The age of trees.

I have always found it fascinating to think about how old a tree might be – when you are standing in front of a majestic Oak it is impossible not to try and guess its age and then to think what it may have ‘seen’ during its lifetime.

I remember as a smallish boy visiting the Natural History Museum in South Kensington and standing in awe in front of a giant section through a tree which had on its surface a catalogue of events that had occurred since the tree germinated.

That tree was a Giant Sequoia  and it was felled in 1891 at the age of 1300 years. Evidently the section of tree has recently been restored and cleaned – perhaps I should return and have another look. To read more about it click Tree section at Natural History museum.

Incidentally I used to visit the museum on my own at the age of about 11, catching the underground from Green Street  in West Ham on the District and Circle Line and then getting off at South Ken., walking along the tunnel to the Museum. I spent many happy hours there during the two years we lived in the East End of London.  My other solo trips were on my bike to Wansted Park, and Epping Forest. I also sometimes went down to the Woolwich Ferry and took a ride across the Thames for only a penny. However the bloke in charge used to get cross with us kids because we then came back on the very next crossing. Would responsible parents allow their 11/12 year old child the freedom to roam London these days?

I have my own collection of tree sections, not quite as grand as the Natural History museum specimen, but then our house would probably not be quite big enough for that one. I have about 15 sections and they are from different trees and shrubs. They are mostly representatives of the species growing in our wood. So I have Oak, Beech, Silver Birch, Rowan, Corsican Pine, Scots Pine, Holly and several more. I rub them down with several grades of sandpaper until they are nice and smooth  and then apply polish and buff them up. It brings out the grain and you can see the growth rings quite clearly on some of them. The Holly is quite distinctive – it is a cream colour  with virtually no grain or discernible rings. It also comes up as the most polished wood. It could easily be mistaken for Ivory.

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So is it possible to tell the age of a tree without cutting it down?  Yes  but before we get on to that I should mention that with some trees even if you cut them down  you would still not be able to determine their age.  For example Yew trees can live to be thousands of years old but with those advanced years the centre of the trunk starts to break down and lose its identity and the growth rings are no longer discernible.

Some trees can be artificially induced to achieve a great age. I am thinking of Ash trees which have been coppiced for hundreds and hundred of years. This process causes the tree to regenerate new mini trunks called poles and as time goes by and with each cutting of the poles the circumference of the trunk expands,  the central core starts to rot and eventually a ring of living ‘units’ all derived from the original tree are produced and as the centuries pass by this circle of ‘units’ grows/moves ever outwards such that it becomes quite difficult to tell that they were all originally from one tree.  In this way an Ash could live almost  indefinitely. I have seen examples of this in some East Anglian Coppice with standard woodlands where you can find five or six coppice stools in a circle all derived originally from one tree. There is no means of accurately estimating the age but obviously the larger the diameter the older it is.

Now on to how to age a tree.

The accurate method is by dendrochronology and that means counting the rings.  You do not have to cut a tree down to count the rings. A small core can be removed and the rings can then be seen and counted. You need a special drill to do this which does not do any major harm to the tree. The diameter of the core is quite small, about 3.5mm,  and the hole is sealed afterwards to stop fungi etc getting access. There are companies who will undertake this on your behalf but obviously it costs quite a lot and you would probably only consider it worthwhile if you had some really old looking tree growing on your property. The other use of this technique is to age timber in houses or large objects like timbers in sunken boats and from archaeological sites.

The cheap, in fact free, method which is less accurate is to measure the girth and thus the diameter of a tree. (Conversion of girth measured in centimetres to diameter is achieved by dividing girth by π.) This is normally done at chest height and then there are lists of tables which give you a factor to multiply the diameter (in centimetres) by to get an estimate of the age.  Obviously the factor will be different for different species of tree, so you will have to be able to identify your tree. Also the factor should vary for where the tree is growing. Trees in the open will grow quicker than those in a woodland which are surrounded by other trees. Poor soil or higher altitude will also affect how big they grow. Furthermore as a tree gets past maturity and into older age its growth will slow down. However the information is out there and here are links to a couple of sites. The Forestry Commission has  very detailed and perhaps the most accurate information about tree ageing.  Another less complicated system but probably a less accurate one is provided by the National Parks. I am not sure how accurate they are  but they will give you a reasonable idea of the age. The following is their information.

Trees can live to be very old.
The table below shows the life-span of some of our most common trees:
Name of tree            Life Span
Alder                          120
Ash                             200
Beech                         350
Birch                          100
Cherrry                      50
Hawthorn                 300
Hazel                          70
Holly                          300
Hornbeam                300
Maple                        120
Oak                            800
Rowan                      120
Scots Pine                500
Willow                      400
Yew                           5000

The height of the tree does not always help us guess how old it is. To find out the age of a tree:
1. Measure the girth of the tree at shoulder height in centimetres (cm)
2. Divide the girth by the correct number in the table below.
3. Round your answer up or down to the nearest whole number. This gives you the approximate
age of your tree!
Name of tree Divide girth (cm) by this number:
Oak 2
Hazel, Elm, Ash, Beech 2.5
Holly, Yew 1.25
Pine, Spruce 3.25
Sycamore 2.75

Note that this is slightly different to the method that I referred to in the text above in that it uses girth and that is measured at shoulder not chest height.  All the systems vary and there are lots to choose from.

Finally, you might be wondering which are the oldest trees on earth. The oldest are the Bristlecone Pines found in California. One is called Methuselah and until 2012 it was thought to be the oldest plant on earth at 4,849 years. Then, another one growing close by was investigated using the core ring counting technique described earlier and it was found to be 5,067 years old. There is a species of Cypress called Patagonian Cypress  Fitzroya cupressoides and one of these has been found to be 3,646 years old. The famous Giant Sequoias  come in at just over  3,000 years. However many species have not been investigated using the dendrochronology technique, so the ages quoted are estimates. Yews are often estimated in the thousands of years as are Olives. This is a photo of an Olive tree growing outside a restaurant in El Rocio in southern Spain where I had a very good meal and the tree is reputed to be 1,000 years old.

Finally I will mention what are termed as Colonal trees of which the Quaking Aspen Populus tremuuloides  is the most famous with an estimated age for the colony of between 80,000 and 100,000 years old. Each tree does not achieve a great age but due to new trees developing from suckers the colony just goes on and on and estimates of its age are based on the size of the colony and the fact that all members are genetically identical so all members have arisen from one original plant that started to grow back i 78,000 BC  give or take a few 1000 years.

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