How trees grow

Trees are the tallest free-standing organisms in the world. They live longer and grow larger than any other living organism on earth. The world's tallest known living tree is a coast redwood sequoia sempervierns. Measuring 115.61 meters (379.3 ft) in height, the 'Hyperion' is recognised as the tallest tree on earth.

All plants grow from actively dividing cells called meristem found at the tips of buds and roots, but in most woody plants there is also a layer of cells called the cambium, which wrap around every stem and limb of the plant like a glove. The cambium allows perennial woody trees and shrubs to grow outwards as well as upwards in each growing season, allowing them to reach considerable sizes.

Tree hug

© Forestry Commission

If you were to look inside a living tree trunk you would see several different layers of cells. The first outer layer is the bark; a tough, waterproof layer which protects the tree from the elements, from insects, pests and fungal diseases and helps it to retain its moisture. As the trunk grows fatter the bark spreads and cracks, often giving it a gnarled appearance.

Tree core Underneath the bark there is a layer of tissue called the phloem which transports sugary sap made during photosynthesis from the leaves to all the other parts of the tree.

The next layer is the cambium which, as mentioned, is responsible for the outward growth of the tree. Next you will find a number of layers of woody tissue called xylem. The first few layers of xylem are known as sapwood and are responsible for transporting water from the roots to the rest of the tree. The innermost layers of xylem are found right in the centre of the tree and are known as heartwood. The heartwood is dense and strong and is thought to provide stability as the tree grows.

 

One growth cycle

In places like the UK, with a seasonal climate, most growing activity occurs in spring and summer. In every growing season the cells of the cambium divide to produce layers of new cells:

  • On the outer side of the cambium new cells are added to the phloem; the part of the tree responsible for transporting sugars, produced by the leaves to other parts of the tree.
  • On the inner side of the cambium new woody cells are added to the xylem. The xylem cells made in spring are wide with thin walls. These wide cells help the tree transport large volumes of water from the roots to the trunk and branches to support the growth of new leaves and flowers.

Toward the end of the growing season the cambium produces xylem cells that are narrow, with thicker walls. The tree needs less water at this time of the year because growth activity is slowing; instead the tree needs to produce strong, dense wood to help support its new growth so the cells continue to narrow and thicken until the autumn. All the xylem cells produced by the cambium during the year contribute to the outward growth of the tree.

As summer ends, trees stop growing and divert their resources to produce new buds in preparation for next year’s leaves. During late autumn a reduction in daylight hours and temperature signals the tree to enter a state of dormancy. Evidence of this annual growth cycle can be clearly seen in growth rings on a cut tree trunk.

Oak in summer
Oak in summer © Debbie Cotton

Oak in winter
Oak in winter © Debbie Cotton

If you would like to know how to age a tree using growth rings see our page on Tree ageing.