Annual rings, heartwood and sapwood · Biology Class 11–12 | StemPy Lab
Course 6.3·Lesson 3 of 3
Annual rings, heartwood and sapwood
Spring wood is light, with wide vessels. Autumn wood is dark, with narrow vessels. A pair of them makes one annual ring, and counting rings gives the age of a tree. The old inner wood becomes dark, dead heartwood, and the outer sapwood carries the water.
about 25 min1 simulation1 practice question1 Python program5 questions to check yourself
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Spring wood and autumn wood
In places with seasons, the vascular cambium does not work at the same speed all year.
In spring, water is plentiful and new leaves need it. The cambium is very active. It makes xylem with many wide vessels. The wood is light in colour and has a low density. It is called spring wood, or early wood.
In autumn, the cambium is less active. It makes xylem with fewer and narrower vessels. The wood is darker and has a higher density. It is called autumn wood, or late wood.
A strip of wood cells cut along a radius, with three annual rings from the pith on the left to the bark on the right. In each ring the first cells made, the spring wood, are wide and pale. The last cells made, the autumn wood, are narrow and dark. The dark autumn wood of one year touches the pale spring wood of the next year, which makes a sharp line between the rings. Brackets above mark each annual ring.
The annual ring
One layer of spring wood and the layer of autumn wood that follows it form one annual ring. Winter then stops the work. Next spring, pale wood begins again against the dark edge of the old ring. This sharp change from dark to pale draws the line between two rings.
You can see the rings in a cross-section of a trunk, a branch or a root. They look like circles round the pith.
Reading the age of a tree
The number of rings tells the age of the tree. Count the dark autumn lines from the pith to the bark. The study of the rings is dendrochronology.
A wide ring means a year of good growth, with plenty of water.
A narrow ring means a poor year. It may have been dry or cold.
Rings in old timber can be matched with the rings of living trees, which dates the timber.
Take care in tropical places. The climate there changes little, and the rings can be faint. A tree may also make a false ring if a dry spell cuts one season short.
Add years and watch the trunk grow outward. Increase the rings of sapwood and the heartwood shrinks.
Heartwood and sapwood
In an old tree the xylem is not all the same.
Heartwood is the central, oldest wood. Its cells are dead, and they no longer carry water. They fill with tannins, resins, oils, gums and other aromatic substances. This makes the wood dark. It is hard, it resists attack by fungi and insects, and it gives the trunk strength. Timber is mostly heartwood.
Sapwood is the outer, younger wood next to the cambium. It is lighter in colour. It carries water and minerals up the tree.
A tree can survive when its heartwood rots away and the trunk becomes hollow. Only the sapwood carries water, and the sapwood is still alive.
A cross-section of an old woody stem with its layers labelled. From the middle outwards: the pith, a dark heartwood core with annual rings, a lighter ring of sapwood, a thin ring of vascular cambium, the secondary phloem, the phelloderm, the cork cambium, and the cork on the outside. The cork, cork cambium and phelloderm together are the bark layers outside the secondary phloem. A small lenticel breaks the cork at the top. Thin radial lines, the medullary rays, run from the pith to the cambium.
Try it in Python
The program takes the width of each ring of a stump and finds its age and radius.
Practise with new numbers
Common mistakes
Counting spring wood and autumn wood as two rings. Together they make one ring.
Saying heartwood carries water. Heartwood is dead and does not conduct. Sapwood conducts.
Thinking heartwood is the youngest wood. It is the oldest wood, next to the pith.
Mixing up the colours. Spring wood is light with wide vessels. Autumn wood is dark with narrow vessels.
Remember these
Spring (early) wood: wide vessels, light colour, low density. Autumn (late) wood: narrow vessels, dark colour, high density.
One annual ring is one spring layer with one autumn layer. The rings give the age of a tree.
Wide rings mean good years, and narrow rings mean poor years.
Heartwood is dark, dead and hard. It has tannins, resins and oils, and it does not conduct.
Sapwood is light and young. It carries water and minerals.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
How does autumn wood differ from spring wood?
How do you find the age of a tree from its trunk?
Why is heartwood dark?
Which wood carries water in an old tree, the heartwood or the sapwood?
A rotten core hollows an old tree, yet the tree lives on. Why?
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