Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea, not measured data.
Tap the buttons and see what happens.
Drag the years. Does the nail move up as the tree grows taller?
Scrub 20 years. The trunk thickens around the nail, but the nail never rises: height is added only at the tips.
Slide it. Does the nail go up?
Tap the bud to grow a new stretch of twig. Count the scar bands.
Each tap is one growing season: the apical meristem extends the shoot and leaves a band of bud-scale scars.
Tap the button. The tip grows!
Segment lengths are illustrative.
Pick wet, normal or dry years. How wide is each ring?
Each season adds one early-wood + late-wood pair. Wet years give wide rings; drought gives narrow ones.
Tap rainy or dry. Watch the rings!
Ring widths are illustrative, not measured.
Slide through the five years. Which gets heavier: the tree or the soil?
5 lb willow, 200 lb dry soil, only water for five years. Compare the gain with the soil loss.
Slide it. What gets heavier?
Slide the sun. How fast does water climb the trunk?
Raise evaporative demand: faster transpiration increases tension and speeds flow up the xylem.
Slide the sun. Watch the water go up!
A simplified picture; flow speeds are illustrative.
Pick wet or dry, then grow year by year. Count the rings and check the nail.
Add seasons: height from the tips, girth from the cambium, one ring each year, nail fixed.
Tap grow. Watch it get big!
A toy model: sizes and ring widths are illustrative.
A tree grows in two ways. It gets taller at the very tips of its branches. And it gets fatter, just under its bark.
If there were a nail in a young tree, as high as your shoulder, it would still be there when the tree is big. The trunk does not stretch. Only the tips go up. (Please don't put nails in real trees. It hurts them.)
Every year the tree adds a new layer of wood under its bark. Each layer makes a ring. Count the rings on a tree stump, and you know how old the tree was! Rainy years make fat rings. Dry years make thin rings.
Where does all the wood come from? Not the dirt! Leaves use sunshine to turn air and water into food. The tree builds its wood from that food. So a tree is made mostly from air.
Roots spread out wide, near the top of the ground. Tiny fungus threads help them find water. The water goes up the trunk to the leaves, like a very long straw.
Want more? See how leaves make food in how photosynthesis works, and why leaves change color in autumn.
A tree grows in only a few places. It gets taller at the tips of its branches, where buds hold busy growing cells, and its roots get longer at their tips. It gets wider because a thin layer just under the bark, the cambium, adds a new layer of wood every year.123 Those yearly layers are the growth rings you can count on a stump.
And the stuff a tree builds with? Mostly air and water. Leaves take in carbon dioxide and use sunlight to turn it, together with water, into sugar. The tree turns that sugar into wood, and roughly half the weight of dry wood is carbon that came from the air.678 Water comes up from the soil too, but of the solid stuff in wood, the soil gives only a tiny pinch of minerals.
From the top, or more exactly from the tips. Here's a puzzle that shows it. Hammer a nail into a young tree at shoulder height. Come back twenty years later. Where is the nail? Most people guess it has been carried up high. It hasn't. It's still at shoulder height.1
Foresters see this all the time. When they paint a mark on a boundary tree and come back years later, the mark is at exactly the same height.1 The trunk doesn't stretch. Only the ends of the branches move up, and the trunk just gets fatter around the nail. (Please don't really hammer nails into trees; wounds let in rot and pests.)
Grass is the opposite. It grows from its base, which is why a lawn keeps growing after you mow it. Chopping the top off a tree, called "topping", is harmful, and professional tree-care groups like the International Society of Arboriculture strongly discourage it.1
Look at the end of a twig in winter and you'll find a bud. Packed inside are tiny leaves, a short piece of new stem, and sometimes flowers. The tree built them the summer before, ready for next year.2 In spring the bud scales fall away, the new shoot starts to lengthen, and the little leaves unfold.1
The lengthening happens in a zone of dividing cells at the very tip, called the apical meristem ("apical" means at the apex, the tip). Botanists call this primary growth. Root tips have apical meristems too, which is how roots push further through the soil.13
Each spring's bud leaves a little band of scars on the twig when its scales drop off. So on many twigs you can measure how far the twig grew in each of the last few years, just by finding the bands.
How tall a tree can get depends on its genes, its climate and its soil.1 That's why an apple tree stays small while a coast redwood can pass 300 feet.
Just under the bark, wrapped around the trunk, every branch and every root, is a layer of dividing cells too thin to see: the vascular cambium.213 It works in two directions at once. On its inside it adds new wood (xylem, the water pipes). On its outside it adds new inner bark (phloem, the food pipes).133 This widening is called secondary growth.3
| Layer (outside → in) | What it does |
|---|---|
| Outer bark | Protects the tree from weather, pests and injury; renewed from within |
| Inner bark (phloem) | Carries sugar from the leaves to the rest of the tree; lives briefly, then becomes outer bark |
| Cambium | The growing layer: makes new phloem outward and new wood inward |
| Sapwood (xylem) | Young wood that carries water and minerals up from the roots |
| Heartwood | Older, inactive wood in the center that holds the tree up |
Based on the Arbor Day Foundation and Texas A&M Forest Service.52
Every one of those layers is made of plant cells, each wrapped in a stiff cell wall. That wall is one of the things that makes plant cells different from animal cells, and it is why wood is so strong.
