Ageing

In places like the UK with a marked difference or seasonal climate, trees do not grow all year. The seasonal growth pattern is shown by growth rings in the wood. These are visible if the trunk is cut horizontally. These rings occur because trees lay down a larger proportion of wide, thin-walled cells in spring to help water transport up the stem; later in the year, narrow, thick-walled cells are laid down which help strengthen the growing trunk. And most are dormant in winter.

In climates with a marked summer and winter, there is an annual cycle of tree growth. The first new activity is below ground. In spring, as the soil warms, new tiny, fine root hairs grow and begin absorbing water and dissolved nutrients to pass up the rest of the buds, shoots and leaves. In the leaves, the green pigment chlorophyll captures solar energy and uses it to make carbohydrates from water and carbon dioxide through photosynthesis.

These carbohydrates from the leaves are transported to other parts of the tree and are the building blocks for making more complex products like starch or cellulose.

Every growing season, the cambial cells divide to produce multiple layers of new cells - to the inside these form the new wood cells or xylem - to the outside they make the thinner phloem or bast.

The first new xylem cells or spring wood each year are large with thin walls. By mid-summer the cells added become smaller and often darker - the summer wood - and by autumn these can be tiny, dark and thick-walled. The system then shuts down for the winter and starts up again with a new layer of spring wood the following year.

So if you cut a tree trunk, a clear annual ring is evident.

This happens each year, so by counting the annual rings you can find out how old a tree is or was.

But ageing by ring counts often means felling the tree. Foresters do use increment borers to take a core sample about the width of a pencil from the outside to the centre of a living tree - that core or plug is then carefully extracted and the annual rings counted. The hole is then plugged again.

You can also get a pretty good idea of a tree's age from the girth of its trunk - how fat it is. Trees will grow at different rates depending on how good soil, shelter or climate are and if they enjoy space to grow freely or are hemmed in by neighbours. Trees grow faster when young and slow down in later life. Some species of tree are inherently fast growing.

But as a good guide, broadleaved trees like oak, ash, beech and sycamore in UK woodlands put on about 1.5 to 2cm in circumference or girth per year. In open conditions, like parkland, the growth rate will be nearer 2.5cm annually. So by measuring the trunk circumference and dividing by 1.5, 2 or 2.5, you can make an informed guesstimate of age.

This is Mitchell's Rule, named after the late world famous UK tree expert, Alan Mitchell.

If there are tree stumps or felled trunks nearby, count the annual growth rings and measure the trunk circumference to find local growth rates.

With conifers, these commonly show a regular pattern of growth, producing a whorl of branches each year. So if you follow the main trunk up, you can see a whorl of branches all radiating out from the trunk at the same level, followed by a clear section, then another whorl and so on. Counting the number of whorls up the trunk, gives an idea of the tree's age and also how fast it has grown. This is easier in younger trees.