale, or early-turning leaves, thinning branches, or dieback starting in one part of the crown. This delay also makes checking the trunk base for old wounds worthwhile when a tree is declining for no clear reason.
When you see a fresh bark wound, trim ragged loose bark cleanly without widening the wound. Skip wound paint, since it doesn’t help the tree heal. Remove the cause, whether that’s a tight tie or a trimmer habit, and keep a mulch ring around the trunk to hold mowers away.
The Second Cambium That Makes Bark
Bark comes from its own growth layer. This second layer, the cork cambium (from here, the bark-making layer), makes cork toward the outside and a thin living layer toward the inside.12Biology LibreTexts. What Is the Cambium in a Plant and What Does It Do? Those three together form the outer bark that covers older stems and roots.13Biology LibreTexts. What Is the Cambium in a Plant and What Does It Do? As a stem thickens, its original thin skin can’t keep up and is replaced by this tougher covering.14PubMed. The Making of Plant Armor: The Periderm Cork cells are waterproofed and act as armor against pests, drying, and other stresses.15PubMed. The Making of Plant Armor: The Periderm A wound triggers a new patch of cork to seal the injury.16PubMed. The Making of Plant Armor: The Periderm
A smooth young trunk turns rough and furrowed as it thickens. As the cambium adds wood inside, the trunk’s circumference grows and the outer bark gets pulled apart. The cracks, ridges, and plates on an old trunk are the outer bark splitting, with fresh cork forming underneath to fill the gaps. How the bark splits and renews depends on how the cork-making layer behaves, so each species has its own pattern. Beeches keep smooth bark because their bark-making layer keeps pace by growing and stretching along with the trunk.
The cambium can’t do some jobs for itself. It is made of soft, thin-walled cells that dry out quickly, so it depends on bark to hold in moisture. Bark is also a physical barrier that insects and fungal spores must chew or work through before reaching the food-carrying tissue. And it insulates against sudden temperature swings, which is why a trunk’s surface can get far hotter or colder than the living layer beneath. Where bark is lost, the exposed cambium dries, freezes, or sunburns, so a bare wound is much more dangerous than a covered one.
Young trunks have thin bark, so small knocks or sunscald can reach the cambium easily. Bark thickens with age and gives more protection.
Thick bark can even save the cambium in a fire. How well an oak survives fire depends on how thick and dense its bark is, because bark slows heat reaching the cambium.17CEC Research. A comparison of bark thickness and fire resistance of three Californian oak species Cork oak bark thicker than about 3.7 cm kept the cambium below killing temperatures in intense fires.18Forest Ecology and Management. Quercus suber cork as a keystone trait for fire response: A flammability analysis using bench and field scales So cork oaks can resprout after fires that kill thinner-barked trees.
Can the Cambium Grow Back After a Wound
Often it can. When a ring of bark is stripped, called girdling, the cambium and inner bark go with it, leaving bare wood.19PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research Young wood cells near the surface can then start dividing again and form a rough healing tissue called callus within about 2 to 6 days.20PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research New food-carrying cells appear within about a week, before the new cambium forms.21PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research A continuous new cambium formed in about 12 days in poplar and about 21 days in a slower species.22PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research The regrown cambium then resumes making wood inward and inner bark outward.23PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research
The new layer starts as a patch rather than an unbroken cylinder, because it forms from cells that were once part of the wood. Patches join as callus spreads, which is why a continuous layer takes days to weeks. A wide wound makes the new layer bridge a larger gap, so size changes the timing. Healing is quickest in a tree that is actively growing and slowest in dormancy, so the season of the wound matters too.
Regrowth needs the exposed wood to stay alive and moist.24PubMed Central. Plant in situ tissue regeneration: dynamics, mechanisms and implications for forestry research Canker fungi block callus and inner-bark regrowth that would otherwise happen,25Europe PMC / Elsevier (Physiological and Molecular Plant Pathology). Fungal pathogens of canker disease trigger canopy dieback in poplar saplings by inducing functional failure of the phloem and cambium and carbon starvation in the xylem
On your tree, healing looks like a raised rim of new tissue rolling in from the wound edges over months to years. The old wood under the wound doesn’t heal; it gets covered over. Small wounds close fully, while large ones may never close.
To help a wounded tree, avoid letting fresh exposed wood dry out or get further damaged, and water during drought so the tree has resources to regrow. Watch for oozing, sunken bark, or fungal growth as warning signs. For a complete ring or a large trunk wound, call an arborist.