In spring, the cambium makes wood with big, thin-walled cells. This early wood looks pale. Later in the season it makes smaller cells with thick walls. This late wood is denser and darker.32 One pale band plus one dark band is one annual ring. Count the rings on a stump and you have the tree's age. Their size and wall thickness can also hint at the climate in each year.3
A wide ring usually means a good year with plenty of water. A narrow ring may mean drought, crowding by other trees, or some other stress.2 That's why the ring widths in our player's toys are only illustrative: real rings depend on the species, the site and the weather.
Tree rings are a bit like the yearly layers that build up in glaciers and ice sheets: each layer is a record of one year, so you can count back through time.
This is the biggest surprise about trees. A big tree holds a huge amount of wood, so where did all that stuff come from? Most people say the soil. It's a sensible guess, and it's wrong.
Here's what really happens. Leaves take in carbon dioxide through tiny pores called stomata. Using the energy of sunlight, they join the carbon dioxide with water to make sugar. That's photosynthesis.72 The inner bark carries the sugar to every growing part of the tree, even down to the root tips.2 There the tree uses it as fuel and as raw material for cellulose and lignin, the stuff wood is made of.
Dry wood is roughly 50% carbon by weight (measured values for temperate and boreal trees run from about 43% to 56%).8 Every bit of that carbon came out of the air. Most of the rest is oxygen and hydrogen, which came from carbon dioxide and water. The soil adds water and small amounts of minerals such as phosphorus, which matter a lot for health but add little weight.69
Roots grow longer at their tips, just like branches.2 They also thicken each year, because each year a new layer of wood is added around the trunk, branches and roots.2 But their shape surprises people. Most tree roots are not a deep mirror image of the crown. They are typically found in the top three feet of soil, and they spread well past the edge of the branches, often across an area two to four times the size of the crown.5 Some trees do have a taproot that grows straight down, with side roots branching out.2
Roots anchor the tree, soak up water and minerals, and store food for winter.5 They also need oxygen from the soil, which is why packing the ground hard, or flooding it, can harm most trees.2
Most trees don't do this job alone. Their fine roots team up with fungi in a partnership called mycorrhizae ("fungus roots"). The fungus threads, or hyphae, are far thinner than roots and reach into tiny soil pores, well beyond the area the roots have already drained. They bring back minerals such as phosphorus. In return the tree feeds the fungus sugar; the fungi can take up to about 20% of the carbon the plant takes in.9
You may have heard that forests are linked by a "wood wide web" through which trees share food and warnings, with big "mother trees" feeding their young. Be careful with that one. The fungi are real, and a single fungus can link more than one plant.14 But a 2023 review in Nature Ecology & Evolution argued that popular claims about trees sharing resources and signals through these networks go well beyond the field evidence.10 Other researchers disagree with that review, so the question is still open.14
A tree has no heart and no pump, yet it lifts water higher than a 30-story building. The trick is transpiration. Water evaporates out of the leaves through their stomata. That creates a strong pull, or tension, on the water left behind in the leaf, which pulls on the water in the tubes below it.4
Those tubes are the xylem in the sapwood. Water molecules cling tightly to each other (cohesion) and to the tube walls (adhesion), so the whole thin column of water moves up like a chain being pulled from the top, all the way from the roots. Scientists call this the cohesion–tension theory.4 On a dry, sunny day evaporation is fast and the pull is strong; in humid air it's much weaker.4
The champions are coast redwoods. A large redwood moves hundreds of gallons of water a day from its roots to its crown, powered by evaporation from its foliage.11 The tallest known tree, a coast redwood in California's Redwood Creek watershed, is about 379 feet tall.12 Redwoods even drink from the air: fog that drips off their needles provides about 40% of their moisture.11
There's no single answer, because it depends on the species, the climate and the soil.1 Most of a year's growth happens in late spring and early summer.2 A tree also doesn't simply "finish": as long as it's alive and healthy, its cambium keeps adding a ring each year, so trees keep getting wider even after they stop getting much taller. And every one of those rings is mostly air, turned into wood. That wood eventually becomes things like lumber and paper, and in autumn the tree's leaves put on their own show: see why leaves change color.
From the top, or more exactly from the tips. New height and new branch length come from growing tips called apical meristems, inside the buds at the ends of twigs. The trunk itself doesn't stretch, so a mark or nail on it stays at the same height forever. Grass is the opposite: it grows from its base.
Sunlight, carbon dioxide from the air, water and a small amount of soil minerals. Leaves use sunlight to turn carbon dioxide and water into sugar through photosynthesis. The tree then uses that sugar as fuel and as building material for new wood, leaves and roots, which are made in its buds, root tips and cambium.
A thin growing layer under the bark, the vascular cambium, adds new wood on its inside and new inner bark on its outside every growing season. Each year's wood makes one growth ring: pale, wide-celled spring wood and darker, denser late wood.
It depends hugely on the species, climate and soil. Height is set by genetics, climate and soil, and some trees keep adding width for centuries. Most of a year's growth happens in late spring and early summer.
From seeds that fall, blow, float or get carried by animals, then sprout when conditions are right. Some trees, like coast redwoods, can also sprout new trunks from stumps, roots or burls.
Mostly from the air. About half the weight of dry wood is carbon, and that carbon came from carbon dioxide the leaves took in. Most of the rest is oxygen and hydrogen, which come from carbon dioxide and water. Soil supplies only a small amount of minerals, which is why van Helmont's willow gained about 164 pounds while its soil lost just 2 ounces.
It is pulled, not pumped. Water evaporates from the leaves (transpiration), and because water molecules cling to each other, that evaporation tugs the whole column of water up through tiny tubes in the sapwood, called xylem. This is the cohesion-tension theory, and it works even in 379-foot redwoods.