Why Grafting Depends on Matching the Cambium
A successful graft starts with the cambium of the top piece lined up with that of the rootstock.26PubMed Central. Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants The cut surfaces start a wound-healing response and grow a bridge of callus.27PubMed Central. Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants A new cambium forms across the bridge and makes new wood and inner bark that join the two pieces.28PubMed Central. Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants If the green layers don’t touch, there is nothing to bridge and the graft fails.
The cambium is only a thin ring just under the bark, so the two pieces touch correctly only if their outer edges meet. Lining up the centers isn’t what matters. Even a partial match can work, because new tissue spreads from the contact point.
In practice, make clean, flat cuts with a sharp knife. Wrap tightly so the pieces don’t shift while the bridge forms, and seal against drying so callus can grow.
Timing matters too. Grafts are usually made when the top piece is still dormant and the rootstock is starting to wake up, so the cambium cells are ready to divide. Cuttings for the top piece are often collected in winter and kept cool and moist until grafting. If the top piece sprouts leaves before the bridge forms, it can dry out because it has no water from the roots.
A graft has taken when buds on the top piece swell and leaf out and a smooth bump forms where the pieces joined. Withered, dry buds after a few weeks mean failure.
How the Cambium Makes Tree Rings
In cool and temperate places, the cambium rests in winter and grows in the warm season.29PubMed Central. Biological Basis of Tree-Ring Formation: A Crash Course In the tropics it may rest in the dry season and grow in the wet season.30PubMed Central. Biological Basis of Tree-Ring Formation: A Crash Course In white oaks it restarted in early April and stopped by early August.31Forests. Seasonal Cambial Activity and Formation of Secondary Phloem and Xylem in White Oaks (Quercus alba L.) Growth usually peaks around midsummer, when wood shifts from light spring wood to denser late wood.32PubMed Central. Biological Basis of Tree-Ring Formation: A Crash Course Cooler stem temperatures help switch the cambium to late wood and then to rest.33PubMed Central. Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers So each light-and-dark pair is one year’s growth.
The two looks come from cell structure. Spring wood is made of larger, thinner-walled cells that move water quickly, so it looks lighter and softer. Late wood is made of smaller, thicker-walled cells, so it looks darker and denser. The sharp line where dark late wood meets next spring’s light wood is what the eye reads as the edge of a ring.
On a stump, count the dark bands to estimate age. Wide rings often reflect good growing years, and narrow ones can reflect stress such as drought, though other factors also affect ring width. Rings can be uneven from side to side when a tree leaned or was crowded. Tropical trees may show faint or no clear rings.
Patterns also differ among tree types. In many conifers and oaks, the contrast between light and dark is strong, so rings are easy to count. In some trees with evenly spread vessels, rings are faint and hard to see without sanding.
Do All Plants Have a Cambium
Even small non-woody plants can have one. Most flowering annuals with broad leaves have a cambium with the same two kinds of starter cells as trees.34PubMed Central. Regulating the Vascular Cambium: Do Not Forget the Vascular Ray Initials and Their Derivatives The small weed thale cress makes a little wood at the base of its stems.35PubMed Central. Rays, intrusive growth, and storied cambium in the inflorescence stems of Arabidopsis thaliana Heynh So the gap between a herb and a tree is partly how active the cambium is.36PubMed. Control of cambium initiation and activity in Arabidopsis by the transcriptional regulator AHL15 Grasses, palms, and lilies lack this kind of cambium. Some, like dragon trees and yuccas, thicken with a different growth layer that adds support or stores food and water.37PubMed Central. Lateral Meristems Responsible for Secondary Growth of the Monocotyledons: A Survey of the State of the Art
Without a cambium, a palm has its trunk width set early, when the growing tip builds the stem. The trunk gets taller but not thicker over the years, apart from some cell enlargement. Its water and food pipes are scattered through the trunk rather than in a ring under the bark, so there is no single layer to damage and regrow.
A wound in a palm trunk stays as a scar for the life of the palm. Damaging the single growing tip at the top, rather than the trunk surface, is what usually kills a palm. Woody shrubs, by contrast, respond to wounds like small trees.
Dragon trees and yuccas, despite being in the lily-related group, build thicker trunks using a different growth layer. It lies outside the bundles of pipes and adds tissue both for support and for storing food and water. The result looks woody, but it is laid down differently from an oak’s rings.